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Critical Reviews in Environmental Science andTechnology

ISSN: 1064-3389 (Print) 1547-6537 (Online) Journal homepage: http://www.tandfonline.com/loi/best20

Development of metal-matrix composites fromindustrial/agricultural waste materials and theirderivatives

A. Bahrami, N. Soltani, M.I. Pech-Canul & C. A. Gutiérrez

To cite this article: A. Bahrami, N. Soltani, M.I. Pech-Canul & C. A. Gutiérrez (2015):Development of metal-matrix composites from industrial/agricultural waste materialsand their derivatives, Critical Reviews in Environmental Science and Technology, DOI:10.1080/10643389.2015.1077067

To link to this article: http://dx.doi.org/10.1080/10643389.2015.1077067

Accepted online: 04 Aug 2015.Publishedonline: 04 Aug 2015.

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Development of metal-matrix composites from industrial/agricultural waste materials and their derivatives

A. Bahrami, N. Soltani, M.I. Pech-Canul and C. A. Guti�errez

Centro de Investigaci�on y de Estudios Avanzados del IPN Unidad Saltillo, Av. Industria Metal�urgica,Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila, M�exico

ABSTRACTIn this contribution, authors present a review on the state-of-the-art in the utilization of industrial and agricultural wastematerials for the development of metal�matrix composites(MMCs), providing, through the judicious analysis of an ampleand varied references source � from the oldest to the newestones � an insight into the challenges and opportunities forthe exploitation to their full potential. In addition to itstopicality, the novelty of this contribution lies in thepresentation of key statistical, technical, and property-relatedinformation of a comprehensive variety of waste materialsclassed into two main groups, namely, fly ash reinforcedMMCs and MMCs derived from other waste materials.Although fly ash has been exploited in a broad range ofapplications, the attention paid for its use in thedevelopment of MMCs seems to be insufficient. A purposelydesigned chart helped to pinpoint the more demanding andprofitable applications of fly ash, and establish strategicopportunity areas. With the exception of the recent utilizationof fly ash for the automotive industry, virtually no other wastematerial has been reused for a specific industrial application.In this context, by identifying five reasons for this observeddelay, an essential goal of this review is to arouse the interestof academicians, scientists/technologists, and industrialists inthe use of those materials for the fabrication of MMCs. In thecase of agricultural materials, a twofold perspective mayapply, because while on the one hand, certain chemicalelements have to be removed for specific applications, on theother hand, recovery of certain elements might be moreattractive. Based on the significant progress observed so far,in terms of scientific and technological research, a promisingfuture can be anticipated. The proper use of industrial andagricultural waste materials entails knowledge generation asa prerequisite for incubation of pilot-plant andindustrialization stages, culminating with all related benefitsto society.

KEYWORDSagricultural waste materials;industrial waste materials;metal�matrix composites

CONTACT M.I. Pech-Canul martin.pech@cinvestav.edu.mx Centro de Investigaci�on y de EstudiosAvanzados del IPN Unidad Saltillo, Av. Industria Metal�urgica, No. 1062, Parque Industrial Saltillo-Ramos Arizpe,Ramos Arizpe, Coahuila 25900, M�exico.Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/best.© 2016 Taylor & Francis Group, LLC

CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY2016, VOL. 0, NO. 0, 1�66http://dx.doi.org/10.1080/10643389.2015.1077067

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1. Introduction

The ever growing amount of agricultural and industrial waste materialsincreases the environmental pollution concerns, with the ensuing threat ofintoxication to humans and animals, as well as the contamination of plantsthemselves. Moreover, regardless the region, the consumption level is alreadyhaving a significant impact on the environment across the globe. Solid wastesare the unwanted or useless solid materials generated from combined residential,industrial, and commercial activities. They can be classed according to its origin(domestic, industrial, commercial, construction, or institutional); to its contents(organic material, glass, metal, plastic, paper, etc.); or to its potential hazard(toxic, non-toxic, flammable, radioactive, infectious, etc.). Therefore, the tran-scendental decisions must be underpinned with a careful systematic and meth-odological analysis of the potential use and applications of wastes directly orwaste by-products. Recycling refers to the removal of items from the wastestream to be used as raw materials in the manufacture of new products. Thus,recycling can be conducted in two forms: first, the recyclables may be useddirectly and second, waste materials may be used as raw material for other newproducts. Recycling reduces the need for raw materials such as metals, forests,and oil and so reduces human’s impact on the environment.

There are several environmental benefits that can be derived from wastereduction and waste reuse/recycling methods. They reduce or prevent green-house gas emissions, reduce the release of pollutants, conserve resources, saveenergy, and reduce the demand for waste treatment technology and landfillspace. Therefore, it is advisable that these methods be adopted and incorporatedas part of the waste management plan.

Extracting virgin materials is a key cause of global habitat loss. For example,demand for paper and cardboard is threatening ancient woodlands. Virginmaterials need to be refined and processed to create products, requiring vastamounts of energy and the use of polluting chemicals. For example, making 1ton of aluminum needs 4 tons of chemicals and 8 tons of bauxite (the mineralore), and it takes 95% less energy to make a recycled aluminum can than it doesto make one from virgin materials (Brubaker, 1967). As another timely example,synthesizing silicon carbide (SiC) as a common reinforcement in aluminummatrices through Acheson process requires very high temperatures to achieve acomplete reaction. Next, the fabricated SiC needs to be ground to yield the clas-sified granulated end-product, thereby expending large amounts of energy, inaddition to the environmentally contentious issue of CO generated by the pro-cess (Heimann, 2010).

Besides the promising applications explored in different fields, like in energygeneration, the outstanding physical and mechanical properties of some agricul-tural and industrial wastes � like fly ash (FA) and rice-hull ash � make them to

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be placed foreground for the potential production of advanced materialsamongst which, metal�matrix composites (MMCs) have become a benchmark.From their initial utilization, exploiting only their structural or mechanicalproperties, MMCs have evolved in different directions, including new concepts,and even, its definition. In the most recent literature, an MMC has been definedas a material that after an optimum processing, results in the consolidated com-bination or mixture of two or more engineering macroconstituents, microcon-stituents or nanoconstituents that differ in geometry/morphology (particles,fibers, flakes, platelets, etc.) and chemical composition. One constituent playsthe role of the metallic matrix and the other or others act as strengthening, ther-mal and/or functionality phases that can be “ex-situ” or “in-situ,” or both in ori-gin, and that can vary in size (with monomodal, multimodal distribution, or sizeratio); the entire assembly can be designed as a monolith or as a multilayer panelor as films/coatings with sandwich or functionally graded structure, where thelayer thickness can be equal or variable within the assembly. Synergistically, thecomposite leverages and maximizes the characteristics of each of the constitu-ents, resulting in a (functional, structural or multifunctional) material withproperties not achievable by the constituents individually (Pech-Canul andValdez, 2015; Pech-Canul and Aifantis, 2014; Mallik and Ekere, 2013).

With the exception of the recent utilization of FA for the automotive industry(ULTALITE®, ReNu®-ceramics, etc.), virtually no other waste material has beenreused for a specific industrial application. Herein, five main reasons of why lit-tle progress has been observed regarding the application of agro-industrial wastematerials in the past decades for the fabrication of metal MMCs are mentioned:

1. Several decades ago, there were no regulations that prevented the incinera-tion of agricultural waste materials (for instance, rice husk (RH)) and thelandfill of industrial wastes (for instance, waste glasses and metallurgicalslags).

2. Unlike polymer�matrix composites and concrete, fabrication of MMCsrequires the use of high temperature. This fact causes some challengesthat should be addressed in order to accelerate utilization of agro-indus-trial waste materials in MMCs:� Finding an adequate calcination temperature to remove organic com-ponents and keep all the necessary chemical elements or constituentsfor strengthening the matrix.

� Finding one or more reactants in chemical treatment to remove theunwanted chemical elements.

� Choosing an applicable fabrication method for production of MMCs(process temperature, time, pressure, matrix chemical composition,reinforcement percentage, etc.) to keep the desired properties from thewaste materials.

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Several decades ago there was no motivation to address these issues because withthe conventional materials (alloys, wood, and plastics) most of the industrialrequirements were satisfied.

3. For a long time, there has been a debate in that calcination of agriculturalwaste materials is a threat for environment. However, as it has beenreported (Hall and Scrase, 1998), that calcination of agricultural wastematerials does not necessarily impose an extra amount of CO2 to theatmosphere. This issue is discussed in detail in the revised version of themanuscript, Section 4.2.1).

4. Lack of information on the attributes of agro-industrial waste materialsthat leads to a lack of confidence by customers. Although, in most cases,composites fabricated from waste materials show equal or even better per-formance than conventional composite materials, customers are oftenconcerned about their quality and efficiency.

5. The above-mentioned challenges requires an incubation time for basic sci-entific research and technology development, followed by scaling to pilot-plant and the consequent industrial level.

In spite of the numerous research articles on FA for different applications, upto now, no review paper with an ample and varied references source has beenpublished analyzing and discussing its use in MMCs. Accordingly, the aim ofthis work is to present a review on the state-of-the-art in the utilization of indus-trial and agricultural waste materials for the development of MMCs, discussingfrom advantages and disadvantages of synthesis methods, the physical andmechanical properties of waste materials and obtained composites, up to theirwear and corrosion behavior. This contribution is divided into two main parts,namely, FA reinforced MMCs and MMCs derived from other waste materials,including a review including important statistical information not publishedpreviously in papers on the subject, from the oldest to the most recent publica-tions. The reason for including FA in the first section is because of its commer-cial relevance nowadays. In the second part, based on the use of other wastematerials (agricultural and industrial), the composites are categorized into threegroups. (1) Composites having waste materials as matrices, (2) Composites inwhich the reinforcements are waste materials, and (3) Composites in whichboth, the matrix and the reinforcement are waste materials. This categorizationis depicted in Figure 1.

The review includes a number of natural and industrial materials, like glass,not addressed in previous works. Although, the work by Lancaster et al. (2013)discusses the utilization of industrial wastes in MMCs, the range of materialsincluded is narrower as compared to those included in the present work; fur-thermore, it attributes some uses to certain waste materials, like palm oil fuelash (POFA) and maize stalk ash, stating that it can be used both in the concreteor polymer industry and in MMCs, but no evidence is shown regarding the lat-ter. What is more, in the paper by Lancaster, a conclusion about MMCs from

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results observed in concrete with POFA is drawn. The synthesis methods,advantages and disadvantages, the physical and mechanical properties ofobtained composites, as well as their wear and corrosion behavior are discussedin detail in the current review. Because of its topicality component, the selectedtopics in each of the sections and the number of assorted references on the sub-ject, this paper is aimed at meeting the needs of a wide variety of readers, includ-ing metallurgists, chemical engineers, researchers on composites, environmentalscientists, and other materials related scientists and technologists.

2. Fly ash

Coal is used as a major source of energy throughout the world. In order to pro-duce energy, pulverized coal is generally burned. During the combustion pro-cess, the volatile matter and carbon burn off, and the coal impurities, such asclays, shale, quartz, and feldspar, mostly fuse and remain in suspension (Mehta,1989). These fused particles are carried along with the flue gas. As the flue gasapproaches the low temperature zones, the fused substances solidify to form pre-dominantly spherical particles which are called FA. The remaining matterswhich agglomerate and settle down at the bottom of the furnace are called bot-tom ash. There are many good reasons to consider coal ash as a resource, ratherthan a waste. Recycling it conserves natural resources and saves energy. In manycases, products made with coal ash perform better than products made without

Figure 1. Classification of metal matrix composite materials fabricated from waste materials.

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it. Figure 2 shows significant growth in the production and consumption of coalFA since 1966 up to 2012 (American Coal Ash Association, 2013) in the UnitedStates of America. Before the year 2008, a remarkable increase in the productionand use of FA was observed for a period of about 15 years. Due to a general eco-nomic stagnation, a decrease in coal use and to regulatory uncertainties, after2008 a notable decrease was observed in the production and use during the fiveconsecutive years. However, a similar behavior was observed in the case of per-cent use but only for a couple of years, after which a recovery was noted, makingit promising for several applications.

Mainly composed of silicate, aluminate, and calcium compounds, FA hasmechanical and chemical properties that make it a valuable ingredient in a widerange of concrete products. Roads, bridges, buildings, concrete blocks, and otherconcrete products commonly contain FA. Concrete made with coal FA is stron-ger and more durable than concrete made with cement alone. By reducing theamount of manufactured cement needed to produce concrete, FA accounts forabout 10 million tons of greenhouse gas emissions reductions each year. Othermajor uses for FA include constructing structural fills and embankments, wastestabilization and solidification, mine reclamation, and use as raw feed in cementmanufacturing. Figure 3 is a chart reflecting the percentage distribution of theuse of FA in the US in 2012, amongst 11 different and well defined industriesand applications. From the major to the minor consumers, like concrete indus-try and oil field services, all of them are represented in the development of eco-nomic activities which undoubtedly impact and benefit to society. However, asit can also be observed in the graph, inclusion of the application of FA in metal-lurgy industry and related areas has been neglected. Most likely certain activityis carried out in that field, but in a non-significant fashion, perhaps included in

Figure 2. Production, use, and use percentage of coal fly ash in the United States from 1966 to2013.

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the twelfth category, miscellaneous/others. But the bottom-line is that in metal-lurgy and ceramics engineering fields, it offers a new window of opportunity, asin the development of MMCs. Some significant work has been done in that field,to the point of commercializing specific FA aluminum�matrix composites withcertain attractive engineering properties.

In order to have an insight into the factors make this waste material as aresource, its physical, chemical, and mechanical properties should be discussed.The specific gravity of FA usually ranges from 2.1 to 3.0, while its specific areamay vary from 170 to 1000 m2/kg. FA can be captured by mechanical separators,electrostatic precipitators, or bag filters. During coal combustion, the organicmatter in coal is utilized to produce heat and as a result, the concentrations oftrace elements are increased relative to those in the source coal. Several trace ele-ments such as As, Se, Cd, Cr, Ni, Sb, Pb, Sn, Zn, and B are enriched in coal com-bustion by-products. These impurities have a negative impact on FA utilizationdue to environmental restrictions (Lokeshappa and Dikshit, 2011).

FA collected from power plants is a mixture of particles varying in shape, size,and composition. These particles can be classified as carbon from unburnt coal,thin-walled hollow spheres, and their fragments, magnetic iron containingspherical particles (Berry et al., 1989; Berry and Malhotra, 1980). FA particlesare generally spherical in shape and range in size from 0.5 to 100 mm (Hrairi

Figure 3. Percentage distribution of the use of fly ash in the US (year 2012), 1 � concrete/con-crete products/grout, 2 � blended cement/feed for clinker, 3 � flowable fill, 4 � structural fills/embankments, 5 � road base/sub-base, 6 � soil modification/stabilization, 7 � blasting grit/roofing granules, 8 � mining applications, 9 � waste stabilization/solidification, 10 � agricul-ture, 11 � oil field services, and 12 � miscellaneous/other.

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et al., 2009). In general FA can be classified into two categories, precipitator, andcenosphere. Precipitator fly ash is solid and has a density of about 2.1 g cm¡3.Cenosphere particles are separated from precipitator by using flotation methodsand are hollow spheres with an apparent density ranging from 0.4�0.6 g cm¡3.Cenospheres are classified based on their size and density to obtain appropriateproperties in resulting composites. However, the hollow particles of differentsizes may have the same density because of a difference in their wall thickness.Representative micrographs of cenosphere and participator FA are presented inFigure 4 (Matsunaga et al., 2002; Rohatgi et al., 2011).

The carbonaceous material in the FA is composed of angular particles. On theother hand, FA is generally gray in color (its color is depending on the amountof unburned carbon in the ash), abrasive, mostly alkaline, and refractory innature. Pozzolans, which are siliceous and aluminous materials that togetherwith water and calcium hydroxide form cementitious products at ambient

Figure 4. (a) Fly ash cenospheres on a substrate (imperfect structure and defects are observed insome particles) and (b) fly ash precipitator ((a and b) Matsunaga et al., 2002; (inset of part a)Rohatgi et al., 2011).(a and b) © Elsevier. Reproduced by permission of Elsevier. Permission to reuse must beobtained from the rightsholder; (inset of part a) © Springer. Reproduced by permission ofSpringer. Permission to reuse must be obtained from the rightsholder.

Table 1. Chemical compositions (wt.%) of fly ash (types C and F) and glass spheres (Rohatgi,1994).

Material SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO SO3 FeO/LOI�

Type C 33�61 8�26 4�10 14�37 1�7 0.3�2.0 0.4�6.4 0.9�2.8 0.5�7.3 0.2�1.4Type F 33�65 11�33 3�31 0.6�13 0�5 0.7�5.6 0�3.1 0.7�5.6 0�4 1�12Glass 72.5 0.4 � 918 3.3 0.1 13.7 � � �

�LOI � Loss on ignition.

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temperatures, are also admixtures (Ahmaruzzaman, 2010; Eary et al., 1990; Royet al., 1981; Tolle et al., 1982).

ASTM C-618 categorizes coal combustion FA into two classes: Class F andClass C. Typical chemical compositions of Type C and Type F FA are given inTable 1. The Class F FAs are normally generated due to combustion of anthra-cite or bituminous coal. The Class C FAs are produced due to burning of ligniteor sub butiminous coal. Most FAs are rich in SiO2, Al2O3, and Fe2O3, and con-tain significant amounts of CaO, MgO, MnO, TiO2, Na2O, K2O, SO3, etc.ASTM Class C FAs (high-lime FAs) typically contain CaO in excess of 10% upto 40%, and Class F FAs (low-lime FAs) generally contain less than 10% CaO.Therefore, Class C FAs are classified as cementitious and pozzolanic admix-tures/additives and Class F FAs as normal pozzolans for use in concrete. How-ever, after some required beneficiation (FA burning, triboelectric separation,etc.), both types can also be suitable for the synthesis of MMCs (Bittner et al.,1997; Hwang et al., 1994; Naik and Singh, 1998; Rohatgi, 1993, 1994).

FA is also sometimes classified according to the type of coal from which theash was derived. Based on heating value, chemical composition, ash content,and geological origin there are basically four types or ranks of coal. The fourtypes of coal are anthracite, bituminous, sub-bituminous, and lignite. The prin-cipal components of bituminous coal FA are silica, alumina, iron oxide, and cal-cium with varying amounts of carbon, as measured by the loss on ignition(LOI). Lignite and sub-bituminous coal FA is characterized by higher concentra-tions of calcium and magnesium oxide, as well as lower carbon content, com-pared with bituminous coal FA. Very little anthracite coal is burned in utilityboilers, so there are only small amounts of anthracite coal FA. Table 2 comparesthe normal range of the chemical constituents of bituminous coal FA with thoseof lignite coal FA and sub-bituminous coal FA. From the table, it is evident thatlignite and sub-bituminous coal FA has a higher calcium oxide content andlower loss of ignition than FA from bituminous coals. Lignite and sub-bitumi-nous coal FA may have a higher concentration of sulfate compounds than bitu-minous coal FA (Ahmaruzzaman, 2010).

Table 2. Normal range of chemical composition for fly ash produced from different coal types(Ahmaruzzaman, 2010).

Components (wt.%) Bituminous Sub-bituminous Lignite

SiO2 20�60 40�60 15�45Al2O3 5�35 20�30 10�25Fe2O3 10�40 4�10 4�15CaO 1�12 5�30 15�40MgO 0�5 1�6 3�10SO3 0�4 0�2 0�10Na2O 0�4 0�2 0�6K2O 0�3 0�4 0�4LOI 0�15 0�3 0�5

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It should be considered that unlike synthetically prepared reinforcements thathave closely controlled compositions and sizes, FA, being a waste byproduct,varies a great deal in its chemical composition (are influenced to a great extentby the properties of the coal being burned and the techniques used for handlingand storage), particle size, impurity content, and degree of porosity of individualparticles. The FA from different sources will have to be suitably beneficiated(Hwang et al., 1994; Rohatgi, 1994) and reproducible and obtainable propertyranges with different sources and types of FA will have to be established. Thepercentage of unburned carbon present in FA (measured by LOI) is one of thoseconcerns that should be controlled for any further utilization of it in MMCs orconcerts. Factors that determine the variability of LOI include boiler load,burner design, the efficiency of pulverizers, and operating characteristics of clas-sifiers. A plasma-assisted combustion enhancement can achieve better ignitionand a more stable flame for less combustible coals. Since most of the unburntcarbon concentrates on large particles, it is possible to separate them by a simpleclassification operation. Froth flotation has been used to recover disposed FAinto a useful FA product and an unburnt carbon-based solid fuel. Triboelectro-static separators are chemical free, reliable, and also compact. Thermal processescan recover the thermal energy from unburnt carbon through combustion,fusion, or steam gasification. Carbon surface modification is often consideredfor FAs with relatively low unburnt carbon content, because the other carbonreduction methods are not economically viable for these ashes (Dong, 2010).The need to produce composites with properties within narrow ranges with avariable resource like FA is a major challenge. A comparison of properties oftypical FA with other commonly used particulates in composites is shown inTable 3, along with the properties of a typical metal�matrix like aluminum

Table 3. Physical and mechanical properties of fly ash and other materials (LaBotz and Mason,1963; Razaghian et al., 2012; Rohatgi, 1994; Shackelford and Alexander, 2010; Soltani et al.,2014a).

Material

Truedensity(g/cm3)

Meltingpoint (�C)

Poisson’sratio

Modulus(GPa)

Thermalconductivity[W/(m¢k)]

Electricalresistance(V¢cm)

Fly ash Precipitator 1.6�2.7 >1300 — 143�310 0.06�0.16 109�12

Cenosphere 0.4�0.6 — — — 0.07�0.35 —Al2O3 3.9 2250 0.25 380 100��5�30��� —SiC 3.2 2730 0.3 420 100��4�20��� 1000SiO2 2.6 1580 — 94 — 1£1016

Mg2Si 1.99 1102 — 120 23.4 —TiB2 4.52 3230 0.28 366.9 60�120 14£106���

Mullite 3.2 1850 0.25 140 <5.9 —GlassSphere

Hollow 0.15�0.4 589� — — — 6£1012

Solid 2.45�2.5 — — 68 1.5 —Sodalime

Glass2.5 460���� 0.23 60�70 — 2.5£106

(at 250�C)Aluminum 2.7 660 0.33 69�79 237 3.15£10¡6

�Softening point, ��Single crystal, ���Polycrystal, ����Maximum service temperature.

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(LaBotz and Mason, 1963; Razaghian et al., 2012; Rohatgi, 1994; Shackelford andAlexander, 2010; Soltani et al., 2014a).

The ash can be also produced from biomass. The ash that was derived frombiomass as fuel does not contain toxic metals like in the case of coal ash. Thechemical composition of this type of ash is diverse depending on the type of bio-mass, type of soil, and harvesting (Thy et al., 2006). In general, the major ashforming inorganic elements in biomass fuels are Ca, K, Na, Si, and P and someof these act as important nutrients for the biomass (Masi�a et al., 2007). However,some biomass fuels have high silicon content (e.g., RH) while some have highalkali metal content (wood). While the elemental composition of the ash isdetermined by the inorganic constituents in the parent biomass, the crystallinityand mineralogy depends on the combustion technique used. Typically, FA fromneat biomass combustion has more alkali (Na and K) and less alumina (Al2O3)than coal FA (Llorente and Garc�ıa, 2006; Thy et al., 2006). As a class, biomassfuels show more variation in both composition and amount of inorganic materi-als than that typically observed in coal. Therefore, biomass FA varies more thancoal FA, which depends on the varieties of origin from woody to herbaceousand other resources (Bridgeman et al., 2007; Masi�a et al., 2007; Wiselogel et al.,1996). Many types of biomass FA have similar pozzolanic properties as coal FA,such as those from RH, wood, wheat straw, and sugarcane straw (Martirenaet al., 2006; Naik and Kraus, 2003; Yu et al., 1999) among which have beenadded in concrete as mineral admixture, improving the performance of concrete.

3. Fabrication of aluminum-fly ash composites

FA was introduced to MMCs industry almost about 20 years ago (Pond, 1989;Rohatgi et al., 1995a, 1993), and during these two decades, numerous investiga-tions have been conducted on these newMMCs. Nowadays, FA has found a num-ber of industrial applications such as covers, shrouds, casings, manifolds, valvecovers, garden furniture, and engine blocks in the automotive, small engine, andelectromechanical industry sectors (Rohatgi et al., 1995a, 1996). Even more, somecompanies own the patents or have commercialized products such as ULTA-LITE®, which use Al/FA composites to fabricate brake drums, brake discs, engineblocks, cylinder heads, pistons, con rods, oil pumps, transmission components,and many other automotive parts. Also NuForm® Company is providing ReNu-ceramics materials which are recycled from an energy by-product making theman environmentally friendly solution to many materials needs. The addition ofReNu® ceramics to metallic matrices can provide cost and weight savings whileimproving hardness, strength, and wear properties in automotive and recrea-tional vehicles, aerospace, consumer goods, and self-lubricating bearings.

There are different techniques to produce MMCs. Some factors such as quan-tity and distribution of the reinforcement components (particles and fibers),matrix alloy, and application of MMC can determine the fabrication process. By

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altering the manufacturing method, the processing and the finishing, as well asby the form of the reinforcement components it is possible to obtain differentcharacteristic profiles, although the same composition and amounts of the com-ponents are involved (Nodooshan et al., 2014; Soltani et al., 2013a).

Powder metallurgy, stir casting, infiltration techniques, squeeze casting, capil-lary driven or pressureless infiltration, ultrasonic cavitation based solidification,compo-casting, and friction stir processes are some commonly employed meansby which Al/FA composites are fabricated (Chew et al., 2011; Huang et al., 2011;Jailani et al., 2009; Jiang et al., 2012; Luong et al., 2011b; Marin et al., 2012;Ramesh et al., 1991; Rohatgi et al., 2009, 1994; Uju and Oguocha, 2012; Zahiand Daud, 2011).

3.1. Powder metallurgy

Powder metallurgy (P/M) is a special manufacturing approach to metal compo-nent fabrication that draws its attractiveness from several distinct properties.Notably, the ability to fabricate complex parts to close tolerances is a key eco-nomical benefit. However, this low-cost, high-volume manufacturing methodimposes very special quality assurance requirements. The P/M process can bebroken down into three main manufacturing steps: the mixing of the powder,the compaction to produce the green-state, and the sintering. It is the compac-tion process that offers the highest pay-off for quality control through non-destructive evaluation (NDE) techniques. Detection of compaction-related prob-lems in the green-state would permit early process intervention and thus preventthe creation of potentially significant numbers of faulty parts before sintering.The problem of forming defects in green parts during compaction and ejectionhas become more prevalent as parts producers have started to use higher com-paction pressures in an effort to achieve high-density and high-performanceP/M parts. It has been shown that the physical properties of P/M parts, espe-cially the fatigue strength, are always improved by increasing the density (Hrairiet al., 2009; O’Brien, 1988).

Using powder metallurgy, it is possible to obtain a homogeneous distributionof the reinforcement in the matrix (Bahrami et al., 2015b; Bhanuprasad et al.,1991; Soltani et al., 2013b, 2014b). However, the successful production of com-ponents by powder metallurgy processing depends on the characteristics of boththe metal and reinforcement particles, and their compacting behavior. Guo et al.(1997) have used metallurgy powder technique in order to prepare of alumi-num�FA particulate composite. They have reported that during sintering pro-cess some cenosphere particles are broken into pieces during the compactionconditions. This adversely affects their capability of reducing the density of thecomposite by virtue of their hollow shape and low density. Because the ceno-sphere particles break during compaction and are not able to maintain their hol-low shape, they carried out a detailed sintering study on aluminum�precipitator

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FA compacts, because precipitator particles are mostly solid and do not breakduring compaction. It has been found that green and sintered density of thecompacts decrease with increasing weight per cent of FA. Also, the strength ofthe sintered compacts decreased with increasing weight per cent of FA underthe experimental conditions; however, the hardness was found to increaseslightly up to 10 wt.% FA, beyond which it decreased.

In the work by Marin et al. (2012), two different ASTM C 618 Class C FAswere used for the production of aluminum MMCs using powder metallurgytechnology. FAs were milled in order to reduce the mean particle diameter andAl�FA composites containing 10% and 20% of FA were then prepared, com-pacted, and sintered. The electrochemical behavior was investigated by open cir-cuit potential (OCP) measurements and potentiodynamic polarization, whilethe corrosion mechanisms were complemented with studied by SEM observa-tions after different times of immersion in a mild corrosive medium. In all theconsidered cases it can be stated that although FA addition into the Al matrixdetermines a remarkable increase of mechanical properties and hardness withrespect to pure sintered aluminum, it also negatively affects the corrosion behav-ior of the composite samples with respect to the pure aluminum ones. The deg-radation phenomena occurring on the FA containing samples might be relatedto the following mechanisms: (I) partial detachment or dissolution of the FA sol-uble phases, in particular based on Si, Fe, and Ca; (II) dissolution of the Almatrix surrounding the FA particles due to crevice corrosion; and (III) Al local-ized dissolution due to galvanic coupling between the Fe-rich intermetallics andthe matrix.

Kumar et al. (2010) have studied the effect of FA particulate on high tempera-ture dry sliding wear resistance of AA6061, developed by powder metallurgy andhot extrusion. The dry sliding wear behavior of the prepared composites wasinvestigated at an applied load of 14.6 N for various temperatures (100�C,200�C, and 300�C). The study at room temperature was also carried out forcomparison purpose. The results showed a transition from mild-to-severe wearfor the unreinforced alloy in the temperature range of 200�300�C. With theaddition of FA to AA6061, the mild-to-severe wear transition was not noticedeven up to 300�C. This behavior was attributable to the formation of protectivetransfer layers of comminuted reinforcing particulates and transferred steeldebris from slider counterpaces. The absence of severe wear phenomena in thiscomposite was due to the “particulate hardening” of subsurface layer in the tem-perature range investigated.

Effects of Pb and sintering temperature on the mechanical properties andmicrostructure of aluminum�FA composites were investigated by Reddy et al.(2013). They added different concentrations of Pb (0�20 wt.%) to the Alumi-num/10 wt% FA powder mixtures and after compacting the samples were sin-tered at the temperatures of 500�C, 530�C, 560�C, and 590�C in argon gasatmosphere. The experimental results demonstrated that the density of sintered

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specimens, hardness, and compressive strength increase with the increase incompaction pressure. In addition, by increasing in Pb content the densityincreases whereas the hardness and compressive strength decreases. Changes inphysical and mechanical properties of fabricated composites in the temperaturerange of 500�560�C were negligible, however beyond 560�C they weresignificant.

3.2. Stir casting

Amongst the various processing techniques, stir casting appears to be the mostpromising route for the production of aluminum matrix composites because ofits simplicity and ability to manufacture composites on an industrial scale eco-nomically. In any production process, superior quality of the product can beachieved only when the process is run with the optimum parameters. Themechanical properties of composites are influenced by the size, shape, andweight fraction of the reinforcement materials, as well as potential reactions atthe matrix/reinforcement interface. Interfacial strength between the matrix andreinforcement plays a significant role in determining the properties of MMCs.These aspects have been discussed by many researchers (Bahrami et al., 2012;Shanmughasundaram et al., 2011).

An important feature of the aluminum alloy�FA composite casting is the distri-bution of FA particles in the casting. Using the stir-casting technique, FA particlestend to segregate along the aluminum grain boundary due to particle pushing. On amicroscopic scale, the FA particles are present in the interdendritic regions betweena-aluminum dendrites due to lack of nucleation of a-aluminum on FA particles anddue to pushing of FA particles by growing a-aluminum dendrites during solidifica-tion. In this respect, the FA particles behave in a manner similar to graphite, siliconcarbide, and alumina particles, which are also pushed into interdendritic regions ofa-aluminum during solidification of their respective composites.

In addition, the distribution of the FA particles is influenced by the tendencyof particles to float due to density differences, interactions with the solidifyingmetal (Rohatgi, 1994) and high surface tension which leads poor wettability(Shanmughasundaram et al., 2011). It is, therefore, a strong function of thesolidification rate and the geometry of the casting. Since the gas layers at the sur-faces of the particles can cause the buoyant migration, mechanical stirring canbe done in a semi-solid state rather than in the completely liquid state in orderto break away the gas layers thereby reducing surface tension. Wettability can beimproved by increasing the surface energies of the solids, decreasing the surfacetension of the liquid matrix alloy and decreasing the solid/liquid interfacialenergy at the reinforcement matrix interface. Magnesium which acts as a power-ful surfactant as well as a reactive element, in the aluminum alloy matrix seemsto fulfill all the above three requirements. An important role played by magne-sium during the composite fabrication is the scavenging of the oxygen from the

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dispersoid surface, thus thinning the gas layer and improving wetting actionwith the surface of the dispersoids. As concluded from previous investigationsthe strengthening of aluminum alloys with a dispersion of fine particulatesstrongly increases their potential in tribological and structural applications(Shanmughasundaram et al., 2011). Sarkar et al. (2010) studied Al/FA compo-sites produced by impeller mixing and concluded that up to 17 wt.% FA couldbe incorporated by liquid metallurgy route and that the addition of magnesiumincreases the wettability, which leads to improved mechanical properties such ashardness, tensile strength, and the wear resistance.

Using an alloy of Al�7Si�0.35 Mg, Rajan et al. (2007) studied the effect ofthree different stir-casting routes on the microstructure and properties of FA(13 mm average particle size) composites. Among liquid metal stir casting, com-pocasting (semi-solid processing), modified compocasting followed by squeezecasting routes evaluated, the latter resulted in a well-dispersed and relativelyagglomerate and porosity free FA particle dispersed composites. Interfacial reac-tions between the FA particles and the matrix leading to the formation ofMgAl2O4 spinel and iron intermetallics are more evident in liquid metal stir-cast composites than in compocast composites.

The strain rate dependence of compressive response is determined for alumi-num alloy/hollow FA cenosphere composites by (Luong et al., 2011a). A4032alloy was used as the matrix material. Quasi-static and high strain rate compres-sion tests are conducted on the matrix alloy and the composite. While the matrixalloy does not show any While the matrix alloy does not show any perceptiblestrain rate sensitivity, the composite shows higher strength at higher strain rates.The energy absorption capability of A4032/FA cenosphere composites is foundto be higher at higher strain rates.

Mahendra and Radhakrishna (2007) prepared composites with Al�4.5% Cuvarying FA contents in 5, 10, and 15 wt.%, using stir casting. They studied theeffect of FA on the fluidity of the composites. Density, hardness, impactstrength, dry sliding wear, slurry erosive wear, and corrosion performance wereevaluated. The results showed an increase in hardness, tensile strength, compres-sion strength, and impact strength with increasing the FA content. While thedensity decreases with increasing FA content, the resistance to dry wear andslurry erosive wear augment. Corrosion increases with increasing FA content.Ramachandra and Radhakrishna (2007) investigated the effect of FA on slidingwear, slurry erosive wear, and corrosive behavior of aluminum matrix compo-sites produced by stir-casting method. It was reported that Al (12 wt% Si) �15 wt% of FA particulate composite improved the abrasive wear resistance how-ever the corrosion resistance decreased as FA content increases.

Surappa (2008) synthesized A356 Al�FA particle composites and studied drysliding wear properties of A356Al�FA under different applied loads and rein-forcement contents. Compared to the unreinforced alloy, favorably, it was foundthat composites with 6 vol.% FA particles showed lower wear rates at low loads

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(10 and 20 N) and even better, 12 vol.% FA reinforced composites showed lowerwear rates in a higher load range (20�80 N).

3.3. Pressure infiltration

Pressure infiltration casting is a unique form of liquid infiltration which utilizespressurized inert gas to force liquid metal into a preform of reinforcement mate-rial. The methods and equipment used for pressure infiltration casting allow forinexpensive development of composite materials, prototypes, and net-shapecomponent production. With this technique it is possible to prepare MMCswith high volume fraction and uniform distribution of the reinforcement par-ticles in the matrix (Rohatgi et al., 1998). Other techniques such as stir-castingallow to fabricate MMCs having only up to 30% particles by volume and are at adisadvantage compared to the pressure infiltration technique. The pressure infil-tration technique is used for the fabrication of MMCs containing fibrous or par-ticulate reinforcement (Cook and Werner, 1991; Kouzeli et al., 2002; Rohatgiet al., 2006b). The use of pressure infiltration casting of an enclosed die chamberwith controlled pressurization makes it possible to cast in low strength moldswith high infiltration pressures. The development of a number of solidificationsystems has enabled parts to be infiltrated and directionally solidified, producinghigh quality composites. It has been reported that the pressure infiltrationmethod gave better castings than the other techniques developed earlier. Wis-consin Electric Power Company has patented a manufacturing method of castaluminum�FA particle composites (ash alloy) (Ramme and Tharaniyil, 2000).

In addition to the central porosity, the FA cenosphere walls are known tohave spherical pores in the diameter range of 1�13 mm (Rohatgi et al., 1993b).These porosities will affect the processing as well as properties of the resultantcomposite materials. In order to diminish and mask any porosity and cracks onthe surface of the FA particles, Rohatgi et al. (1998) infiltrated beds of nickelcoated and uncoated cenosphere FA by molten aluminum under very low pres-sures. To calculate the contact angel between uncoated and coated cenosphereparticles and molten metal, they determined the threshold pressure experimen-tally. Two different relationships, the Young�Laplace equation and the Wash-burn equation (Murr, 1975; Washburn, 1921) were used in order to calculatethe contact angle between molten pure aluminum and fly ash particles. Theyreported that lower threshold pressures are needed for infiltration of coated FAparticles than for uncoated particles, thus, the contact angle between particlesand molten metal is less for coated particles (»50�) that uncoated particles(»111�).These two degrees are lower than the values reported for practically allceramics under same test conditions (Laurent et al., 1987; Valentine, 1977). Inaddition, nickel coating helps to reduce the presence of uninfiltrated agglomer-ates of FA in composites and it reduces the infiltration of aluminum into thecavities within the cenospheres.

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Rohatgi et al. (2006b) synthesized A356-FA cenosphere composite using gaspressure infiltration technique over a wide range of cenosphere volume fractionfrom 20% to 65%. The processing variables included are melt temperature, gaspressure, and particle size of reinforcement. They found that the density of com-posites increased for the same cenosphere volume fraction with increasing melttemperature, applied pressure, and the size of particles. This comes across as areduction in voids present near particles and therefore, decline of capillary resis-tance by an enhancement of the melt flow in a bed of cenosphere. The compres-sive strength, plateau stress and modulus of the composites increased with thecomposite density. In comparison with A7075-T6 syntactic foam made by pres-sure infiltration of FA cenosphere (Balch and Dunand, 2002), A356/FA foamhas lower plateau stress. This could be related to the higher strength of thematrix alloy of A7075 (»350 MPa) compared to that of A356 alloy (»240 MPa).

Using a dilatometer, Rohatgi et al. (2006a) have measured the coefficients ofthermal expansion (CTEs) of commercially available pure aluminum and alumi-num alloy composites containing hollow FA particles (cenospheres) fabricatedthrough pressure infiltration. Three types of composites were made using thepressure infiltration technique at applied pressures and infiltration times of 35kPa for 3 min, 35 kPa for 7 min, and 62 kPa for 7 min. The CTE of the compo-sites is measured to be in the range of 13.1 £ 10¡6�11 £ 10¡6/�C, which islower than that of pure aluminum (25.3 £ 10¡6/�C). The infiltration processingconditions are found to influence the CTE of the composites. A higher appliedpressure and a longer infiltration time lead to a lower CTE. The theoretical valueof the CTE of FA cenospheres is estimated to be 6.1 £ 10¡6/�C.

Recently, for the first time, high Ca FA (Type C, from the most calcareous tothe most siliceous) preforms were pressure infiltrated with A356 aluminum alloyby (Itskos et al., 2012). Because of the presence of compounds like calcite, lime,and anhydrite, calcareous FA is more crystalline than siliceous FA (mostly con-tains quartz). Although, most of the works were done to utilize fine and groundFA particles into the molten alloys in order to achieve materials with outstand-ing mechanical properties, in their investigation it was tried to synthesize Al-based composites using highly calcareous and simultaneously very fine fly ashparticles. FA particle fraction of 25�40 mm have been ground to break glassphase and subsequently make the active silica of FA particles release in orderto increase the possibility of alloying with aluminum. It was reported thatgrinding of FA aids the successful manufacturing of composites and it alsoimproves their wear properties. Also, they have mentioned that the optimumsiliceous FA particle fraction to be used in A356 composites manufacturing �in terms of friction coefficient � is 25�40 mm, and the size fraction of 25 mmfor highly calcareous ash. Although, the use of fine FA particles can stronglyimprove the properties of composites, due to electrostatic forces, fine FA par-ticles tend to cluster, thus restraining the good dispersion of FA within thealloy matrix.

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3.4. Squeeze casting

Initial investigations employing a squeeze casting process (the application ofexternal pressure on the molten metal) for aluminum�FA MMCs have alsodemonstrated many advantages of this technique (Rohatgi et al., 1993a; 1995b)such as: (a) better compatibility between the metal�matrix and the FA particles,(b) a more improved structure of the matrix alloy, (c) filling of some hollow par-ticles of the FA with metal, and (d) pressure activation of the FA�metal inter-face. Bienia et al. (2003) have studied the microstructure and corrosion behaviorof aluminum�FA composites obtained by gravity and squeeze costing techni-ques. It was reported that in comparison with gravity casting, squeeze castingtechnology is beneficial for obtaining higher structural homogeneity with mini-mum possible porosity levels, good interfacial bonding, and quite a uniform dis-tribution of reinforcements. Whilst FA particles lead to an enhanced pittingcorrosion of the FA composite in comparison with unreinforced matrix, and thepresence of nobler second phase of FA particles, cast defects like pores, andhigher silicon content formed as a result of reaction between aluminum and sil-ica in AK12 (AlSi12CuNiMg) alloy and aluminum�FA composite determinethe pitting corrosion behavior and the properties of oxide film forming on thecorroding surface.

Wu et al. (2006) synthesized aluminum�FA composites by squeeze cast-ing method in order to investigate the damping properties of the compo-sites. Using squeeze casting, a vertical pressure is applied to force moltenaluminum into the FA preform completely. The pressure is maintained fora given time, typically 5 min, until the solidification is completed. Theyused the forced vibration mode and the bending�vibration mode on themultifunctional internal friction apparatus. The damping capacity is themeasure of a material’s ability to dissipate elastic strain energy duringmechanical vibration or wave propagation. The results indicated that thedamping capacity of the FA 6061 Al composites with smaller reinforcementdiameter is higher than those with larger reinforcement diameter in bothvibration modes. In a different investigation, the authors (Wu et al., 2007)developed a new method to predict the compressive strength of cenosphere-�aluminum syntactic foams, showing the relation between the relative wallthickness of the cenosphere and the compressive strength of such foams.Quasi-static compression tests indicated that the annealed cenosphere-�aluminum syntactic foams can deform plastically at relatively higherstresses (»45�75 MPa) and their energy absorbing capacity can reach»20�35 MJm¡3.

In the work by Dou et al. (2007), the cenosphere and precipitator FA particu-lates were used to produce two kinds of aluminum matrix composites with den-sities of 1.4�1.6 and 2.2�2.4 g cm¡3, respectively. The electromagneticshielding effectiveness (EMSE) properties of the composites were measured in

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the frequency range 30.0 kHz�1.5 GHz. The results indicated that the EMSEproperties of the two types of composites were nearly the same. The tensilestrength of the aluminum matrix decreased by addition of cenosphere and pre-cipitator FA particulate and the tensile strength of the composites were 110.2and 180.6 MPa, respectively. The fractographic analysis showed thatAl�cenosphere FA composite fractured in a brittle manner and Al�precipitatorFA composite in a micro-ductile manner.

3.5. Capillary driven or pressureless infiltration

The infiltration process can be categorized into three classes based on the driv-ing force: (I) external pressure assisted, (II) vacuum driven, and (III) capillaritydriven or pressureless. The main drawbacks of pressure-assisted infiltration areexpensive tooling and the difficulty involved in making dies for making com-plex-shaped components, although the process is quick. Pressureless infiltrationis attractive due to its cost effectiveness and near-net-shape capability. Althoughpressureless infiltration of Cu into Fe powder compacts was well established wayback in 1958 (Semlak and Rhines, 1958), in the case of Al alloys it was firstreported in 1989 by Aghajanian et al. (1989) of Lanxide Corporation.

Although presently the utilization of pressureless infiltration for the prepara-tion of FA composite is not widespread, previous reports by the same researchgroup presenting this review suggest promising prospects for this route. Alsoreferred to as spontaneous infiltration, this route offers attractive advantages,such as economic and simplicity, because it depends on capillary phenomena tofill up the interstices in a porous body � which could be made out of FA � withthe liquid metal. Figure 5 is a simplified schematic representation of the pres-sureless infiltration technique, showing the solid metallic alloy before meltingand then, the liquid metal infiltrating spontaneously.

Accordingly, the processing parameters should be optimized previously to thewetting of the solid by the specific metallic alloys, i.e., low contact angle andliquid�vapor interfacial energy. It has been reported that there are some barriersthat prevent this method to display its full potential as a commercial techniqueof composite manufacturing. Amongst these challenges is the lack of wetting ofthe ceramic perform by the molten metal, which in the case of SiC substrateresults from the formation of oxide layer on the surface of Al melt. Unwanted

Figure 5. Schematic representation of the pressureless infiltration method.

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reactions between ceramic particles and molten metal are another hindrance.Undesirable reactions at the Al/SiC interface alter the chemical composition ofthe molten aluminum and lead to the formation of unwanted phases at theinterface, such as Al4C3 and Al4SiC4 (Ortega-Celaya et al., 2007). Pech-Canulet al. (2000b) have reported some treatments in order to increase the wettabilityof ceramic particles and prevent unfavorable reactions at the ceramic/alloy inter-face. For example in Al/SiC composites with adding preferably more than 3%magnesium to the aluminum melt, attaining an internal pressure of 1.2 atm forthe chamber and changing the internal atmosphere to 100% nitrogen leads to asuccessful infiltration process. The thermodynamic analysis indicates that thereis a tendency for the reaction between the molten aluminum and the FA par-ticles. The particles contain alumina, silica, and iron oxide which during solidifi-cation process of Al FA composites or during holding the composites attemperatures above 850�C, are likely to undergo chemical reactions, reported byGuo and Rohatgi (1998). The experiments indicate that there is a progressivereduction between SiO2, Fe2O3, and mullite by Al and the formation of Al2O3,Fe, and Si. The wall of cenosphere FA particles progressively disintegrates intodiscrete particles, as the reaction proceeds. In other investigations, Escalera-Loz-ano et al. (2007) and Pech-Canul et al. (2007) used FA in aluminum compositesprimarily reinforced with SiC with a twofold purpose: prevent the attack of SiCby liquid aluminum and, induce the formation of stable phases that can ulti-mately act as reinforcements, as it is the case for MgAl2O4 and MgO.

Escalera-Lozano et al. (2007) have assessed the corrosion characteristics ofAl/SiCp/spinel composites fabricated with SiCp, FA, and recycled aluminum.They have reported that, for type A composites with matrix composition ofAl�8Si�15Mg (wt.%), the Mg2Si intermetallic precipitated during solidificationacted as a microanode coupled to the matrix (in the presence of condensedhumidity) and led to catastrophic localized corrosion. Although the potentialattack of SiC by liquid aluminum was successfully avoided by the presence ofSiO2 in the FA, Al4C3 was still formed due to the reaction of carbon in the FAwith aluminum. For type B composites, processed with the alloy Al�3Si�15Mg(wt.%) and calcinated FA, the silicon content was low enough to avoid forma-tion of Mg2Si. Moreover, chemical degradation by Al4C3 hydrolysis did not hap-pen either because of the absence of carbon and the presence of SiO2. Thisexplains the physical integrity of type B composites even after 11 months ofexposure to humid environment.

3.6. Ultrasonic cavitation based solidification

Compared with mechanical alloying, the melt processing techniques ofmetalmatrix nanocomposites (MMNCs) which involve the stirring of ceramicparticles into melts, offer some important advantages such as bettermatrix�particle bonding, easier control of matrix structure, simplicity, and low

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processing cost. However, it is extremely difficult for the mechanical stirringmethod and other conventional methods for the fabrication of MMCs to distrib-ute and disperse nano-scale particles uniformly in metal melts due to their largesurface-to-volume ratio and their low wettability in metal melts, which easilyinduce agglomeration and clustering. A number of ongoing research projectsare devoted to accomplish the uniform distribution and good wettability ofnano-sized particles into molten metals. Yang et al. (2004) proposed a new ultra-sonic cavitation technique for better and uniform distribution of nano-sized SiCparticles in the aluminum alloy melt. Ultrasonic dispersion is one of the manyeffectual ways used to reduce agglomeration amongst the particles. Ultrasoundis a sort of mechanical wave with frequency in the range 20�106 kHz, wavespeed of 500 m/s, and wavelength of 10�0.01 cm. The key physical action ofultrasonic dispersion is due to cavitation effect. When ultrasound acts upon themelt, large numbers of micro-bubbles are produced in the solution. The cavita-tion effect is produced during forming and bursting the micro-bubble to formstrong vibration waves, which weakens the reuniting action of particles to effec-tively increase the dispersibility of particles (Cao et al., 2008a; 2008b). In general,as it is shown in Figure 6, the alloy is melted in a graphite crucible using an

Figure 6. Schematics of ultrasonic cavitation based solidification.

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electric resistance heating unit and protected by Argon gas. The ultrasonic probeis made of Niobium (Nb) which can withstand high processing temperaturewith minimum ultrasonic cavitation induced erosion.

In order to fabricate aluminum�FA nanocomposites, Narasimha Murthyet al. (2012) made an attempt to introduce nano-sized FA into molten AA2024alloy through ultrasonic cavitation route. Using high energy ball milling for 30hr, the crystallite size was found to be 23 nm. While the fresh FA particles aremostly spherical in shape, the 30 hr milled FA particles are irregular in shapeand their surface morphology is rough. Scanning electron microscopy images ofnanocomposites reveal a homogeneous distribution of the nano-FA particlesinto the AA 2024 matrix. The authors reported that as the amount of nano-FAincreases, the hardness of the composite also augments. The nano-FA additionleads to improvement in the compression strength of the composites.

3.7. Compo-casting technique

Compo-casting process is the stir casting with reinforcement under the semi-solid state, which has low potential for Mg ignition and oxidation. As magne-sium alloy has high reactivity, compo-casting is a desirable process for magne-sium alloy matrix fabrication (Sasaki et al., 2003). In-situ composites fabricatedusing compo-casting are attractive for many researchers because of the cleaninterface between reinforcement and matrix, excellent properties and simplefabrication procedure.

Huang et al. (2011) have used FA cenosphere to form in-situ Mg2Si particlesin magnesium alloy during compo-casting route. The results demonstrated thatcompared with the matrix alloy, the tensile strength of the composites washigher at both room temperature and 150�C. After the solution treatment, theshape of Mg2Si particles transformed from the Chinese script Mg2Si to sphericalshape, and the tensile strength of the composites was further improved.

David Raja Selvam et al. (2013) used the compo-casting method in order tostrengthen AA6061 alloy with different weight percentages of FA (0, 4, 8, and12 wt.%). FA particles were added to the semi-solid state alloy. They reportedthat FA particles in the fabricated composite have clean interface and goodbonding to the aluminum matrix, which can be attributed to the fact that lowtemperature (610�C) fabrication makes FA thermodynamically stable and pre-vent the particles from reacting with the matrix and forming intermetallics. Theexperimental results show that AA6061/12 wt.% FA AMC exhibited 132.21%higher microhardness and 56.95% higher UTS compared to unreinforcedAA6061 alloy.

3.8. Friction stir welding method

In recent years, several surface modification techniques, such as high-energylaser melt treatment, high-energy electron beam irradiation, plasma spraying,

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cast sinter, and casting, have been developed to fabricate surface MMCs. Itshould be pointed out that the existing processing techniques for formingsurface composites are based on liquid phase processing at high temperatures.In this case, it is hard to avoid the interfacial reaction between reinforcementand metal�matrix and the formation of some detrimental phases (Mishra et al.,2003). As pioneers, Mishra et al. (1999) developed friction stir processing (FSP)for microstructural modification based on the principles of friction stir welding(FSW). Most of the fabricated surface composites have used synthesized mate-rial as reinforcing phase. Recently, Vijay and Dinaharan (2013) used FSP to fab-ricate MMCs reinforced with FA particles. It was observed that during FSP theFA particles undergo fragmentation due to the stirring action of the tool. How-ever, the ceramic particles increase grain nucleation and as a consequence a finergrain size matrix is obtained. The wear experiment results show that the wearrate of the surface composite decreased due to microstructural refinement andOrowan strengthening.

Table 4 summarizes the various processing routes used for the preparation ofFA composites, with some important aspects, such as the percentage of FA used,alloy type, and size range of FA particles (Balch and Dunand, 2006; Bienia et al.,2003; Daoud, 2008; David Raja Selvam et al., 2013; Dou et al., 2007; Escalera-Lozano et al., 2007; Huang et al., 2011; Kountouras et al., 2013; Kumar et al.,2010; Luong et al., 2011a; Marin et al., 2012; Narasimha Murthy et al., 2012; Nel-son et al., 2013; Palanisamy et al., 2013; Reddy et al., 2013; Rohatgi et al., 2006b;Sai et al., 2008; Surappa, 2008; Uthayakumar et al., 2013; Wu et al., 2006; Wuet al., 2007; Zahi and Daud, 2011). It should be made clear that all those differ-ences lead to different physical, mechanical, and corrosion properties.

As expected, a number of advantages and disadvantages may be observed,depending on the processing route. Table 5 shows some pros and cons of proc-essing routes for the production of FA MMCs.

4. Composites from waste material

As pioneers, in 1975 Sato and Mehrabian (1976) used a variety of non-metallic reinforcements from solid waste materials in aluminum alloy matri-ces. These non-metallic materials included SiC, TiC, Si3N4, AI2O3, glass,solid waste slag, and silica sand. They reported that composites containing»10 wt.% or more of these hard nonmetals experience less wear than thepure matrix alloy, but have slightly higher average coefficients of friction.Wear in composites containing soft particles, especially MgO and boronnitride was higher than that of the pure matrix alloy. In general, theyclaimed that all the composites show strength levels comparable to that ofthe matrix alloy. In the case of the finer nonmetallic particles, fibers, andsmall mica flakes, significant improvements in strength or ductility werenoted.

CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 23

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Table4.

Vario

usflyash/alloysystem

sstud

iedby

different

processing

routes.

Fabricationroute

Flyashpercentage

Flyashparticlesize

Matrix

Observatio

nsRef.

Squeezecasting

60vol.%

20�2

50mm

6061

AlAlmostsixtim

esincrem

entindamping

capacityofMMCs

incomparison

totheas

received

alloy

(Wuet

al.,2006)

Squeezecasting

70vol.%

10�7

6mmand

10�3

90mmfor

cenosphere

and

participator

flyash,

respectively

2024

AlElectrom

agnetic

shielding

effectivenesscharacteristics

(Dou

etal.,2007)

Squeezecasting

70�6

590�1

50mm

PureAl

Plastic

deform

ationathigh

erstress

(45�

75MPa)thanpu

reAl

(Wuet

al.,2007)

Squeezecasting

3.7and9.0wt.%

53�7

5mmand

75�1

00mm

AK12

alloy

Betterdistrib

utionof

flyashinsqueeze

castingmethodthan

gravity

casting,an

enhanced

pitting

corrosionof

theAK

12flyash

composite

incomparison

with

unreinforced

matrix

(Bieniaetal.,2003)

Stircasting

6,12

Narrowsize

rang

e(53�

106mm)and

widesize

rang

e(0.5�4

00mm)

A356

Flyashlead

toincrease

inhardness,

elastic

modulus

anddamping

capacityofmatrix

(Surappa,2008)

Stircasting

6�50

Ni-coatedflyash

NovelZnAl22

Superio

rcom

pressive

propertiesand

energy

absorptio

ncapacity

comparedto

thoseofthe

conventio

nalfoams

(Daoud

,2008)

Stircasting

5,10,and

1510

mm

Al�4

.5%Cu

Anincrease

inhardness,tensile,

compression,and

impactstreng

thincomparison

tomatrix

(Mahendraand

Radh

akrishn

a,2007)

Stircasting

0�8

�Al�8

%Mg,

Al�1

0%Mg

Wearb

ehaviorimprovem

entin

comparison

toAl�M

galloy

(Zahiand

Daud,2011)

Twostep

stircasting

10,15,and20

wt.%

<75

mm

PureAl

Betterwearp

ropertiesthan

pureAl

(Palanisam

yetal.,2013)

Stircasting

5,10,and

15wt.%

2�10

mm

AA-6351

Improved

wearp

ropertiescomparedto

AA-6351alloy

(Uthayakum

aretal.,2013)

Pressureinfiltration

20�6

545�2

50mm

A356

Higherstrengthandwiderservice

temperature

rang

ethan

polymeric

syntactic

foam

s

(Rohatgiet

al.,2006b)

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Pressureinfiltration

�15�7

5mm

7075-A

lIncreasesofmacroscopicfoam

stiffness

byover40%,ascomparedto

the

purealum

inum

foam

s

(Balch

andDunand,2006)

Pressureinfiltration

65vol.%

125mm

PureAl

LowerCTEvalueof

composite

(13.1£

10-6-11£

10-6/�C)

than

that

forp

urealum

inum

(25.3£

10-6/�C)

(Rohatgiet

al.,2006a)

Pressureinfiltration

�<65

mm

AA7075

Resulting

microstructuralhomogeneity

(Kountourasetal.,2013)

Powderm

etallurgy

10,20wt.%

<56

mm

(grin

ded)

PureAl

Bettermechanicalpropertiesand

inferio

rcorrosion

resistance

incomparison

topu

reAl

(Marinetal.,2012)

Powderm

etallurgy

0�16

wt.%

Cu-coatedanduncoated

flyash

Cu-5%Sn

Higherd

ensificatio

n,hardness,

compressive

yieldstreng

th,and

dry

slidingresistance

ofcompositewith

coated

flyashthan

uncoated

composites

(Saiet

al.,2008)

Powderm

etallurgy

2,6,and10

wt.%

8.22

mm

AA6061

Highertem

perature

dryslidingwear

resistance

than

AA6061

alloy

(Kum

aretal.,2010)

Powderm

etallurgy

10wt.%

90�3

00mm

Al�(

0�20)w

t.%Pb

Density(sintered)

increasedwhereas

hardnessandcompressive

streng

thdecreasedwith

theadditio

nofPb

(Reddy

etal.,2013)

Pressurelessinfiltration

20vol.%

50�7

0mm

A�3Si�15Mg

Compositeswith

calcined

flyashhave

better

corrosionresistance

than

composite

with

asreceived

flyash

(Escalera-Lozano

etal.,2007)

Ultrasoniccavitatio

n1�

3wt.%

23nm

AA2024

Nano-flyashadditio

nleadsto

improvem

entinthecompression

streng

thandhardnessofthe

composites.

(Narasimha

Murthyet

al.,2012)

Compo-castin

g5wt.%

100mm

AZ91D

Comparedto

thematrix

alloy,the

tensile

streng

thofthecomposites

ishigh

eratboth

room

temperature

and150�C.

(Huang

etal.,2011)

Compo-castin

g4,8,and12

wt.%

�AA

6061

Flyashincorporationimproves

the

microhardnessandUTS

ofthe

AMCs.

(DavidRajaSelvam

etal.,2013)

Frictio

nstirprocess

6,12,18,and24

vol.%

�AA

6360-T6

Improved

dryslidingwearb

ehavior

(Nelsonetal.,2013)

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4.1. Waste materials as matrices

Melting and sintering capabilities of waste metallic materials make them moreinteresting in fabrication of matrices for MMCs. Powder metallurgy nickel alloysgenerally exhibit improved properties over conventional cast and wrought alloyproducts and particularly nickel�iron alloys have attracted much attention dueto their mechanical and magnetic properties (Li and Ebrahimi, 2003). Bose et al.have fabricated Ni3Fe�Y2O3 alloy matrix composites starting from the elemen-tal Ni and carbonyl Fe powders (Bose et al., 1993). Recently, Karayannis andSotiriou (2006) looked at the commercial manufacturing potential of MMCs,using powdered Ni3Fe ferro�nickel alloy powder as the raw material instead ofelemental powders. Ni3Fe is known for its high ductility, insensitivity towardtesting environments, and magnetic properties. In addition, there are orderingtendencies near this composition, generally improving mechanical performance.It should be mentioned that all the materials that have been used are derivedfrom ferrous scrap, a widely available and comparatively low-cost waste mate-rial. The recovery of Ni3Fe in powder form from scrap is realized by a hydromet-allurgical process with many advantages in its steps. They have used Al2O3 asreinforcement because of its high hardness and specific stiffness, low density,electrical resistivity, and thermal expansion coefficient and its stability, provid-ing oxidation and corrosion resistance as well as high temperature mechanicalproperties. Small pieces of waste materials (ferrous scrap classified as stainlesssteel 316) were treated by hydrometallurgy. They reported that Ni and Fe pow-ders can be produced by reduction of the Ni and Fe chlorides with hydrogen,

Table 5. Pros and cons of processing routes for FA-MMCs

Fabrication routes Advantages Disadvantages

Squeeze casting � For high volume fraction of FA� Low levels of residual porosity� Uniform distribution of fly ash� Short processing times

� Deformation or fracture of FA

Stir casting (vortex) � For low and medium volumefraction of FA

� Short processing times

� High/medium processing costs� Segregation and non-uniformdistribution of fly ash (particlepushing and tendency ofparticles to float due to densitydifferences)

Powder metallurgy � Better temperature control� Control on phase distribution� Short processing times

� High/medium processing costsDeformation or fracture of FA(specially for cenosphere fly ash)

Pressureless infiltration � Integrity of fly ash� Uniform distribution of flyash and other reinforcements

� Risk of attack of thereinforcement by the liquid metal

Compo-casting � Uniform distribution of reinforcement� Low degree of reactivity matrix/reinforcement

� Short processing times

� High/medium processing costs

Friction stir processing � Highly grain refiner (matrix)� Uniform distribution of reinforcementthroughout the processing zone

� High/medium processing cost� Deformation or fracture of FA(specially for cenosphere fly ash)

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which results from the dissolution of the scrap with hydrochloric acid and areselectively extracted by Versatic Acid 6 from their acidic solution and then crys-tallized. Ni3Fe alloy powder was produced here from the Ni and Fe chloridesmixtures of the same origin in a similar way. The initial feeding was composedof FeCl2¢4H2O and NiCl2¢6H2O mixtures at weight proportions correspondingFe/Ni D 1:3. The characterization of Ni3Fe powder derived from ferrous scrapare listed in Table 6. It can be seen that their values are generally similar to thoseof typical commercially available powders produced by atomization � encour-aging results for powder metallurgical development.

After synthesizing of Ni3Fe�Al2O3 composites developed from Ni3Fe pow-der, the authors prepared a composite by mixing the appropriate amount of ele-mental Ni and Fe powders (3NiCFe: weight ratio of 3/1) of the same origin andAl2O3 ceramic particles to compare the physical�mechanical properties of twokinds of composites. The result demonstrated that as the percentage of Al2O3 inthe composites increases, apparent density and relative density decreases due tothe lower density of the ceramic versus the metallic constituent. From the elec-trical resistivity test results it can be seen that the lower the percentage of metal-lic content, the greater the electrical resistivity.

(3NiCFe)-Al2O3 composites are more conductive than the Ni3Fe-based onesat each relative matrix-reinforcement composition, because of a severe increasein resistivity that, generally, accompanies alloying in comparison to pure metals.The authors reported that by increasing the amount of reinforcement in com-posite, the hardness increased. The mean hardness values achieved are clearlyhigher in the composites developed starting from the Ni3Fe powder, the harderof the matrix materials considered, than in the metal powder based ones, at eachAl2O3 content. Ni3Fe�Al2O3 composites slightly prevail in rigidity over(3NiCFe)-Al2O3 at a given percentage of reinforcement. The difference in stiff-ness of the matrix materials used and a greater heterogeneity in the metal pow-der based MMCs � composed from three constituents instead of only two inthe case of the alloy powder based ones � must be responsible for these results.

Table 6. Comparison of Ni, Fe, and Ni3Fe powders recovered from ferrous scrap with typicalcommercial grades (Karayannis and Sotiriou, 2006).

Ni powder Fe powder Ni3Fe powder

Characterization

Commercialgrade

(atomization)Recoveredfrom scrap

Commercialgrade

(atomization)Recoveredfrom scrap

Commercialgrade

(atomization)Recoveredfrom scrap

Specific gravity(g/cm3)

8.63 8.83 7.81 7.70 11.50 11.36

Specific surface(cm2/g)

3100 3500 2500 1500

Particle shape Hexagonal Angular Spherical Spherical Spherical AngularApparent density

(g/cm3)3.93 3.35

Purity 99.9% 99.86% 99.85%

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In fact, this heterogeneity influences significant parameters for the composites’elastic behavior, including porosity, the distribution of the reinforcement par-ticles, the nature and strength of the interface, and the development of micro-damage.

In another work by Karayannis and Moutsatsou (2012), Ni powder wasextracted from ferrous scrap and fabricated nickel�alumina composite by thepowder metallurgy route. It was reported that when increasing the % addition ofstiffer ceramic particles, the apparent density clearly decreases while strengthen-ing is progressively achieved.

The tribological behavior of an AlSiMg�SiCp composite, fabricated by pres-sureless infiltration method, using a slider-on-cylinder tribometer, was studiedby (Ceschini et al., 2001). The effect on the wear resistance of both a T6 thermaltreatment and of the use of recycled material in the casting process (50% and100% recycling) was investigated. It was found that at the lowest applied load(10 N) a mild wear regime with high coefficients of friction was always foundfor all the tested composites. This wear regime was characterized by the forma-tion on the worn surfaces of a well adhered iron-oxides transfer layer, producedby wear of the steel counterface. In addition, at the higher loads (20 and 30 N)and low sliding speed (0.3 ms¡1) the as-cast composites exhibited a better wearresistance than the T6 aged ones. In fact, they maintained a mild wear regime,probably on account of the good resistance of the oxides protective layer sus-tained by a strain-hardened subsurface material. On the contrary, the T6 agedcomposites showed delamination wear, caused by crack formation and propaga-tion, both in the mechanically mixed layer and in the subsurface material. Withincreasing sliding speed, instead, both the as-cast and the T6 composites dis-played transition toward a severe wear regime, probably related to a softening ofthe matrix. Finally, the composite with 50% of recycled material showed similarwear resistance as the corresponding composite (30 vol.% SiC, as-cast) notrecycled. The composites with 100% of recycled material (as-cast and T6),instead, displayed a slightly higher wear damage possibly due to the lower (20vol.%) reinforcement content and/or to the inhomogeneous particlesdistribution.

In the work by Samoshina et al. (2008), the fabrication possibility of two typesof composites from Al alloys scraps were investigated. The first type of compos-ite was prepared by mixing the matrix aluminum alloy chips in combinationwith 20 vol.% SiC in a planetary mill and in argon atmosphere. The second typeof composites was fabricated from (Al�6% Mg�0.7% Mn�0.3% Fe)�O bytreating the matrix alloy chips in a vibration mill in a controlled air atmosphere.In case of the former, mechanical alloying led to the formation of the homoge-nous structure with uniformly distributed SiC particles less than 1 mm in size,while in the case of the later, the increase of mechanical alloying time resulted inthe increase of the amount of oxygen in composite material granules. In accor-dance with this, the strengthening oxide particles were found to be synthesized

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in the aluminum matrix. It was found that the full oxidation of magnesium andthe partly oxidation of aluminum occur after 15 hr treatment.

Gronostajski et al. (2001) used FeCr powder to strengthen aluminum chips.AlMg2 chips of two fractions: 0.5�1.0 mm and below 0.5 mm were mixed withdifferent weight percentage (from 6 to 14 wt.%) of FeCr powder, compacted andsintered in vacuum of 10¡3 HPa or in air at 550�C for 2 hr. Finally, hot extrusionwas used as the final operation to bring about diffusion bonding between par-ticles by crushing the layer of oxides and actuating diffusion process under ahigh pressure and temperature (500�550�C). It was concluded that the mechan-ical properties of composites fabricated with finer chips (below 0.5 mm) are bet-ter than those for bigger chips. Also it was reported that, the effect of ferro-chromium content on the flow stress is dependent on the experiment tempera-ture and at higher temperature the best result is obtained for 6 wt.% of reinforc-ing phase.

In the work by Samuel (2003), granulated aluminum (Al-2014) scrap wasreinforced with Al2O3 Safill fibers (2, 5, 10, and 20 vol.%) through hot extrusionwith three different reduction ratios (4:1, 16:1, and 38:1). The mechanical prop-erties at ambient and elevated temperatures, the microstructure, and fiber lengthwere evaluated. The experimental results demonstrated that in terms of mechan-ical properties and microstructure, the best extrusion ratio is λ D 16:1. In highextrusion ratio decohesion of intermetallics particles in the matrix caused areduction in elastic modulus. On the other hand, if the volume fraction is above10%, then a lower reduction ratio (λ D 4:1) causes residual porosity. Addition of10 vol.% Al2O3 Saffil fibers caused increment of ultimate tensile strength (UTS)and yield strength (YS) at 270�C by 77% and 86%, respectively. The Vickershardness increased by 43% and the elastic modulus increased by 27% at ambienttemperature. This outcome can be attributed to the fact that the composites con-taining fibers up to 10 vol.% have a very good internal cohesion.

4.2. Waste materials as reinforcements

4.2.1. Natural waste materialIn recent years, biomasses, in particular RH, have been used to generate electricpower, but it releases a large number of greenhouse gases and residues. Lately,environmental consideration and public concern are increasingly becomingmore important, striving toward the quality and environmental preservationthrough sustainable development and cleaner technology approach (Yusoff,2006). For instance, the emission of RH ash into the ecosystem has provokedhuge criticisms and complaints, mainly associated with its persistent, carcino-genic, and bio-accumulative effects, resulting in silicosis syndrome, fatigue,shortness of breath, loss of appetite, and respiratory failure (Foo and Hameed,2009). However, it has been reported elsewhere that unlike fossil fuels, RH canbe used for reduce greenhouse effect because trees absorb CO2 as they grow and

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this carbon is released when the biomass is combusted. Thus, the net amount ofCO2 added to the atmosphere during energy generation and ash productionthrough the use of biomass over the entire life cycle is nearly zero (Hall andScrase, 1998) (Figure 7).

In addition, the residual biomass after combustion, enriched of silicon andcarbon can be utilized as ceramics reinforcing or filler in metal� or polymer-�matrix composites, respectively. The advantages of using biomaterials or theirderivatives for ceramics are: (1) the biomaterial is a cheap source of carbon andsilica (or other elements). When the biomaterials are traditionally regarded asbyproducts or wastes, their use is also environmentally benign and (2) the bio-materials or their derivatives provide templates for fabricating ceramics withunique structures non-obtainable by conventional ceramic processing techni-ques. Among ceramics derived from biomaterials, silicon carbide synthesizedfrom various kinds of plants has been most studied. The plants have cellular,anisotropic, and fibrous structure. It is believed that these structures shouldhave reached optimal configuration because all the plants have been evolving formillions of years. Consequently, the cellular, porous ceramics made from plantsare also expected to exhibit optimal properties with unique pore structure of theprecursor plants. These highly porous ceramics can possess relatively high

Figure 7. Schematic of CO2 adsorption and release during growth and conversion of biomass toits ash.

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strength and become an ideal skeleton for ceramic composites or ceramic rein-forcements (Martınez-Fern�andez et al., 2000; Qian et al., 2004a; 2004b; Rodri-guez-Lugo et al., 2002).

4.2.1.1. Breadfruit seed hull ash. The African breadfruit tree (Treculia afri-cana) is native to many tropical countries like west India, Ghana, Sierra, Nigeria,and Jamaica (Nwabueze and Otunwa, 2006). One breadfruit pod may contain upto 100�200 seeds. The breadfruit is about the size of a football or more and isgreenish in color and the seed hull is brown or black, the edible seed is whitishyellow (Obasuyi and Nwokoro, 2006). Breadfruit seed hull (seed coat or seedshell) is an agricultural waste. The waste is produced in abundance globally andposes risk to health as well as environment. Thus their effective, conducive, andeco-friendly utilization has always been a challenge for scientific applications.The approximate composition and mineral elements composition of breadfruitseeds are presented in Table 7 (Atuanya et al., 2012; Bamgboye and Jekayinfa,2006; Osabor et al., 2009).

Recently, Atuanya et al. (2014) investigated the effects of breadfruit seed hullash on the microstructures and properties of Al�Si�Fe alloy/breadfruit seedhull ash particulate composites fabricated through stir-casting technique. It wasfound that the density of the breadfruit seed hull ash is 1.98 g/cm3 which meansthat breadfruit seed hull ash is very light material. The value obtained fall withinthe range of density of FA and bagasse ash which are 1.8 and 2.2 g/cm3, respec-tively (Aigbodion et al., 2010a; Kumar and Swamy, 2011; Prasad and Krishna,2010), currently used in MMCs. XRF studies revealed the presence of hardphases like SiO2, Al2O3, MgO, Cr2O3, and Fe2O3 as major constituents of theash. Some other oxides viz. K2O, Na2O, and MnO were also found to be presentin traces. It was observed that by increasing the weight percent of breadfruitseed hull ash in the matrix, the density of composite decreased while the hard-ness increased. This increment was attributed to the presence of hard phases

Table 7. Components and chemical composition of breadfruit seed before and after incineration(Atuanya et al., 2012; Osabor et al., 2009).

Before incineration After incineration

Approximate compositionMineral elementscomposition Phases

Weightpercentage

Chemical property Percentage Elements Composition(mg/100 g)

Al2O3 35.80

Moisture 8.01§ 0.10 Sodium 7.10 § 0.10 Cr2O3 5.06Crude protein 12.47 § 0.10 Potassium 587§ 0.02 Fe2O3 30.34Fat 4.23§ 0.02 Calcium 165§ 0.01 K2O 0.52Ash 2.26§ 0.02 Magnesium 186§ 0.01 MgO 1.20Fiber 1.62§ 0.02 Iron 1.66§ 0.01 Na2O 0.45Carbohydrate 73.26 § 0.01 Zinc 8.50 § 0.02 SiO2 15.45

Copper 3.67 § 0.01 MnO 0.22ZnO 0.05

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such as SiO2, Al2O3, Cr2O3, and Fe2O3 in the ash and differences between CTEof matrix and reinforcing phase that cause elastic and plastic incompatibilitybetween the matrix and the reinforcement and therefore, increase of dislocationdensity. No porosities and clustering were observed in the microstructure as aresult of good wettability of the breadfruit seed hull ash particles, by the moltenaluminum alloy, which helped the dispersion process of the particles into thealuminum�alloy melt. The authors reported that the addition of breadfruit seedhull-ash reinforcement particles to Al�Si�Fe alloy increased the tensile strengthof composites but slightly reduced the impact energy. By increasing the amountof reinforcement, the fracture mode of composite changes from ductile to brittlemode. Based on these results and comparing with the characterization of otherreinforcements, the authors suggest that breadfruit seed hull ash can be anappropriate reinforcement in the production of light weight MMCs componentwith good thermal resistance.

4.2.1.2. Coconut. Coconut shell and coir are agricultural wastes available invery large quantities throughout the tropical countries of the world. Moreover,coconut is becoming an important agricultural product for tropical countriesaround the world as a new source of energy-biofuel. Previously, coconut shellwas burnt as a means of solid waste disposal which contributed significantly toCO2 and methane emissions. However as the cost of fuel oil, natural gas, andelectricity supply has increased and become erratic, coconut shell has come tobe regarded as source of fuel rather than discarding it. Presently, the Nigeriacoconut shell is used as a source of fuel for the boilers, and residual coconut shellis disposed of as gravel for plantation roads maintenance. Blacksmiths also buythe coconut shell as fuel material in their casting and forging operations (Bamg-boye and Jekayinfa, 2006). Recently, characterization of coconut shell ash forpotential utilization in MMCs was studied by Madakson et al. (2012). The resultshowed that coconut shell ash contains silicon oxide (SiO2), corderite (Mg2Al-Si5O18), quartz (SiO2), and Moissanite (SiC) as the primary compound withSiO2 as the highest percentage of all the compound and element present. Inaddition, XRF studies revealed the presence of hard phases like SiO2, Al2O3,MgO, and Fe2O3 as major constituents. The coconut shell ash can withstand atemperature of up to 1500�C with a density of 2.05 g/cm3. According to theresults and in comparison with other waste material applied as reinforcement inMMCs, the authors have suggested coconut shell ash as a light and hard compo-nent for fabricating of MMCs.

Coir fiber has been used as highly durable fiber in all types of matrices viz.,polymer, bitumen, cement, gypsum, FA-lime, mud, etc. It is characterized by itslightweight, and low thermal conductivity (Asasutjarit et al., 2009). In the workby Maleque et al. (2012), coconut coir was used as a natural fiber, reinforcing Alin composites for automotive brake pads. They mixed Al powder and differentvolume percentages of coconut coir (0, 5, 10, and 15 vol.%) with different

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ingredients such as silicon carbide, alumina, zirconia, graphite, and phenolicresin and then compacted and sintered them. The experimental results showedthat the best properties in terms of higher density, lower porosity and highercompressive strength were obtained from 5% and 10% coconut�fiber compo-sites. The microstructure reveals uniform distribution of resin and coco-nut�fiber in the matrix. It can be concluded that 5% and 10% showed betterphysicomechanical properties compared to other formulations.

4.2.1.3. Bagasse. Bagasse is the matted cellulose fiber residue from sugarcanethat has been processed in a sugar mill. In the past, bagasse was burnt as a meansof solid waste disposal. However, as the cost of conventional fuels has increased,bagasse can be considered as an alternative fuel rather than getting rid of it inthe sugar mills (Pay�a et al., 2002). Barroso et al. (2003) stated that 1 ton of sugar-cane generates 280 kg of bagasse, and that based on economics as well as envi-ronmental related issues, enormous efforts have been directed worldwidetoward bagasse management issues, i.e., of utilization, storage and disposal. Aig-bodion et al. (2010a) investigated the physical properties of bagasse ash andreported that consists of quartz (SiO2), cliftonite (C), moissanite (SiC), and tita-nium oxide (Ti6O) as the primary compounds. The ash is a combination of dif-ferent morphologies such as prismatic, spherical, and fibrous, and each of themis related to different phases. The ash has a density of 1.95 g/cm3 and can with-stand temperatures of up to 1600�C.

Aigbodion et al. (2012) studied the effect of bagasse ash reinforcement on thewear behavior of Al�Cu�Mg/bagasse ash particulate composites. The testswere conducted at varying loads, from 5 to 20 N and different sliding speeds fora constant sliding distance of 5000 m. The result showed that the aluminumalloy reinforced with bagasse ash particles exhibits better dry sliding wear resis-tance than the unreinforced alloy. Wear rate decreases as the amount of bagasseash particles reinforcement increased in the alloy. In addition, the microstruc-ture of the worn surface revealed that a large amount of plastic deformationoccurred on the surface of the unreinforced aluminum alloy. WhileAl�Cu�Mg/BAp reinforced alloy, the worn out surfaces are not smooth, andgrooves, scratches, and parallel lines were observed. By increasing the slidingspeed and the applied load, the wear rate increased.

The age-hardening characteristics of an alloy are generally modified by theintroduction of reinforcements. These modifications are due to the manufactur-ing process, the reactivity between the reinforcement and the matrix, the size,morphology, and volume fraction of the reinforcement (Appendino et al., 1991;Christman et al., 1989). Aigbodion and Hassan (2010) and Aigbodion et al.(2010b, 2011) developed the thermal ageing behavior model of Al�Cu�Mg/bagasse ash particulate composites with 2�10 wt% bagasse ash particles pro-duced by double stir-casting method in terms of weight fraction of bagasse ash,ageing temperature, and time. Hardness measurement values and

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microstructural analysis were used in determining the ageing behavior, aftersolution, and age-hardening heat treatment. The experimental results demon-strated that the bagasse ash is the main parameter in the ageing behavior, fol-lowed by ageing temperature, and that the hardness values decreased as theageing time increased. As the methodology allowed projecting the optimumcombination of the testing parameters, it was found that the predicted hardnessvalues were close to those of experimentally observed ones. The developedmathematical model can be used for optimization of the process parameters ofthe ageing behavior of Al�Cu�Mg/bagasse ash particulate composites withrespect to hardness values. In addition, TEM observations revealed that theaddition of bagasse ash particles to the Al�Cu�Mg alloy can speed up thegrowth rate of precipitates S’ (Cu3Al2, and Al6CuMg4) phases. The acceleratedprecipitation of S’ phases is proposed to be responsible for the enhanced age-hardening of the Al�Cu�Mg/BAp composites.

4.2.1.4. Rice husk. Amongst the various known biomasses, with abundant andrenewable energy sources, RH is not only a potential source of energy, but also avalue-added byproduct (Soltani et al., 2015; Vlaev et al., 2003). The world riceharvest, according to Food and Agriculture Organization of United Nations(FAO) rice market monitor (RMM) in 2013 is estimated in 741.4 million tons.

Figure 8. Global rice paddy production and area (FAO Rice Market Monitor, 2013).

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At the revised level of 741.4 million tons, world paddy production in 2013 wouldstand in 7.8 million tons above the record 2012 harvest, a rather modest increasein comparison with those in 2010 and 2011. Figure 8 depicts the rice productiongrowth through the past 11 years (FAO Rice Market Monitor, 2013).

Considering that 20% of the grain is husk, and 20% of the husk after combus-tion is converted into ash, a total of 29.6 million tons of ash can be obtained.RHs contain organic substances and 20% of inorganic material. The utilizationof these fractions as sources for producing large-scale commodities, such asxylose, activated carbon, and silicon dioxide, can be a good choice.

Due to its attractive properties, RH ashes are now used by various industriesfor different applications, such as coatings, pigments, cement industry, insula-tors, rubber filler, etc. (Chandrasekhar et al., 2006; Fuad et al., 1995; Goncalvesand Bergmann, 2007; Lee et al., 2013; Li et al., 2012; Sobhanardakani et al., 2013;Ugheoke et al., 2006).

There are several reasons why RH is not being used effectively, including (1)lack of awareness of its potential by farmers and industry persons, (2) socioeco-nomic problems, (3) penetration of technology, (4) lack of interest, and (5) lackof environmental concerns. Solution to the problems associated with utilizationof this solid waste needs to be worked out both in quality and quantity aspects.Component percentage, chemical composition, and physical properties of RHand its ash are listed in Table 8. It should be mentioned that the physical andchemical properties of the RH ash depend on the methods used to reach to it(Chuah et al., 2005; Malik, 2003; Patel et al., 1987; Rahman and Ismail, 1993).

For the first time Das et al. (1986) used RH ash as reinforcing phase in alumi-num�silicon alloys to study the effects of second phase on the solidificationbehavior of the alloy. They burned RH at the temperature of 600�C for 12 hrafter washing and drying it. During stirring of molten metal they added Mg inorder to enhance the wettability between RH particles and the alloy melt. It wasnoticed that without the addition of magnesium, the RH ash particles wererejected. They measured the secondary dendrite arm spacing (center-to-centerdistance between two neighboring dendrites). It was reported that the average

Table 8. Typical composition, physical and chemical properties of rice husk and its ash (Chuahet al., 2005; Malik, 2003; Patel et al., 1987; Rahman and Ismail, 1993).

Component percentage

Elementalcomponent

of rice husk (wt.%)

Physical andchemicalproperties

Chemicalcomposition

of rice husk ash

Cellulose 43.3 C 37.05 Bulk density (g/ml) 0.73 SiO2 96.34Lignin 22.0 H 8.80 Solid density (g/ml) 1.5 K2O 2.31D-xylose 17.52 N 11.06 Moisture content (%) 6.62 MgO 0.45Methyl glucoronic

acid6.53 Si 9.01 Ash content (%) 45.97 Fe2O3 0.2

L-Arabinose 6.53 O 35.03 Particle size (mesh) 200�16 Al2O3 0.41D-Galctose 2.37 Surface area (m2/g) 272.5 CaO 0.41Extractives 1.82 Surface acidity (meq/gm) 0.1 K2O 0.08

Surface basicity (meq/gm) 0.45 MnO2 0.074

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secondary dendrite arm spacing of primary a-aluminum is about 12 mm in thealloy without RH ash, while it is 20 mm in the composite. It was observed thatthe aluminum dendrites are oriented differently within the necklace-shaped RHash particles as compared to their orientation outside the necklace, there is nocontinuity in the dendritic structure across the rice husk particles. This can beattributed to the incorporation of molten alloy into the cavity of hull-shaped RHash, and solidification as a result of nucleation events separately from the bulkliquid, outside the ash particles. This filling of hull-shaped RH ash particles withmolten alloy prior to solidification might prevent the ash particles from floating.The apparent density of ash particle was found to be 2.0 g cm¡3 whereas thedensity of the molten aluminum�silicon alloy was 2.65 g cm¡3.

In the work by Deshmukh et al. (2012) and Deshmukh and Pathak (2012),the mechanical properties of Al based MMC reinforced with RHA and metallur-gical grade SiO2 with varying percentage of Mg (0.5, 1, 2.5, and 5 wt.%) wasstudied. To fabricate the composites, amorphous SiO₂ (extracted from RH) wasadded in proportion of 5 wt.%, into the alloy samples by the stir-casting route,repeating the procedure for the metallurgical grade SiO2. The SiO₂ extractedfrom RH was in the range of 32�56 nm while the metallurgical grade SiO₂ wasabout 10 mm average size. It was reported that with the increasing of Mg percentup to 2.5 wt.% to the molten alloy, the hardness values increased in both fabri-cated composites. This can be attributed to a better wettability of SiO2 particlesand formation of hard phase of spinel and a solid solution of Mg in Al matrix inthe presence of 2.5 wt.% Mg. Consequently, excessive lattice distortion of Aloccurs, resulting in the formation of finer grains with higher hardness. Besides,nano-scale SiO2 derived from RH has more reactivity and surface area with mol-ten alloy than metallurgical grade SiO2 and hence, the former results in the for-mation of porosity and the later causes less contact area with the matrix. Thewear loss of Al�2.5 wt.% Mg composite reinforced with RH silica was alsofound to be minimum than that of other samples of composites. From the SEMmicrographs of the worn out particles it was concluded that for the compositewith 2.5%Mg and with RH silica as reinforcement, the minimum worn out par-ticles are seen which is equivalent to maximum hardness and minimum wearloss.

Prasad and Krishna (2010, 2011) investigated the characterization andmechanical properties of A356-RHA composite fabricated by stir casting. Toprepare ash from RH they used similar procedure according to Das et al. (1986)study. The result demonstrated that, as the percentage of RHA particlesincreases, the hardness and UTS of the composites increases while density ofcomposite decreases.

Luangvaranunt et al. (2010) used amorphous RHA in order to fabricate alu-minum�4mass% copper/alumina composites by powder forging. It wasreported that amorphous silica from rice hush ash due to its amorphism andhigh specific surface area, yields higher chemical reactivity compared to

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crystalline silica (Prabriputaloong and Piggott, 1974; Umeda et al., 2009). Amor-phous silica had a structure of a nearly tetrahedral network of SiO¡ 4

4 , with openstructure where large pores surrounding atoms can play the role of vacancy indiffusion process. In order to induce in-situ chemical reaction to form aluminafrom silica and aluminum, the forged billet of Al, Cu, and RH silica after coldcompaction, sintering, and forging was heated at temperatures of 590�C, 630�C,and 650�C for 10 hr. Then fabricated composite was heat treated to obtain maxi-mum hardness of matrix due to precipitation hardening procedure. Theyreported that the occurrence of an exothermic reaction (in-situ formation ofAl2O3 from RH silica) in DTA results of the forged specimens in comparison tothe compacted one, shows that the forging process is necessary to activate theexothermic reaction. Forging results in severe plastic deformation in aluminummatrix and fractured RH ash. Consequently, it generated ultimate contactbetween fresh surfaces of RH ash silica and aluminum powder, promoting thereaction to proceed. Addition of Cu powder is only to facilitate sintering by liq-uid phase sintering mechanism, because based on sessile drop experience thereis no reaction between liquid copper and amorphous silica. Besides, addition ofCu yields a composite which is heat treatable by precipitation hardening.

It was reported that SiO2 could be employed as industrial raw materials tosynthesize magnesium silicide (Mg2Si) via a deoxidization reaction of SiO2 bymagnesium in solid-state. Mg2Si intermetallics are suitable reinforcements ofmagnesium and aluminum alloys because of their superior characteristics, suchas high hardness of 350�450 Hv and high Young’s modulus of 120 GPa (Bah-rami et al., 2011; Bahrami et al., 2012; Soltani et al., 2012). In the case of Mgmatrix composite, Kondoh et al. (2005) and Umeda et al. (2009) have used RHas a starting material for fabricating Mg2Si particle as reinforcing phase throughsolid-state reaction. They chose two kinds of preparation processes for synthe-sizing RH ashes: acid washing and non-acid washing of RH. It was observedthat the crystallization temperature for RH ash that was not washed with acid,during burning, is higher than that for sample that was washed with acid. Thatresult can be attributed to the presence of some alkaline contents in non-acidwashed RH, which reduces the melting temperature of SiO2. The TG-DTAresults indicated that the acid washing treatment is conducive to remove 40%organics of RHs, in particular the thermally stable fixed carbon contents, whichcauses the carbon compound impurities of wastes or ashes after burning. Theresult demonstrated that RH wastes burned at 1000�C after the acid treatmentare more suitable to synthesize Mg2Si via reaction with magnesium alloy pow-der, compared to those without pre-treatment. This is because a high purity andamorphous structure of SiO2 makes them finer and more reactive. Table 9 listssome of ceramic and MMCs fabricated from RH (Adam et al., 2013; Aigbodion,2012; Chen et al., 2010; Das et al., 1986; Escalera-Lozano et al., 2008; Jaroenwor-aluck et al., 2012; Kondoh et al., 2003; Kumagai and Sasaki, 2009; Luangvara-nunt et al., 2010; NIYOMWAS, 2009, 2012; Prasad and Krishna, 2012;

CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 37

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Table9.

Utilizationofricehusk

inceramicandmetalmatrix

composites.

Composite

Fabricationroute

Startin

gmaterial

Observatio

nsRef.

Si�S

iCcomposite

Self-propagatinghigh

temperaturesynthesis(SHS)

Powderm

ixtureofricehu

skash�

activated

carbon�M

gMicrostructureandchem

ical

compositio

n(Niyom

was,2009)

Ferrite

compositeusingSiO2

Athigh

temperaturetreatm

ent

undern

itrogen

atmosph

ere

Pyrolyzedricehu

sk�F

e(NO3)3¢9H2O

Enhanced

adsorptio

nabilityfor

acidorange

II(Chenetal.,2010)

Alalloy-ricehu

skash

Stircastingtechniqu

eA3

56.2Al

alloy

Goodinterfacialbondbetween

matrix

andricehu

skash,

increm

entinUTS,hardn

ess,

andwearresistance

(PrasadandKrishn

a,2010;

Prasad

andKrishn

a,2011)

Alum

inosilicatecomposites

(ASC)

Alum

inum

hydroxide-RH

A�NaO

H,N

a 2SiO3boric

acid

Goodresistance

in3vol%H2SO4

solutio

n(Rattanasaketal.,2010)

Alum

inum

�4mass%

copp

er/

alum

ina

PowderForging

Alum

inum

,copper,andricehusk

ashsilica

Maximum

hardnessof44

HRA

(Luang

varanunt

etal.,2010)

Al�S

i�Fe/ricehu

skash

DoubleStircastingmethod

Ferrosilicon,ricehu

skashparticle,aluminum

Higherstrengththan

thatof

matrix

(Aigbodion,2012)

Alum

ina�

mullite�

silicon

carbidecomposite

Insitu

bycarbotherm

alredu

ction

Al2Si 2O7¢2

H2O,SiO

2powder,ricehu

sk(SiO

2),and

activated

carbon

Obtaining

Al2O

3�Al

6Si 2O

13�S

iCcompositewith

thewhiskers

SiC

(Niyom

was,2012)

Alum

inum

alloy�

ricehu

skash

Casting

Rice

husk,LM13

alloy,Mg

(Das

etal.,1986)

Magnesium

compositewith

magnesium

silicide

Powderm

etallurgy(solid-state

reactio

n)Rice

husk

silicaparticleswith

magnesium

powder

Highdensity

andhardness

(Umedaetal.,2009)

Carbon/Silica

composite

Carbonizingandhot-pressing

Rice

husk

Maximum

bulkdensity

and

Vickershardnessatsintering

temperatureof

800�C.

(Kum

agaiandSasaki,2009)

SiC w/M

oSi 2�

SiC

Liqu

idSiinfiltration(LSI)

Rice

husks�

Mopowder

Highelastic

modulus

andfracturetoug

hness

(Zhu

etal.,2012a)

Al/M

gAl 2O

4Insitu

Microsilica,ricehu

skash,pu

reAl,M

gMicrostructuralobservations

(Sreekum

aretal.,2008)

SiC/Fe�S

iReactiveinfiltration

Cocked

ricehu

sks,SiCpowder,FeSi2alloy

HighVickershardness,elastic

modulus,three-point

flexural

streng

thandindentation

fracture

toug

hness

(Zhu

etal.,2013)

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Nanocom

positesof

RHA�

10Sn10Ti

RHA�

10Sn

RHA�

10Ti

Sol�geltechn

ique

SnCl2¢2

H2O,

HClTiCl3,RH

A,cetyltrimethylammonim

brom

ide,

HNO3,NaO

H

Highph

otocatalyticactivity

(Adametal.,2013)

Nanocom

posite

TiO2�

SiO2

sol�gel

Rice

husk

HighUVabsorptio

nefficiency

(Jaroenw

oralucketal.,2012)

Magnesium

compositealloys

dispersedwith

Mg 2Si

Extrusion

HClsolutio

n,Rice

husk,AZ31alloypowder

Havinghigh

hardnessand

Young’smodulus

(Kondohetal.,2003)

SiCw

/SiC�S

icom

posite

Infiltration

RH,carbon,silicon

HighVickershardness,flexural

streng

th,elasticmodulus,

andfracturetoug

hness

(Zhu

etal.,2012b)

AlSi10Mg/RH

AStircasting

RHAwith

different

size

rang

es(50�

75,75�

100,

and100�

150mm),AlSi10Mgalloy

Betterwearp

roperties;the

coarserand

thehigh

erthe

amount

ofricehu

skash,the

lowerthewearrate

(Saravanan

etal.,2013)

Al/SiC/RHA

PressurelessInfiltration

Recycled

alum

inum

form

beverage

containers,

SiC,RH

AThepresence

ofRH

Aprevents

degradation(viathe

form

ationandsubsequent

hydrolysisof

Al4C

3)of

composites

(Escalera-Lozano

etal.,2008)

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Rattanasak et al., 2010; Saravanan et al., 2013; Siva Prasad and Rama Krishna,2011; Sreekumar et al., 2008; Umeda et al., 2009; Zawrah et al., 2012; Zhu et al.,2012a, 2013, 2012b).

4.2.1.5. Woodceramic. As a part of a plant, wood is a heterogeneous, hygro-scopic, cellular, and anisotropic material, composed of fibers of cellulose andhemicellulose, held together by lignin. Wood exhibits microstructural fea-tures ranging from mm (growth ring patterns) via mm (tracheidal cell pat-terns, macro- and micro-fibril cell wall textures) down to nm scale(molecular cellulose fibers and membrane structures of cell walls). Since allthe cell walls in the wood link together to form a frame, the body of thewood is then divided into innumerable rooms by this frame. Generally,woodceramics are fabricated through three steps: (i) formation of bio-car-bon template by pyrolyzing the wood materials; (ii) infiltration of the bio-carbon template with ceramic precursors; and (iii) calcination to formceramics and remove organic materials. Carbides and carbide compositesare the most reported ones, due to carbonaceous nature of the woodtem-plate, but a range of ceramic materials, including oxides, nitrides, and zeo-lites, have also been produced by employing proper precursors and reactionroutes. Woodceramics is a three-dimension interconnected porous material,so it can be impregnated with metal (e.g., aluminum, magnesium and ortheir alloys) to fabricate ceramic�metal composites with interpenetratingnetwork (Di et al., 2003; Xian-qing et al., 2002; Zhang et al., 2003, 2010).There are millions of species of trees all over the world and they all containunique structures. In the work by Zhang et al. (2010) different structures ofwoodceramics with the composition of titanium carbide (TiC) were illus-trated. This material is supposed to be used in many industrial fields, suchas electrical brush, brake disks and bearing bush, electromagnetic shieldingcabin, and so on.

Xie et al. (2002a) used woodceramics in order to fabricate composites withinterpenetrating networks by high pressure infiltration. It was reported that thecomposites which were infiltrated by Al�Si alloy had much less uninfiltratedregion than those infiltrated by Al. This is supposed to be related with the addi-tion of silicon, which increases fluidity of molten alloy and improves the wetta-bility of woodceramics by molten aluminum. The results showed that thefabricated materials exhibit superior compressive strength and bending strengthwhen compared with uninfiltrated woodceramics. In addition, fractographicanalysis of the composites demonstrated that there is plastic deformation of themetal phase and cleavage fracture of woodceramics, suggesting that crack bridg-ing by the ductile phase is one main toughening mechanism in those materials.In another investigation done by the same authors, the mechanical propertiesand damping behavior of woodceramics/ZK60A Mg alloy composite were stud-ied (Xie et al., 2002b). It was reported that the ZK60A alloy infiltrated most of

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the pores and the WCMs/ZK60A composite has obvious three-dimension inter-penetrating network structure. The bending strength and compressive strengthof WCMs/ZK60A composite are improved remarkably by incorporation of theZK60A Mg alloy. The elastic modulus increases two times compared with thatof woodceramics. WCMs/ZK60A composite has excellent damping asset anddamping values increase with testing temperature and decrease with vibrationfrequency. The damping mechanisms of WCMs/ZK60A composite at low tem-perature may be attributed to intrinsic damping of components and dislocationdamping in the matrix. At elevated temperature, interface damping is thoughtto play a dominant role in the damping of the resultant composite.

4.2.1.6. Palm oil clinker (POC). In the palm oil mill, after it is being processed,the oil shells are used as burning fuel and as a result, the waste produced by thisprocess is called the clinker. The clinker is normally treated as waste and has noeconomical value. Therefore, it will be very useful if the palm oil clinker can berecycled as a form of industrial material, thus eliminating waste and providing acheap option to manufacture concrete. It will be of much benefit to the construc-tion industry as to minimize cost at the same time preserve natural aggregates(Ahmad et al., 2007). Recently, Zamri et al. (2011) used POC as a reinforcingphase in Al matrix composite and studied the wear behavior of fabricated com-posite. The main component of POC is carbon in companion with Si and O.The composite samples were made out by the powder metallurgy route. Theresults showed that the presence of POC particles enhanced the wear resistanceperformance of the aluminum. In addition, the weight percent of POC particlesand applied load had an insignificant effect on the cumulative wear rates of Al/POCp composites. Even though, cumulative wear rate pronounced slightlyincreases with increasing the percentage of POCp at applied load 51. The wearmechanisms of the composite were predominated by subsurface crack andmicro-crack formation around the POCp which cause the wear. The POCp actsas solid lubricant at the contact surface and reduces the crack propagation.

4.2.1.7. Ramie. Ramie fibers are obtained from the bast/stem of the annualshrub (Boehmeria nivea family). They have the highest proportion of celluloseand the lowest proportion of lignin in comparison with bamboo and wood. It iswell known that the tensile strength of plant materials depends mainly on theamount of cellulose present. It has been used as decorative fabrics, wallpaper,sewing thread, furniture covers, and reinforcing fiber for polymer compositesrather than construction material due to their excellent fiber characteristics suchas good resistance to bacteria, mildew, and insect attack, durability, easy dying,high stability in alkaline, and mild acids media. Further, exhibition even greaterstrength when wet can be counted as its advantages. Compared with other natu-ral cellulose fibers, harsh and rigid handling of ramie fiber, poor elastic recovery,and resiliency and easy wrinkling affect its performance and decrease the wear

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properties. In addition, soaking of ramie fibers are complicated process becausethe fibers are surrounded by gummy substances, which have to be removed withchemicals. This process is expensive and difficult to control, often resulting in adamaged fiber (Cheng and How, 1996; Lu et al., 2006; Mohanty et al., 2002). Inthe work by Shihong et al. (1994), ramie fibers were used in order to fabricate anew kind of super-hybrid composite material. Because of strong combination offibers in natural state and in order to prevent the inevitable fiber damage duringthe separation process, the ramie fibers were used directly and in its naturalstate. The authors sandwiched ramie fibers with aluminum sheets and reportedthat the tensile strength of the new material is 24% higher than that of the alumi-num used, but the Young’s modulus is 3.5% lower. If density is taken into con-sideration, the specific modulus of RF/A is 70% higher than that of aluminum,and the specific strength is twice as high. Laminating with ramie fiber decreasesthe density by »40%.

4.2.1.8. Bamboo. Bamboo is an abundant natural resource in Asia and SouthAmerica and is recognized as one of the most attractive candidates for strength-ening polymer or metallic matrices. It has several advantages such as (i) theenvironmental load is small, because it is renewable yearly and grows rapidly,thus it is easy to regenerate after cutting, (ii) the bamboo fiber has relativelyhigh strength compared with other natural fibers such as jute and cotton, and(iii) the high strength with respect to its weight is derived from fibers longitudi-nally aligned in its body. Therefore, bamboo fibers are often called “natural glassfiber” (Okubo et al., 2004). Bamboo was used in the preparation of preforms forinfiltration of Al 2024 alloy and pure Cu by Lin and Lin (2007). In order to formporous SiC with bamboo structure, samples taken from the culm of the bamboowere pyrolyzed and then siliconized by piling Si. The Cu infiltration occurred attemperatures as low as 1100�C and became significant at 1200�C. After infiltra-tion at 1300�C for 4 hr, there was still »5% of residual porosity. The compres-sion behavior of SiC/Cu depended on the degree of metal infiltration. For lowdegree of infiltration (low infiltration temperatures), the composite samples frac-tured like the bamboo-structured porous SiC. For high degree of infiltration, thecomposite samples behaved like copper. For the case of Al alloy, infiltrationoccurred at 900�C. Higher infiltration temperatures would result in significantformation of Al4C3, which gradually decomposes in humid air. For effectivereinforcement strengthening the interface interactions have to be carefully con-trolled. This can best be achieved by balancing the reaction zone (time and tem-perature of fabrication) such that an adequate bond is assured yet excessivereaction is prevented (Sato and Mehrabian, 1976).

Osman and Ahmad (2013) sandwiched bamboo fibers with aluminum. Theyused polyurethane resin (PUR) for making bonds between bamboo’s fiber andaluminum sheets. They reported that the small gap that exists between bamboolaminates seemed to be favorable spot of wrinkling initiation on aluminum

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sheet. The failure of composites during the bending test always initiated fromthe gap of edge joints of bamboo slabs on the upper layer and, consequently wasfollowed by formation of wrinkling on the upper aluminum sheet.

In the investigation by Li et al. (1994, 1996), the fabrication method ofreformed bamboo and reformed bamboo/Al composite and their impact proper-ties were studied. In order to make reformed bamboo, natural bamboo culm wasseparated longitudinally into several parts. Then the bamboo strips were heatedin a container to adjust the moisture content to certain value. The strips werethen compressed with a compressor to obtain the required compressive ratio.Finally, under certain pressure, the strips were pressed for 3 hr for the purposeof fixture. For fabricating the composite, reformed bamboo was applied to rein-force aluminum sheet. Aluminum alloy sheets outside the composite can protectthe reformed bamboo from absorbing water in air and from rotting. The impacttest results showed that the toughness of reformed bamboo decreases and itsstatic strength increases, and this effect becomes stronger when the reformedbamboo has a higher compressive ratio. On the other hand, the addition of alayer of glass fiber can further increase the impact toughness of reformed bam-boo. From the viewpoint of impact, the composition of reformed bamboo withaluminum alloy sheets improves the overall performance.

In the work of Alaneme and Adewuyi (2013), recently, the ash produced dur-ing the incineration of bamboo leaf was used as a reinforcing phase incorporat-ing with Al2O3. The main component of bamboo leaf ash are SiO2 (76.4 wt.%),Al2O3 (5.04 wt.%), MgO (2.05 wt.%), K2O (5,76 wt.%), CaO (6.68 wt.%), andFe2O3, which are stable phases or can be converted to other stable and desirablephases during fabrication rout. Bamboo leaf ash with Al2O3 powders with theratio of 0:10, 2:8, 3:7, and 4:6 wt.% were introduced to molten alloy throughdouble stir-casting method. It was reported that, although increasing the weightpercentage of bamboo leaf ash leads to decrement of hardness and tensilestrength, this decrease is insignificant. On the other hand, the fracture toughnessof all hybrid composite compositions (Al/BLA/Al2O3) was superior to that of thesingle reinforced composite. Bamboo leaf ash is cheaper than alumina, thus partof alumina in the Al/Al2O3 composites can be replaced with bamboo leaf ash.

4.2.1.9. Rattan. Rattan is a tropical plant with slender stems 2�5 cm diameterand long internodes. It is a very good material mainly because it is lightweight,durable, and flexible. Rattan has three different sizes of interconnecting pores:»300 mm (metaxylem vessel), 6�20 mm, and 1�3 mm. The total porosity of rat-tan is larger than those of wood or bamboo. Therefore, it is expected thatceramics with rattan structure would be easier for pressureless infiltration ofmolten metal and become a good reinforcement for metal�ceramic composites.Lin and Lo (2011) have used rattan as starting material in order to fabricate ofSiC porous with rattan structure.

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The bio-carbon can be used as a reactant as well as a template in the reactionsynthesis of carbide. During the synthesis, the structure of the bio-materials isretained. Nevertheless, oxide ceramics can also be fabricated via sol�gel process-ing (Sieber et al., 2001). So far, the most widely used bio-materials for carbidesynthesis are wood and bamboo because of their availability and their unique,hierarchical, anisotropic pore structures (Martınez-Fern�andez et al., 2000; Qianet al., 2004b). The rattan cane was carbonized at 1100�C in Ar atmosphere for 4hr. The product is charcoal with rattan structure. The rattan charcoal (bio-car-bon) was then placed in alumina crucibles with Si powder (98% purity) andreacted in a tube furnace in flowing Ar at 1500�C for 1�16 hr. After the reac-tion, the charcoal would convert into SiC.

Eventually, porous SiC was pressureless infiltrated by 2024 aluminum alloy at1400�C for different times under flowing Ar. The infiltration depended on thealloy composition and reaction temperature. Increasing amounts of Si in Alalloys increases the wettability of the Al�Si melts on the ceramic surface (Frageet al., 2003; Pech-Canul et al., 2000c). Spontaneous infiltration is promoted bycapillary forces. Furthermore, higher Si concentration in Al prevents the decom-position of SiC and prohibits the formation of Al4C3 (Frage et al., 2003; Pech-Canul et al., 2000a; Viala et al., 1997). However, high Si concentration willembrittle Al alloys. The results of compression and bending tests demonstratedthat the SiC/Al�20 at.% Si composites reacted for 5 hr and had bulk density of2.3 g/cm3, open porosity of 19%, compressive strength 316 MPa and bendingstrength of 138 MPa. This is because the infiltration is better and the porosity ofthe samples becomes lower.

4.2.1.10. Kenaf. Kenaf is a warm season annual fiber crop closely related to cot-ton and jute. In the past, kenaf was used as a cordage crop to produce twine,ropes and sackcloth. Nowadays, kenaf fibers exhibit good potential as a rawmaterial for usage in composite products, paper products, building materialsand absorbents. The combination of excellent flexural strength and tensilestrength of kenaf fiber makes it a suitable material for a wide range of extruded,molded and non-woven products (Edeerozey et al., 2007). Although its maindisadvantage is high moisture absorption in the core, it offers some advantagessuch as rapid growth, large longitude, high yield, and good tensile strength. Atefiet al. (2012) used kenaf short and long fibers as core and aluminum sheets asskins to fabricate sandwich panels through layer method (hand lay-up). Theexperimental results showed that kenaf long fiber with rubber exhibited greatimprovement in tensile strength and compressive strength.

4.2.1.11. Eggshell. Previous studies have proved that chicken eggshell (ES) is anaviculture byproduct that has been listed worldwide as one of the worst environ-mental problems, especially in those countries where the egg product industry iswell developed. In the US alone, about 150,000 tons of this material is disposed

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in landfills (Shuhadah et al., 2008). Eggshell is a biomaterial containing 95% byweight of calcium carbonate and 5% by weight of organic materials. Waste ES isoften turned into a low protein supplement for animal feed making it a margin-ally profitable venture with supply exceeding demand. In addition the ES is anabundant byproduct of poultry industry. Eggshell also has a relatively lower den-sity compared to mineral calcium. These characteristics qualify ES as excellentcandidates for bulk quantity, inexpensive, low-load bearing composite applica-tions such as the automotive industry, trucks, homes, offices, and factories(Toro et al., 2007a, 2007b).

Recently, Hassan and Aigbodion (2013) used eggshell as a reinforcing phaseinto Al�Cu�Mg alloy. The eggshell was ground and calcined at 1200�C toobtain carbonized particles. Carbonized and uncarbonized particles were addedto the molten alloy by a two-step stir-casting method. The results showed thataddition of eggshell particles reinforcement to Al�Cu�Mg alloy increased thetensile strength (from 98.28 to 106.79 and 112.84 N/mm2 for Al�Cu�Mgalloy, composite with uncarbonized and carbonized eggshell, respectively), thehardness value of the Al�Cu�Mg/eggshell particulate composites (from 59.12to 65.41 HRF and 74.17 HRF for Al�Cu�Mg alloy, composite with uncarbon-ized and carbonized eggshell, respectively) with a slight reduction in impactenergy.

4.2.2. Industrial waste materialNumerous manufacturing and industrial processes, including electric utilitygenerators, steelmaking plants, aluminum extraction and refining plants createhundreds of millions of tons of nonhazardous industrial materials that can bereused or recycled in an environmentally responsible manner as substitutes forraw materials in the manufacture of consumer products, composite materials,roads, bridges, buildings, and other construction projects. Those nonhazardousindustrial materials, such as coal bottom and FA, foundry sand, broken andwaste glasses, construction and demolition materials, slags, electric arc furnacedust, and gypsum, are valuable by-products which as raw materials can evenimprove the quality of a product. This way, for instance, coal FA can enhancethe physical and mechanical properties of aluminum alloys. These actions canlead to save resources and energy, reduce greenhouse gas emissions, and con-tribute to a sustainable future.

In addition, industrial materials recycling preserves the natural resources bydecreasing the demand for virgin materials and saving the natural ones, conserv-ing energy through a decrease in the demand for products made from energyintensive manufacturing processes. In summary, this results in the saving ofmoney.

In this section the industrial waste materials which have been used in the fab-rication of MMCs will be introduced, and the physical and mechanical proper-ties of the final composites will be discussed.

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4.2.2.1. Waste glasses. Kondoh and Luangvaranunt (2003) and Tsuzuki andKondoh (2006) used waste glasses to fabricate Mg2Si/MgO/Mg composites.Waste glasses were used to synthesis Mg2Si and MgO phase through a solid-state reaction with Mg powder. It was found that Mg and waste glass compos-ite billets with a fine structure can be produced via rapid plastic working(RPW). The starting temperature of solid-state synthesis between Mg andSi�O decreased due to the refinement effect of the composite billets. Mg2Si/MgO dispersed Mg composites with an ultrafine structure were producedfrom the billets after preheating at a low temperature. The properties of thecomposites produced with waste glass were almost identical to those of Mgcomposites produced by using conventional SiO2 particles. This is due to theeffects of refinement and dispersion produced by the combination of hotextrusion and RPW.

4.2.2.2. Electric arc furnace dust (EAFD). Slag and dust are generated duringthe manufacture of steel via the smelting of scrap metal. EAFD is considered ahazardous solid waste because it contains small amounts of Pb, Cr, and Znoxides which are evaporated at high temperatures above the steel bath and con-densed in the off-gas systems. Because of the large quantities of slag and dustproduced and the present expensive manner by which it is discarded, new meth-ods for treating and using these solid wastes are required. Flores-V�elez et al.(2001) fabricated Al/EAFD MMC through powder metallurgy powder tech-nique. The hardness and compressive strength of the sintered compacts wereinvestigated to compare the mechanical properties of the composite material asa function of the EAFD content. The XRD result considered EAFD particles asfranklinite (ZnO ¢ Fe2O3) and cincite (ZnO), with some small amount of hema-tite (Fe2O3). The EAFD contained small amounts of Pb, Cu, Si, and As oxidesand was mainly composed of nanometric spherical particles with diameterbelow 300 nm. Chemical reaction between EAFD constituents such as SiO2,ZnO, and FeO and Al matrix cause the formation of some intermetallic com-pounds in the matrix. The possible chemical reactions between the aluminumpowder and the constituents of the solid wastes are as follows:

36 2 FeO sð Þ C Al sð Þ ! 16 2 Al2O3 sð Þ C 36 2 Fe sð Þ (1)

36 2 ZnO sð Þ C Al sð Þ ! 16 2 Al2O3 sð Þ C 3 6 2 Zn lð Þ (2)

36 4 SiO2 sð Þ C Al sð Þ ! 16 2 Al2O3 sð Þ C 36 4 Si sð Þ (3)

16 2 Fe2O3 sð Þ C Al sð Þ ! 16 2 Al2O3 sð Þ C Fe sð Þ (4)

Based on thermodynamic considerations the occurrence of these chemicalreactions during sintering at 620�C is possible. The outcomes showed thatincreasing the amount of EAFD up to 10 wt.% results in an increment of hard-ness and compressive strength of composite to 74 HV and 248 MPa,

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respectively. Beyond this limit, increasing the ceramic particle content weakensthe material.

4.2.2.3. Slag. Besides electric arc furnace dust, slag is one of the major wasteproducts of the steelmaking industry, generated each year in significant volumesof thousands of tons. This gives the potential for manufacturing added valueend products. In the mentioned work done by Flores-V�elez et al. (2001), granu-lated slag (GS) was added as a reinforcing phase to the Al matrix through metal-lurgy powder route. Slag particles were irregular with a mean particle size of35 mm. The major components in the GS were CaO, SiO2, and FeO, with vari-able amounts of MgO, Al2O3, and MnO. In general, the slag coming from thesteelmaking industry always contains small amounts of iron. Therefore, in addi-tion to mentioned chemical reactions (1�4), formation of the FeAl3 intermetal-lic compound can be expected:

16 3 Fe sð Þ C Al sð Þ ! 16 3 FeAl3 sð Þ (5)

Flores-V�elez et al. (2001) reported that the observed increment in the hard-ness and compressive strength values of composite with 10 wt.% GS can beattributed to the presence of the Al�Fe intermetallic compounds produced dur-ing the heat treatment of composites. The result showed that the mechanicalbehavior of the Al/EAFD composites was inferior to that observed using GS asreinforcement.

The investigation by Mantry et al. (2013) showed the solid particle erosionresponse of plasma sprayed composite coatings using an industrial waste prod-uct (i.e., copper slag) and aluminum. The influence of five operating parameterssuch as impact velocity, erodent size, erodent temperature, impingement angle,and aluminum content of feed stock with four different levels each, on perfor-mance output (i.e., erosion rate) were studied by using of Taguchi’s L16 orthogo-nal array design. The results demonstrated that impact velocity is the mostinfluencing factor on the erosion wear rate of coated samples. Maximum erosiontook place at an impingement angle of 60� showing the semi-ductile response ofthe coating to solid particle erosion.

4.2.2.4. Red mud. Red mud is the caustic insoluble waste residue generated byalumina production from bauxite by the Bayer’s process at an estimated annualrate of 66 million tons in the world. Based on the Bayer process, it is assessedthat 3.45 tons of bauxite will yield 1 ton aluminum and 1.45 tons red mud.Under normal conditions, when 1 ton of alumina is produced nearly a ton of redmud is generated as a waste. In terms of metal production, the ratio of aluminumto red mud is 1:2. This waste material has been accumulated at an increasing ratethroughout the world. In general, application of red mud in industry is restrictedto recovery of some metal values like titanium, vanadium and zinc as well as the

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production of ceramics, cement, bricks, pigments, glazed sewer, etc. (Grigorevaet al., 1977; Prasad and Acharya, 2006; Thakur and Das, 1994).

The tendency for production of MMCs with the low-cost constitutes as astarting material stimulated the interest toward utilization of red mud whichcontains major constituents like Al2O3, Fe2O3, TiO2, and Na2O as reinforcingphase in metallic matrices.

An investigation by Acharya et al. (2008) reported that Al composites rein-forced with red mud has better wear resistance than aluminum, and that 30% byweight results in improved erosion properties compared to 10% and 15% byweight of red mud sample. In addition, the influence of impingement angle onerosive wear of aluminum red mud composite under consideration exhibits brit-tle behavior with maximum wear rate at 45� impingement angle. It can beunderstood from optical microscopic examination of the eroded surface thatmaterial removal takes place more easily at low impact angles owing to plough-ing and cutting.

Rajesh et al. (2012) used the Taguchi method for minimizing the specific wearand coefficient of friction in aluminum-based red mud MMCs. The compositessamples were fabricated by the powder metallurgy route and an experimentalplan, based on an L27 Taguchi standard array. The orthogonal array, signal-to-noise ratio and analysis of variance (ANOVA) were used to investigate the influ-ence of the parameters, applied load, sliding velocity, % of reinforcement, andhardness of the counterpart material. It was reported that the lowest specificwear occurs at high load, low sliding velocity, low percentage of reinforcement,and medium level counterpart material hardness. The results of ANOVArevealed that reinforcement percentage has the greatest influence on the specificwear rate and coefficient of friction. In the work of (Prasad and Acharya, 2006),red mud was introduced to pure aluminum by the stir-casting route. Theobtained results indicate that increasing the volume fraction of red mud causesreducing the wear coefficient of the fabricated composites, whilst increasing theload-on-pin and rotating speed make it increase.

4.2.2.5. Industrial sludge. Automotive painting operations generate variouswastes: exhaust spray-booth air that contains volatile organic compounds(VOCs); spent spray-booth scrubber water that contains VOCs; and paintsludge. Almost 2555�4380 tons of paint sludge are produced annually in anauto-manufacturing plant (Khezri et al., 2012; Kim et al., 1996). One of themain factors that make transportation of sludge costly procedure is the volumeof sludge. In order to reduce sludge volume, some automotive assembly plantshave been drying the sludge. Another process that could be used for volumereduction is pyrolysis of sludge. The pyrolysis of paint sludge not only results ina smaller volume than that achieved by drying, but also could produce a char.The char could be put back into the scrubber water as an adsorbent to removeVOCs (especially nonpolar VOCs) from the exhaust booth air or be used in

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MMCs or polymer�matrix composite as a reinforcement or filling phase,respectively. Paint sludge which is pyrolyzed under N2 or NH3 atmosphere con-tain crystalline CaTiO3, BaTiO3, TiO2, amorphous alumina, carbon, and crystal-line phase of titanium nitride (Kim and Pingel, 1989; Nakouzi et al., 1998).

For the first time, Nakouzi et al. (1998) used pyrolyzed paint sludge as a rein-forcing phase in Al matrix. A typical sample of paint sludge consists of approxi-mately 35.1 wt.% ceramic composites, 36.5 wt.% offgases, and the rest arevolatile liquid compounds. In order to pyrolyze paint sludge, three methods �listed in Table 10 � were used. The ceramic powders obtained from pyrolyzingthe paint sludge were analyzed by XRD. The phases AlN, BaTiO3, TiN, and andTiO2 were detected.

The composites were fabricated by the powder metallurgy route, using10�70 wt.% of pyrolyzed paint sludge. It was found that the density of theMMCs increased as the ceramic content was augmented from 10 to 30 wt.%, butdecreased for the 50 wt.% composition due to poor compaction. When theceramic content was increased to 70 wt.%, the fabrication method failed. Itshould be mentioned that during characterization, the presence of Al4C3, whichinitiates corrosion in Al based matrix composite, was not detected in the fabri-cated composites.

Using the spark plasma sintering technique, Fukumoto et al. (2006) investigatedthe effect of addition of a-alumina derived from heat treated sludge on the mechani-cal properties of Al matrix composites. It was reported that reinforcement content,pressure, and heating temperature affected the bending strength of composites. Ata-alumina content of 2 wt.%, the composite material showed the highest bendingstrength, because the agglomerated sludge obstructed the crack propagation.

4.2.2.6. Recycled hard particles. As industry responds to public demands thatresources should be conserved and the environment protected, recycled materi-als are becoming increasingly important. Hardmetal wastes are a part of metalscrap. With hardmetal prices rising, their recycling has become an urgent issue.One of the old ways of materials recycling is to produce powder materials fromworn products. In this context, secondary producers have to pursue new tech-nologies and other innovations (Kawatra and Ripke, 2002; Zimakov et al., 2003).

�Zikin et al. (2013) used recycled hard particles of WC�Co, TiC�NiMo, andCr3C2�Ni as a reinforcing phase to enhance the wear properties of NiCrBSi

Table 10. Pyrolysis procedures for obtaining ceramic phases from paint sludge (Nakouzi et al.,1998).

Name ofsamples Procedure

I/600 Pyrolysis of dried paint sludge up to 600�C at an approximate rate of 5�C/min with a hold of 2 hrI/N2/1000 Pyrolysis of I/600 sample at 1000�C (5�C/min, 4 hr hold time) in a dynamic nitrogen atmosphereI/NH3/1000 Pyrolysis of I/600 sample at 1000�C (5�C/min, 4 hr hold time) in a dynamic ammonia

atmosphere

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composite hardfacings via plasma transferred arc (PTA) process. The NiCrBSimatrix powder was mixed with recycled cermet particles (WC�Co, TiC�NiMo orCr3C2�Ni) with ratio of 60/40 vol.%. NiCrBSi hardfacing with 40 vol.% of WC/W2C hard particles was used as reference material. Wear and erosion tests weredone for all the samples to characterize the composite coatings. The experimentalresults showed that under 3-body abrasive wear conditions and in comparison withWC/W2C reinforced system, hard particle reinforced NiCrBSi hardfacings showaverage wear resistance. This can be explained with lower hardness values of cermetzones and high interparticle distances inside the matrix. In addition, under impact/abrasive conditions, high wear resistance of cermet particle reinforced hardfacingswas achieved. This can be attributed to the microstructural features of the cermetreinforced hardfacings. Cermet particles are lower severe stress concentrators com-pared toWC/W2C particles. The erosion wear results also showed almost no depen-dence of the TiC�NiMo reinforced coating on oblique and normal impact angles.In both cases a high wear resistance was obtained. This can be ascribed to the doublestructuring of the hardfacing, where spherical shape fine TiC-based precipitationssupport and increase resistance of thematrix material.

4.2.2.7. Mines waste colliery shale. In the mining of coal there is generally pro-duced a considerable quantity of shale or other argillaceous substance whichordinarily is valueless and is discarded, forming the dumps that are to be seen atnearly all collieries. In some few instances this shale has been found suitable forbrick-making by the usual process and some other minor uses have been foundfor small quantities of it, but ordinarily it is valueless and is an expense to dump.Furthermore, because of the combustible it contains the heaps of shale generallytake fire and slowly burn, with attendant disadvantages of smoke nuisance, etc.(Knibbs and Pehrson, 1939).

Recently, Siva et al. (2013a, 2013b) introduced colliery shale into aluminumto investigate the mechanical properties and machinability of composites pro-duced by the vortex melt technique, and compared them with those of Al/SiCand Al/Al2O3 composites. The colliery shale was thermally treated in high tem-perature plasma reactor in argon atmosphere to convert the material to a usefulin-situ ceramic composite. The chemical compositions of colliery shale are pre-sented in Table 11.

Colliery shale is basically alumino-silicate. At high temperature in plasma orin a tubular furnace, the material dissociates into alumina and silicon carbideand carbon. Possible chemical reactions that occurred during carbothermal reac-tion in the plasma reactor/tubular furnace are shown in Eqs. (6) through (8):

C sð Þ C SiO2 sð Þ ! SiO gð Þ C CO gð Þ (6)

2C sð Þ C SiO gð Þ ! SiC sð Þ C CO gð Þ (7)

SiO2 C 3C ! SiC C 2CO (8)

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The results of mechanical testing show that stiffness, tensile strength, andelongation values of the Al�colliery shale composites are better when comparedwith those of the Al�Al2O3 composites. Analyses of the stress�strain curvesshow that the Al�colliery shale composites exhibit higher toughness than theircounterparts of Al�Al2O3. In addition, the wear resistance offered by theAl�colliery shale composite is greater than that of the Al�Al2O3 compositesand pure aluminum. The machinability of the prepared samples in comparisonwith Al/SiC and Al/Al2O3/SiC composites was investigated by measuring thevariables such as radial force, feed force, cutting force, power consumption, andsurface roughness. The results demonstrated that the extent of tool wear is lessand surface roughness is minimum for specimens prepared with in-situ ceramiccomposites, in comparison to other composites made with Al2O3 and Al2O3—SiC, for the same volume percentage. Thus, the AMC made with colliery shaleas reinforcing phase showed better machinability in comparison to other AMCsfabricated in the work by Venkata Siva et al.

4.3. Waste material as reinforcing phase and matrix

Yoshikawa et al. (2005) have used aluminum lumps and soda-lime glass beads asstarting materials for fabricating Al�Al2O3 composite. They reported that achemical reaction occurred during the infiltration of molten Al to soda-limeglass beads as shown below:

4Al Lð Þ C 3SiO2 Sð Þ ! 2Al2O3 Sð Þ C 3Si in Al Lð Þð Þ (9)

Although the compressive strength of composites prepared from waste mate-rial is lower than that of Al�Al2O3 composites from pure material, this compos-ite has potential to be modified with varying some parameters such asinfiltration temperature, glass bead size, etc.

In another work, Yoshikawa et al. (2003) utilized Al scrap from windowframe and Al2O3 in order to fabricate Al (alloy)/Al2O3 composites. The compos-ite material is a product of displacement reaction between molten Al scrap andSiO2, as represented in reaction 9. The authors compared the mechanical prop-erties of Al (alloy)/Al2O3 composites with those of Al/Al2O3 and (Al�Fe)/Al2O3

composites fabricated using pure Al, and Al�Fe alloy, respectively. The micro-structural studies showed that the reinforcements were uniformly distributedthroughout the specimens examined. In addition, it was reported that the

Table 11. Chemical analysis of colliery shell before and after magnetic separation (Siva et al.,2013b).

Constituents Fixed carbon Al2O3 SiO2 Fe2O3 CaO MgO Volatiles

Before magnetic separation 18�20 25�28 35�40 4.5�5.0 0.4�0.5 0.4�0.5 21�22After magnetic separation 19�21 26�29 37�42 0.4�0.5 0.3�0.4 0.2�0.3 20�22

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composite materials prepared using scrap exhibited a little higher strength andhardness than Al/Al2O3 composites, and did not break into pieces upon fracture,as occurred in the case of (Al�10 at.% Fe)/Al2O3 samples.

Considering Al scrap and glass waste as starting material, Zhang (2012) fabri-cated Al scrap-waste glass composites by the stir-casting route. The mechanicalproperties and wear behavior of the composites were studied. The results dem-onstrated that by increasing the weight percent of glass particles in matrix up to10%, the tensile strength and hardness values increased while wear weight lossdecreased. The utilization of finer particles resulted in better mechanical proper-ties and enhanced wear resistance of composite.

Escalera-Lozano et al. (2008) used recycling aluminum from beverage con-tainers and RH ash in combination with SiC particles as starting material forfabrication of Al/SiCp composites. The degradation of composites in a humidenvironment was investigated using XRD, SEM, EDX, FTIR, and ICP. Theresults revealed that the use of RHA was beneficial to avoid degradation throughthe formation and subsequent hydration of the Al4C3 phase. However with con-densed moisture acting as an electrolyte, localized corrosion took place withaggressive damage manifested by the disintegration of the composite into a pow-dery mixture. Although other phases such as SiC, Si, and MgAl2O4 could alsowork as microcathodes, the relevant corrosion mechanism was mainly attributedto microgalvanic coupling between the Mg2Si intermetallic compound and thematrix.

The aluminum (AlMg1SiCu) and steel (AISI 1040) chip which are theremaining materials from sawing process were used by Guluzade et al. (2013),in order to fabricate aluminum matrix composite reinforced with steel chips.Different weight ratios of steel chips were added to the aluminum powdermatrix and after cold pressing the samples were sintered in a tube furnace filledwith sand and clay. This novel method of sintering was used in order to absorbthe metal gases and humidity and unify the heat transfer throughout the tube.The results show that increasing the steel chips (40 wt.%) enhance compressivestrength of the composite up to 640 MPa, while the composite containing20 wt.% steel chips has maximum fracture toughness.

5. Summary and scope for future studies

In the present critical review, an attempt has been made to provide an insightand information that might be proven useful for future investigations on thedevelopment of MMCs using industrial and agricultural waste materials. Onaccount of their attractiveness such as unique structure/morphology, chemicalcomposition, unlimited availability, and low-cost, industrial, and agriculturalwaste materials constitute viable alternatives to replace matrices and/or reinforc-ing phases in MMCs. Their performance depends strongly not only on their ori-gin, chemical composition, and morphology, but also on the choice of

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processing route (chemical or mechanical). It was found that with the exceptionof FA, up to date, no industrial-scale endeavor has been undertaken to use wastematerials in MMCs. Consequently, plenty of opportunities can be envisioned.For example, the use of cenosphere FA, Celceram® (Cellular Ceramic Materials,a specific fraction of power plant coal FA) could be a window of opportunity inthis field. Metal�cenosphere FA was introduced to the industry by the Centerfor Composite Materials and Center for Advanced Materials Manufacture, Uni-versity of Wisconsin-Milwaukee for the fabrication of Crumple zones, framemembers, frame reinforcements, pedestrian impact zones, batteries (lead�FAcomposite), intake manifolds, accessory brackets, low-load brackets, oil pans,valve covers, alternator covers, water pumps, etc. (Macke et al., 2012). Becauseof the unique properties of cenosphere FA, addressed in a comprehensive man-ner in Section 2, Celceram® seems to have potential as a reinforcing constituentfor fabrication of MMCs. However, in terms of high volume utilization, quitelikely, a series of economic barriers will have to be faced and overcome. Forinstance, diminishing the content of some deleterious constituents throughchemical modification in FA will result in an enhancement of the final physicaland mechanical properties of the composites. Most of the ceramic industrialwaste materials contain valuable oxides, such as Al2O3, SiO2, MgO, and Fe2O3,which can act as a reinforcing phases in their original composition or by a modi-fication by means of suitable composites processing routes. Furthermore, interms of chemical composition, reutilization of metallic industrial wastesrequires lower production costs. And although agricultural waste materials can-not be used in their original form in MMCs, choosing the most efficient proce-dure can yield ceramic phases with unique and valuable structure. As a richsource of silica, RH ash has been used in several investigations, but dependingon fabrication route (powder metallurgy, stir casting, etc.) its unique and valu-able structure can be negatively altered, or even worse, be destroyed. Therefore,besides the benefits of its chemical composition, setting a different and appropri-ate route can help keep its original morphology. Even though FA has alreadybeen industrialized as a reinforcing phase, RH ash and woodceramics as inher-ent porous materials with unique reinforcing architectures, still remain valuableopportunities for research and development. Likewise, the investigations aimedat controlling the chemical composition of ceramic phases obtained from indus-trial or agricultural waste materials and as a consequence, the physical andmechanical properties of the resultant composites, are limited and constituteanother window of opportunity. From this critical review it is apparent that dueto an underestimation or lack of information of the real potential of industrialand agricultural wastes, their application in the development and fabrication ofMMCs has been neglected. Although this endeavor perhaps might not competein volume terms, the generation of high added value products is by itself a signif-icant achievement. In addition, an environmentally friendly approach will becultivated by not having to produce synthetic phases.

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In conclusion, in order to facilitate their application toward new and innova-tive areas of economic interest and exploit their full potential as valuable resour-ces, industrial and agricultural wastes must be the subject of strong andsystematic studies. Currently, the research group on “Composite and AdvancedMaterials” at Cinvestav IPN-Saltillo is conducting an extensive investigation(Bahrami et al., 2015a; Soltani et al., 2015) on the use of rice-hull ash in MMCsand advanced ceramics, specifically for the preparation of aluminum matrixcomposites reinforced with rice-hull ash and the synthesis of silicon nitride andoxynitride for MMCs or porous ceramic-matrix composites (CMCs).

FundingMs. Niloofar Soltani and Mr. Amin Bahrami gratefully acknowledge Conacyt (NationalCouncil of Science and Technology, in Mexico) for granting a doctoral scholarship. Theauthors are also thankful to Cinvestav IPN-Saltillo for support in the research activities inthe field of composite materials. Cinvestav-IPN is the Spanish acronym of Center forResearch and Advanced Studies of the National Polytechnic Institute. Saltillo is the campuscity, in the State of Coahuila, Mexico.

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