CHAPTER 8CERAMIC/METAL

NANOCOMPOSITES

Noraiham Mohamad, PhDDepartment of Engineering Materials,Faculty of Manufacturing Engineering,

Universiti Teknikal Malaysia Melaka

Content Introduction Ceramic Matrix Nanocomposites

• Type of CMN• Processing

Metal Matrix Nanocomposites• Type of MMN• Processing

Introduction of Nanocomposites

Nanocomposites- solid structures with nanometer-scale dimensional repeat distances between the different phases that constitute the structure

Typically consist of:• Inorganic(host) solid containing an organic

component or vice versa• OR consist of 2 or more inorganic/organic phases in

some combinatorial form. Constraint- at least one of the phases or features be in nano size

• Extreme example: can be porous media, colloids, gels and copolymers

Core concept:• Combination of nano-dimensional phases with

distinct differences in structure, chemistry and properties.

Nanocomposites- can demonstrate different mechanical, electrical, optical, electrochemical, catalytic, and structural properties than individual component

Multifunctional behavior often more than the sum of individual components.

Introduction of Nanocomposites

Why Nanocomposites? Efforts to develop high-performance ceramics

for engineering applications:• Gas turbines• Aerospace materials• Automobiles and etc.

Conventional processed ceramics pose many unsolved problems:• Relatively low fracture toughness & strength• Degradation mechanical properties at high T• Poor resistance to creep, fatigue and thermal shock

Attempts to solve incorporating second phases

Second phases Either:

• Particulates• Platelets• Whiskers• Fibers

Disappointment in the use of micron-size range fillers

To solve the problem nano-size fillers• Passive control of microstructures by

incorporating nanometer-size second-phase dispersions into ceramic matrices.

Ceramic Matrix Nanocomposites

How to produce? Incorporation of a very small amount

of additive into ceramic matrix 2 mechanisms:

• Additive segregates at the grain boundary with a gradient concentration OR

• Precipitates as molecular or cluster sized particles within the grains or at the grain boundaries

Crucial: process optimization

Types of Dispersion 2 major type: Intragranular

• Aim to: generate and fix dislocations during the processing,

annealing, cooling, and/or the in-situ control of size and shape of matrix grains

• Role of nanodispersoids esp in nano scale, important in oxide ceramics, some of which become ductile at high T.

Intergranular nanodispersoids• Must play important roles in control of the grain

boundary structure of oxide (Al2O3, MgO) and nonoxide (Si3N4, SiC) ceramics which improves their high T mechanical properties

New Concept of Ceramic Metal Nanocomposites

MicrocompositeToughness increase, strength decrease, High Tem mech properties decrease

Microsize particle, platelet, whisker or fiber

• High strength & reliability, • excellent high temp mech properties

New Functions• Machinibility•Superplasticity

Composites powder for ceramic matrix nanocomposites

Eg. AL2O3/W, Mo, Ni, Cu, Co, Fe; ZrO2/Ni, Mo; MgO/Fe,Co,Ni

Processing method:• Conventional powder metallurgy• Solution chemical processes (eg. Sol-gel and

co-precipitation method) Powders- sintered in reductive

atmosphere to give homogenous dispersions of metallic particles within ceramic matrices.

Advantages Dispersing metallic 2nd phases into ceramics

improves mechanical properties (eg. Fracture toughness, strength and/or hardness)• Due to- microstructural refinement by the nanodispersions and

their plasticity.• Eg. Incorporating a small amount of ceramic or metallic

nanoparticles into BaTiO3, ZnO or cubic ZrO2. Improve mechanical strength, hardness and toughness ( important in

creating highly reliable electric devices operating in severe environment conditions)

• Dispersion of soft materials: adding hexagonal boron nitride to silicone nitride ceramic can enhance its fracture strength not only at Troom but also at high T up to 1500C. Some exhibit superior thermal shock resistance and machinibility

because of the characteristic plasticity of one of the phases and the interface regions between soft materials and hard ceramic matrices.

Advantages Wide variety of properties (magnetic, electric

and optical properties can be tailored)• Due to- size effect of nanosized metal dispersions.• Eg. Dispersing conducting metallic nanoparticles or

nanowires can enhance the electrical properties Ferromagnetism- value added for transition

metal particle dispersed oxide ceramic composites

Good magnetic response to apply stress- found in ceramic/ferromagnetic metal nanocomposites• Allow the possibility to remote sensing of initiation

fractures or deformations.

Example of ceramic nanocomposites

Silicon nitride (Si3N4) and silicon carbide/silicon nitride (SiC/Si3N4) composites

SiC/Si3N4 composites• Crystallites of microcrystals of Si3N4 and nanocrystals of SiC.• Produced from amorphus silicon carbonitride (obtained from

pyrolysis of compacted polyhydridomethylsilazane [CH3SiH-NH]m[(CH3)2Si-NH]n at about 1000C.

• Can withstand high temperature without oxidation or degradation (it is arises from the formation of a thin (few microns) silicon oxide layer.

Application: motor engines, gas turbines, catalytic heat exchangers and combustion system (aircraft and spacecraft applications)

Phase Diagram of system Si/B/C/N

Nanocomposites by mechanical alloying

• Originally invented to form small-particle (oxide, carbide, etc.) dispersion-strengthened metallic alloys

• High energy ball milling process (eg. Planetary mill)

• Advantages:• Simplicity & possibility of

scaled-up manufacturing.• Disadv: • Low purity and less

homogeneity of the structures

Planetary mill

Welding - solid state reactionCracking - crystal refinementMgB2-Magnesium diboride (superconductor)

Schematic of Mechanical Alloying

Principle of Mechanical Alloying Alloying occurs as a result of repeated breaking up and

joining (welding)• Can prepare highly metastable structures such as amorphus

alloys and nanocomposites structures with high flexibility. In addition of erosion and agglomeration milling

provoke chemical reactions- induced by the transfer of mechanical energy• Can influence the milling process and product properties• Idea is used to prepare magnetic oxide-metal nanocomposites

via mechanically induced displacement reactions between a metal oxide and a more reactive metal.

Induce chemical changes in nonmetallurgical systems, including silicates, minerals, ferrites, ceramics and organic compounds.

Displacement reactions Also called “Replacement reactions” Chemical reaction in which a less reactive element is

replaced in a compound by a more reactive one. For example, the addition of powdered zinc to a solution

of copper(II) sulphate displaces copper metal, which can be detected by its characteristic colour:

Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)

The copper is taken out of the solution and is deposited as a solid (s).

Principle of Mechanical Alloying May initiate self-propagating combustive

reaction by mechanical stresses developed during high impact hits. The nature depends on:• Thermodynamic parameters (esp. in highly exothermic

systems melting the reaction mixture & destroying ultrafine (nanocrystalline) microstructure)

• Microstructure of the reaction mixture• The way microstructure develops during the milling

process Milling mixture of ceramic and metal powders-

can induce mechanochemical reaction (an efficient way to produce nanocermet).

Reaction mixture reaction mixture (chemistry) A product of the act or process or an

instance of mixing and reacting ( COMBINING) two or more substances together causing reaction(s); mixture causing chemical transformation or change; the interaction of entities :• the state resulting from such a reaction• a process involving change in atomic nuclei

A product formed by the combination of two or more elements, compounds or substances together causing reactions and transformation of original items or substances.

Factor affecting reactions Thermodynamic of the metal/metal oxide

systems Kinetics of the exchange (displacement)

reactions during processing. Eg. Reduction of metal oxides with aluminium

nanocomposites of Al2O3 and metallic alloys (Fe, Ni, Cr; particularly binary alloy systems)• Ceramics with ductile metal inclusion produce toughened

materials with superior mechanical properties• Have better thermomechanical properties (higher

thermal shock resistance due to better metal-ceramic interfacial strength)

Eg. Nanocomposites by Mechanical Alloying

Nanoparticles of iron embedded in insulating alumina matrix (Nanocomposites with magnetic phases)• Mixture of iron and alumina powder• Process: direct milling using ball milling• Ave particle size: 10 nm range• Magnetization of iron particles: 25-40% less

than expected in bulk iron.

Fe3O4 particles dispersed in Cu (smaller magnetic particles embedded in nonmagnetic matrix)• Prepared by high-energy ball milling of Fe3O4 and Cu

powders OR mixture of CuO and metallic iron.• Both results in: magnetic semi-hard nanocomposites

with a significant superparagamagnetic fraction (due to very small particles sizes of the dispersed magnetic phase)

Hard magnetic SmCoFe phases in soft magnetic (Nanocomposite magnet)• Prepared by mechanical milling and heat treatment

Eg. Nanocomposites by Mechanical Alloying

Metastable nanocrystalline/amorphus structures • Result from repeated deformation and fracture

events during collisions between powders and balls.

• Plastic deformation in powders: formation of shear bands reach high dislocation densities shear bands degenerate into randomly oriented subgrains.

• Large surface area of nanocrystalline grains help in transformation of crystalline amorphus

Eg. Nanocomposites by Mechanical Alloying

What is shear band?? A shear band (or, more generally, a

'strain localization') is a narrow zone of intense shearing strain, usually of plastic nature, developing during severe deformation of ductile materials.

two X-shaped shear bands are clearly visible (see also the sketch on the right, where initial vertical scratches on the external surface help understanding the shear deformation).

Nanocomposites by Sol-Gel

Ideal starting materials: Aerogels Aerogels:

• High-porosity structures (nanosize pores)• Made by sol-gel polymerization of selected

silica, alumina or resorcinol-formaldehyde monomers in solution

• Extremely light (~0.5-0.001 g/cc-1)

Sol-Gel Generation of a dispersion of colloidal particles suspended in Brownian

motion within a fluid matrix. Colloids are suspension of particles of linear dimensions between 1nm and 1

m. The colloidal suspensions can subsequently convert to viscous gels and then to solid materials

Sol-gel preparation leads to the greatest possible homogeneous distribution of reinforcing phases in the host matrix.

Products have high purity and homogeneity, ease of processing and composition control

Sol-gel synthesis involving following steps

Ageing

Gelation

Drying

Densification

Sol-Gel

Drying Freeze drying or lyophilization is a drying method where the

solvent is frozen prior to drying and is then sublimed, i.e., passed to the gas phase directly from the solid phase, below the melting point of the solvent. It is increasingly applied to dry foods, beyond its already classical pharmaceutical or medical applications. It keeps biological properties of proteins, and retains vitamins and bioactive compounds. Pressure can be reduced by a high vacuum pump (though freeze drying at atmospheric pressure is possible in dry air). If using a vacuum pump, the vapor produced by sublimation is removed from the system by converting it into ice in a condenser, operating at very low temperatures, outside the freeze drying chamber.

Supercritical drying (superheated steam drying) involves steam drying of products containing water. This process is feasible because water in the product is boiled off, and joined with the drying medium, increasing its flow. It is usually employed in closed circuit and allows a proportion of latent heat to be recovered by recompression, a feature which is not possible with conventional air drying, for instance. The process has potential for use in foods if carried out at reduced pressure, to lower the boiling point.

Natural air drying takes place when materials are dried with unheated forced air, taking advantage of its natural drying potential. The process is slow and weather-dependent, so a wise strategy "fan off-fan on" must be devised considering the following conditions: Air temperature, relative humidity and moisture content and temperature of the material being dried. Grains are increasingly dried with this technique, and the total time (including fan off and on periods) may last from one week to various months, if a winter rest can be tolerated in cold areas.

In a typical phase diagram, the boundary between gas and liquid runs from the triple point to the critical point. Regular drying is the green arrow, while supercritical drying is the red arrow and freeze drying is the blue.

Nanocomposites derived from Aerogel

The product consists of:• A substrate (eg. Silica aerogel)• One or more additional phases (of any

composition or scale) Most commonly made- silica-based

nanocomposite system (can be used to other aerogel (alumina, etc.) precursor

A flower is on a piece of aerogel which is suspended over a bunsen burner. Aerogel has excellent insulating properties, and the flower is protected from the flame

Microstructure of aerogel-encapsulated phase nanocomposite

Fabrication of aerogel composites

Depending on when second phase is introduced:• Can be added during the sol-gel processing

(before supercritical drying)• Or added through the vapor phase (after

supercritical drying)- vapor phase infiltration• Or chemical modification of the aerogel backbone

(may be effected through reactive gas treatment)• Non-silica material is added to the silica sol

before gelation Produce varieties of composites

Second phase Can be:

• Soluble organic• Inorganic compound• Insoluble powder• Polymer• Biomaterial etc.

Must withstand the subsequent processing steps used to form aerogel (alcohol soaking, supercritical drying)

Prevent from settling before gelation (for bulk insoluble materials)

2nd phase- soluble organic or organic compounds

2 criteria• Added component MUST NOT interfere with

the gelation chemistry of aerogel precursor• MUST NOT leaching out during the alcohol

soak or supercritical drying steps Big problem- if a high loading 2nd

phase is desired

2nd phase- metal complex Better to use chemical binding agent (can bind to

the silica backbone & bond the metal complex) Many use to prepare nanocomposites of silica

aerogels or xerogel Nanocomposites of aerogel with metal atoms or

ions uniformly (atomically) dispersed throughout the material

Thermal post-processing- creates nanosize metal particles within aerogel matrix

Eg. Application: • Catalysts for gas-phase reactions• Catalysts for catalyzed growth of nanostructures

Vapor phase infiltration Through open pore network of aerogels Create various forms of aerogel-base

nanocomposites Almost any compound can be deposited

uniformly throughout aerogel Absorbed material in silica aerogel solid

phases by thermal or chemical decomposition

Eg. CNT have been deposited within pores of zeolites to create superconductor

Nanocomposites of silica-base aerogel matrices

Silica aerogel/carbon composites• Decomposition of hydrocarbon gases at T (200-450 C)• Carbon loadings from 1-800% (at low loading-C desposited

uniformly, at high-loading-C begins to localize at the exterior surf of composites)

• Have interesting electrical conductivity (> certain level) & higher mech strength than aerogel

Silica aerogel/silicon composites• Thermal decomposition of various organosilanes on silica gel

forms deposits of elemental silicon• Rapid decomposition of the silane precursor leads to deposits

localized near the exterior surface of the aerogel substrate• Nanocomposites with 20-30 nm diameter silicon particles

exhibits strong visible photoluminescence at 600nm

Nanocomposites of silica-base aerogel matrices

Silica aerogel/transition metal composites• organo/transition-metal complexes can be

used to thermally deposit metal compounds intermediate composites

• Intermediate composites can be converted to metal oxides, sulfides or halides

• Control loading- can be changed by repeated deposition steps

• Crystals of desired metal (range; 5-100 nm diameter)

Example Property

Metal Matrix NanocompositesMetal matrix composites (MMC) Materials in which rigid ceramic reinforcements are

embedded in a ductile metal or alloy matrix Combine metallic properties (ductility & toughness) with

ceramic characteristics (high strength and modulus), greater shear strength, compression strength, higher service temperature capabiilties.

Interest in MMCs increased over the past 30 years (aerospace and automotive industries)

Resulted from:• Availability of inexpensive reinforcements• Development of various processing routes- results in reproducible

microstructure and properties• Availability of standard or near-standard metal working methods

(can be utilized to fabricate the composites)

Discontinuous reinforced Metal matrix composites (MMC) Type of reinforcements: particulate, whiskers

or short fiber Most glamour: particulate (ease of fabrication,

lower costs and isotropic properties) Traditional processing routes: powder

metallurgy, spray deposition, mechanical alloying (MA) and various casting techniques.

All process: based on addition of ceramic reinforcements to the matrix (molten or powder form).

Weakness of MMCs Ex situ process- reinforcing phases are prepared

separately prior to composite fabrication Scale of the reinforcing phase limited to starting

powder size, normally ~µm (10µm – 1µm) Less interfacial reactions between

reinforcements and matrix Poor wettability between reinforcements-matrix

(due to surface structure & contamination) Properties highly controlled by size, volume

fraction of reinforcements and nature of matrix/reinforcements interfaces

Optimum: when fine, thermally stable

ceramic particulates dispersed uniformly in

metal matrix

MM Nanocomposites @ in-situ MMC

Reinforcements are formed in a metal matrix by exothermal chemical reactions between elements or between element and compound during fabrication of the composite.

Wide range of matrix materials (aluminium, titanium, Cu, Ni, Fe)

Second phase particles (borides, carbides, nitrides, oxides and mixtures)

Advantages of in-situ MMC Formation of reinforcements that

thermodynamically stable in matrix Reinforcement/matrix interfaces that

clean strong interfacial bonding Formation of reinforcing particles with

finer size, more uniform distribution in matrix yields better mechanical properties

Processing Variety of processing due to great

potential of in situ MMC for widespread applications:

Solidification process (most attractive)• Reinforcements formed in situ in molten

metallic phase prior to its solidification Controlled crystallization

• Crystallization of amorphus solid under proper heat treatment conditions

Mechanical alloying

Solidification Why attractive? (simple, economy

and flexibility) Factors affecting structures &

properties• Selection of solidification processing

techniques (solidification conditions)• Matrix alloy composition• Dispersoids

Microstructure refinement- rapid solidification processing (RSP)

RSP (rapid solidification processing)

Reduce solute segregation Enhance dispersion hardening (by reducing size of

reinforcing phases & modify their distribution) Ability to produce alloy composition not obtainable

by conventional processing Materials have excellent compositional

homogeneity, small grain sizes, homogenously distributed fine precipitates or dispersiods.

Eg of RSP:• Large undercooling & high cooling rate of Ti/B or Ti/Si alloys

in situ Ti-based nanocomposites containing large volume fraction of particles

• In-situ TiC particulate-reinforced Al-based composites

RSP in TiC reinforced AlMaster Material Ingot

Al, Ti, graphite powder(Tmelt, graphite lined

furnace, Ar)

Direct Chill CastChill block melt spinning

(rapid solidify in ribbon form)

MillingRibbons mill into powder

(100±250 µm)

Canned & DegassedExtruded(into rods)

ResultTiC particles of 40±80 nm

distributed uniformly in Al matrix with grain sizes of 0.3±0.85 µm

Weaknesses• presence of agglomerates TiC

(0.2±1.0 µm) accumulate at Al subgrain or grain boundaries

Control Crystallization of Amorphus Solid

Nanocrystalline materials either single or multi-phase polycrystals with grain sizes in nm.

Properties are different from and often superior to conventional polycrystals or amorphus solid:• Increase strength or hardness• Improve ductility or toughness• Reduce elastic modulus• Enhance diffusivity• Higher specific heat• Enhance CTE• Superior soft magnetic properties

How to produce? Crystallization of amorphus solid via heat treatment

Complete crystallization nanocrystalline materials Controlled crystallization of amorphus alloys

partially crystallized materials with nanosized crystallites embedded in residual amophus matrix (special nanocrystal/amorphus nanocomposite structure)

Excellent mechanical & magnetic properties Eg. Fe/Cu/Nb/Si/B alloys (BCC Fe solid solution and

10 nm diameter nanostructures embedded in amorphus matrix):• Excellent soft magnetic properties• High saturation flux density• Low magnetostriction• App: magnetic devices such as choke coils & transformers

Control Crystallization of Amorphus Solid

Example Property

Example Property