262 A. Nur Maizatul Shima, B.J. Shamsul and D. Mohd Nazree

© 2012 Advanced Study Center Co. Ltd.

Rev. Adv. Mater. Sci. 30 (2012) 262-266

Corresponding author: Nur Maizatul Shima Adzali, e-mail: shima@unimap.edu.my

MECHANICAL PROPERTIES, CORROSION BEHAVIOR,AND BIOACTIVITY OF COMPOSITE METAL ALLOYS

ADDED WITH CERAMIC FOR BIOMEDICAL APPLICATIONS

Nur Maizatul Shima Adzali1, Shamsul Baharin Jamaludin2

and Mohd Nazree Derman3

1Schools of Materials Engineering, University Malaysia Perlis (UNIMAP),Kompleks Pusat Pengajian Taman Muhibah, Jejawi, 02600, Arau, Perlis, Malaysia2Sustainable Engineering Research Cluster, University Malaysia Perlis, Malaysia

3Institute of Nano Electronic Engineering, University Malaysia Perlis, Malaysia

Received: February 29, 2012

Abstract. In recent years, the development and use of biomaterials are expected to continuouslyincrease over the coming decades as a result of ageing populations in the world. The biomaterialscommunity is producing new and improved implant materials and techniques to meet this demand.A lot of new materials that have been produced in biomaterials field such as cobalt, magnesium,and titanium based alloys reinforced with ceramic. Many types of ceramics that suitable asreinforcement for these alloys such as bioactive glass, hydroxyapatite (HA), wollastonite, calciumpyrophosphate, boron carbide, silicium nitride, fluoroapatite (FA) and so many others. This paperreviews the research works carried out in the field of composite metal alloys reinforced withceramic with special reference to their mechanical properties, corrosion and also bioactivitybehavior.

1. INTRODUCTION

Variety of materials including metals, polymers,ceramics and composites with the appropriatephysical properties and biocompatibility are chosenfor the fabrication of biomaterials. Metallic materialsare commonly used for load bearing implants andinternal fixation devices [1]. Current metallicbiomaterials such as cobalt based alloys (primarilyCo-Cr-Mo), austenitic stainless steels, titanium andtitanium base alloys and magnesium based alloysare the common materials used in orthopedics.

Recently, cobalt based alloys have become oneof the important metallic biomaterials because itexhibites low levels of corrosion and this alloyremains as a fixture in orthopedic surgery to thisday. Co based alloys are well known for their high

Young’s modulus, fatigue strength, wear resistance,good biotolerance and high corrosion resistance.The most of Co-based alloys have been fabricatedby using casting technique. This technique hasprovided lower initial costs. However, noticeablerestrictions that encountered with casting are coarsegrain size, non-uniform microstructural segregationand also lower tensile and fatigue strength.Fabrication via hot forging and powder metallurgyare expected to give better properties [2]. The rigidityof sintered Co based alloy is significantly lower thananalogue cast alloy, which is very importantrequirement from the point of view of bone restorationprocess.

Most of the research reports that pure porousCo based alloys have a lower resistance to pittingand crevice corrosion than solid materials [3-5]. It

263Mechanical properties, corrosion behavior, and bioactivity of composite metal alloys added with...

Sample Elastic modulus (GPa)

Bone 3.14Cast Co-Cr-Mo alloy 240Sintered Co-Cr-Mo alloy 8.1Composite with 10%volume addition of:Ca2P207 (calcium 37pyrophosphate)B4C (Boron caribide) 12Si3N4 (Silicon Nitride) 3.8

Table 1. The comparison of elastic modulus valuesof bone and investigated materials (GPa) [14].

happened because of the increased contact area oftheir surface with the electrolyte, which results inthe polarization current density being shifted towardshigher values. In general, corrosion resistanceincreases with increasing the density of material,which can be realized by introducing appropriateadditives such as ceramic [6].

In particular, the second very important group ofbiomaterials is ceramics, with much attention hasalready paid to calcium based biomaterials likehydroxyapatite (HAP with chemical formulaCa10(PO4)6(OH)2), due to their excellentbiocompatibility, structural and chemical similaritywith bone mineral compositions [7]. Otherbioceramics may be bioinert (alumina, zirkonia),bioresorbable (tricalcium phosphate), bioactive(hydroxyapatite, bioactive glasses, and glassceramics), or porous for tissue in growth(hydroxyapatite coating and bioglass coating onmetallic materials) [8-10]. As reported by Navarroet al. [11], the application of this ceramic materialas bone substitutes started around the 1970s andhas mainly used as bone defect fillers. HAP is highlystable in body fluids, interacts with bone tissue uponimplantation and enhances bone cell proliferation.

Little attention has been focused on bio-composite material based on metal alloys matrixfilled with ceramic, but the results shows that thisbio-composite is a promising candidate for implantsto improve bioactivity, mechanical properties andcorrosion properties [6,12,13]. Therefore, thisresearch presents the effect of ceramic addition inmetal alloy produced by various method. Furtherstudies are still of necessity for metal alloys toachieve significant improvement in both mechanicalproperties and corrosion resistance.

2. Co BASED ALLOYS COMPOSITES

The application of Co-Cr-Mo alloys as femoralcomponents have been widely used due to theirmechanical properties, good wear and corrosionresistances as well as biocompatibility [14].Gradzka-Dahlke et al. [14] have studied theinfluence of ceramic additives such as calciumpyrophosphate, boron carbide and silicon nitride onmechanical properties of the composite Co-Cr-Moalloy. These composites successfully fabricated viapowder metallurgy method. The results showed thatthe effect of examined fillers were different. The mostadvantageous properties were obtained for thecomposite with calcium pyrophosphate, with thehighest compactibility and good mechanicalproperties compared to two others. The compressive

Fig. 1. SEM microstructures of the specimens: (a)]ure Co–Cr–Mo alloy; (b alloy with 5 wt.% ofbioglass; (c) alloy with 10 wt.% of bioglass; (d) alloywith 15 wt.% of bioglass. Reprinted with permis-sion from [6], © 2009 Elsevier.

strength and yield point values are comparable withproperties of cast Co-Cr-Mo alloys. The influence ofthe other two additives (boron carbide and siliconnitride) on composite properties was insignificant.The influence of these three additives on the rigidityof composites is shown in Table 1, with the valuesof elastic modulus of the samples are lower thananalogous cast alloys. This characteristic isimportant for implants biomechanics characteristic.The rigidity of metallic prostheses is usually muchhigher than bone. Therefore, the load is mainlycarried by the implant. Their results show that thesecomposites can be alternative materials forbiomedical applications.

Another successful material that can act as ad-ditive to Co based alloys is bioglass [6], which thecomposite has been prepared by powder metallurgymethod. The addition of 5 wt.%, 10 wt.%, and 15wt.% of bioglass to the Co–Cr–Mo matrix, as wellas applying the rotary cold repressing and heat

264 A. Nur Maizatul Shima, B.J. Shamsul and D. Mohd Nazree

Fig. 2. Compressive stress-strain curves for AZ91(magnesium alloys) and AZ91-20FA (magnesiumreinforced with 20 wt.% FA) samples. Replotted from[13].

Fig. 3. Tafel plot for sample AZ91 (magnesium al-loys) and AZ91-20FA (Magnesium alloys with 20wt.% FA). Replotted from [13].

treatment have a significant influence on the micro-structure, mechanical properties and corrosion re-sistance of the composite in comparison with theCo–Cr–Mo sintered alloy. Fig. 1 shows themicrostructure of the composites after heattreatment, which composed of the matrix alloy ingrey areas, bioglass particle (white regions) andpores (dark regions). These elements are distributeduniformly throughout the material structure.

3. Mg BASED ALLOYS COMPOSITES

Razavi et al. [13] presented new nanocompositemade of AZ91 magnesium alloy added with nanoparticles of fluoroapatite (FA) which has beensuccessfully prepared by blending-pressing-sintering method. This composite is more suitablefor large degradable implants in load bearingapplications compared to a biomaterial with lowercompressive yield strength than natural bone [13].This composite was proved to have good mechanicalproperties and low corrosion rate. Fig. 2. shows theresults for compressive strength for AZ91-20FA andAZ91. AZ91-20FA have higher compressive strengthvalue (107±4 MPa , com]ared to AZ91 (88±5 MPa .The AZ91-20FA sample seemed to be in higheragreement than AZ91 magnesium alloy with thecharacteristics of the natural bone, havingcom]ressive yield strength of 130–180 MPa [15].

4. CORROSION AND BIOACTIVITYBEHAVIOR

4.1. Corrosion

Corrosion occurred as a result of chemical andelectrochemical reactions with its surroundingenvironment. Implants materials installed in humanbody are generally exposed to harsh aqueous

environment containing various anions such as Cl-,HCO3

-, HPO42-, cations such as Na+, K+, Ca2+, Mg2+,

organic substances, and dissolved oxygen [16-18].According to Yoshiaki et al. [19], Bishop et al. [20],and Sameljuk et al. [21], the incorporation of secondreinforcing phase into metal matrix not alsoenhances the physical and mechanical propertiesof a material, but it also can alter the material’scorrosion behaviour.

It was reported by Oksuita et al. [6] and Gradzka-Dahkle et al. [14] that the addition of filler to Co-Cr-Mo based composite can enhance the corrosionresistance. Although Co-Cr based alloys shows lowlevel of corrosion, however, when implanted into thehuman body, the metal-in-metal joint will experiencetribocorrosion, a form of metal degradation, whichmay lead to the release of metals ions into the body[22]. So, the addition of Ca and phosphate fromhydroxyapatite is known to reduce the susceptibilityof Co-Cr-Mo alloys to corrosion [6].Thus bioceramicas well as HAP play a twin role both in preventingthe release of metal ions (rendering it more corrosionresistant) and also in making the metal surfacebioactive [17].

In the research conducted by Razavi et al. [13],they have found that the addition of fluoroapatite (FA)nanoparticle reinforcements to magnesium alloyshas reduced the corrosion rate (Fig. 3). As seen inFig. 3, the corrosion resistance of AZ91-20FA ishigher than pure magnesium alloys (AZ91), whichconfirmed by the values of their corrosion currents(Icorr) and corrosion potentials (Ecorr).

4.2. Bioactivity behavior

Bioactivity test is an evaluation of apatite-formingon implant material in SBF (simulated body fluid).When a bioactive material is implanted in a animal

265Mechanical properties, corrosion behavior, and bioactivity of composite metal alloys added with...

Fig. 4. SEM microphotographs of all composite materials before and after immersion in SBF for various]eriods of time. Re]rinted with ]ermission from [6], © 2011 Ashding Publishings.

or human body, a thin layer rich in Ca and P formson its surfaces. This apatite layer can connects thisbioactive material to the living tissue. Materials ofvarious kinds bind to living bone through this apatitelayer which can be reproduced on their surfaces inSBF, and that apatite formed is very similar to thebone mineral in its composition and structure. Asbioactivity increases, apatite forms on the materialsurface in a shorter time in proportion to thisincrease. The formation of this apatite layers havemostly been analyzed by thin film X-ray diffraction(TF-XRD) [23,24], fourier transformed infraredspectroscopy (FTIR) [25,26] and scanning electronmicroscopy coupled engergy dispersive X-rayspectroscopy (SEM/EDXA) [27].

Roumeli et al. [28] have studied about bioactivitybehavior of SiO2–CaO–MgO glass-ceramic (51.6%SiO2, 35.6% CaO, and 12.8% MgO) andhomogeneous mixtures of 75% HA/25% SiO2–CaO–MgO (75:25) and 50% HA/50% SiO2–CaO–MgO(50:50) that were obtained by Transferred ArcPlasma (TAP) melting method. These compositeswere immersed in simulated body fluid (SBF)solution for various immersion periods of time. Theresults show that all materials showed the formationof an HCAp layer on top of every pellet. Both 50:50and 75:25 composites show rather thick apatite layer(5-6 m), while pure glass pellet shows much thinnerlayer (1-2 m). The microstructure of thesecomposites (Fig. 4) reveals for longer reaction times,the apatite layer covered a greater fraction of thepellet surface. As seen in Fig. 4, the apatite layer

consists of spherulitic apatite aggregates even af-ter 6 days of immersion in SBF solution.

The study about biomimetic apatite formation ona Co-Cr-Mo alloy by using wollastonite, bioactiveglass or hydroxyapatite has been conducted byCortes et al. [29]. This biomimetic method used topromote a bioactive surface on a Co-Cr-Mo alloy(F75). This metallic samples were chemically treatedand immersed for 7 days in SBF solution on a bedof these three bioactive materials (wollastonite,bioactive glass and hydroxyapatite), followed by animmersion in 1.5 SBF for 7 or 14 days withoutbioactive system. As a results, the surface of allsamples were formed a bonelike apatite layer, withwollastonite presented a higher rate of apatiteformation. By using XRD, hydroxyapatite wasdetected only on the samples treated withwollastonite after 7 days of immersion. This provedthat rate of apatite formation increases by usingwollastonite.

5. CONCLUSION

The composite metal alloys added with ceramic forbiomedical applications were reviewed from the pointof microstructure and mechanical properties andalso corrosion and bioactivity behavior. Investigationsabout these composites have found ceramic as aadditive can enhance the mechanical properties,reduce the corrosion rate, and accelerate theformation of an apatite layer on the surface, whichprovides improved protection for the metal based

266 A. Nur Maizatul Shima, B.J. Shamsul and D. Mohd Nazree

alloys matrix. Thus, we conclude that those com-posites can be good alternative materials for bio-medical applications.

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