JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Effect of sintering temperatureon tribological behavior ofCe-TZP/Al2O3-aluminum nanocomposite

A Bahrami1, N Soltani1,2 and MI Pech-Canul1

Abstract

10Ce-TZP/Al2O3 nanoparticles as reinforcement can be a good substitute in aluminum matrix composites prepared

through powder metallurgy. In this work, the effects of sintering temperature on the hardness, friction, and wear

characteristics of Al-10Ce-TZP/Al2O3 composites have been investigated. Ce-TZP/Al2O3 nanocomposites were synthe-

sized by the aqueous combustion method. About 7 wt% 10Ce-TZP/Al2O3-aluminum composites in the form of cylin-

drical samples were prepared at the sintering temperatures of 400�C, 450�C, and 500�C under an applied pressure of

600 MPa for 60 min. The experimental results show that the distribution of Ce-TZP/Al2O3 nanocomposite into the metal

matrix is homogenous and that at 450�C the hardness and wear resistance improved considerably. Formation of an oxide

layer is a manifestation that in nanocomposites produced at 450�C, worn surfaces were smoother and the total depth of

deformations were not severe.

Keywords

10Ce-TZP/Al2O3 nanocomposite, metal matrix composite, tribological behavior

Introduction

Aluminum metal matrix composites (MMCs) withhard ceramic particles are attractive structural mater-ials in defense and aerospace applications because oftheir low density, higher specific strength, stiffness,heat resistance, and resistance to wear.1,2 Differentceramic or intermetallic particles have been used asreinforcing materials such as Al2O3, SiC, andMg2Si.

3–5 Nanocomposites have found considerableattention due to their unique mechanical behavior.6,7

It has been reported that the mechanical properties ofaluminum matrix composites can be improved bydecreasing reinforcement particle from micro to nano-meter sized, but the clustered and agglomerated regionalso increases.8 As a new class of composite materials,ceramic–ceramic nanocomposites have both specificadvantages of ceramic and nanoparticles such as neu-trality, extraordinary high strength, as well as goodthermodynamic properties.9 Several zirconia–aluminacomposites have been recently developed as bearingcomponents.10 Y2O3 or ceria (CeO2) owing to theirstabilizing effect in tetragonal zirconia polycrystalline(TZP) prevent transformation of the tetragonal tomonoclinic phase under the external applied stresses

at ambient temperature.11 Recently, Nawa et al.12

developed a Ce-TZP/Al2O3 nanocomposite (withboth phases being of nanometer scale). Ce-TZP/Al2O3 nanocomposite possesses more flexibilitystrength compared to Ce-TZP. The homogeneous dis-persion of alumina phase in the TZP matrix not onlydoes improve mechanical properties but also increaseshydrothermal stability of tetragonal Zirconia.13

Powder metallurgy method has been considered asan effective method for improving the reinforcementparticle distribution not only in microcomposites butalso in nanocomposites.14 Various cast aluminummatrix composites (AMCs) reinforced with zirconare being studied but there have been fewer

1Centro de Investigacion y de Estudios Avanzados del IPN Unidad Saltillo,

Av. Industria Metalurgica, No. 1062, Parque Industrial Ramos Aripe-

Saltillo, Ramos Arizpe, Coahuila, Mexico, 259002Department of Materials Science and Engineering, Sharif University of

Technology, Iran

Corresponding author:

N Soltani, Department of Materials Science and Engineering, Sharif

University of Technology, Tehran, Iran.

Email: niloofar.soltani@cinvestav.edu.mx

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investigations on fabricating them by powder metal-lurgy techniques.12 The use of liquid metal method hasdisadvantages like contamination of the matrix mater-ial, segregation due to the relatively high specific grav-ity of zircon, hard to be controlled, and the highmelting points usually makes the liquid techniqueinadequate.15 This technique also is not economicalfor expensive process equipment but powder metal-lurgy techniques are low-cost methods for synthesizingcomposite.16 Many studies have examined the effect ofreinforced materials on the wear resistance of MMC;especially, the aluminum matrix under the dry slidingconditions. SiC, TiC, TiB, B4C, and Mg2Si are used asceramic reinforcement particles that limit the amountof plastic deformation in the matrix.17–19 ZrO2 has thehighest fracture toughness of any advanced technicalceramic. Its toughness, mechanical properties, andcorrosion resistance make it ideal for medical andselected wear applications. Al2O3 also shows goodmechanical and electrical properties, wear resistance,and corrosion resistance.20 Uniform distribution ofalumina into tetragonal polycrystalline zirconia alsoresults in improvement of flexibility. In spite of themuch attention lately devoted to increase the wearresistance of AMCs, no report has been found in therecent literature on the improvement of wear proper-ties of aluminum reinforced with 10Ce-TZP/Al2O3

phases. Many experimental studies on the wear behav-ior of AMCs have shown that the improvement in thewear resistance of aluminum matrix depends on thereinforcement’s particle type, particle size, andvolume fraction of the particles.21,22 The wear testsare usually conducted under sliding wear conditionsusing pin-on-disk or block-on-ring tests. A novelmethod for manufacturing a new generation ofAMCs by the addition of 10Ce-TZP (tetragonal zirco-nia polycrystal)/Al2O3 nanoparticles to aluminum viapowder metallurgy is proposed. The objective of thepresent research work is to study the effect of sinteringtemperature on the microstructure, mechanical prop-erties, and wear resistance of Al-Ce-TZP/Al2O3 nano-composites produced for the first time by the powdermetallurgy method, and establish the optimal process-ing conditions for enhanced mechanical properties.

Experimental

Aluminum nitrate, cerous nitrate, and zirconyl nitratewere the starting materials as the oxidizers, and alsoused as a source of cerium, zirconium, and aluminumfor the combustion synthesis. Glycine, urea, ammo-nium acetate, hexamethylenetetramine, and ammoniumnitrate were used as fuels. The production method of10Ce-TZP/Al2O3 nanocomposite powders were dis-cussed in detail by Soltani et al.7

The ball milling of aluminum and 10Ce-TZP/Al2O3

powders was conducted using a planetary apparatus toproduce composite powder particles. The powders andhardened 10mm diameter steel balls were sealed in ahardened stainless steel vial at room temperature. Theball-to-powder weight ratio and rotational speed were10:1 and 250 r/min, respectively. To prevent excessivewelding of particles in the vial, 1wt% stearic acid wasadded to the system and used as the process controlagent (PCA). To avoid oxidation, the entire processwas performed in argon atmosphere. The milledpowder was hot compacted uniaxially at the pressuresof 600MPa at 400�C, 450�C, and 500�C for 60min.Archimedes and relative density values of compactsamples were measured for microstructural study.Scanning electron microscopy (performed in aVEGA\\� Tescan SEM operated at 15 kV) was usedfor microstructural observation. Brinell hardness wasdetermined at 31.25 g load for 30 s of dwell time. Atleast five indentations were measured for each material.The dry wear tests were performed according to theASTM G99 standard procedures23 on a unidirectionalpin-on-disk aperture wear tester with a counter face ofAISI 52100 steel. Samples were cleaned thoroughlyusing an ultrasonic device with acetone and driedprior and after the tests, then weighed to determinethe wear loss using an electronic balance with an accur-acy of 0.1mg. All data on wear were measured from thepins. The same test parameters were used for all sam-ples: 10N and 20N for the normal load, 0.25m/s forthe sliding speed, 1000m for the sliding distance at25�C. Finally, the morphology of the worn surfaceswas analyzed using SEM. The wear rate was estimatedby measuring the weight loss of the specimen after eachtest. The volume loss was calculated by taking theweight difference of the sample before and after eachtest. The weight loss is defined as

�w ¼ ðwa � wbÞ ð1Þ

where �w is the weight loss expressed in gram and wa

and wb are the weight of the sample after and before thewear test in gram. The volume loss of the compositerelated to density (�) was calculated using theexpression

Volume loss ðmm3Þ ¼weight lossðgÞ

Density ðg=mm3Þ� 1000 ð2Þ

The volumetric wear rate Wr (mm3/km) was calculatedusing the expression

Wr ¼Volume loss ðmm3Þ

Sliding distance ðkmÞð3Þ

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Results and discussion

The crystallinity of the samples was evidenced by theXRD peaks corresponding to the monoclinic Al2O3

phase and the tetragonal Ce0.12O2Zr0.88 phase. TheWilliamson–Hall method and the Scherrer equationwere used to determine the sizes of crystallites. Thesizes of the Al2O3 and Ce0.12O2Zr0.88 crystallites calcu-lated from the X-ray diffraction (XRD) patterns are25 and 31 nm, respectively. XRD patterns andTransmission electron microscopic (TEM) images ofthe 10Ce-TZP/Al2O3 powders have been shown by thesame research group elsewhere.7

Figure 1 shows SEM representative photomicro-graphs of the nanocomposite with 7wt% 10Ce-TZP/Al2O3 sintered at 400�C, 450�C, and 500�C for60min. It can be observed that by increasing tempera-ture up to 450�C the bonding between the particlesgreatly improves and very few isolated porosity andvoids are present. This observation can be ascribed tothe increase in the driving force for sintering and, thus,in an increment in the self-diffusion and inter-diffusionof atoms. As aluminum can diffuse faster and fill thepores in the specimen, this results in a densification ofthe composites.

Furthermore, representative photomicrographs ofthe nanocomposites indicate that the distribution of10Ce-TZP/Al2O3 particles in the matrix is reasonablyuniform with lesser agglomeration. As it can beobserved in Figure 1(c), by increasing temperature upto 500�C there are some nonuniformly distributedregions, with agglomeration and pores. These particlesremarkably reduce the bonding strength between thematrix and reinforcement particles.

It was found that with increasing temperature up to450�C the relative density of the composites increasesfrom 0.88 to the maximum point of 0.92. This can beattributed to large interface areas between the particlesand the matrix that decrease the concentration of

vacancies in the matrix. It is postulated that the relativedensity of the compacts reduces to 0.90 as a result ofthe formation of porosities and agglomeration ofreinforcement particles at 500�C. The difference inhardness between the samples should be attributed tothe better distribution of particulate reinforcements andthe reduction in clustered regions. At constant concen-tration of reinforcements, the uniform distribution ofparticles causes the reduction of distance between 10Ce-TZP/Al2O3 particles. This phenomenon is associatedwith a higher degree of collision and blockage betweenparticles in the composite and, thus, the magnitude ofstresses developed at the metal/reinforcement interfaceare higher, so greater stresses will be expected to hindersome plastic deformation. In other words, greater stres-ses will be required to produce further plastic deform-ation on going from composite with coarser particles tocomposites with fine particles.

Typical plots of weight loss versus sliding distance atdifferent sintering temperatures, under 10N and 20Napplied loads are presented in Figure 2(a) and (b),respectively.

It is observed that the weight loss increases almostlinearly with increasing sliding distance, whereas thewear rate has a declining trend up to a critical value,after which it tends to increase again. As expected, theweight loss and wear rate for all samples under 20Napplied load are greater than those for 10N load. Thiscan be attributed to the fact that at higher loads theseparation of surface asperities, and hence the numberof contact asperities increase and consequently thematerial’s worn surface increases.24 It is generallyaccepted that to a great extent, the contribution ofhard particles to aluminum alloys results in animprovement of the wear resistance of the basealloy.11

The main reason for the enhancement in the com-posite’s wear resistance is clearly related to the hardness

Figure 1. SEM micrographs of the Al-7 wt% 10Ce-TZP/Al2O3 composites sintered at: (a) 400�C; (b) 450�C; and (c) 500�C.

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enhancement of nano-sized 10Ce-TZP/Al2O3 rein-forced samples, according to the Archad equation25

Q ¼ KW

Hð4Þ

In this equation, Q is the wear rate (mm3/km), thevolume of worn material per distance, K is a constantcalled wear coefficient, and H is the hardness of thespecimen in Vickers scale (kg/mm2).

By increasing temperature above 450�C, the wearresistance diminishes (Figure 3) due to nonuniform dis-tribution and agglomeration of particles, as well as toporosity regions. It seems that the high likelihood ofparticle agglomeration, the lack of homogeneity in the

structure of consolidated composites, and the abun-dance of porosities in the microstructure are the mainreasons for the increased wear rates after increasing thetemperature up to 500�C. Based on the weight lossdata, the specimens sintered at 450�C exhibit the high-est wear resistance amongst all the tested samples (seeFigure 2).

The variations of the friction coefficients (m) of thesliding materials for steady-state sliding against thesteel counterpart at the sliding speed of 0.25m/s,under 10N and 20N applied loads, are shown inFigure 4(a) and (b), respectively. For most of the spe-cimens, initially the friction coefficients have a rapidincrease with sliding distance to an approximatelysteady range after about 50m. This increase in m cor-responds to an increase in the apparent contact area,until full contact occurs across the full diameter of thepin. According to Figure 4(a), the average friction coef-ficient of Al-7wt% 10Ce-TZP composites at theapplied load of 10N exhibits a minimum value of 0.2at the sintering temperature of 450�C, after which itincreases up to 0.3 at 500�C. The highest average fric-tion coefficient (0.5) occurs at the lowest temperature,400�C. A slight increase is observed for the correspond-ing friction coefficient values at the applied load of20N. This behavior can be attributed to the densifica-tion state of the composites, depending on the sinteringtemperature. At 400�C, there is a definite lack of sin-tering of aluminum. At 500�C (500�C & 74% meltingpoint of aluminum), two possible factors may explainthe observed outcome. On the one hand, because of thedifference in thermal expansion coefficient between alu-minum and the reinforcement, some levels of porositiescharacterize the metal–ceramic interface. On the other

Figure 2. Wear weight loss versus sliding distance for Al-7 wt% 10Ce TZP/Al2O3 composites at different sintering temperatures

under: (a) 10 N and (b) 20 N applied loads.

Figure 3. Hardness and wear rate versus sintering temperature

under 10 N and 20 N applied loads.

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hand, a possible flow of aluminum in the semi-solidstate produces an agglomeration of particles, thus redu-cing the wear and mechanical properties. At 450�Cnone of the above behaviors is observed, thus resultingthe optimum microstructure conditions and, conse-quently, wear response.

A reason for improvement in the friction coefficientmay correspond to the reinforcement particles that sup-port the load to lessen the touch area between the pin,and counter disc surface. It is postulated that thereinforcement particles pulled out from the aluminummatrix, together with the debris from the wear test,form a layer that acts as a solid lubricant which spreadsout on the composite and counterpart surfaces, hinder-ing the direct contact between the two metallic surfaces.This leads to a reduction of the friction coefficient andwear rate.26 The increased coefficient of friction of thesamples synthesized at 500�C can be attributed to thefracture of agglomerated particles in to smaller particlesduring sliding. Thus, these particles cannot support theload to lessen the touch area between the pin and coun-ter disc surface. On the other hand, the 10Ce-TZP/Al2O3 particles become ineffective to resist the deform-ation developed in the wear surface so the cracks canpropagate in the matrix, causing them to pull out fromthe matrix, spread out on the composites and counter-part and act as hard abrasive particles against thecomposite.

Scanning electron micrographs of worn surfaces ofthe samples under 10N and 20N applied load are pre-sented in Figure 5(a) and (b), respectively.

As it can be seen from Figure 5(a-1) the major wearmechanism is delamination wear and no sign of groovesare seen on the worn surface of the nanocomposites.Suh,27 stated that delamination in worn surfacesoccurred by the following sequential steps: (1) cyclicplastic deformation of surface layers by normal and

tangential loads; (2) crack or void nucleation in thedeformed layers at inclusions or second-phase particles;(3) crack growth nearly parallel to the surface and;finally (4) formation of thin, long wear debris particlesand their removal by extension of cracks to the surface.The worn surfaces exhibited damages and cracksformed by plastic deformation and some pieces of frac-ture composites began to peel off in a Fake-shape. Asthe load increases from 10N to 20N, the delaminatedand adhesive zones became more abundant and the pitsformed were found to be quite deeper and larger.Under the load of 20N, due to the poor bondingbetween the matrix and the reinforcement, the frag-ments of reinforcement are pulled off and attached tothe surface due to accumulation of energy in the plasticzone between the asperities during the relative motion.Variation in the coefficient of friction by increasing theload from 10N to 20N confirms the change of wearmechanism from de-lamination to adhesive form,shown in Figure 4(a) and (b).

As it can be seen from Figure 5(a-2) at the appliedload of 10N, for the composite synthesized at 450�C, athin atmospheric layer formed between the materialpair acts like a solid lubricant. Also, there are nosigns of adhesive or de-lamination wear on the wornsurface under 10N applied load. The oxygen present inthe energy-dispersive X-ray spectroscopic spectrum ofFigure 6 of indicates that oxidizing wear occurred andthe presence of Fe suggests the transfer of a smallamount of material from the counterpart to the wornsurface. By increasing the applied load from 10N to20N, the air foil breaks and the unbearable oxides dis-connect from the surfaces.

As seen in Figure 5(a-3), due to the lower hardnessand less uniform distribution of 10Ce-TZP/Al2O3

reinforcements, a greater portion of adhesive mechan-ism, rougher worn surface and quite deeper total depth

Figure 4. Change in the coefficient of friction versus sliding distance for Al-7 wt% 10Ce-TZP composites sintered at 400�C, 450�C,

and 500�C under: (a) 10 N and (b) 20 N applied loads.

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of deformation is observed in the nanocomposites pro-duced at 500�C.

As the load increases from 10N to 20N (Figure 5(b-3)) the formed debris on the surface during the wear,rewelded, and embedded into the aluminum matrix andact as abrader elements. Thus, the wear rate is con-trolled by the particle, matrix and counter face. As itcan be seen, the morphology of composite worn

surfaces in comparison with applied load of 10Nbecame rougher and the adhesive zones increased.

Figure 7 compares scanning electron micrographs ofthe wear debris of 7wt% reinforcement composites sin-tered at 400�C and 450�C, both worn at 20N appliedload. The numerous uneven flake-like debris shown inFigure 7(a) might be evidence of debris generation byplastic deformation. It seems that plowing over the

Figure 5. Comparison of SEM images of the Al-7 wt% 10Ce-TZP/Al2O3 composites under: (a) 10 N and (b) 20 N applied loads at the

sintering temperatures of: (1) 400�C, (2) 450�C, (3) 500�C.

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AMC surface by hard asperities without cutting willproduce ridges, which can then be Fattened by furthercontact, forming extrusions or lips. These lips are even-tually broken off to the uneven Fat wear Fakesobserved.

The debris morphology of the composite Al-7wt%10Ce-TZP/Al2O3 sintered at 450�C (Figure 7(b)) indi-cates the formation of fine and irregular particles. It canbe due to the uniform distribution of hard reinforce-ment constituent and, consequently, surface discontinu-ity of matrix material and the slight depth of weardeformation on the nanocomposite surfaces.

Conclusions

In this investigation, Al-7wt% 10Ce-TZP/Al2O3 nano-composites were successfully produced by powdermetallurgy. Characterization of friction and wearbehavior of fabricated composites at different tempera-ture was investigated. The following conclusions aredrawn:

1. The optimum sintering temperature to attain anincrease in hardness and a decrease of both wearweight loss and wear rates of the specimens, is450�C.

2. Worn surface observations suggest that the domin-ant wear mechanism can be classified into twomodels. The first model refers to adhesive wearand it dominates the material sintered at 400�Cand 500�C. The second one refers to the formationof an oxide layer and it manifests that in nanocom-posites produced at 450�C, the worn surfaces weresmoother and the total depth of deformations werenot severe.

Conflict of interest

None declared.

Funding

This research received no specific grant from any

funding agency in the public, commercial, or not-for-profitsectors.

Figure 6. EDS analyses of worn surface of composite with

7 wt% 10Ce-TZP/Al2O3 under 10 N applied load, sintered at

450�C.

Figure 7. Scanning electron micrographs of the wear debris of 7 wt% reinforcement composites sintered at: (a) 400�C and

(b) 450�C, both worn at 20 N applied load.

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