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INTRODUCTION

THE current state of research on the microstructures, properties, and fabrication of particulate-reinforced aluminum matrix composites has been reviewed by Clyne and Withers tn and Lloyd.I2] As a result of the improved traderstanding of their mechanical and physical properties, it is now becoming possible to set up design guidelines for these materials for applications requiring a specific thermomechanical performance.[3] These applications include components such as drive shafts and connecting rods in automotive engines where high specific strength and modulus are important. Other examples include high-speed rotating or reciprocating items such as cylinder bores or liners and brake rotors,I4,5I for which materials resistance against sliding wear and seizure is the principal design criteria. Although these components are often required to operate at temperatures 0.4 to 0.8 of the melting temperature of the matrix alloy, the high-temperature sliding wear behavior of aluminum matrix composites has started to receive attention only recently.I6,7~These studies, although new, are also important for delineating friction and wear effects during J. S1NGH, Graduate Student, and A.T. ALPAS, Associate Professor, are with the Department of Mechanical and Materials Engineering, University of Windsor, Windsor, ON, Canada N9B 3P4. Manuscript submitted August 28, 1995 METALLURGICALAND MATERIALSTRANSACTIONSA

high-temperature processing of the composites, e . g . , die wear in hot extrusion, t8] and microstructural development in diffusion bonding.m] Most studies on the wear of metal matrix composites have focused on the effects of load and sliding velocity[1~ 17] on the wear rates. When the applied loads and test temperatures are low, ceramic particles can support the applied load without fracturet15.161 and consequently may provide more than an order of magnitude improvement in the wear resistance of the matrix alloy. Increasing the particle size and volume fraction of particles improves the wear resistance in this wear regime, vS] When the contact stresses are high enough to fracture particles at the surface, large local strains and strain gradients are generated at the material layers adjacent to the contact surface and wear proceeds by mechanisms such as subsurface delamination, which involves crack propagation along the particle matrix interfaces, tm,2~ or third body abrasion induced by fragmented hard ceramic particles.t2q In this type of wear, particle-reinforced composites show almost no or little improvement over the unreinforced alloys. It is also commonly observed that above a critical load or speed, aluminum alloys show a transition to severe wear that is characterized by gross plastic deformation and large-scale material transfer to the counterface. Zhang and Alpas E181showed that the transition from mild to severe wear in 6061 AI-20 pct AI203 occurs when the friction-induced heating raises the contact surface VOLUME 27A, OCTOBER 1996--3135

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