Wear and friction behavior of metal impregnated microporous carbon composites
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INTRODUCTION
METAL-MATRIX composites, reinforced by nonmetallic fibers, whiskers, and particulates, are being seriously examined for structural, thermal-management, and wear applications. Copper matrix–carbon composites are specially suited for thermal-management applications, because of their low coefficient of thermal expansion, high thermal conductivity, and low density.[1] They are also attractive for wear applications, such as sliding electrical contacts, bearings, and bushings.[2] Solid-state consolidation techniques, such as sintering[3] and hot pressing,[4] and casting processes,[5] have been used to prepare these composites. However, the casting techniques, such as stir casting,[6] compocasting,[7] and various forms of melt infiltration,[8] are more attractive because of their net shape capability and low processing cost. For the most part, graphitic carbon has been used for copper matrix–carbon composites in wear applications, because its addition (graphite-particle volume fraction . 0.2)[2] reduces the coefficient of friction and increases the wear resistance, as compared with the matrix. Easy glide of the basal planes under ambient conditions is responsible for the lubricity and antiseizure characteristics of graphite. Glassy (amorphous) carbon, on the other hand, is much harder, about 8 Moh.[9] One would, therefore, expect a higher strength and wear resistance from a copperalloy composite containing glassy carbon. There is only one study on copper-amorphous carbon composite,[9] where copper wires (30 to 50 mm in diameter) were introduced into an organic material, after which polymerization and pyrolysis yielded a glassy carbon matrix. This composite GULTEKIN GOLLER, formerly UNIDO Fellow, Chemical Engineering Department, Cleveland State University, is Graduate Student with the Department of Materials Science, Istanbul Technical University, Istanbul, Turkey. D.P. KOTY, formerly Graduate Student, Chemical Engineering Department, Cleveland State University, is with Applied Materials, 3100 S. Vista Ave., Suite 140, Boise, ID 83705. S.N. TEWARI, Professor, is with the Chemical Engineering Department, Cleveland State University, Cleveland, OH 44115. M. SINGH, Senior Research Engineer, is with NYMA Inc., NASA–Lewis Research Center Group, Cleveland, OH 44135. A TEKIN, Professor, is with the Department of Materials Science, Istanbul Technical University, Istanbul, Turkey. Manuscript submitted March 22, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
showed low wear rates and friction coefficients, comparable to those typically observed in graphite and copper matrix– graphite composites. Graphite sliding against itself yields a low coefficient of friction, about 0.1, independent of the sliding distance, but the friction coefficient between two amorphous carbon surfaces increases from 0.1 to a steadystate value of about 0.8.[10] Intuitively, one would therefore expect that metal-matrix composites containing amorphous carbon would show similar high friction coefficients. However, the observed behavior is to
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