Tribological properties of centrifugally cast copper alloy-graphite particle composite
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I. INTRODUCTION
LEADED copper alloys have been widely used as bearing components due to their good tribological properties and machinability. The addition of lead to copper alloys reduces chip size during machining and improves the tribological properties of copper alloys. However, the contamination of lubricating oil by lead, when leaded copper alloys are used for bearing applications, creates environmental problems. Melting, casting, and machining of leaded copper alloys also leads to environmental problems due to the lead released in the environment. Considerable research has been done to replace lead by bismuth and selenium.[1–5] Additions of bismuth and selenium to copper alloys improves their machinability presumably due to the formation of eutectic microstructures, but both of them are expensive and may be toxic. Rohatgi and co-workers[5–8] have incorporated graphite particles into the copper alloy matrix using stir-mixing techniques to obtain copper alloys with graphite particles dispersed across the entire cross section; these cast copper alloys containing dispersed graphite particles have machinability, solderability, and plateability, which is comparable to the copper alloys containing lead.[6] Metal matrix composites (MMCs) have been shown to posses unique tribological properties as compared to the matrix alloys due to the presence of dispersed particles in J.K. KIM, Research Associate, M. KESTURSATYA, Graduate Student, and P.K. ROHATGI, Professor, are with the Materials Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53211. Manuscript submitted November 23, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
the matrix. Incorporation of zircon, SiC, and TiO2 particles in the matrix of aluminum alloys was found to improve the abrasion resistance of these alloys.[9] The dispersion of graphite particles in an aluminum alloy matrix was also found to improve the tribilogical properties especially scuffing resistance and galling resistance due to the formation of the graphite film on the counterface, which also leads to a decrease in the friction coefficient.[10,11] In view of this, the concentration of graphite particles near the inner periphery of copper alloy centrifugal castings was postulated to impart good tribological properties to the inner periphery of these cylindrical castings. In this study, centrifugal casting techniques are used to impart better tribological properties to the inner periphery of copper alloy (C90300) cylindrical castings, where graphite particles are segregated. This technique uses the following steps: (1) the addition of graphite particles into copper alloy melt along with a wetting agent titanium, which presumably forms titanium carbide on the surface of the particle; (2) mixing of the particles in the melt; and (3) the pouring of the melt containing graphite particles into a rotating mold in the centrifugal casting machine. Although there is some prior research on the tribological properties of powder metallurgy produced copper-graphite alloys, to date, no significa
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