Friction and wear behavior of a centrifugally cast lead-free copper alloy containing graphite particles
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I. INTRODUCTION
IN the past, several different types of materials have been studied for bearing applications. To date, various copper alloys, including leaded bronzes, aluminum bronzes, and phosphorus bronzes, have been used extensively.[1] The bearings were tested in both dry and lubricated conditions against a steel counterface, which is used as the shaft material in most applications. Under lubricated conditions, the presence of the lubricating film prevented contact between the mating surfaces, resulting in desirable tribological properties. When used in dry conditions, the presence of soft phases, including elemental lead, helps to maintain lubrication.[2–8] Due to its harmful effects, restrictions have been imposed on the use of lead. This has prompted researchers to find alternative materials that have tribological properties and a machinability similar to lead, for use as additives to copper alloys. Several materials, including graphite, bismuth, and selenium, have been proposed as replacements for lead. The addition of graphite to aluminum matrices reduces the friction coefficient significantly, as was previously shown,[9,10,11] which was attributed to the formation of a graphite film on the surface of the matrix. Previous work on copper-graphite composites, produced by the powder metallurgy (P/M) technique,[13–16] showed that the presence of graphite particles in the matrix reduces the friction coefficient.
Casstevens et al.[15] reported that the tribological properties depend on the structure of the matrix and also on the distribution of the graphite in the matrix. The structure of copper-graphite alloys obtained by the centrifugal casting method will be very different than the structure of the same alloy produced by the P/M process. Prior work[19] has shown that, for the same composition, the statically cast coppergraphite castings have better tribological properties than the composites produced by the P/M technique. This was partly attributed to the concentration of graphite in the interconnected interdendritic region, which facilitates the transfer of graphite to the tribosurface. This makes it necessary to characterize the tribological behavior of the centrifugally cast copper-graphite composites, which are in contact with a standard shaft material like 1045 steel. This study focuses on the measurements of friction coefficients and wear rates of a base-alloy (C90300) and coppergraphite composites against a 1045 steel counterface. Special attention has been paid to identify the effect of graphite particles on the element transfer, weight change in the pins and the counterface, and debris generation. In addition, a new isostrain model, to estimate the friction coefficient of a metal matrix composite containing soft particles like graphite, has been proposed.
II. EXPERIMENTAL PROCEDURE M. KESTURSATYA, formerly Graduate Student, Materials Department, University of Wisconsin–Milwaukee, is Materials Engineer with Helwig Carbon Products, Inc., Milwaukee, WI 53224. J.K. KIM, formerly Postdoctoral Fellow, Ma
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