Tribological characteristics of aluminum-50 Vol Pct graphite composite
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I.
INTRODUCTION
IN recent years, considerable work has been done on metal-matrix graphite particle composites which exhibit low friction, low wear rate, and excellent antiseizing properties, t~] In these composites, graphite presumably imparts improved tribological properties to the composite through the formation of a graphite-rich film on the tribosurface which provides solid lubrication. The formation of the solid lubricating film depends on the matrix characteristics like its deformability, helping the process of transfer of graphite to the tribosurface, adhesion of graphite film to the matrix, and the presence of an environment which permits graphite to spread in the form of a film and act as a solid lubricant. In metalmatrix graphite composites, graphite particles are embedded in the matrix, and the formation of a graphite film will take place by transfer of the graphite from the particles to the tribosurface during initial periods of sliding.t 2,31 The objective of the present investigation is to determine the friction and adhesive wear behavior of aluminum alloy base composites containing large volume fractions of graphite. The influence of the shape of graphite particles--both granular and f l a k e - - o n the friction and wear of the composite has been investigated. The friction and wear in composites containing flake graphite depend on the orientation of the flakes relative to the sliding direction. The origin of this orientation dependence and the general mechanism of adhesive wear have been investigated in a study of the subsurface region and the thickness of the graphite film on the tribosurfaces. II.
EXPERIMENTAL PROCEDURE
Aluminum graphite samples were prepared using pressure infiltration of graphite powders which were packed Y. LIU is with the Research Faculty, Department of Mechanical and Production Engineering, National University of Singapore, Singapore 0511. S. RAY, Reader, Department of Metallurgical Engineering, University of Roorkee, Roorkee 247667, India, and P.K. ROHATGI, Professor, are with the Department of Materials, University of Wisconsin-Milwaukee, Milwaukee, WI 53211. Manuscript submitted March 10, 1992. METALLURGICAL TRANSACTIONS A
in a die by molten aluminum alloys. The die used for the process was made of electrode-grade hard graphite with 10-cm OD and 5-cm ID. Graphite powders with an average size of 300 to 5 0 0 / x m were used, and the preform was made by pouring the powders into the die and tamping the powder bed into a preform. The top of the preform was covered by a cloth made of A1203 fibers. 2014 aluminum alloy was melted at 760 ~ before pouring into the die. After pouring, a pressure of 2.2 MPa was applied to the liquid alloy and held for 300 seconds. The liquid alloy was thus infiltrated into the preform in the die and solidified under pressure, before the casting was ejected. The graphite content of the composite was determined by quantitative metallography. Wear tests were carried out under dry sliding conditions with three pin-on-disk configurations in a Falex-
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