TRIBOLOGY OF ALUMINUM METAL-MATRIX COMPOSITES - LUBRICATION BY GRAPHITE

A study has been performed to examine the tribological compatibility of commercial aluminum metal-matrix composites (MMCs) with graphite as a solid lubricant. Two commercial composites, namely a 2014 aluminum alloy reinforced with 20 vol.% of beta-SiC whiskers and an Al-Si alloy dispersed with 20 vol.% of SiC particles, were chosen as substrate materials. The solid lubricant films were powder burnished on metallographically polished surfaces of composites and matrix alloys. The durability of these burnished films was assessed by performing long-term friction experiments using a ball-on-disc apparatus. The nature of the adhesion between the graphite films and the MMC substrates was examined using a scanning electron microscope equipped with a wavelength dispersive X-ray spectroscope. Films burnished on Al-Cu alloy surfaces withstood much higher loads than those burnished on Al-Si alloys. The silicon protrusions in the etched surfaces of Al-Si alloys anchored the graphite films. Silicon carbide particles in unburnished composite surfaces abraded the steel counterface extensively, whereas the whiskers acted in a more benign manner as a polishing medium.