Tribological Properties of Magnesium Nano-Alumina Composites under Nano-Graphite Lubrication

Abstract

The tribological behavior of magnesium metal matrix composites lubricated with graphite nanoparticles as additives in paraffin oil was investigated using a pin-on-disc wear tester. The results of nano-graphite-lubricated conditions are compared with the results of the tribotests carried out with micro-graphite additives in paraffin oil, pure paraffin oil, and dry sliding conditions. Wear tests were conducted at loads ranging from 15 to 60 N and sliding speeds varying from 1.25 to 3.15 m/s for a constant sliding distance of 1,600 m. The worn surfaces of the composites were analyzed through scanning electron microscopy (SEM) and energy dispersive X-ray (EDAX) spectroscopy. Characterization of the tribolayer and subsurface deformation layer of worn-out composites was accomplished using a novel focused ion beam (FIB) milling technique. The wear resistance of the metal matrix composite was observed to decrease with increasing severity of testing conditions (i.e., applied load and sliding speed) when tested under a dry sliding condition and paraffin oil lubrication. Nevertheless, the wear rate of the composites when tested under nano- and micro-graphite lubrication conditions showed very little variation over the entire range of loads and sliding speeds. The low-order coefficient of friction for the nano-graphite-lubricated condition arose from the formation of a thick continuous graphite layer at the contact zone. The operating wear mechanism for micro-graphite and nano-graphite lubrication was established as a combination of smearing, forming, and transfer of the graphite layer. © 2012 Copyright Taylor and Francis Group, LLC.