Cryotribology of diamond and graphite

An experimental study was carried out on the tribological behaviour of materials of interest in cryogenic applications, focusing on diamond and graphite. Both natural diamond (referred in the text as diamond) and chemical-vapour-deposition (CVD) diamond (CVD-diamond) were used. The experiment was carried out using a pin-on-disc tribometer capable of operating at cryogenic temperatures, from 4.2 to 293 K. Two basic scenarios of testing were used: (1) frictional coefficient (mu) versus velocity (v) characteristics at constant temperatures; and (2) mu versus temperature (T) behaviour at fixed sliding speeds. For diamond, graphite, and stainless steel pins, each set of pins against CVD-diamond disc, values of mu are virtually velocity independent. For each of diamond, alumina, and graphite pins, each set against graphite disc, the delta mu/delta v characteristic is favorable, i.e. positive. For diamond and graphite pins against CVD-diamond discs, values of mu are nearly temperature independent in the range 77-293 K. Each mu(T) trace for pin materials sliding on graphite discs has a peak at a temperature in the range 100-200 K. As is the case with diamond/stainless steel pairs, there is a definite correlation between friction and material hardness: mu is inversely proportional to material hardness. Thus mu is higher at higher temperatures, because hardness generally decreases with temperature. (C)1998 Published by Elsevier Science Ltd.