Friction of diamond on diamond in ultra-high vacuum and low-pressure environments

Effects on diamond-diamond friction of ultra-high vacuum (UHV) ( approximately 4*10^-9 Torr) and low pressure ( approximately 1*10^-5 Torr) gases including hydrogen, oxygen and nitrogen in their molecular and atomic states have been investigated. The friction coefficient for diamond sliding on diamond in UHV was found to be between 0.6 and 1.0, which is about ten times that measured in air. Among molecular gases studied at pressures of approximately 1*10^-5 Torr, only oxygen caused a significant reduction of the friction coefficient. When a heated filament was used to dissociate the gases in the vacuum chamber, atomic hydrogen produced the largest decrease in friction coefficient. The reduction in friction coefficient increased with increasing exposure time before sliding started. These results suggest that passivation of diamond surfaces by atomic hydrogen and oxygen as well as molecular oxygen is effective in reducing the carbon-carbon bond formation during sliding and consequently the friction coefficient.