TRIBOCHEMISTRY OF DIAMOND-LIKE CARBON COATINGS IN VARIOUS ENVIRONMENTS

Diamond-like carbon films deposited on silicon wafers by r.f.-plasma-a ssisted chemical vapour deposition were friction tested in controlled atmospheres in a reciprocating pin-on-plate configuration using a stee l sphere. Friction experiments were carried out in a vacuum range from 10(-7) to 50 Pa, in dry nitrogen and in ambient air. Analytical inves tigations of the wear process were peformed using transmission electro n microscopy-electron energy loss spectroscopy and secondary ion mass spectroscopy. In all cases a transfer film was observed to form on the steel pin during the first 100 cycles, associated with relatively hig h values of the friction coefficient (0.2-0.3) at this stage. Beyond N =100 cycles the friction coefficient decreased to 0.006-0.008 in a vac uum below 10(-1) Pa and to 0.01-0.07 in a vacuum of 10-50 Pa and in dr y nitrogen. The shearing ability of the interfacial film depends stron gly on the nature of the atmosphere during friction, which affects the surface composition of the sliding counterfaces. A high vacuum is ass ociated with ultralow friction and low wear. A poor vacuum and an iner t atmosphere are associated with low friction and moderate wear. Ambie nt air is associated with relatively high friction and severe wear, co upled with the formation of roll-shaped debris of amorphous carbon con taining iron oxide precipitates.