Effect of hydrogen content in a-C:H coatings on their tribological behaviour at room temperature up to 150 °C

Hydrogenated amorphous carbon (a-C:H) coatings deposited onto steel substrates by plasma assisted CVD, using different precursor gases (1 < H/C ratio < 4) were tested for their tribological behaviour. The H content in these coatings ranged from 25 to 29 at.%. Fretting mode I tests were performed on different couples consisting of coated and/or uncoated first bodies. Some tests were performed after a heat treatment of the coatings. As-deposited a-C:H/corundum couples tested at 23 °C and 50% RH showed lowering of the coefficient of friction at increasing normal load. Graphitisation is taking place in sliding contacts at high normal loads. For a-C:H/corundum couples a clear minimum in the coefficient of friction was noticed at 100 °C for coatings containing 27 at.% H. The coefficient of friction recorded on such couples is high compared to the one recorded on as-deposited a-C:H/a-C:H couples. However for the a-C:H/a-C:H couple, a lowering of the coefficient of friction with increasing fretting test temperature was noticed. The decreasing coefficient of friction was accompanied by an increasing wear. Graphitisation is causing severe degradation of a-C:H coatings at high test temperatures. An energetic analysis of the wear is finally reported. It appeared that the wear volume recorded at RT on as-deposited a-C:H coatings varies linearly with the cumulative dissipated energy. The wear rate coefficient decreases with increasing H-content. A stabilization of the sp3 bonds with increasing H-content might explain this behaviour. Confirmation was found by performing high temperature fretting tests. Interesting is the finding that fretting tests at RT performed after a thermal treatment of a-C:H coatings at either 100 or 150 °C, show a friction and wear behaviour identical to the ones recorded on as-deposited coatings tested at RT.