Evolution of transfer layers on steel balls sliding against hydrogenated amorphous carbon coatings in ambient air

The evolution of transfer layers derived from a-C:H coating/steel ball tribopairs sliding in ambient air is tracked by performing one set of overlapping tribotests on a ball-on-plate device with acceptable repeatability of friction coefficients. After sliding tests, counterparts were separated and probed by optical microscopy, SEM/EDX and XPS. Carbon-based transfer layers were built up in the first cycle of sliding. Except for the typical graphitization of transferred carbon materials, a chemisorbed layer rich in C/O groups was also found on the top surface of transfer layers, and considerably affected the friction behavior of a-C:H coating/steel tribopairs in ambient air.