Role of the Transfer Film on the Friction and Wear of Metal Carbide Reinforced Amorphous Carbon Coatings During Run-in

The relationship between friction, wear, and transfer films of three metal carbide-reinforced amorphous carbon coatings (TiC/a:C, TiC/a:C-H, and WC/a:C-H), sometimes referred to as metal-doped diamond-like carbon coatings, has been investigated. Tribological tests were performed in an in situ tribometer with sapphire or steel hemispheres run against coated flats in dry or ambient air. The sliding contact interface was observed and recorded by optical microscopy during reciprocating sliding tests. The friction and wear behavior during run-in depended on the number of sliding cycles to form a stationary transfer film on the hemisphere. Stationary transfer films formed rapidly (within ten cycles) and the friction coefficient fell to 0.2 (ambient air) or 0.1 (dry air), except with sapphire against WC/a:C-H in dry air; with the latter, a stationary transfer film required nearly 100 cycles to form, during which the friction remained high and the wear rate was from 10 to 100 times higher than the other two coatings. For all coatings, three velocity accommodation modes (VAM) were observed from run-in to steady-state sliding and were correlated with the friction and wear behavior. The delayed adherence of the transfer film to sapphire from WC/a:C-H coatings in dry air is discussed in terms of equilibrium thermochemistry. Friction and wear behavior during run-in, therefore, depended on transfer film adherence to the hemisphere and the VAM between transfer films and the coating.