The response of PACVD TiN coatings to tribological tests with different counterparts

Recently, a low-friction effect with friction coefficients of below 0.2 against ball-bearing steel has been reported for chlorine-containing TiN coatings grown by plasma-assisted chemical vapor deposition (PACVD). This low-friction effect has been attributed to the formation of a chlorine-containing lubricant layer on top of the coating in humid air which can be regarded as a self-adaptation process of the coating to the given tribological situation. Within this study we show that in dry sliding ball-on-disc experiments against different counterparts, the response of the coating to the tribological conditions is determined by the hardness of the counterpart material. For the softest ball material used, i.e. austenitic stainless steel, the friction coefficient drops to the low-friction regime after a running-in distance which is more than 35 times longer compared to that of the hardest counterpart material, i.e. alumina.