Control of the tribological moisture sensitivity of diamond-like carbon films by alloying with F, Ti or Si

A specific feature of diamond-like carbon (DLC) tribological behaviour is the increase of its friction coefficient with increasing ambient humidity levels and the loss of its self-lubricating properties at very high humidity. In a previous study it was shown that alloying DLC with small amounts of Si provided a significant reduction in friction coefficient at high relative humidity and allowed the friction coefficient to be stabilised over a wide humidity range. For example, the addition of approximately 4 at.% Si allowed friction coefficient to be held approximately constant at 0.08±0.01 over the humidity range 5–85% R.H. sliding against a steel counterface [Surf. Coat. Technol. 133–134 (2000) 437]. In this study, the possibilities for optimising and tailoring the tribological humidity response of DLC has been further investigated for the alloying elements F and Ti and the results compared with those obtained for Si. The addition of Ti was found to lower friction coefficient slightly both under dry and humid conditions. The addition of F was found to produce a dramatic increase in friction coefficient especially at low humidity, resulting in a friction coefficient as high as 0.9 against steel.