On the significance of running-in of hard nc-TiC/a-C:H coating for short-term repeating machining

The importance of the often omitted running-in phase of tribological testing was studied on hard nc-TiC/a-C:H coating system selected as a representative of protective tribological coatings. The tribological measurements were periodically interrupted and the counterpart ball was replaced by a new one after every interrupted test to simulate machining applications, where the coatings are repeatedly in contact with a new material. The changes in the coefficient of friction and the wear rate of the counterpart ball were found to be dependent mainly on the roughness of the coating at any stage. It was found out that after substantial smoothening of the wear track of the measured coating, the coefficient of friction stabilized at the steady-state value attained in an uninterrupted long tribotest. This steady state value was reached at the very beginning of the test and initial instabilities in the coefficient of friction were no longer observed. It was also concluded that only after the nc-TiC/a-C:H coating gets smoothened enough within the wear track, the steady-state coefficient of friction can describe even applications where the counterpart is being periodically replaced by a new one.