Fretting wear of Ti(CxNy) PVD coatings under variable environmental conditions

Abstract

Fretting wear as a specific type of degradation is defined as an oscillatory motion at small amplitude between two nominally stationary solid bodies in mutual contact. Under external stresses, the interface is being damaged by debris generation and its successive ejections outside the contact area. A potential protection against fretting damage by means of hard coatings is being offered by different surface engineering techniques. For this study, TiC, TiN, and TiCN hard coatings manufactured by a physical vapour deposition method have been selected and tested against smooth polycrystalline alumina ball. A fretting test programme has been carried out at the frequency of 5 Hz, 100 N normal load, 100 μm displacement amplitude, and at three values of a relative humidity (RH): 10, 50, and 90 per cent at 295-298 K temperature. It turned out that the intensity of wear process was depending not only on loading conditions but also on environmental ones. A significant impact of RH on wear rate and friction behaviour of the coatings under investigation has been observed. Two different damage mechanisms have been identified and related to the phenomena of debris oxidation and debris adhesion to the counterbody surface. In the latter case, the debris deposited onto the surface of the alumina ball lead to a change of stress distribution at the interface and as a result to accelerated wear. In this work, experiments with variable RH increasing from 10 to 90 per cent within a single fretting test have been completed. It follows from these experiments that there exists an intermediate value of the RH at which the friction coefficient changes rapidly. Finally, a dissipated energy approach has been applied in the work to quantify and compare fretting wear rates of different hard coatings.