Evaluation of adhesion and fatigue of MoS2-Nb solid-lubricant films deposited by pulsed-dc magnetron sputtering

The thin film deposition for tribological applications becomes more and more widespread. The tribological performance of the overlay coatings correlates with coating-substrate adhesion. Hence, it is important to measure adhesive strength. The scratch adhesion test for thin films is extensively used. In this work, MoS2-Nb coatings deposited by closed-field unbalanced magnetron sputtering (CFUBMS) have been scratch tested in two modes. A multi-mode operation was used as sliding-fatigue, like multi-pass scratching in the same track at different fractions of critical load (unidirectional sliding) and a standard mode using progressive load operation. Failure mechanisms are discussed according to examination of response of very dense microstructure and the adhesion value. The critical load to the first failure (LC1) was 15 N but the final adhesion value from the film and substrate interface was 120 N (LC2) as function of the coating thinning. The coefficient of friction (COF) from the multi-scratch for MoS2-Nb started at a very high value of around 0.067, 0.073, and 0.093 under 5, 8, and 15 N loads respectively and then drops to 0.006, 0.035, and 0.065 at the end of the 1000 cycles. The most significant finding in the test is that when the multi-scratch passes reached to 1000 cycles, micro scale fatigue failures disappeared.