Different tribological behavior of MoS2 coatings under fretting and pin-on-disk conditions

The performance of tribological coatings is often evaluated by fretting or pin-on-disk measurements. However, only a few papers dealt with the comparison of the two methods on the same coating. In this paper, a comparison of the tribological behavior of sputtered MoS2 coatings in fretting and pin-on-disk tests under different conditions is studied. The relationship between the mechanical properties of the coatings and their wear performance in both type of sliding tests is discussed. MoS2 coatings were deposited by magnetron sputtering from a MoS2 target. The coating thickness and adhesion were evaluated by respectively the ball crater and scratch test. The hardness as well as the toughness were obtained by a Vickers microhardness test. After the tribological tests the surface morphology of the wear scar in the coating was observed by scanning electron microscope. Also transfer films on the counterbody were investigated. It appears that the fretting and pin-on-disk tests can give considerably different friction and wear results. Coatings that show excellent performance in fretting tests have shown a low endurance in pin-on-disk tests. Sliding speed appears to be a crucial factor. Also the material of the counterbody, steel or corundum, influences the results in both tests in a different way. The different behavior of a coating in both tests is related to the toughness of the coating and to the way that the debris is involved in the friction and wear mechanisms.