Effect of Atmosphere and Temperature on the Tribological Behavior of the Ti Containing MoS2 Coatings Against Aluminum

Abstract

MoS2 coatings exhibit low coefficient of friction (COF) when sliding against aluminum; however, the magnitudes of their COF show high sensitivity to environmental conditions. Ti could reduce the sensitivity of the frictional behavior of MoS2 coatings to moisture. This study examines the tribological properties of Ti containing MoS2 coating (Ti–MoS2) tested against an aluminum alloy (Al-6.5% Si) in ambient air (58% relative humidity, RH), dry oxygen, dry air and dry N2 (< 4% RH) atmospheres. The Ti–MoS2 coating exhibited similar COF values under an ambient (0.14), dry oxygen (0.15) and dry air (0.16) atmospheres. It was found that oxidation of MoS2 to MoO3 was responsible for high COF under these testing conditions as revealed by Energy-dispersive X-ray Spectroscopy (EDS) and micro-Raman spectroscopy. However, a low and stable COF of 0.07 was observed under a dry N2 condition. This work further showed that the tests performed at elevated temperatures, up to 400 °C in a dry N2 atmosphere sustained the low and stable COF of the Ti–MoS2 coatings. The sliding tests performed under a dry N2 atmosphere prevented the formation of MoO3 and as a result, the Ti–MoS2 coatings maintained low COF values. Low COF values were also attributed to the formation of MoS2 transfer layers.