Tribological degradation of MoS 2 –Ti sputtered coating when exposed to elevated temperatures

Abstract

The use of solid lubricants is an effective way to control friction and wear in applications where traditional lubricants such as oils and greases cannot be used. MoS₂ is a popular solid lubricant which has been widely used in many applications, especially space applications in view of its good performance in vacuum. Recent developments in physical vapour deposition technology have led to the development of sputtered MoS₂ films doped with different metals to improve their durability and reduce the detrimental effects of oxidation and humidity on their tribological performance. In this work, a MoS₂–Ti coating deposited on a hot forming tool steel substrate has been studied at ambient and elevated temperatures. The objective was to investigate how the friction and durability of the MoS₂–Ti coating are affected after exposure to elevated temperatures. The results have shown that low friction values of ∼0·02 were obtained at room temperature and low relative humidity of 25%. An increase in relative humidity to 40% led to an increase in friction by almost 100%. There was a very significant degradation in frictional characteristics as well as durability after the MoS₂–Ti coating was exposed to 400°C. The wear of the counter surface also increased when sliding against the MoS₂–Ti coating exposed to elevated temperatures. This has been attributed to interaction of the counterbody with hard abrasive molybdenum oxides formed on the MoS₂–Ti coating due to exposure to elevated temperatures.