The wear and friction behaviour of engineering coatings in ambient air and dry nitrogen

Abstract

This work reports on the structural and wear properties of a range of engineering coatings including TiN, TiAlN, CrAlN, MoS2/Ti and a number of different DLC coatings, deposited on tool steel substrates. The tribological properties of the coatings were characterised by sliding wear tests in different environments of humid air and in dry nitrogen. Microstructural assessment was performed using scanning electron microscopy and atomic force microscopy (AFM). DLC coatings produced the lowest friction coefficient in dry nitrogen and in humid air, demonstrating their versatility. The coefficient of friction can be attributed to the oxidation of MoS2 at the wear track to form MoOx that is known to cause an increase in the friction coefficient. In the case of DLC the main effect of changing atmosphere was both friction coefficient and local wear mode/nature of debris produced; in general moist air produced far lower friction coefficients and wear rates; attributed to surface absorbates, which in turn modify local ductility and friction at asperity contacts. Delamination occurred in an IBAD (ion beam assisted deposition)-produced DLC, which was subjected to further investigation by scratch testing. AFM was used to investigate in detail the wear track morphology in the IBAD DLC and a reference DLC. The highest friction coefficients were generated by the TiAlN and CrAlN coatings.