The effect of substrate surface roughness on the friction and wear of sputtered MoS₂ films

This paper examines how the durability and friction of thin films of molybdenum disulphide are affected by the surface roughness of the substrate to which they are applied. Three types of substrate, representing a variety of bearing materials (bearing steel, titanium alloy and hot-pressed silicon nitride), were prepared with surface roughnesses of nominally 0.04, 0.1, 0.2 and 0.4 µm (centre line average values). Onto these substrates were deposited 1 µm thick films of sputtered MoS₂. Pin-on-disc tests undertaken in high vacuum indicated that both film friction and film durability varied appreciably with substrate surface roughness. The most durable films were those applied to silicon nitride substrates. With this film/substrate combination, the onset of film failure was more gradual than that seen with the films applied to metal substrates, and the MoS₂ film was observed to recover its low friction properties several times before complete failure. Results from these experiments are presented and a theoretical model, based on the numerical simulation of friction between contacting rough surfaces, is introduced and is used to explain qualitatively the variation in MoS₂ film friction with the surface roughness of metallic substrates. Possible reasons for the poor agreement between the model and those results obtained with silicon nitride substrates are discussed.