Structural and environmental dependence of superlow friction in ion vapour-deposited a-C : H : Si films for solid lubrication application

Abstract

Understanding the tribochemical interaction of water molecules in humid environment with carbonaceous film surfaces, especially hydrophilic surface, is fundamental for applications in tribology and solid lubrication. This paper highlights some experimental evidence to elucidate the structural and environmental dependence of ultralow or even superlow friction in ion vapour-deposited a-C : H : Si films. The results indicate that both surface density of silicon hydroxyl group (Si–OH) and humidity level (RH) determine the frictional performance of a-C : H : Si films. Ultralow friction coefficient μ (∼0.01–0.055) is feasible in a wide range of RH. The dissociative formation of hydrophilic Si–OH surface and the following nanostructure of interfacial water molecules under contact pressure are the origin of ultralow friction for a-C : H : Si films in humid environment. The correlation between contact pressure and friction coefficient derived from Hertzian contact model is not valid in the present case. Under this nanoscale boundary lubrication, the friction coefficient tends to increase as the contact pressure increases. There even exists a contact pressure threshold for the transition from ultralow to superlow friction (μ ∼ 0.007). In comparison, when tribotested in dry N2, the observed superlow friction (μ ∼ 0.004) in the absence of water is correlated with the formation of a low shear strength tribolayer by wear-induced phase transformation.