Tribological Response of Silicon Oxide-Containing Hydrogenated Amorphous Carbon, Probed across Lengthscales

Abstract

This work examines the structure and properties of silicon-oxide containing hydrogenated amorphous carbon (a-C:H:Si:O) thin films, and how the structure and properties are responsible for the fundamental tribological response of the material. The films are studied through a range of spectroscopic techniques, focused on the surface-sensitive X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopies. The tribological response is studied at several lengthscales: using macroscale ball-on-flat tribometry, at the nanoscale with sharp diamond-like carbon-coated AFM probes, and at the microscale with steel colloids affixed to AFM cantilevers. The spectroscopic study reveals that the films contain a high fraction of SiOx which leads to a structure rich in sp3 carbon-carbon bonding that affords strong protection against oxidative attack at the elevated temperatures in aerobic environments, which is important for demanding applications. At the macroscale, low friction coefficients are achieved upon the formation of an inherently lubricious, soft and polymeric tribofilm whose composition and structure depends heavily on the sliding environment, while the lubriciousness of the resulting tribofilm does not depend on the environment in which it was formed. Nanoscale experiments demonstrate that the shear strength of a sharp, single asperity contact sliding on a-C:H:Si:O is at least an order of magnitude higher than those estimated from macroscale sliding, raising questions about whether the surface passivation theory of DLC lubricity is sufficient to explain macroscale lubricity. Colloidal AFM experiments show, in situ, that low friction is achieved with the growth of the tribofilm via a combination of reduced adhesion and a precipitous drop in the shear strength, which offset a simultaneous increase in the real area of contact. The compilation of results suggests a model of lubrication which relies on both surface passivation of the counterfaces and the soft and viscoelastic properties of the tribofilm, which reduce the effect on friction of nanoasperity pinning