Amorphous silicon carbide coatings grown by low frequency PACVD: Structural and mechanical description

Hard coatings have received considerable attention for many mechanical applications, such as aeronautics, space and high precision mechanics. Material properties, like high hardness, low friction and low wear versus metals, are required for this purpose. Diamondlike carbon films (a-C:H, DLCs) fulfill these requirements. Nevertheless, they are limited in use as their microstructure is strongly changed at high temperature. Doping DLCs with elements such as silicon (up to 30 at.%) should extend their applications to high temperature environments (T > 675 K). In this work, hard silicon carbide based films (a-Si:C:H) grown from tetramethylsilane/argon plasma are presented. Such coatings are obtained in a capacitively coupled low frequency (ν = 50 kHz) PACVD device, at surface temperature ranging from 623 to 853 K. The evolution of their microstructure, observed by Infrared spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS) and EDS is described in regard to variations of i) surface reactivity, through substrate temperature, ii) nature of plasma species through gas residence time and TMS content, iii) energy of ions impinging the growing film through surface DC bias. As generally observed, an increase of surface temperature leads to an improvement of film crosslinking (Si–C content increase, H content decrease) as an increase of gas residence time in the reactor leads, through recombinations of gaseous species in the discharge, to a reduction of bonded hydrogen and silicon contents in the deposits. Finally, the energy of impinging ions during growth is of major importance as it limits films growth rate through selective sputtering phenomena. Comparisons between characterizations of films grown at various DC bias in Ar / TMS mixture, and films post-treated in argon plasma without precursor, lead to the fact that an ion energy domain exists which limits silicon and C–Csp2 contents to the benefit of C–Csp3 (XPS) and sp3CH2 (FTIR) environments. Determination of hardness (H) and Young modulus (E) of those films by nanoindentation technique, shows that exploiting ion bombardment can significantly improve hardness (from 20 to 30 GPa). Corresponding ion energies lead to an improvement of the ratio H3 / E2, giving rise to low friction coefficients against steel (μ # 0.15).