Influence of CH(4) partial pressure on the microstructure of sputter-deposited tungsten carbide thin films

Tungsten carbide thin films have been prepared by reactive rf sputtering from a tungsten target in various Ar-CH(4) mixtures. The composition, structure, microstructure and chemical state of the films have been investigated by the complementary use of RBS, NRA, XRD, GIXRD, TEM and XPS analyses. These characteristics of the films were then correlated to their mechanical properties determined by hardness (H), Young's modulus (E(r)) and friction coefficient measurements. Under low CH(4) pressures, the formation of a mixture of nanocrystalline WC(1-x) and W(2)C phases has been observed. A pure WC(1-x) phase was observed in films having a composition close to W(1)C(0.9). With increasing CH(4) pressure, the amount of carbon in the films increases, leading to a progressive amorphization of tungsten carbide deposited layers. Nanocomposite films appeared to be formed, with WC(1-x) nanograins (<3 nm) dispersed in an amorphous carbon matrix. The film deposited at 30% of CH(4) exhibits a-C:H phase. The nature of the phases present in the films plays an important role on their mechanical properties, as shown by the wide domain of variation of the films' hardness (between 22 and 5.5 GPa) and the plastic deformation parameter H(3)/E(r)(2) (between 0.08 and 0.04).