Tribological characteristics of WC1-x, W2C and WC tungsten carbide films

Thin film coatings with dominant phases of WC1-x, W2C and WC, were prepared by magnetron sputtering. The microstructures and chemistry of the coatings were characterized by X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS). The surface morphology and mechanical properties were examined by atomic force microscopy (AFM) and nanoindentation techniques. The tribological behaviour of the coatings was assessed by means of a microtribometer. Low hardness and indentation modulus of the W2C coatings were found to be from the high surface roughness. The W2C coatings were not wear resistant and were worn through in less than 15,000 cycles under a normal load of 40 mN. The WC1-X coatings were textured with the (200) plane parallel to the surface of substrate. The hardness and indentation modulus of the WC1-x coatings were ∼19 GPa and ∼600 GPa, respectively. A thick transfer layer was observed at the surface of the sphere employed in the sliding tests of the WC1-x coatings. The build-up and maintaining of this transfer layer was found to be important in determining the friction behaviour of the WC1-X coatings. A maximum hardness of ∼23 GPa and the lowest friction coefficient of 0.19 was obtained from the WC coating. In contrast to the large variation of the friction coefficients in the other coatings, the friction coefficient of WC was stable in the long-time running process.