Microstructural, mechanical and tribological properties of tungsten-gradually doped diamond-like carbon films with functionally graded interlayers

A series of tungsten-gradually doped diamond-like carbon (DLC) films with functionally graded interlayer were prepared using a hybrid technique of vacuum cathodic arc/magnetron sputtering/ion beam deposition. With ‘compositionally graded coating’ concept, the deposition of wear-resistant carbon-based films with excellent adhesion to metallic substrate was realized. In the films, a functionally graded interlayer with layer sequence of Cr/CrN/CrNC/CrC/WC was first deposited onto the substrate, and then, a DLC layer doped with gradually decreasing content of W was coated on. The W concentration gradient along depth of the film was tailored by adjusting the W target current and deposition time. The characterized results indicate that the microstructural, mechanical and tribological properties of these films show a significant dependence on the W concentration gradient. A high fraction of W atom in carbon matrix can promote the formation of sp² sites and WC_1 − x nanoparticles. Applying this coating concept, strongly adherent carbon films with critical load exceeding 100 N in scratch test were obtained, and no fractures or delaminations were observed at the end of the scratched trace. The hardness was found to vary from 13.28 to 32.13 GPa with increasing W concentration. These films also presented excellent tribological properties, especially significantly low wear rate under dry sliding condition against Si₃N₄ ball. The optimum wear performance with friction coefficient of 0.19 and wear rate of 8.36 × 10⁻⁷ mm³/Nm was achieved for the tungsten-gradually doped DLC film with a graded W concentration ranging from 52.5% to 17.8%. This compositionally graded coating system might be a potentially promising candidate for wear-resistant carbon-based films in the demanding tribological applications.