Effect of substrate bias on microstructure and mechanical properties of WC-DLC coatings deposited by HiPIMS

The hardness and friction coefficient of the molding tools are the two key factors in influencing their performance during the cutting process. In this regard deposited by physical vapor deposition (PVD), WC-DLC nanocomposite hard coatings featuring high hardness and low friction coefficient are highly preferred to be used as protective coatings. As a newly developed PVD technology, high power impulse magnetron sputtering (HiPIMS) is quite advantageous in terms of the deposition of hard coatings. Substrate bias voltage exerts significant influences on the discharge characteristic of HiPIMS, plasma energy, chemical composition and the microstructure of the deposited coatings, which subsequently affect the coating's mechanical properties and performance in production. This article aims at investigating the effects of substrate bias on the plasma discharge characteristic of HiPIMS and on the mechanical properties, surface morphology, deposition rate, cross-sectional morphology, element concentration, crystal phase composition and tribological properties of the deposited coatings. The results show that the peak discharge current rises up from 57 A to 76 A with the increase of substrate bias from −40 V to −200 V. By controlling the bias voltage, WC-DLC coatings with different microstructures, mechanical properties and tribological properties have been produced. Meanwhile, the C concentrations of deposited coatings decline and the composed phase of the coating is transformed from hexagonal α-C at low bias voltage to equiaxial β-WC_1−x and then hexagonal β-W₂C accompanied by the rising bias voltage. The deposited WC-DLC coatings exhibit a decrease in surface roughness from Ra 16.1 nm to Ra 9.2 nm. Crystal phase evolutions also play a part in addition to the biased voltage upon the grain size and the hardness of the coating. It is found that the minimum grain size of 6 nm and the maximum hardness of 40.1 GPa appear at −160 V bias voltage when the coating is composed mainly of equiaxial β-WC_1−x phase and mixture of β-WC_1−x and β-W₂C. The friction coefficient of the WC-DLC coating measured by ball-on-disk test increased correspondingly with the increase of the bias voltage except that the wear rate reaches the lowest at −120 V bias voltage, indicating that the wear rate is related not only to friction coefficient but also to coating hardness.