Effects of HiPIMS pulse-length on plasma discharge and on the properties of WC-DLC coatings

Abstract

High power impulse magnetron sputtering (HiPIMS) has been employed to deposit WC-DLC coatings on cemented carbide substrate from a tungsten target in acetylene (C2H2) reactive atmosphere. Pulse length of HiPIMS was varied from 50 μs to 200 μs at constant frequency to investigate its influences upon plasma discharge characteristics, target poisoning in the reactive HiPIMS process, and upon the chemical composition, microstructure, mechanical and tribological properties of the deposited coatings. Consequently an oscilloscope (OSC) was used to record the discharge current during the experiments. The results exhibit that the discharge current shows a significant drop at short pulses but a slow descent at long pulses during the deposition. The optical emission peak intensity of W (target material) monitored by optical emission spectroscopy (OES) displays the same tendency. The deposition rate and cross-sectional morphology of WC-DLC coatings were investigated by scanning electron microscope (SEM). Surface morphology and roughness were detected by atomic force microscopy (AFM). The chemical composition, crystal structure and carbon phase were analyzed by means of energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Raman spectroscopy respectively. The glow discharge optical emission spectroscopy (GDOES) was used to demonstrate the depth profiles of the WC-DLC coatings. The results verify that coatings of different depths deposited at longer pulse length exhibit better uniformity of elemental concentration. The hardness obtained by nano-indentation tester also shows significant changes from 35 GPa at 200 μs to 18 GPa at 50 μs pulse length of the deposited coatings. The friction coefficients and wear rates of WC-DLC coatings measured by ball-on-disk test and laser scanning confocal microscope (LSCM) show a strong dependency upon the amorphous carbon composition of regions close to surface, which is influenced by the pulse length of HiPIMS.