Fabrication of tungsten nitride thin films by superimposed HiPIMS and MF system: Effects of nitrogen flow rate

Abstract

The high-power impulse magnetron sputtering (HiPIMS) technology starts a new era in the thin film fabrication, which already attracts researchers and industries' interest on its further application in many fields. For improving the low deposition rate of HiPIMS, the superimposition concept by introducing mid-frequency (MF) pulses during the off-time of HiPIMS pulsing was adopted for the fabrication of transition metal nitride hard coatings. Tungsten nitride coating is characterized by its high hardness, good thermal stability and low electrical resistivity. In this study, tungsten nitride coatings were fabricated using five different N2 gas flow rate ratios by a superimposed HiPIMS-MF power system. The W target showed a hysteresis-free behavior during the reactive sputtering process of WNx coating, where x indicated the chemical composition ratio between W and N elements. The peak current and peak power density values of both HiPIMS and MF powers decreased with increasing N2 flow rate ratio. The nitrogen content of WNx coating increased with increasing nitrogen flow rate ratio, whereas the tungsten concentration shows a decreasing tendency. The x value and the nitrogen content of WNx coating increased from 0.05 to 0.60 and from 5.1 to 36.7 at.%, respectively, when the nitrogen flow rate increased from 13 to 37 sccm. The crystalline structure of WNx coatings changed from a pure BCC-W phase to a mixture of BCC-W and FCC β-W2N phases when the x value increased from 0.05 to 0.12. A pure FCC β-W2N phase structure was obtained when the x values of WNx coating reached 0.36, 0.43 and 0.60. The hardness values of WNx coatings were ranging from 26.7 to 31.6 GPa. The WN0.12 coating with a low coefficient of frictions of 0.33, a high hardness of 31.5 GPa and high H/E and H3/E2 ratios were achieved due to the solution hardening and smoother microstructure induced by the substrate bias and the higher energy of superimposed HiPIMS-MF system.