Microstructure evolution and mechanical properties of refractory molybdenum-tungsten nitride coatings

Abstract

This study focused on the microstructure and the mechanical properties of MoWN coatings fabricated by radio frequency reactive co-sputtering technique. The modulation of input power, which was kept at 150 W for Mo and adjusted from 25 to 125 W for W target, was manipulated. The Ar/N2 inlet gas flow and the background temperature were set at 12/8 sccm/sccm and 350 °C, respectively. The MoWN possessed a multiphase polycrystalline structure featured with Mo(110), MoN2(200), B1-MoN(111) and δ-WN(100) phases. With the increase of W input power, the content of W rose from 4.1 to 24.2 at. %, and a gradual growth of the δ-WN(100) phase was observed. The 150/100 W/W deposited MoWN coating possessed 20.1 at. % W and was dominated by δ-WN(100) phase. From TEM analysis, the longer and wider nano columnar WN(100) grains could be confirmed. The growth of δ-WN(100) phase elevated the compressive residual stress in MoWN films from −4.25 to −7.18 GPa and enhanced the hardness of coatings from 19.7 to 25.7 GPa. This phenomenon also improved the wear resistance of thin film. The wear track maintained smooth and intact after a tribological test of 100 m wear length for MoWN films. Both mechanical and tribological properties were significantly improved, as compared to the MoN film. This could be attributed to the addition of W, which caused the phase transformation and competition between B1-MoN(111) and δ-WN(100) phases. The 150/100 W/W manufactured MoWN exhibited a highest hardness of 25.7 GPa and showed a superior anti-wear behavior with least damage on wear track.