A study on pre-treatment with rotation accelerated shot peening in multi-NbN films by double glow plasma surface metallurgy

Abstract

Multi-NbN films manufactured by a hybrid process comprising pre-treatment rotation accelerated shot peening (RASP) and double glow plasma surface metallurgy (DG technique) and designed to protect titanium alloys were assessed to determine their tribological and mechanical properties. The surface microstructures of a modified TA15 alloy were characterized using X-ray diffraction, scanning electron microscopy, 3D-profilometer and micro-Vickers hardness testing. The results reveal that the distribution of N concentration on layers was influenced by the pre-treatment with RASP, in which a dense nitride gradient layer was formed inside the films and a new phase β-Nb2N occurred at the film surface. This is because a large number of defects on the surface layer were generated by the pre-treatment with RASP. While multi-NbN films on the original sample without new phase detected can be divided into three zones—NbTi interdiffusion layer, Nb-Ti-N interdiffusion layer, and NbN layer. Strong increase in surface hardness was verified in correspondence of elevated peening velocity, and the hardest RASP-60 m/s sample is up to 30.1803 GPa. However, the thickness of films cannot keep increasing trend in agreement with increasing shot peeing velocity. The thickest films occurred on the RASP-40 m/s sample, is 5.85 μm. The results show that the RASP-treated samples exhibited more serious damage than the untreated sample, due to the formation of β-Nb2N phase with brittle structure which bonding on the course surface are incapable of overcoming the damage of shearing stress from counterparts. And once starting to crack, the films that are supposed to protect the substrate are turned into hard abrasive particles that adversely affect the wear resistance of the substrate.