Effect of Substrate Pre-carburizing on Properties of TiN (Ti) Hard Coatings Deposited on Ti-6Al-4V Alloy

Interface fracture often occurs in systems of soft substrate and hard coatings during the service process, which is related to the weak interface strength and the unstable expansion of cracks along the interface induced by the residual thermal stress. The residual thermal stress mainly comes from the mismatch of thermal physical properties between matrix and hard coatings. In this study, a gradient carburized layer was prepared on TC4 substrate before the deposition of TiN(Ti) coatings using double glow plasma carburization. And then the mono- and multilayer TiN(Ti) coatings were synthesized on the carburized layer, forming composite hard coatings. The effect of substrate carburizing on properties of coatings was studied. The results show that the composite coatings' hardness can be increased nearly 2 times and the bonding strength is enhanced to over 80 N, compared with that of mono-and multilayer TiN(Ti) coatings. Also the interface brittle fracture tendency is restrained obviously by the hardened substrate, and the coordinative deformation ability of the coating at extra load is optimized. The composite coating composed of pre-carburized layer and TiN hard coating shows a higher strength and toughness.