Effect of plasma interface treatment on the microstructure, residual stress profile, and mechanical properties of PVD TiN coatings on Ti-6Al-4V substrates

Abstract

Interface engineering is essential to enhance and to maintain the performance of protective coatings on metallic substrates. Plasma pre-treatments and coating deposition processes have shown to be an ideal solution to improve Ti-6Al-4V alloy mechanical and tribological properties, while enhancing components' durability. In the present work, we study model Titanium Nitride (TiN) coatings with three interface engineering surface treatment approaches using: a) Argon plasma, b) Titanium implantation, and c) plasma surface nitriding. In particular, we investigate the influence of the plasma pre-treatments on the microstructure, mechanical properties, especially residual stress (RS) and adhesion, of TiN coatings on Ti-6Al-4V substrates. X-Ray Diffraction, Transmission Electron Microscopy, and Transmission Kikuchi Diffraction were used as complementary techniques to evaluate the crystallographic and microstructural properties of the interfaces created by the three pre-treatment methods in order to elucidate their effect on the evolution of microstructure, hardness, RS and adhesion. The RS study involved surface and depth profiles through the coating-substrate system. Compressive RS values were found to vary between −1 GPa and −4 GPa throughout the TiN coatings, and from −0.2 to −0.8 GPa across the different interfaces. In addition, a strong RS anisotropy in dependence on the way how the substrate has been fixed to the holder (rigidly clamped or freely attached) shows a significant difference in the RS with a deviation of >100% in dependence on the orientation of coated samples during the XRD measurements. Experimental results show that Ar plasma and Ti implantation form interfaces with (200) and (111) preferential orientations within the TiN layers that exhibited a hardness of up to ~29 GPa and a Young's modulus of ~350 GPa.