Effect of Plasma Spraying Process on Microstructure and Microhardness of Titanium Alloy Substrate

High-temperature titanium alloys are considered as good candidate materials for many aerospace applications. In order to increase the usable temperatures and oxidation resistance of titanium alloys, plasma spraying thermal barrier coatings (TBCs) on the titanium alloys is considered as an effective method. The effect of plasma spraying process on microstructure and microhardness of the titanium alloy (Ti-6.6Al-3.61Mo-1.69Zr-0.28Si in wt.%) was investigated by scanning electron microscope, energy dispersion analytical X-ray spectroscopy (EDAX) and microhardness test. The results show that the microstructure of the titanium alloy inside the substrate keeps unchanged after plasma spraying, and no interaction and atomic diffusion happen evidently at the bond coat/substrate interface. However there exists a thin layer of plastic deformation zone in the substrate beneath the bond coat/substrate interface after plasma spraying. The residual stresses are induced inside the titanium alloy due to the thermal expansion mismatch and the temperature gradient inside the substrate during plasma spraying, and lead to generating microcracks in the surface beneath the bond coat/substrate interface and the increase of microhardness in the substrate.