Air plasma sprayed thermal barrier coatings on titanium alloy substrates

Titanium alloys as lightweight structure materials have been shown more interest in the use at moderately elevated temperatures. However, their poor oxidation resistance at temperature above 600 °C limits a wide application. Consequently, thermal protection becomes a concern. 8 wt.% yttria partially stabilized zirconia thermal barrier coatings (TBCs) were air plasma sprayed on titanium alloy substrates (Ti–6.6Al–3.61Mo–1.69Zr–0.28Si in wt.%). The microstructures and mechanical properties of the coating system were studied by using SEM, XRD, MIP, tensile test and Vickers microhardness. The results show that the as-sprayed ceramic coating consists of the non-transformable tetragonal t′ phase of zirconia. The microstructure of the internal substrate keeps unchanged, no reaction and interdiffusion happen obviously at the bond coat/substrate interface during plasma spraying. However, there exists a thin layer of plastic deformation zone in the substrate beneath the bond coat/substrate interface. The surface connected porosity of the as-sprayed ceramic coating shows a typical bimodal pore size distribution. The adhesive strength of the TBCs decreases with increasing the thickness of the as-sprayed ceramic coatings, which is attributed to the residual stresses induced during thermal spraying. The anisotropy in the mechanical properties between the cross section and the top surface of the ceramic coatings is examined.