High-temperature oxidation behavior of Ti2AlNb alloy with PEO/hBN composite coating at 1000 °C

Abstract

The oxidation resistance of Ti2AlNb alloy is not ideal when the service temperature is higher than 800 °C. In this work, plasma electrolytic oxidation (PEO) coating and PEO/hBN composite coating were prepared and used to improve the high temperature performance of Ti2AlNb alloy. The microstructure and chemical composition of these coatings were systematically analyzed by FESEM, XPS and XRD. Meanwhile, the high temperature oxidation behaviors of bare and coated Ti2AlNb samples at 1000 °C were studied by isothermal oxidation tests. The results showed that the PEO coating was porous and contained rutile-TiO2, anatas-TiO2, Al2O3, a little Nb2O5 and WO3. PEO/hBN composite coating was denser and also had hBN phase in addition to the original components. The bare sample had the biggest oxidation reaction rate constant (K) and poor oxidation resistance. After being oxidized at 1000 °C for 100 h, the mass gain reached to 12.08 mg/cm2, and a multi-layered oxide scale consisting of loose Al2O3 outer layer, R-TiO2 and AlNbO4 mixed oxide layer and oxygen diffuse zone was formed. PEO coating improved the initial oxidation resistance by hindering the diffusion of oxygen, but the coating began to fail after 30 h due to the presence of micro-pores. In contrast, PEO/hBN composite coating had better protection effect against Ti2AlNb matrix. The oxidation kinetic curve of this sample followed the quasi-parabolic law and did not degrade at 1000 °C up to 100 h. The dense coating structure and the filling effect of nitride (AlN, TiN, etc.) formed during high-temperature process on coating defects were considered to be the main reasons for the excellent oxidation resistance of PEO/hBN coatings.