Preparation and oxidation behavior of SiO2–Al2O3–glass composite coating on Ti–47Al–2Cr–2Nb alloy

A SiO2–Al2O3–glass composite coating was prepared on Ti–47Al–2Cr–2Nb alloy by air spray technique and subsequent firing. The Ti–47Al–2Cr–2Nb alloy with and without composite coating were oxidized in air for 100 h at 800 °C and 900 °C, respectively. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and electron probe microanalysis (EPMA) technique have been employed to study the microstructures of the specimens before and after oxidation tests, X-ray diffraction (XRD) was used to analyze the oxide phase. For the bare Ti–47Al–2Cr–2Nb alloy, a typical layered structure oxide scale formed, the outer layer is TiO2, the intermediate one is an Al2O3-rich layer, the inner one is a mixture of TiO2 and Al2O3, a small amount of dissolved Cr and Nb ion being preferentially segregated in the inner layer. The composite coating provides good oxidation resistance for the Ti–47Al–2Cr–2Nb alloy. An interfacial reaction of the composite coating and the substrate occurred, forming an Al2O3/Ti5Si3/Al2O3/Z-phase Ti5Al3O2 layered structure in the interfacial zone. The composite coating/Al2O3 bilayer act as oxygen diffusion barrier and benefit oxidation resistance of the Ti–47Al–2Cr–2Nb alloy. Al2O3 and quartz were instable in the glass phase; Al2O3 and quartz inclusions partially dissolved into the glass phase, and some quartz particles transformed into cristobalite.