Oxidation behaviour of gamma titanium aluminides with EB-PVD thermal barrier coatings exposed to air at 900 °C

Abstract

The oxidation behaviour of two γ-TiAl-based alloys coated with thermal barrier coatings (TBCs) was investigated in air at 900 °C for up to 1000 h under quasi-isothermal conditions. Samples bare or with a 4 μm thick Ti–51Al–12Cr (at.%) protective layer were coated with yttria-stabilized zirconia using electron-beam physical vapour deposition. Prior to TBC deposition, the specimens were pre-oxidised in air and low-pressure oxygen. Thermal barrier coatings on samples of the alloy Ti–45Al–8Nb–0.2B–0.15C (at.%) did not spall off during the maximum exposure length, whereas failure occurred with pre-oxidised specimens of the alloy Ti–45Al–6Nb–1Cr–0.4W–0.2B–0.5C–0.2Si (at.%). Scanning electron micrographs revealed an outer oxide scale with a columnar structure grown by outward diffusion of Ti and Al cations. Below this columnar oxide layer, a broad porous zone formed consisting predominantly of titania. A discontinuous nitride layer was observed in the transition region between oxide scale and substrate. Spallation of the TBC was associated with cracking in the porous titania-rich layer. The zirconia top coat exhibited an excellent adherence to the thermally grown oxide scales. Pre-exposure in low-pressure oxygen did not improve the oxidation resistance. Post-oxidation microstructural examinations of specimens coated with Ti–Al–Cr revealed that the protective layer was severely deteriorated. On the degraded Ti–Al–Cr coating, an outer scale with columnar structure formed beneath the TBC.