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

The oxidation behaviour of two gamma-TiAl-based alloys coated with thermal barrier coatings (TBCs) was investigated in air at 900 degrees C for up to 1000 h under quasi-isothermal conditions. Samples bare or with a 4 mu m 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 lowpressure 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. (c) 2007 Elsevier B.V. All rights reserved.