Investigation on thermal stability of Ta ₂ O ₅ , TiO ₂ and Al ₂ O ₃ coatings for application at high temperature

In this paper, tantalum pentoxide (Ta ₂ O ₅ ), titanium dioxide (TiO ₂ ) and aluminum oxide (Al ₂ O ₃ ) coatings are deposited on silicon substrates by ion beam sputtering (IBS). The influences of the thermal exposure at high temperature in air on the surface morphology, roughness, and the structure were investigated. The results indicate that the chemical composition of the as-deposited TiO ₂ and Ta ₂ O ₅ coatings are apparently close to the stoichiometry ratios and both of them are amorphous structures. The peaks corresponding to anatase TiO ₂ appear at 400 C while the anatase-to-rutile transformation is not observed after 800 C and 1000 C bake. Ta ₂ O ₅ coating crystallizes at 800 C and 1000 C to form the hexagonal structure and orthorhombic structure, respectively. TiO ₂ and Al ₂ O ₃ single layers all develop catastrophic damage at 400 C in the form of noted spallation or blisters, whereas there is no visible damage for the Ta ₂ O ₅ coating even at 1000 C. To understand possible damage mechanisms, the thermal stress distributions through the thickness of Ta ₂ O ₅ and TiO ₂ coatings and the influence of the microstructure transformation are discussed. Finally, some possible approaches to improve the thermal stability are also proposed.