Improvement of the High-Temperature Oxidation Resistance of γ-TiAl by selectively Pre-treated Si-based Coatings

Kurzfassung

The aerospace as well as the automotive industry permanently intend to reduce the weight of applied structural materials. Therefore, γ-TiAl is an attractive material fulfilling that requirement due to its low mass density, generally, and good mechanical properties in the temperature range between 700°C and 900°C. However, application of titanium aluminides above 750°C service temperature is limited by poor oxidation behaviour caused by formation of fast growing and porous titania. To decrease the oxidation rate of TiAl it is essential to prevent the formation of non-protective TiO₂ scales. In this study the affinity of titanium to oxygen was lowered by forming titanium silicides. Si-based coatings with a thickness of 10µm were deposited on the TiAl substrate by means of magnetron sputter technique. The coating system had to be pre-treated to form thermodynamically stable titanium silicides. A detailed study of phase formation in bare γ-TiAl under various exposure conditions, such as oxygen and nitrogen partial pressure, temperature and time, led to an optimized pre-treatment process of 100h at 1000°C at 10^-6 mbar. Thus no oxides and nitrides were formed during this procedure but phase alterations occurred caused by interdiffusion processes. Therefore this pre-treatment was successfully used to form titanium silicides in the coatings. Annealing at 1000°C under high-vacuum conditions resulted in the formation of a Ti₅Si₃ layer, whereas less thermodynamically stable titanium silicides (Ti₅Si₄, TiSi and TiSi₂) formed when exposed to air at 750°C for 100h. With the latter pre-treatment, a mixed oxide scale was also found on the top of the coating. The oxidation behaviour of the pre-treated coatings on γ-TiAl was studied by cyclic testing at 900°C and 950°C in air. Phase formation and microstructure evolution of the samples were investigated after 10, 100 and 1000 cycles by SEM and EDX as well as XRD. Notably, the oxidation rate of the high-vacuum annealed samples is lower than that of the pre-oxidized ones. All specimens exceeded a lifetime of 1000 cycles at both test-temperatures at minimum.