Improvement of the High Temperature Oxidation Resistance of γ-TiAl by means of Si-based Coatings

Abstract

Recent trends in aerospace as well as in automotive industry lead to the use of light weight materials especially applied in highly stressed components. Thereby γ-TiAl becomes more and more interesting because of the attractive mechanical properties under service conditions at temperatures up to 900°C. However, the oxidation resistance of γ-TiAl alloys decreases rapidly at temperatures above 750°C due to a fast formation of TiO₂. Reducing the formation of non-protective titanium oxide is the primary objective to improve the resistance of titanium aluminides against oxidation. In this study the affinity of titanium to form oxides was reduced by forming thermodynamically stable titanium silicides. Two coatings, pure Si and Ti-75at.%Si, were deposited on the γ-TiAl base material by means of magnetron sputter techniques. To induce formation of silicides, the coating systems were pre-treated by air exposure for 100h at 750°C or annealed in vacuum (pressure of 10^-6 mbar) for 100h at 1000°C. The oxidation resistance of the coatings was tested at 900°C and 950°C using a cyclic testing procedure: 1h heating and 10 min cooling down to 60°C. After 10, 100 and 1000 cycles (or failure) the samples were investigated by means of SEM and EDX as well as XRD to analyse the microstructure evolution and phase formation in course of time. After pre-treatment at low atmospheric pressure the most thermodynamically stable TiSi phase, i.e. Ti₅ Si₃, has been formed. It resulted in a minimum lifetime of the coating systems of 1000 cycles at 900°C and 950°C. Uncoated γ-TiAl already failed after 520 cycles at 900°C and after 40 cycles at 950°C. The samples pre-oxidised at 750°C showed TiSi₂, TiSi and Ti₅Si₄ phase formation, which are less stable than Ti₅Si₃. Additionally, a thick oxide scale was formed which grows much slower during cyclic testing. Despite a higher oxidation rate of the pre-oxidised samples, only one specimen failed after 440 cycles when exposed at 950°C.