Amorphous Si–B–C–N coatings for high-temperature oxidation protection of the γ-TiAl alloy

The present work focuses on the preparation of amorphous Si–B–C–N coatings to protect γ-TiAl intermetallic alloys against oxidation above 750 °C that otherwise leads to surface degradation and embrittlement. The film fabrication process included the deposition of an amorphous Si interlayer at a temperature of 700 °C to promote adhesion. Subsequently, Si–B–C–N coatings were prepared by pulsed dc and rf magnetron co-sputtering from Si and B4C targets in Ar + N2 gas mixtures. Specifically, we studied the oxidation behavior of the coatings and the evolution of the microstructure, composition, and mechanical properties upon an isothermal oxidation test performed in laboratory air at 800 °C for 100 h. We found that an optimized Si40B5C2N47 (at.%) coating protects the γ-TiAl alloy substrate against oxidation by a combination of several effects: a) Interdiffusion of Si and Ti results in the formation of TixSiy phases on the film-substrate interface promoting adhesion of the coating, b) Diffusion of Ti to the surface is effectively inhibited and no fast-growing TiO2 is formed at the outer surface, and c) Stable amorphous structure of Si–B–C–N coatings hinders inward oxidation of O. Investigation of the oxidation kinetics at 800 °C up to 1000 h proved that Si–B–C–N coating serves as an efficient oxidation barrier demonstrated by the significantly reduced mass gain and the parabolic oxidation rate constant (1.05 × 10−13 g2·cm−4·s−1) compared to the uncoated γ-TiAl alloy (5.65 × 10−13 g2·cm−4·s−1).