Protective TixSiy coatings for enhanced oxidation resistance of the ɣ-TiAl alloy at 900 °C

Abstract

Due to well-balanced mechanical properties and density ratio, ɣ-TiAl alloys have been extensively investigated for potential aircraft jet engine applications. However, their use is restricted by limited oxidation resistance at temperatures above 750 °C. The present study focuses on the development of titanium silicide protective coatings using different Si reservoirs (3.0 μm, 6.0 μm and 9.5 μm of initial Si thickness) deposited by magnetron sputtering, followed by different periods (2 h to 24 h) of vacuum thermal annealing at 950 °C. The growth of titanium silicides and evolution of the composition and microstructure following oxidation was investigated using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD) techniques. The initial Si thickness and annealing time have an influence on the formation of different zones of composition, assigned as the TixSiy layer, interdiffusion zone and TiAl2 zone, which are correlated with their oxidation behavior at 900 °C in laboratory air after 100 h. The ɣ-TiAl substrates with Si-based coatings presented a protective inner layer of TixSiy. The coatings with initial 3 μm of Si show formation of mixed Si and Ti oxides as well as minor Al content, and traces of Cr at the oxide scale. The oxidation process is described by the parabolic oxidation coefficient, kp, with values of kp = 3.5–8.9 × 10−12 g2·cm−4·s−1 for the coated surfaces, one order of magnitude lower than for the bare substrate. In comparison, the coating systems possessing a higher initial Si thickness (6.0 μm and 9.5 μm) mainly exhibit the mixed formation of dense Si and Al oxide scales with significant Cr enrichment.