Ti-Al-Nb-Cr-Zr and Ti-Al-Nb-Cr-Zr-B-La alloys were used as substrates to be protected from oxidation with APS and HVOF M-Cr-Al-X coatings. High-temperature 920 °C isothermal annealing during 100 h was carried out on these samples to study the oxidation products, diffusion zones formation, mechanical properties and thermal cycling resistance.
All the tested coatings impart good oxidation protection to both alloys forming mixed oxide TGO consisting of CoNi (AlCr)2O4 and α-Al2O3 main phases. Only Ti-Al-Nb-Cr-Zr alloy with HVOF coatings TGO contains minor TiO2 amount (0.5-0.6 mass.%). TiO2 presence in TGO (XRD) and Ti concentration curves (EDX) confirms high diffusion activity of Ti through the HVOF M-Cr-Al-X coatings independent of Ta addition. Ti diffusion activity depends on structure and chemical composition of substrate together with coating application technique. Feedstock in APS was subjected to much higher oxidation during spraying compared to HVOF, leading to oxide stringers formation in coating serving as diffusion barriers. Two diffusion zones were formed between substrates and coatings during annealing: Heusler phase AlNi2Ti in zone 1 and γ-Ni0.35Ti0.30Al0.35 and Ti3Al (TiAl) mixture in zone 2.
The DMA results showed complex elastic modulus decrease at room temperature of as-sprayed samples and further decrease after isothermal annealing due to diffusion zones formation and phase composition changing. Complex elastic modulus temperature dependence confirmed the decrease in values compared to uncoated alloy due to diffusion zones formation. All the tested samples showed no spallation, cracks or other coating defects during thermal cycling tests at the burner rig setup.
Further investigations on mechanical properties should be performed. The data obtained in this study may be helpful in choosing the coating composition and application technology for γ-TiAl alloys oxidation protection.
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