Isothermal Oxidation Studies of βNiAl Coatings for Aeroengine Applications

Nickel aluminide (βNiAl) coatings were prepared by in situ aluminising technique onto CMSX-4 superalloy substrates and isothermally oxidised at 1150°C for 100 h. The factors influencing the spallation resistance of βNiAl coatings were examined. The thermally grown oxide formed during oxidation was found to be multiphase (TiO2, Ni2AlO4, and θ-Al2O3) along with pure α-Al2O3. Coatings and substrate underwent massive interdiffusion resulting in (i) segregation of alloy-precipitates at thermally grown oxide and coating interface, (ii) sulphur migrated from substrate was present in such precipitates and (iii) Ni influx and Al depletion rendering βNiAl → γ'-Ni3Al transformation. Moreover, the degree of spallation and thermally grown oxide residual stresses increased with the oxidation time.