First Stages of Oxidation of Pt-Modified Nickel Aluminide Bond Coat Systems at Low Oxygen Partial Pressure

The θ-Al2O3 → α-Al2O3 phase transformation was investigated in thermally grown oxide formed on β-(Ni,Pt)Al bond coats during isothermal exposures at 900–1,200 °C in an argon atmosphere stream with the O2 partial pressure of 1 × 10−5 atm. Local curve fitting was used to evaluate the evolution of the parabolic rate constant, k p , using a general kinetic model (t = A + BΔm + CΔm 2), during the first 5 h of oxidation. All net mass–gain curves exhibited deviations from the classic parabolic model, Δm = k p t ½; a steady state regime was established only after 4 h of exposures, except for the sample oxidized at 1,100 °C.