Oxidation resistance of Pt-modified γ/γ′ Ni-Al at 1150 °C

Nickel aluminide coatings are used to protect the surfaces of turbine engine hot-stage components. Increasing service temperatures are driving the need for improved performance coatings, which grow and retain thin, protective alumina scales. This paper describes the oxidation resistance of platinum-modified, two-phase (γ/γ′) Ni-22Al materials at 1150 °C in air. Under isothermal conditions, a composition of Ni-22Al-30Pt (at.%) oxidises with the same parabolic rate constant as β-NiAl. The addition of hafnium reduces the rate by an order of magnitude. The protective oxide formed is in all cases α-Al2O3. Focused ion beam (FIB) milling was used to examine the very thin scales. In 100 h of reaction, a hafnium-free Ni-Al-Pt material grew a roughly uniform scale 4–5 µm thick, decorated with numerous oxide whiskers. At the same time, a coating containing 0.5 at.% Hf grew a very thin (< 0.5 µm) oxide over which developed a network of ridges about 1 µm high. The effects of hafnium are discussed in terms of oxide transport properties. Possible explanations for the beneficial effects of platinum are also considered.