High-temperature diffusion barriers for protective coatings

An effective diffusion barrier between the superalloy and aluminide coating would reduce coating degradation by lowering the rate of Al loss to the substrate by interdiffusion, and by inhibiting diffusion of substrate elements (such as Cr, Re, Ta, W) into the coating, which degrade its corrosion and oxidation resistance. In this preliminary study, sputtered Hf–Ni and Hf–Pt were evaluated as potential high-temperature diffusion barrier layers beneath NiAl or NiPtAl coatings. A Ni 3Hf-type layer formed during aluminizing of Ni–Hf thin films, and appeared to result in reduced β-NiAl depletion during oxidation testing at 1150 °C. Some Hf-containing simple aluminide coatings also showed very good oxidation resistance. However, the Ni–Hf phase disrupted coating microstructure and displayed limited high-temperature stability. Heat treatment of Hf and Pt thin films formed a compound similar to HfPt 3-type, which subsequently dissolved during aluminizing.