A combinatorial investigation of palladium and platinum additions to β-NiAl overlay coatings

A combinatorial approach was used to investigate the effects of higher-order additions to NiAl on the oxidation, phase stability and interdiffusion behavior of the coatings. Overlay coatings with compositions (in at.%) in the range Ni-38Al-5Cr-(0.1-0.5)Hf-(2-8)Pd and Ni-36Al-5Cr-(0.1-0.5)Hf-(2-8)Pt were deposited on René N5 single crystal substrates via ion plasma deposition (IPD). Cyclic oxidation experiments on ten different Pd + Hf- or Pt + Hf-modified NiAlCr coatings at 1100 °C revealed similar weight gain and oxide scale thickness. While the thickness of alumina scales formed on Pd- and Pt-modified coatings were comparable, a higher density of oxide spalls were observed in the former. High-temperature transformations from β → γ′ → γ + γ′ occurred during extended exposure in both Pd- and Pt-modified coatings owing to Al depletion to substrate and surface oxide. On cooling, Al-depleted β-NiAl transformed to two martensite phases, 3R and fine-twinned 7R. Rumpling or surface roughening occurred for some coating compositions. Microprobe measurements showed that the difference in penetration depths of Pd into the substrate was not as large as expected from the higher interdiffusion coefficient of Pd in β-NiAl and γ-Ni.