Influence of processing variables on the properties of dc magnetron sputtered NiAl coatings containing Hf additions

In this study a series of NiAl-0.1Hf coatings were deposited onto Ni-base superalloy (CMSX-4) substrates at various substrate temperatures and working gas pressures, using direct current (dc) magnetron sputtering. Postdeposition microstructural and chemical analyses using scanning electron microscopy, transmission electron microscopy, and electron probe microanalysis techniques showed the as-deposited coatings to consist of single phase columnar NiAl grains. It was additionally observed that the structural morphology of the coatings could be altered by changing the substrate temperature and Ar pressure. The general trends were in accord with the structure zone model proposed by Thornton [J. A. Thornton, Annu. Rev. Mater Sci. 2, 239 (1977)], however the temperature ranges for each microstructure occurred at lower temperatures. Annealing in argon at 1273 K for 1 h resulted in some grain growth and in the formation of a small interdiffusion zone between the coating and substrate. After oxidation at 1323 K, the β phase gradually transformed to γ' due to Al depletion caused by oxidation and interdiffusion. Coatings deposited at higher substrate temperatures were observed to have larger grains and to exhibit higher resistance to oxidation. This improvement is attributed to a lower volume fraction of grain boundaries in those coatings