Comparison of thermal expansion and oxidation behavior of various high-temperature coating materials and superalloys

The thermal expansion mismatch between a metallic substrate and its external oxide scale generates a strain on cooling that is a primary cause of spallation of protective oxide scales. This study compares thermal expansion behavior and cyclic oxidation performance of the two major composition classes of high-temperature commercial coatings for protection of single-crystal superalloys. The thermal expansion of cast MCrAlY (M = Ni and/or Co) alloys and cast aluminides (NiAl, (Ni,Pt)Al and Ni₃Al) was measured at temperatures up to 1300°C and compared to that of a single-crystal Ni-base superalloy. The tendency for scale spallation from each alloy was evaluated by cyclic oxidation testing at 1150°C. The coefficients of thermal expansion for the aluminides were lower than those of the MCrAlY-based alloys at all temperatures and scale adherence to the Hf-doped aluminides was generally superior. Scale adherence to the various compositions of MCrAlY-type alloys did not directly correlate to their thermal expansion behavior or substrate strength. For both types of materials, the presence of a reactive element (Y,Hf, etc.) had no detectable effect on thermal expansion but a major effect on scale adherence. There was no obvious influence of Al content on the thermal expansion of β phase Ni-Al compositions. The addition of Pt resulted in a lower average thermal expansion for hyperstoichiometric (Ni,Pt)Al at temperatures above 930°C, but this effect was not observed in hypostoichiometric (Ni,Pt)Al.