Surface instability of platinum modified aluminide coatings during 1100 °C cyclic testing

Abstract

Platinum additions to aluminide coatings have been reported to be beneficial in both hot corrosion and oxidation environments. The data, however, are somewhat limited and strongly dependent upon structure and test conditions. Therefore, a study of the effects of surface structure under 1100 °C cyclic oxidation conditions of the platinum modified and the unmodified aluminide coatings on the IN-738 substrate was initiated. During the early stages of this evaluation surface upheavals on the order of 50 μm, often called ‘‘rumpling,’’ were observed for both systems. The degree of rumpling was greater in the platinum modified coatings but was observed to a significant extent with the unmodified coatings as well. The amount of rumpling observed during cyclic testing can be attributed to a number of possible effects including (i) coefficient of thermal expansion mismatch, (ii) thermal gradient across the coated specimen, and (iii) coating mechanical properties. An effect of coating thickness is also observed although it must be noted that for diffusion-type coatings such as the Pt–Al system, coatings with different thicknesses have different compositions. Therefore, the resulting performance of the coating is a complex interplay of the above-mentioned variables. Excellent Al2O3 adherence is seen for all the platinum modified aluminides tested, even on the highly convoluted surfaces. The role of rumpling in the high temperature protectively is not known but should be given more consideration as a potential high-temperature degradation mechanism of these coatings.