Plastic instability of aluminide and platinum modified diffusion coatings during 1100˚C cyclic testing.

Abstract

Platinum modified and unmodified aluminide diffusion coatings, on a nickel base superalloy (IN-738), were prepared to test the pre-aluminizing surface roughness effect on coating oxide scale adherence. A preliminary study of coating microstructure and surface structure changes during cyclic oxidation at 1100˚C was begun. During this testing, significant surface deformation described as rumpling was observed and attributed to plastic instability produced during the cycling. Rumpling is found to be a function of the number and type of thermal strain cycles, thermal expansion mismatch, coating strength, and coating thickness. The role of oxide adherence observed for the Pt modified coatings cannot be determined from the data. Previous mention of this effect was not found in the literature. A similar surface rumpling phenomena, however, has been observed in overlay coating systems under similar conditions. The mechanical and protectivity impact of rumpling can only be inferred due to the limited available data .