THERMAL CYCLING OF IRIDIUM COATINGS ON ISOTROPIC GRAPHITE

The surface morphology observed on the iridium-coated isotropic graphi te substrate varied widely between 300 and up to 2173 K thermal cyclin g and heat-treatment testing. The columnar structure was retained afte r thermal cycling between 300 and 1873 K. At high temperatures between 300 and 1973-2173 K thermal cycling, the columnar grain structure was replaced by dense equiaxed grains and the grain size increased with t ime and temperature. The structure obtained contains pores going outwa rds from the coating after thermal cycling between 300 and 1873 K; on the other hand, high-temperature thermal cycling porosities were dimin ished. Transmission electron microscopy of the specimens showed very l ittle difference in grain size between as-deposited coatings and those thermally cycled between 300 and 1873 K. X-ray diffraction analysis i ndicated that the preferred orientation of columnar structure was dest royed by thermal cycling. In addition, there was no loss in weight aft er thermal cycling and heat treatment testing in nitrogen.