Thermal cycling of iridium coatings on isotropic graphite

Abstract

The surface morphology observed on the iridium-coated isotropic graphite substrate varied widely between 300 and up to 2173 K thermal cycling and heat-treatment testing. The columnar structure was retained after 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 time and temperature. The structure obtained contains pores going outwards from the coating after thermal cycling between 300 and 1873 K; on the other hand, high-temperature thermal cycling porosities were diminished. Transmission electron microscopy of the specimens showed very little difference in grain size between as-deposited coatings and those thermally cycled between 300 and 1873 K. X-ray diffraction analysis indicated that the preferred orientation of columnar structure was destroyed by thermal cycling. In addition, there was no loss in weight after thermal cycling and heat treatment testing in nitrogen.