Temperature Estimation Based on Microstructural Change of Coatings for a Gas Turbine — Influence of Deposition Process on Microstructural Change and Estimated Temperature —

Abstract

CoNiCrAlY coatings are deposited by the air plasma spraying (APS), the high velocity oxygen fuel spraying (HVOF) and the low pressure plasma spraying (LPPS). Oxides and pores are formed in the coatings during the deposition process, and their content in APS coating is the largest among the three types of coatings while that in LPPS coating is the smallest. In order to estimate the service temperature of a gas turbine component by means of the microstructural change of the coatings, these three types of specimens are exposed to high temperature in air. A diffusion layer grows at the boundary between the coating and the substrate due to the interdiffusion during the exposure. The layer thickness increases in proportion to the square root of the test time. The temperature estimation based on the relation is applicable to HVOF coating as well as to LPPS coating. However, it is difficult to apply the method to APS coating since its layer thickness is much smaller than those in the other coatings. The exposure test is also conducted in air by means of thermal barrier coatings with CoNiCrAlY bond coats prepared by HVOF and LPPS. HVOF bond coat contains larger amount of the oxides and pores than LPPS one. The bond coat is oxidized during the test, and the Al content decreases in the layer at the vicinity of the surface. The layer thickness increases in proportion to the square root of the test time. The applicability of the temperature estimation based on the relation is clarified for HVOF bond coat as well as for LPPS one.