Chemical compatibility of coated systems at high temperatures

Abstract

Abstract Increasing the gas inlet temperatures in industrial gas turbines and aero engines requires careful selection of coatings which offer sufficient chemical compatibility with low interdiffusivity. Rapid diffusion of aluminium and cobalt from Co(Ni)CrAlY coatings into nickel-based alloys, with subsequent precipitation of the more stable β-(Ni,Co)Al in the interdiffusion zone and diffusion of nickel into the coating, limits the maximum operation temperature of such coatings in industrial gas turbines to about 950 °C. The degradation of aluminide and Ni(Co)CrAlY coatings is characterized by the diffusion of nickel into the coating, the subsequent inward diffusion of aluminium and the precipitation of Cr23C6 carbide in the interdiffusion zone which can be explained by the higher chromium activity in this zone. Therefore, these coatings have a higher temperature capability than Co(Ni)CrAlY coatings. Oxide-dispersion-strengthened (ODS) alloys as well as single-crystal alloys have a high susceptibility to Kirkendall void formation. In ODS alloys porosity formation occurs in the interdiffusion zone which can be explained by lower condensation energies of vacancies at incoherent phase boundaries between dispersoids and the sorrounding matrix. Porosity formation in single-crystal alloys is favoured in the coating or at the coating-substrate interface. Grain boundaries may act as sinks for vacancies with subsequent porosity formation.