Effect of Long Term Thermal Aging on Microstructure of Gas Turbine Blade Coatings
Akihiro ITO, Yuji SUGITA, Shigeo SAKURAI, Jun KAMEDA
Abstract:The degradation of coatings and substrates would be a great influence on the integrity of gas turbine blades for electric power generation. This paper describes the effect of thermal aging at 870 and 920 for 8000h in air on the microstructure and chemical composition in aluminized CoCrAlY coatings and directionally-solidified Ni-base superalloy substrates of advanced gas turbine blades. The unaged and thermally aged coatings consisted of four layers with different microstructure and chemical composition. Thermal aging led to a little oxidation, nitridation and decrease in the Al content in near-surface regions. In the adjacent region, coarse Cr rich precipitates formed during the thermal aging. During the thermal aging, Ni diffused extensively from the substrates into near-surface regions. The interdiffusion of Ni/Co resulted in the formation of Al/Ni rich intermetallic phases in various coating regions. The thermal aging formed the coating/substrate interdiffusion zone, which comprised Al/Ni rich elongated precipitates. Key Words:Gas turbine blade, MCrAlY coating, Aluminizing, Low-pressure plasma spray, Nickel-base superalloy, Directionally solidified alloy, Thermal aging, Scanning auger microanalysis