Effects of coating material and thermal aging on fatigue strengths of a Ni base superalloy, IN738LC, with protective coatings
Okazaki Masakazu; Mutoh Yoshiharu; Sadasue Teruki; Fujita Atsushi; Saitoh Masahiro
Abstract:The effect of coating material on the high cycle fatigue strength of a Ni base superalloy, IN738LC, was studied at room temperature and 800 deg.. Through the work specific attention was paid to understand not only the fatigue lives themselves but also the interaction between the coatings, the substrate and the diffusion zone. The IN738LC specimens with three kinds of protective coatings were prepared and specified: one was the IN738LC with CoCrAlY alloy overlay coating of 230mm in thickness, the second was the IN738LC with CoNiCrAlY alloy overlay coating of 230mm in thickness, and the third was the combined coatings in which the aluminide coating of about 10 mm in thickness was conducted on the CoNiCrAlY overlay coated IN738LC. The CoNiCrAlY and CoCrAlY alloy coatings, and the aluminide coating, were performed by high velocity oxygen fuel (HVOF) spraying, and chemical vapor deposition (CVD) technique, respectively. It was shown that there was a significant effect of the coating material on the fatigue lives of the protective coatings. The fatigue test results also indicated that the fatigue strength at room, or, low temperature should be as important as that at high temperature. The effect of long term isothermal aging on the fatigue lives of the protective coatings was also studied and clarified, investigating the inward and outward diffusion of main alloy elements between the substrate and the coatings. Key Words:MCrAlY alloys, aluminide coating, combined coating, Ni-base superalloy, fatigue, long term aging, inward and outward diffusion of alloy elements