Original paper(Vol.45 No.2 pp.218)

Aluminizing properties of plasma-sprayed MCrAlY coating

Itoh Yoshiyasu; Tamura Masataka; Harada Yoshio; Takeuchi Jun-ichi

Abstract:A low-pressure-plasma spray (LPPS) process is used for the overlay coating of MCrAlY alloy to protect it against high temperature corrosion and oxidation. This coating process has been found to be very effective for gas turbine components. On the other hand, a diffusion coating process has been applied for many years to improve similarly the environmental resistance by enriching the surface of a substrate in Cr, Al, or Si. Recently, aluminizing on MCrAlY coating is used for improving further the high temperature oxidation resistance. However, the aluminizing properties of plasma-sprayed MCrAlY coating, which have an important effect on the coating performance, have not been clarified. In this study, five kinds of plasma-sprayed MCrAlY (CoCrAlY, CoNiCrAlY, CoNiCrAlY+Ta, NiCrAlY, NiCoCrAlY) coating were selected for the aluminizing tests. The heat treated MCrAlY specimens (1393 K, 2h, Ar atmosphere) were pack-aluminized at 1173, 1223 and 1273 K for 5, 10 and 20 h, respectively.
The experimental results showed that the aluminum rich layer of NiAl or CoAl phase was formed by aluminizing. It was also indicated that the thickness of the aluminum rich layer showed parabolic time dependence. There is a tendency that the reaction diffusion rate by aluminizing increases with increasing nickel content in the MCrAlY coating.

Key Words:low-pressure-plasma spray, McrAlY coating, aluminizing, reaction diffusion, parabolic time law, diffusion rate constant, nickel aluminide, cobalt aluminide