Microstructural evolution of CoCrAlY bond coat on Ni-based superalloy DZ 125 at 1050 °C

Abstract

CoCrAlY alloy has been widely used as metallic protective coatings or the bond coats in thermal barrier coatings (TBCs) to protect the underlying superalloy from oxidation and hot-corrosion. In this paper, the TBC consisting of yttria stabilized zirconia (7YSZ) ceramic top coat and CoCrAlY bond coat was deposited onto directionally solidified nickel based superalloy DZ 125 by electron beam physical vapor deposition (EB-PVD). The microstructural evolution of the bond coat on this superalloy was investigated after thermal exposure for 100h at 1050°C. Due to a significant inward diffusion of Al, Co and Cr from the coating and outward diffusion of Ni, Hf, W and Ti from the substrate, the phase transformation from the Co-based Al-rich β-CoAl phase to the Al-deficient γ-CoNi solid solution phase occurred in the bond coat. Simultaneously, a large amount of Ni-based β-NiCoAl phase was present in the bond coat. In addition, the particles containing substrate strengthening elements Hf and/or W are abundant in the thermally grown oxides (TGO) and within the bond coat. The mechanism for the microstructural evolution is discussed.