Microstructure and lifetime of EB-PVD TBCs with Hf-doped bond coat and Gd-zirconate ceramic top coat on CMSX-4 substrates

Abstract

Gadolinium zirconate (GZO) with its lower thermal conductivity and higher thermal stability compared to the industrial standard 7YSZ is a new promising material for thermal barrier coating (TBC) applications. In this study, top coats of GZO and 7YSZ were deposited on NiCoCrAlY and Hf-doped NiCoCrAlY bond coats with CMSX-4 as substrate material. The bond coats as well as the ceramic top coats were manufactured by electron-beam physical vapor deposition (EB-PVD). The lifetimes of these new TBC systems were investigated by thermal cycling at 1100 °C. In comparison to the standard 7YSZ TBC, GZO deposited on the NiCoCrAlY bond coat exhibited a significantly enhanced lifetime. Doping NiCoCrAlY with 0.6 wt.% Hf resulted in around 10 times increase in the lifetime for the 7YSZ top coat. However, no significant difference in lifetimes was observed when 7YSZ is replaced by GZO on NiCoCrAlY-Hf bond coats. During thermal cycling, a chemical reaction between GZO and the thermally grown oxide (TGO) formed on the NiCoCrAlY bond coat was observed; however, such a chemical reaction did not occur when GZO was deposited on NiCoCrAlY-Hf bond coats. A faster TGO growth has been observed for the Hf-based systems, resulting in TGO thicknesses as large as 20 μm. The role of Hf-doping in the bond coat, the individual TGO microstructure, and the diffusion of refractory elements from the substrate into the bond coats are discussed along with the lifetime measurements of the different TBC systems.