Electron beam physical vapor deposited YbGYZ thermal barrier coatings: Phase stability, thermo-physical properties and thermal shock behavior

Abstract

The rare earth oxide ceramics designed as 5.5 wt% Yb2O3-4.5 wt% Gd2O3-12 wt% Y2O3-78 wt% ZrO2 (YbGYZ) is a candidate material for thermal barrier coating (TBC) which will be suitable for application at higher temperatures. YbGYZ ceramic powders and bulks are fabricated by solid-state synthesis at temperatures above 1673 K, whose powders still have no phase transformation and exhibit excellent thermal stability despite long-term heat treatment at two different temperatures. In the temperature range from room temperature to 1573 K, the averaged coefficient of thermal expansion of YbGYZ ceramic bulk is 11.12 × 10−6 K−1, which meets the demand of novel ceramic materials of TBCs. Meanwhile, the averaged thermal diffusivity and thermal conductivity of YbGYZ ceramics are approximately 2.3% and 7.6% lower than those of the conventional YSZ bulk material respectively. The YbGYZ ceramic coatings are directly manufactured on the surface of (Ni, Pt)Al bond coat by means of electron beam physical vapor deposition (EB-PVD), whose phase structure consists primarily of cubic phase with co-existing of excess Y2O3 and ZrO2. A large number of regularly distributed “mud-like” microcracks appear on the surface of YbGYZ ceramic coating when it has experienced long-term thermal shock at 1373 K. The transverse microcracks originating in the ceramic coating have elongated to the interface of ceramic coat and thermally grown oxide (TGO) film that further cause the degeneration and separation of the interface. The spalling position of the YbGYZ coating where mainly occurs at the upper and lower adjacent interfaces of TGO layer. The serious rumpling, undulation, cross-linking, stress accumulation and rapid relaxation within TGO layer are the critical factors to accelerate interfacial delamination and spallation failure of YbGYZ TBCs.