Thermal shock behavior of EB-PVD thermal barrier coatings based on YSZ ceramic coat and (Ni, Pt)Al metallic bond coat

Abstract

Three different chemical vapor deposition (CVD) aluminizing processes were adopted for preparing of (Ni, Pt)Al metallic coatings, whose microstructure, element content, phase constituent and gas hot corrosion performance were evaluated and comparatively analyzed. YSZ ceramic coatings were subsequently fabricated via electron beam physical vapor deposition (EB-PVD) technique on top of the optimized (Ni, Pt)Al bond coat surface, and thermal barrier coating (TBC) specimens were subjected to 1373 K long-term thermal shock. The surface of aluminized sample with a single external reaction generator is relatively compact, and no defects such as micropores and microcracks are observed. Only a large number of protruding sediments are detected at grain boundary as well as the degree of rumpling and cross-links of these sediments is more obvious. A large amount of Pt is enriched at that grain boundary, while elemental content of Al is lower at the same location. Meanwhile, such sample basically displays β-(Ni, Pt)Al phase, and only three very weak diffraction peaks belonging to ζ-PtAl2 coexist. After long-term thermal shock, the exposed region of bond coat shows two kinds of micro-morphology. The biggest distinction between them is that the composition of Pt and Al elements is evidently different. The interfacial separation of TBCs is mainly concentrated at interface between thermally grown oxide (TGO) and bonding layers, and a very small amount of TGO layer is merely adhered to bond coat surface. It demonstrates that TGO layer is densified and thickened with the extension of thermal exposure period, and then the mismatching of thermal expansion between Al2O3 and (Ni, Pt)Al bond coat occupies the dominant factor. In addition, the grain boundary ridges and undulations on bonding layer surface can also induce accumulation of the tensile stress and further accelerate degradation of the critical interlayer interface during cooling stage of thermal shock.