Studies of microstructure and phase of EB - PVD TBCs exposed to elevated temperatures

A NiCrAlY bond coat had been deposited on a Ni-base single crystal superalloy by magnetron sputtering, and then a top coat (7 mass % Y₂O₃ - ZrO₂) had been deposited by electron beam - physical vapor deposition (EB - PVD). The phase transformation of ceramic top coat was investigated and the surface morphology change of the samples was observed. It was found that the ZrO₂ - 7 mass % Y₂O₃ ceramic top coat deposited by EB - PVD showed columnar grains which formed dense clusters and the surface was relatively dense. However there were gaps or holes among grain clusters, which allowed the lateral expansion and contraction of the ceramic columnar grains and allowed free expansion of the substrate over a relatively large range. After exposure to elevated temperatures, the gaps among columnar grain clusters became larger. The relatively large gaps interconnected into one another and formed microcracks. With thermal cycling going on, the microcracks extended across and through the top coat. Also the Y₂O₃ content in nonequilibrium tetragonal phase decreased gradually, and the nonequilibrium tetragonal phase decomposed to equilibrium tetragonal and cubic phases. After 300 thermal cycles at 1050°C, small amount of monoclinic phase was formed from the decomposition of the tetragonal phases.