High temperature mechanical characterization of plasma-sprayed zirconia–yttria from conventional and nanostructured powders

In this study the high temperature oxidation behavior of NiCoCrAlTaY coatings modified by a PVD (physical vapor deposition) surface treatment was evaluated: the coatings, produced by HVOF (high velocity oxygen fuel) technique, were protected by an Al2O3 PVD sputtered overlay prior to top coating with an YSZ ceramic layer, and were exposed in air at 1100 °C up to 400 h. The oxidation kinetics were evaluated by measuring the thickness of the TGO (thermally grown oxide) scale at several exposure times and the microstructural evolution induced by the heat treatments was investigated. The effect of the presence of the Al2O3 PVD overlay, in terms of microstructure and oxidation resistance, was assessed by comparing the performance of modified TBCs with unprotected NiCoCrAlTaY systems. Experimental evidence was gathered showing that the presence of an Al2O3 PVD overlay enhances the oxidation resistance of NiCoCrAlTaY coatings and promotes the adhesion between bond and top coat. A thermo-mechanical finite element model of the modified TBC system was also developed: the growth of the oxide scale was modeled and the previously assessed oxidation kinetics were implemented in order to estimate the effects of the PVD film on the mechanical stress field of the TBC system.