Phase transformation and bond coat oxidation behavior of EB-PVD thermal barrier coating

Thermal barrier coating, consisting of an electron beam physical vapor deposited partially stabilized zirconia (PSZ, 7 wt.%Y2O3) and a sputtered NiCrAlY bond coat, was investigated by thermal cycling at 1050 °C in air. Experimental characterization was conducted using X-ray diffraction analysis and scanning electron microscopy equipped with energy dispersive analytical X-ray spectroscopy. The lattice parameters of the tetragonal phase were determined as a function of thermal cycles. It was found that the c/a ratio increases with thermal cycling, correspondingly to a decrease in the content of Y2O3 of the tetragonal phase. The nonequilibrium tetragonal phase (T′) of the PSZ slowly decomposed to tetragonal phase with relatively low Y2O3 content and cubic phase. After 300 cycles small amount of monoclinic phase was observed, which resulted from the tetragonal to monoclinic phase transformation during cooling. Al2O3 formed preferentially along the bond and top coat interface. After 200 cycles the thermally grown oxide (TGO) was very thin and consisted of only Al2O3. With Al-depletion in the bond coat, internal oxidation occurred and some other oxides such as NiO and Ni(Cr,Al)2O4 spinels were observed in the TGO near the bond coat.