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.%Y₂ O₃) 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.