Thermocyclic behavior of sputtered NiCrAlY/EB-PVD 7 wt.%Y2O3-ZrO2 thermal barrier coatings

Yttria partially stabilized zirconia (YPSZ) ceramic top coat was deposited on a sputtered NiCrAlY bond coat by electron beam physical vapor deposition (EB-PVD) and the thermal cycling behavior was studied at 1050 deg C. Experimental characterization was conducted by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM), equipped with energy dispersive analytical Xray spectroscopy (EDAX). The results showed that the TGO was only Al2O3 for the TBCs after 200 cycles. With aluminum depletion in the bond coat, NiO, Ni(Cr, Al)2O4 and other spinel formed near the bond coat. The degradation of the TBCs was mainly at the interface between the bond coat and the TGO, due to the spallation of the YPSZ ceramic top coat from the bond coat with TGO scale still adhering to YPSZ. The failure mechanism of the EB-PVD TBCs was proposed as follows: during thermal cycling, microcracks are initiated preferentially in the ceramic top coat along columnar boundaries and then extend through and along the top coat. The growth stress of TGO together with thermal stress imposed by cycling leads to the separation at the bond coat/TGO interface. Once longitudinal cracks penetrate through the ceramic top coat and leads damage to the nearby TGO, the ceramic top coat spalls from the cracked bond coat/TGO interface with the TGO scale still adhered. [Substrate: nickel superalloy].