Oxidation Behaviors Of NiCrAlY/ZrO₂ Thermal Barrier Coatings On A Single-crystal Superalloy

In this paper, a bond coat (Ni-3OCr-l2A1-0.3Y (wt%)) had been deposited on a Ni-base single crystal superalloy by magnetron sputtering, and then a top coat (7wt%Y203-Zr02) had been deposited by electron beam-physical vapor deposition (EB-PVD). The microstructures and high temperature oxidation behavior of sputtered NiCrAIY coating, the effect of vacuum heat treatment on oxidation behavior of sputtered NiCrAIY coating, and the high temperature oxidation behavior of EB-PVD TBCs had been investigated by using thermogravimetry analysis (TGA), x-ray diffraction (XR[)), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The main results are summarized as following: The oxide scales formed on sputtered NiCrAIY coatings after oxidation at 900,1000, and 11000(2 consisted of only A1203 with excellent adhesion. During the oxidation, a O梸ct-Al2O3 phase transformation happened, and the higher the oxidation temperature, the faster the phase transformation. The formation process of a-A1203 on NiCrAIY coating would be accelerated and promoted by vacuum heat treatment, and the formed cx-A1203 scale had a better adherence to the N1CrA1Y coating. Consequently, the oxidation resistance of sputtered NiCrAIY coating was improved significantly by vacuum heat treatment. During high temperature oxidation of TBCs, the non- equilibrium tetragonal phase (t?decomposed gradually to equilibrium tetragonal (t) and cubic (c) phase. After 300 thermal cycles at 10500(2, it was observed that small amounts of monoclinic phase was transformed from tetragonal phase. By the microstructure observation, EDX and XRD analysis, it was found that micro-cracks initiated preferably in the YPSZ topcoat along columnar grain boundaries, and then propagated through the whole topcoat down to the TGO layers. Consequently, oxygen transportation was accelerated and caused sever internal oxidation in the bond coat. With the increasing of thermal cycles, Al was depleted, and NiO and Ni(Al,Cr)204 spinel oxides were formed near the interface between TGQ and bond coat. Stress concentration due to oxide growth and phase transformation between tetragonal and II monoclinic phases would weaken the combination between topcoat and bond coat, and then cause TBCs to spall from the substrate.