Compatibility of mixed zone constituents (YAG, YAP, YCrO3) with a chromia-enriched TGO phase during the late stage of TBC lifetime

The effects of chromia accumulation in the TGO scale of late-stage MCrAlY-based EB-PVD TBC systems are addressed by comparing mixed zone microstructures from isothermal as well as cyclic oxidation tests at 1100 °C with phase relationships derived from the 1100 °C isothermal section of the ternary system Al2O3–Cr2O3–Y2O3. For this approach the c-ZrO2 phase zirconia coexisting with mixed zone constituents is considered an excess component. Compacted powder pellets were processed via co-decomposition of nitric salt solutions. The reaction products were characterized by X-ray diffraction and Raman spectroscopy. teristics of the ternary Al2O3–Cr2O3–Y2O3 relevant for the mixed zone include (i) a continuous solid solution series between the orthorhombic phases YAlO3 (YAP) and YCrO3, (ii) a limited Cr vs. Al substitution (16 mol%) in Cr–YAG Y3(Al1 − xCrx)5O12 and (iii) extended phase fields involving Cr-YAG and sesquioxide phases which represent typical late-stage mixed zones in the excess-zirconia model scenario. The YCrO3 phase was found to be limited to very chromia-rich environments only. A high chromium enrichment of the mixed zone may be established upon extensive cycling through the miscibility gap in the alumina–chromia system. Later in TBC lifetime the thickness of the mixed zone is increasing due to outward growth of the Al2 − xCrxO3 phase into the bottom TBC layer which results in a morphological instability of the TGO/TBC interface.