Comparison Of Sintering And CMAS Corrosion Induced Microstructure Characteristic Of APS Thermal Barrier Coatings

Thermal barrier coatings(TBCs)are extensively applied to protect metallic components of aero engines,which play a key role in improving the engine efficiency and increasing the operating temperature.High temperature oxidation,high temperature sintering and CMAS corrosion are three main failure mechanisms inducing the failure of thermal barrier coatings in-service.Both high-temperature sintering and CMAS corrosion can induce structural changes to the YSZ coatings.However,a clear understanding of the structural degradation induced by high-temperature sintering and CMAS corrosion is still lacking.An in-depth knowledge in the microstructure feature of the YSZ coating between the thermal sintering effect and the CMAS corrosion is necessary for clearly differentiating the degradation mechanisms of the TBCs.These results may have practical implication for exploring YSZ sintering resistance and corrosion resistance.In the present work,the sintering and CMAS corrosion induced microstructure evolution in the APS-7YSZ is comparatively studied by a combination of complementary tools including scanning electron microscopy(SEM),X-ray diffraction(XRD),and transmission electron microscopy(TEM).The temperature of the coating surface is around 1250? and the temperature of the superalloy substrate is around 800?.The samples are held with this temperature gradient for 8 h.The obtained results are as follows:Firstly,the top ceramic layer of the as-sprayed sample,the thermal gradient sintering sample and the thermal gradient CMAS corrosion sample are characterized by XRD.It can be seen that the as-sprayed sample consists of t-ZrO2,the sintered and the CMAS corroded YSZ coatings still keep the t-ZrO2 phase as the as-sprayed YSZ coating.The XRD analyses indicate that there are no obvious phase transformations in the ceramic coating after the sintering and CMAS corrosion..Secondly,the cross-section morphology of the as-sprayed sample,the thermal gradient sintered samples and CMAS corroded samples are observed by SEM.As-sprayed YSZ has a lamellar structure and the lamellas are composed of closely aligned columnar crystals.The sintered YSZ remains the lamellar structure with reduced interlamellar gaps and grain coarsening.After CMAS corrosion,the lamellar structure is not obvious and some micro-cracks are present in the YSZ layer.CMAS penetrates into the YSZ/TGO interface,but not into the TGO/BC interface.Finally,in order to further compare the microstructure characteristics of the YSZ coating,the as-sprayed,sintered and CMAS corroded TBCs are analyzed by TEM.After high temperature sintering,it can be seen that the sintered sample remains lamellar structures with some micro-cracks between them.The gaps between the adjacent lamellar structures become smaller than those of the as-sprayed coating.It indicates that high temperature sintering for a short time has only a little effect on the TBC sample.After the CMAS corrosion,the top layer of the YSZ coating keeps its lamellar structure consisting of some columnar grains with the CMAS infiltration into the intergrain gaps and the formation of strip-morphology Zr2Y2O7.The typical lamellar structure transforms into more equiaxed grains in the middle and bottom layers of the ceramic coating with significant infiltration of amorphous CMAS and the anorthite formation in the bottom layer owing to the high content of Ca and Al.Some thermally grown oxide(TGO)forms between the ceramic coating and the bonding coating(BC).A micro-crack appears in the TGO.These results indicate that the effect of CMAS corrosion on the microstructure of YSZ thermal barrier coating is very significant under the thermal gradient condition for a short time.Furthermore,the closer to the bond coat,the greater the effect is.