Calcium-rich CMAS Corrosion Induced Microstructure Development Of EB-PVD Thermal Barrier Coatings

Thermal barrier coatings(TBCs)are key materials of aeroengine for protecting end components and improving engine efficiency.With the increasing of operating temperature,the failure problem of TBCs is more and more prominent.The main failure modes of TBCs are high temperature oxidation,CMAS corrosion,erosion,sintering and phase transition etc.High temperature CMAS corrosion is an inevitable and most complex failure form.The diversity of CMAS composition,the uncertain deposition quantity and the complicated corrosion environment lead to the diversity of CMAS corrosion characteristics and the complicated mechanism of CMAS corrosion.However,the CMAS components reported in the literature are mainly based on CMAS with a small CaO component,while few researches are focused on calcium-rich CMAS corrosion.The variation in the calcium content in CMAS leads to a dramatic difference in the degradation behavior of the YSZ coating.However,a deeper analysis of the microstructural feature and the fundamental mechanism for the composition effect on the YSZ corrosion is still lacking.In this paper,the microstructural characteristics of 7YSZ TBCs prepared by electron beam physical vapor deposition(EB-PVD)method,with calcium-rich CMAS(50CaO-10MgO-10AlO1.5-30SiO2(mol.%))corrosion at 1400 oC for 4h is studied by scanning electron microscopy(SEM),X-ray diffraction(XRD),Raman spectroscopy(Raman)and transmission electron microscopy(TEM).The main contents are as follows:Firstly,CMAS with the component of 50CaO-10MgO-10AlO1.5-30SiO2 is artificially prepared in the laboratory.The experimental scheme about CMAS corrosion is designed.The characterization methods and the process of preparing TEM sample are introduced.Secondly,the microscopic morphology of TBCs samples is characterized by SEM and TEM before and after CMAS corrosion.The results indicate that the ceramic layer of the as deposited sample has a columnar crystal structure,and the edge of the columnar grains is feathery.There are gaps between the adjacent columnar grains.After CMAS corrosion,the columnar structure of the ceramic layer collapses and becomes a large equiaxed granular structure.By means of XRD,EDS and selective area electron diffraction(SAED)analysis,we find that there exists some magnesium aluminate spinel(MgAl2O4)in the gaps between bulk particles and inside spherical zirconia particles besides the original tetragonal zirconia.There are also some anorthite(CaAl2Si2O8)and amorphous CMAS between spherical zirconia particles.Finally,the microstructure of EB-PVD 7YSZ TBCs after the high temperature CMAS corrosion is analyzed in detail by TEM.In addition,the composition of the top,middle,and bottom of YSZ coating are analyzed using EDS in TEM.The results indicate that the formation of anorthite and magnesium aluminate spinel is relevant to the local chemical compositions during CMAS and YSZ mutual reaction.When it meets the desired environmental conditions,anorthite and magnesia-alumina spinel are generated.The mechanism model of Ca-rich CMAS corrosion is established based on the above analysis.