Characterization Of The Growth Feature Of EBPVD TBC And Molten CMAS Thermochemical Reaction Product ZrSiO₄

Thermal barrier coating（TBC） is one of the key materials of aero engine owing to its ability for obviously improving the service temperature and increasing the work efficiency. However, with increase of the temperature of the engine gas inlet, CMAS corrosion-induced failure problem of TBC has become particularly prominent. A number of studies have demonstrated that CMAS will melt at high temperature and rapidly infiltrate the opening structure of TBC. Thermochemical reaction between CMAS and TBC will occur and produce some new phases such as zirconium silicate, anorthite and spinel, inducing the structure and property degradation of TBC. However, few studies focused on the growth characteristics of these new phases, which is the key to control and avoid CMAS corrosion. Among these new phases, the generation of zirconium silicate involves the direct participation of zirconia during the thermochemical reaction, which leads to the TBC microstructure degradation. Its influence on thermal barrier coating is very serious. Therefore, study on growth characteristics of zirconium silicate microstructure and analysis of zirconium silicate impact on the performance of thermal barrier coatings is necessary.In the present work, the thermo-chemical interaction between 7YSZ TBCs and molten deposits Ca10Mg7Al1370Si70 is investigated by simulating the high temperature corrosion process. The microstructure features of the samples are characterized by XRD, SEM, TEM and EDS. The results are as follows:First, CMAS powder is prepared in the laboratory, whose particle size is mainly about 15μm. XRD analysis of CMAS powder shows that the CMAS powder is amorphous. DSC test confirms that the melting temperature of CMAS powder is about 1160℃. According to the research objective and the literatures, an experimental scheme is designed with CMAS amount of 10mg|cm2depositing on YSZ TBC at 1250 oC for 24 h.Secondly, the micro morphologies of the as-deposited samples and the corroded samples are characterized using SEM and TEM. The results show that as-deposed TBC are columnar crystals. The edges of the columnar crystals are feather-like structures. Moreover, there are a lot of gaps between the columnar crystals. After CMAS corrosion, the feather-like structures of columnar crystals disappear. Moreover, the different corrosion features are found at the top layer, the middle of the layer and the YSZ/Al₂O₃ interface, respectively. The samples are analyzed by XRD, EDS and selective area electron diffraction（SAED）. The results show that a new phase of zirconium silicate is found along the entire ceramic layer. Anorthite is also found at the YSZ/Al₂O₃ interfaces.Finally, combining TEM observation, SAED analysis and EDS analysis, the results show that ZrSiO₄ preferentially forms in the corrosion-induced voids and gaps between the YSZ columnar crystals. Moreover, the growth characteristics of zirconium silicate in the microstructure are varied. Due to the different reaction rates of CMAS and YSZ in different regions, the residual zirconia particles surrounded by zirconium silicate and amorphous CMAS between zirconia and zirconium silicate are found in the samples, respectively. The formation of zirconium silicate may make the TBC microstructure degradation and generate the volume expansion and the thermal mismatch stress, inducing TBCs failure.