Study Of Pre-corrosion On The High Temperature Corrosion Resistance Of Gd₂Zr₂O₇ Thermal Barrier Coatings Against Cmas And The Mechanism

As the thrust-to-weight ratio of aero engines continued to rise,their operating temperatures far exceeded the melting points of high-temperature alloys,and thermal barrier coating technology must be used to protect these hot-end components from such a demanding service environment.In recent years,the thermal barrier coating under the synergistic effect of high temperatures can be eroded by Ca O-Mg O-Al₂O₃-SiO₂（CMAS）causing severe sintering and cracking of the ceramic layer seriously under the synergistic effect of high temperatures such as sand,volcanic ash,and industrial waste gas,which greatly reduced the service life of the aero-engine.It was Gd₂Zr₂O₇who reacted rapidly with CMAS to form a dense apatite phase barrier layer so that it can be one of the most promising next-generation thermal barrier coating materials.In the paper,the pre-designed powders were used to coat the surface of Gd₂Zr₂O₇blocks,including Al₂O₃-TiO₂-SiO₂-Ca O-Gd₂O₃（ATSCG）and Al₂O₃-TiO₂-SiO₂-Ca O-Gd₂O₃-Y₂O₃（ATSCGY）,which called pre-corrosion process,followed by calcination and CMAS high temperature corrosion tests.The effects of pre-corrosion temperature and composition on the performance of CMAS permeation were investigated,and the corrosion mechanism of CMAS permeation behavior was analyzed.The main findings are as follows:（1）CMAS high-temperature corrosion experiments of ATSCG-Gd₂Zr₂O₇ at1250℃/3 h were carried out to study the corrosion behavior and mechanism.The results showed that CMAS resistance of ATSCG-Gd₂Zr₂O₇tended to increase and then decrease with the increase of the pre-corrosion temperature in the range of 900℃-1300℃,and the optimal pre-corrosion temperature is 1100℃,whose the depth of the reaction layer is 48.21μm.The reaction between ATSCG-Gd₂Zr₂O₇ and CMAS produced high melting point stable phases called Ca₂Gd₈（SiO₄）₆O₂（apatite phase）and Ca Al₂Si₂O₈（calcium feldspar phase）increasing the denseness of the upper reaction layer,which blocked the diffusion channel of CMAS and limited the penetration of CMAS melt.In addition,the titanates generated by Ti elements in ATSCG at the early stage of the reaction were mainly distributed in the upper reaction layer,which provided beneficial nucleation conditions for the self-crystallization behavior of CMAS and the formation of crystalline phases such as apatite phase and calcium feldspar phase.（2）CMAS high-temperature corrosion experiments of ATSCGY-Gd₂Zr₂O₇ at1250℃/3 h were carried out to study the corrosion behavior and mechanism.The results showed that the high temperature CMAS resistance of ATSCGY-Gd₂Zr₂O₇ increased and then decreased with the pre-corrosion temperature in the range of 900℃-1300℃.The optimum pre-corrosion temperature was 1200℃ and the depth of the reaction layer was26.39μm.The Y-apatite-rich phase dominated reaction zone mainly,which enriched in the upper interface of the reaction layer,inhibiting the diffusion of CMAS as a barrier layer and reducing the consumption of Gd elements in the original Gd₂Zr₂O₇ matrix.In addition,the ability of Y element and CMAS to form Y-rich garnet phase is also an important factor for the improvement of the anti-CMAS performance.（3）Langwierig CMAS corrosion and related physical properties studies of Gd₂Zr₂O₇,1100°C-ATSCG-Gd₂Zr₂O₇ and 1200°C-ATSCGY-Gd₂Zr₂O₇ were carried out.The results showed that the CMAS corrosion penetration was described into three stages;the pre-corrosion-rapid penetration reaction stage with a fast growth rate;the middle-stable penetration stage with a stable and almost constant growth rate and the later-inhibited CMAS penetration stage with a slow growth rate.The growth rate of the reaction layer of the specimens after pre-corrosion was less than that of Gd₂Zr₂O₇ in the whole stage of corrosion.The CMAS resistance of ATSCGY was the most obvious,due to its growth rate was the slowest in the middle stable penetration stage.The fracture toughness and hardness tests showed that it made the pre-corroded specimens have better resistance to CMAS.High-temperature wettability tests showed that the enhancement of the high-temperature contact angle was attributed to the change its surface chemistry,which was also a factor in the improved CMAS resistance,as the pre-corroded specimens changed from a pro-CMAS state to a sparse CMAS state.