Si-Yb_(2)O_(3)/Yb_(2)Si_(2)O_(7)/Yb_(2)SiO_(5) EBC涂层组织结构及熔盐腐蚀行为研究

Microstructure and Resistance to Molten Salt Corrosion of Si-Yb_(2)O_3/Yb_(2)Si_(2)O_(7)/Yb_(2)SiO_(5) EBC

目的 研究Si-Yb_(2)O_(3)/Yb_(2)Si_(2)O_(7)/Yb_(2)Si O_(5) 3层EBC涂层的抗熔盐腐蚀性能。方法 采用真空等离子喷涂工艺在试样表面单面制备Si-Yb_(2)O_(3)/Yb_(2)Si_(2)O_(7)/Yb_(2)Si O_(5) 3层结构的EBC涂层,选用尺寸为Ф25 mm、厚度为5 mm的SiC_(f)/SiC复合材料基体试样,在900℃、NaCl(质量分数为50%)+Na_(2)SO_(4)(质量分数为50%)混合盐中进行连续100 h的熔盐腐蚀试验,采用光学显微镜、扫描电镜、X射线衍射仪等观察和测试3层EBC涂层熔盐腐蚀前后的形貌,分析其组织结构以及该EBC涂层在熔盐腐蚀过程中的化学反应机理。结果 经过连续100 h、900℃的熔盐腐蚀,基体SiC_(f)/SiC复合材料被完全腐蚀掉;底层Si和Yb_(2)O_(3)出现了部分缺失、涂层不完整的现象;中间层Yb_(2)Si_(2)O_(7)在熔盐腐蚀后形成了大量孔洞,但依然基本保持了其原有的涂层框架尺寸;面层Yb_(2)Si O_(5)在涂层厚度尺寸上与熔盐腐蚀之前的尺寸基本一致,熔盐腐蚀后结构致密,维持了自身的化学稳定性。结论 SiC_(f)/SiC复合材料本身不具有抗熔盐腐蚀能力,底层Si和Yb_(2)O_(3)涂层的抗熔盐腐蚀性能较差,体系中Yb_(2)Si_(2)O_(7)中间层具有一定的抗熔盐腐蚀性能,而Yb_(2)SiO_(5)面层具有良好的抗熔盐腐蚀性能,因此在Si-Yb_(2)O_(3)/Yb_(2)Si_(2)O_(7)/Yb_(2)Si O_(5) 3层结构EBC涂层体系中,按抗熔盐腐蚀能力由低到高排序为:底层Si和Yb_(2)O_(3)<中间层Yb_(2)Si_(2)O_(7)<面层Yb_(2)Si O_(5),本体系中的EBC涂层具有良好的抗熔盐腐蚀性能。

SiC_f/SiC composite materials have high specific strength,high specific modulus and other excellent mechanical properties at high temperature.SiC_f/SiC composites have excellent oxidation resistance due to the reaction with oxygen in the air to form a dense layer of SiO_(2) protective film.However,molten salts from the environment dissolve the SiO_(2) protective film,resulting in rapid corrosion of SiC_f/SiC composites.The most common molten salt impurities in aero-engine combustion chambers,turbines,and tail nozzle environments are molten Na_(2)SO_(4) and NaCl.Environmental barrier coatings(EBCs)are generally used to resist oxidation and corrosion of composite materials by the gas environment.Plasma spraying technology is the most important EBC preparation technology,which has the characteristics of fast deposition rate,high production efficiency,strong adaptability to specimen shape and size,and large controllable range of coating thickness.A three-layer EBC was prepared by vacuum plasma spraying process,the EBC with rare earth Yb_(2)O_(3) doped Si as the bottom layer,Yb_(2)Si_(2)O_(7) as the middle layer,and Yb_(2)SiO_(5) as the surface layer.The three-layer structure EBC was investigated at 900℃.The corrosion behavior of molten salt was researched in an environment mixed with NaCl and Na_(2)SO_(4) at a mass ratio of 1∶1.The Si-Yb_(2)O_(3)/Yb_(2)Si_(2)O_(7)/Yb_(2)SiO_(5) three-layer structure EBC was prepared on one side of the sample surface by vacuum plasma spraying process.Continuous 100 h molten salt corrosion test was carried out in+50wt.%Na_(2)SO_(4) mixed salt.Optical microscope,scanning electron microscope and X-ray diffractometer were used to observe and test the morphology of the three-layer EBC before and after molten salt corrosion,analyze its structure and chemical reaction mechanism in the molten salt corrosion process.After continuous molten salt corrosion at 900℃ for 100 h,the matrix SiC_f/SiC composite was completely corroded;the bottom layer Si and Yb_(2)O_(3) were partially missing and the coating was incomplete;the intermediate layer Yb_(2)Si_(2)O_(7) formed a large number of holes after molten salt corrosion.However,the original coating frame size was still basically maintained;the thickness of the surface layer Yb_(2)SiO_(5) was basically the same as that before the molten salt corrosion.After the molten salt corrosion,the structure was dense and its own chemical stability was maintained.The test results showed that since the EBC was prepared on one side of the SiC_f/SiC substrate,and the SiC on the other side was completely exposed to molten salt.After molten salt corrosion,the substrate SiC was completely corroded,indicating that SiC itself did not have the ability to resist molten salt corrosion.After 100 h of molten salt corrosion in 50wt.%NaCl+50wt.% Na_(2)SO_(4) mixed salt at 900℃,a large number of holes were formed in the Yb_(2)Si_(2)O_(7) interlayer,indicating that the molten salt was immersed in the interlayer during the corrosion,but its chemical stability was still basically maintained.The original coating frame size shows that the Yb_(2)Si_(2)O_(7) intermediate layer has certain anti-corrosion properties of molten salts.After 100 h molten salt corrosion in 50wt.%NaCl+50wt.%Na_(2)SO_(4) mixed salt at 900℃,the thickness and size of Yb_(2)SiO_(5) surface layer in EBC coating system are basically the same as before molten salt corrosion,the structure is compact,and its chemical stability is maintained,with good resistance to molten salt corrosion.SiC_f/Si C composites have no resistance to molten salt corrosion.The underlying Si and Yb_(2)O_(3) coatings have poor resistance to molten salt corrosion.In the system,the YB_(2)Si_(2)O_(7) interlayer has certain resistance to molten salt corrosion,while the Yb_(2)SiO_(5) surface layer has good resistance to molten salt corrosion.Therefore,in the three-layer EBC coating system of Si-Yb_(2)O_(3)/Yb_(2)Si_(2)O_(7)/Yb_(2)SiO_(5),the order of molten salt corrosion resistance from low to high is:the bottom layer Si and Yb_(2)O_(3)interlayer YB_(2)Si_(2)O_(7)surface layer Yb_(2)SiO_(5).The EBC of this system has high molten salt corrosion resistance.