物相组成对硅酸镱涂层显微结构和耐蚀性能的影响研究

Influence of Phase Composition on Microstructure and Corrosion Resistance of Ytterbium Silicate Coatings

陶瓷基复合材料(Ceramic matrix composites CMCs)被视为新一代航空发动机热端部件的主要候选材料。然而,陶瓷基复合材料在服役过程中会受到高温水蒸气腐蚀,从而导致材料性能急剧下降。在CMCs表面制备环境障碍涂层(Environmental barrier coating,EBC)可有效解决这一难题。稀土硅酸盐具有高熔点、与CMCs匹配的热膨胀系数和良好的耐蚀性能等特点,是最具应用潜力的环境障碍涂层材料。大气等离子体喷涂技术是制备稀土硅酸盐环境障碍涂层的常见方法。本文通过固相反应法制备了不同物相组成的硅酸镱粉体,并采用大气等离子体喷涂方法制备了富Yb_2O_3(YS0.75)和富Yb_2Si_2O_7(YS1.25)两种涂层,比较研究了涂层的相组成、微观结构和耐高温水蒸气腐蚀性能。研究发现,YS0.75涂层主要由Yb_2O_3和Yb_2SiO_5相组成,结晶度较高,层状结构明显,涂层内有较多裂纹。YS1.25涂层主要由Yb_2SiO_5和Yb2Si2O7相组成,结晶度较低,片层间结合紧密,涂层含有较多球型气孔。不同物相组成的硅酸镱涂层经1400oC高温水蒸气腐蚀后表面均生成Yb_2SiO_5层。富Yb_2O_3涂层具有更好的耐水蒸气耐蚀性能。

Ceramic matrix composites (CMCs), especially SiCf/SiC, are promising candidates for applications to hot sections of gas turbine engines since they have the characteristics of low density, excellent high-temperature mechanical properties and good stability in dry air environment. However, the corrosion resistance of CMCs in combustion environments containing high-temperature water vapor is relatively poor. The lack of environmental durability in combustion environments obstructs the applications of these materials. Therefore, environmental barrier coatings (EBCs) have been developed to protect CMCs from water vapor corrosion and extend the service life of hot components. Rare-earth silicates have been regarded as one of the most promising EBC materials due to their excellent environment durability, relatively suitable coefficients of thermal expansion (CTE) and superior phase stability. Atmospheric plasma spray (APS) technique is an effective method for the preparation of EBCs. The plasma spray process has the characteristics of high temperature, high speed and fast cooling rate, which lead to the great difference of the phase composition, microstructure and properties of plasma-sprayed coatings compared with bulk ceramics. Therefore, it is necessary to study the influence of the phase composition on microstructure and properties of the rare-earth silicate coatings. In this work, two kinds of ytterbium silicate powders were synthesized by solid-state reaction method using Yb2O3 and SiO2 as raw materials in 1:0.75 and 1:1.25 molar ratios, respectively. The YS0.75 and YS1.25 coatings were fabricated by APS. The phase composition and microstructure of the coatings were characterized. The corrosion resistance to high-temperature water vapor were investigated. The relationship among the phase composition and microstructure and corrosion resistant properties of the coatings were analyzed. The results showed that ytterbium silicate powders with different compositions exhibited well fluidity and uniform distribution of particle size. The as-sprayed YS0.75 and YS1.25 coatings were mostly composed of fully melted particles. Both as-sprayed coatings contained amorphous phase which was resulted from rapid cooling of molten particles during plasma spray processes. The Yb2O3-rich ytterbium silicate (YS0.75) coating was composed of Yb2O3 and Yb2SiO5 phases, which was characterized by higher crystallinity, Yb2O3 columnar grains, obvious interfaces between splats and more microcracks. However, the Yb2Si2O7-rich ytterbium silicate (YS1.25) coating was composed of Yb2SiO5 and Yb2Si2O7 phases, which was characterized by well bonded splats with more spherical pores and lower crystallinity. The porosity of the as-sprayed YS0.75 and YS1.25 coatings were 5.1% and 4.8%, respectively. A relatively dense Yb2SiO5 layer was formed on the surface of both YS0.75 and YS1.25 coatings after 1400℃/150 h water vapor corrosion test. It is presumed that different chemical reactions took place on the coating surface. The Yb2O3 phase in the YS0.75 coating could react with water vapor and the product Yb(OH)3 volatilized, resulting in the formation of the Yb2SiO5 layer. However, the Yb2Si2O7 phase in the YS1.25 coating was corroded by steam to form the Yb2SiO5 layer. The dense Yb2SiO5 layer could effectively prevent the diffusion of corrosive substance into the coating interior. Our experimental results clearly reveal the impurities, such as Yb2O3 and Yb2Si2O7, have great influence on the microstructure and properties of ytterbium silicate coatings. This work gives a clue for designs and applications of plasma sprayed rare earth silicate coatings as EBCs.