Search

SiC ceramic matrix composites (CMC) are potential materials for hot section gas turbine components due to their high heat resistance and low density. However, SiC CMC degrades over time in steam oxidation environments. Therefore, environmental barrier coatings (EBC) are necessary to protect SiC substrates. Yb2Si2O7 is one of the most potent materials for barrier coatings. In this study, self-healing property is further added to Yb2Si2O7 EBC by incorporating SiC in the form of Yb2Si2O7/SiC granulated powders. We investigated the effect of different SiC contents and particle sizes on the long-term oxidation behavior of Yb2Si2O7/SiC coatings. The self-healing property was observed in the microstructural changes of artificial cracks induced by Vickers indentation. The samples were oxidized at 1300°C, and the structural change before and after the oxidation was evaluated by SEM, XRD, and EDX. The cross-sectional SEM images show that SiC was entirely oxidized in the first 50 h. Meanwhile, the surface SEM results show that small particle size and high SiC content exhibited a significant volume expansion during the oxidation of SiC. In addition, the self-healing property was significantly active in the initial stage of oxidation, but drastically deteriorated as the oxidation progressed. [ABSTRACT FROM AUTHOR]

Compared with nickel based alloys used as hot section materials in traditional aviation engines, silicon carbide materials have better physical and chemical properties at high temperature. However, protective coatings are required on the silicon carbide surface in order to reduce the exfoliation and deformation in gases such as oxygen and water vapour in the engine environment. This research has developed silicon/mullite/rare earth silicate three-layered environmental barrier coatings as protective coatings for a silicon carbide substrate. The preparation of the three-layered coating is applied by the sol-gel method and plasma spray process. Measurements show that the coefficient of thermal expansion becomes larger gradually layer by layer from the under layer to the top one, which mitigates the interfacial stress. SEM indicates the Si layer improves the porous structure of the SiC surface. Though some cracks exist after calcination in the mullite layer, the rare earth silicate layer can cover the crack. The weight of SiC rarely increased at high temperature in the oxidation test. Therefore, the coating has excellent anti-oxidation properties to protect the silicon carbide substrate effectively. [ABSTRACT FROM AUTHOR]

Abstract: Y2−x Tb x SiO5 and Y2−x Eu x SiO5 nanophosphors with seven different kinds of silicate sources were synthesized by sol–gel method. The structures have been investigated to be composed of nanometer-size grains of 30–60nm through X-ray diffraction (XRD) and scanning electron microscopy (SEM) was used to compare the different morphology of patterns from seven different silicon sources. The photoluminescence of Y2−x Tb x SiO5 was investigated as a function of silicate sources and the results revealed that these nanometer materials showed the characteristic emission 5D4 → 7F J (J =6, 5, 4, 3) of Tb ions. The characteristic emission 5D0 → 7F J (J =1, 2, 4) of Eu ions was also found in the materials of Y2−x Eu x SiO5. [Copyright &y& Elsevier]