HIGH TEMPERATURE INTERACTIONS BETWEEN ENVIRONMENTAL BARRIER COATING (EBC) CERAMICS AND CALCIA-MAGNESIA-ALUMINA-SILICATE (CMAS) GLASS

摘要

Ceramic-matrix-composites (CMCs) are being researched to replace current metallic hot-section components, which would allow for higher operating temperatures. Due to the oxidation of CMCs (usually SiC-based) in the presence of water vapor, dense environmental barrier coatings (EBCs) are needed. At temperatures above 1200 °C, silicate particles (sand, volcanic ash, fly ash, etc.) enter the engine, melt on the hot surfaces and form calcia-magnesia-alumina-silicate (CMAS) glass deposits. The molten CMAS glass can penetrate grain boundaries and cause dissolution, which leads to premature failure. New coatings are needed to protect CMCs from CMAS attack. A new model, based on optical basicities, has been used to predict the reactivity between CMAS and potential EBC ceramics. Based on this analysis, several potential EBC ceramics have been identified: yttrium aluminate (YAIO_3), yttrium disilicate (γ-Y_2Si_2O_7), ytterbium disilicate (β-Yb_2Si_2O_7), and scandium disilicate (β-Sc_2Si_2C_7).High-temperature (1500 °C) interactions of these four promising dense, polycrystalline EBC ceramics with a CMAS glass have been studied systematically. Although the optical basicities of all the EBC ceramics and the CMAS are similar, Y-bearing EBC ceramics react more with the CMAS. In Si-free YAIO_3, the reaction zone is small and it contains three regions of reaction-crystallization products: (i) needle-like Y-Ca-Si apatite(ss) grains, (ⅱ) blocky grains of YAG(ss) or Y_3Al_5O_(12)(ss), and (ⅲ) a mixture of Y-Ca-Si apatite(ss) and YAG(ss) blocky grains. In contrast, only Y-Ca-Si apatite(ss) forms in the case of Si-containing γ-_2Si_2O_7, and the reaction zone is an order-of-magnitude thicker. These CMAS interactions are analyzed in detail, and are found to be strikingly different than those observed in Y-free EBC ceramics β-Yb2Si2O7 and β-Sc_2Si_2O_7). This is attributed to the presence of the Y in the YAIO3 and γ-Y_2Si_2C_7 EBC ceramics. There is little or no reaction found between the Y-free EBC ceramics and the CMAS. In the case of β-Yb_2Si_2O_7, a small amount of reaction-crystallization product Yb-Ca-Si apatite(ss) forms, whereas none is detected in the case of β-Sc_2Si_2O_7. Instead, the CMAS glasspenetrates the grain boundaries and triple junctions of both Y-free EBC ceramics, and they suffer from a new type of 'blistering' damage comprising of large and wide cracks (Figure 1, C and D). This is attributed to the through-thickness dilatation-gradient caused by the slow grain boundary penetration of the CMAS glass. Based on this understanding, a 'blistering' damage-mitigation approach is devised and successfully demonstrated, where 1 vol% CMAS glass is mixed into the β-Yb2Si2C7 powder prior to sintering. The resulting EBC ceramic does not show the 'blistering' damage, as the presence of the CMAS-glassy phase at the grain boundaries promotes rapid CMAS-glass penetration, thereby eliminating the dilatation-gradient.