Abstract Environmental barrier coating (EBC) systems are applied to the surface of silicon-based composites exposed to high temperature combustion gas flow paths in gas turbine engines. They reduce the rate of composite oxidation, its volatilization by reactions with water vapor, and the temperature of the composite as their thermal conductivity decreases. Current EBC systems consist of a silicon bond coat covered by an ytterbium disilicate (Yb2O⋅32SiO2: YbDS) permeation resistant and low silica activity barrier. When applied by atmospheric-plasma spray deposition, this outer layer contains 10–15% of a secondary ytterbium monosilicate (Yb2O⋅3SiO2: YbMS) phase. YbDS has a coefficient of thermal expansion (CTE) of 4–5 × 10−6 °C−1, similar to those of the silicon and the silicon-based composite, but a relatively high thermal conductivity of 5–7 W m−1 K−1. YbMS has a higher steam volatility resistance than YbDS, and it has a much lower thermal conductivity ( ∼ 2–2.5 W m−1 K−1) at ambient temperature compared to YbDS, but its higher, highly anisotropic CTE (3–11 × 10−6 °C−1) results in channel cracking which reduces environmental protection. Here, we use a combination of scanning electron beam and laser-based thermoreflectance methods to spatially map the distribution of the silicate phases and thermal conductivity at ambient temperature in a Si-ytterbium disilicate EBC system exposed to thermal cycling in water vapor. We show that during thermal cycling, diffusion of silica from the thermally grown oxide on the Si bond coat surface to nearby YbMS regions transforms this phase to YbDS, thereby reducing the risk of thermomechanical coating failure but decreasing its effective thermal resistance. We also show that as silica is volatilized at the water vapor–ytterbium silicate interface, YbDS is transformed to YbMS, restoring some of the thermal protection of the coating system lost by its reduction in thickness and the YbMS to YbDS transformation near the bond coat.

中文翻译：

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多功能环境屏障涂层系统的微观结构和热导率的演变

摘要 环境屏障涂层 (EBC) 系统应用于暴露于燃气涡轮发动机中高温燃烧气体流路的硅基复合材料的表面。它们降低了复合材料的氧化速率，通过与水蒸气的反应而挥发，并随着热导率的降低而降低了复合材料的温度。当前的 EBC 系统由覆盖有二硅酸镱 (Yb2O⋅32SiO2: YbDS) 防渗透和低二氧化硅活性屏障的硅粘合涂层组成。当通过大气等离子喷涂沉积应用时，该外层包含 10-15% 的次级单硅酸镱 (Yb2O⋅3SiO2: YbMS) 相。YbDS 的热膨胀系数 (CTE) 为 4–5 × 10−6 °C−1，类似于硅和硅基复合材料的热膨胀系数，但相对较高的热导率为 5-7 W m-1 K-1。与 YbDS 相比，YbMS 具有更高的蒸汽挥发性阻力，并且与 YbDS 相比，它在环境温度下的热导率要低得多（~ 2-2.5 W m-1 K-1），但其更高的、高度各向异性的 CTE（3-11 × 10−6 °C−1) 导致通道开裂，从而降低环境保护。在这里，我们使用扫描电子束和基于激光的热反射方法的组合在空间上绘制暴露于水蒸气中的热循环的 Si-Ytterbium 二硅酸盐 EBC 系统中的硅酸盐相的分布和环境温度下的热导率。我们表明，在热循环过程中，二氧化硅从 Si 键涂层表面上热生长的氧化物扩散到附近的 YbMS 区域，将该相转化为 YbDS，从而降低热机械涂层失效的风险，但降低其有效热阻。我们还表明，随着二氧化硅在水蒸气 - 硅酸镱界面处挥发，YbDS 转化为 YbMS，恢复了涂层系统的一些热保护，因为涂层系统的厚度减少和 YbMS 到粘合涂层附近的 YbDS 转化而失去了一些热保护。