Environmental Barrier Coatings for Ceramic Matrix Composites

CMCs are a game-changer for a number of applications because of their lighter weight, higher temperature capability, and resistance to oxidation. It has been estimated that aircraft designs relying on CMCs can decrease fuel consumption by 10% by 2020. EBCs are used to protect CMCs from water vapor and other corrosive gases inside engines and other extreme environments. The current state of the art for EBCS features a silicon bond coat that is not viable beyond its melting point of 1482°C. By contrast, Glenn's EBCs have demonstrated a steam oxidation life of at least 500 hours at 1482°C, making them ideal durable coatings for next-generation CMCs. These EBCs are slurries, with either a mullite-based bond coat or a rare earth disilicate-based bond coat comprising at least three and two layers, respectively. Mullite is often used as a refractory material for furnaces, reactors, etc. because of its high melting point (1840°C). Rare earth disilicates also have high melting points (~1800°C). These bond coats can be fabricated by preparing a mixture of a coating material, a primary sintering aid, at least one secondary sintering aid, and a solvent. The mixture is then processed (e.g., in a milling media) to form a slurry that can be deposited to a CMC substrate. The sintering aids have two primary functions: 1) densifying deposited slurry by generating liquid phases via reactions with the coating material and other sintering aids, so that the liquid fills gaps between particles of coated material; 2) enhancing bonding and performance of the coating by generating reaction products that enhance those qualities. One great advantage of this EBC is that it can be fabricated via various low-cost methods - including dipping, spinning, spin-dipping, painting, and spraying - in addition to plasma-spraying. Glenn's innovation rises to meet the need for a new class of EBCs that can keep up with CMCs' increasing ability to withstand higher temperatures and stresses than ever before.