Environmental degradation of high-temperature protective coatings for ceramic-matrix composites in gas-turbine engines

Environmental degradation of high-temperature protective coatings for ceramic-matrix composites in gas-turbine engines Nitin P. Padture 0 School of Engineering, Brown University , Providence, RI 02912 , USA PERSPECTIVE The need for higher efficiencies and performance in gas-turbine engines that propel aircraft in the air, and generate electricity on land, is pushing the operating temperatures of the engines to unprecedented levels. Replacing some of the current hot-section metallic components with ceramic-matrix composites (CMCs) is making that possible. A high-temperature ceramic coatings system, that includes environmental-barrier coatings (EBCs), are needed to protect CMCs. However, these coatings undergo degradation in the highly hostile environment of the gas-turbine engine consisting of a combination of high gas temperatures, pressures, and velocities. In addition, there is the ubiquitous presence of steam (a combustion by-product) and occasional ingestion of calciamagnesia-aluminosilicates (CMASs) in the form of dust, sand, or ash from the environment. Steam can cause corrosion of EBCs, and the molten CMAS deposits can react with the EBCs resulting in their failure. This article provides a perspective on the understanding of these degradation mechanisms, and possible approaches, guided by that understanding, for mitigating the degradation. An outlook on the future challenges and opportunities is presented. - Gas-turbine engines that propel aircraft and generate electricity have a huge impact on the transportation, energy, and defense sectors of the global economy.1 It has been estimated that cumulative sales of gas-turbines engines in the period 2017?2031 will approach US$2 trillion.2 While current engines perform admirably and are highly efficient, there is always demand for even higher performance, better fuel efficiency, and lower NOx emissions. Typically, this can be accomplished by elevating the gas-inlet temperature in the hottest part (hot-section) of the gasturbine engine, where the core-power and the efficiency scale with that temperature.1 But the high-temperature capability of the current hot-section materials is the ?bottleneck.? Improvements in superalloy, and the use of ceramic thermal barrier coatings (TBCs) in conjunction with internal air-cooling and air-film cooling of the superalloys components, have resulted in unprecedented gas-inlet temperatures in today?s gas-turbine engines.1,3?5 However, TBCscoated superalloys are reaching their temperature-capability limit, and it is not clear if they will be able to achieve the >1700 ?C gasinlet temperature goal.1 In this context, ceramic-matrix composites (CMCs) offer a ?quantum jump? in temperature capability.1,6,7 Gasturbine engines with hot-section components made of CMCs, comprising continuous fibers and matrices made of SiC, are already in-service commercially, both for aircraft propulsion and electricity generation. Unfortunately, SiC-based CMCs undergo active oxidation and recession in the high-temperature, high-pressure, high-velocity gas stream of the gas-turbine engine which invariably contains steam, a combustion by-product.8,9 Thus, dense, crack-free environmental barrier coatings (EBCs) are needed to protect SiCbased CMCs from this environmental degradation by blocking diffusion/ingression of oxygen/steam.1,10?13 For achieving crackfree EBCs, they must have an excellent coefficient of thermal expansion (CTE) match with SiC-based CMCs (~4.5 ? 10?6 ?C?1), which limits the choice of ceramics that can be used as EBCs.1,12?14 EBCs must also have low volatility for minimizing steam-induced corrosion/recession, and be resistant to degradation by molten calcia-magnesia-aluminosilicate (ingested dust, sand, or ash; commonly referred to as CMAS) deposits, among several other requirements, including high-temperature capability; phase stability; chemical compatibility with other layers; and mechanical robustness (high hardness and toughness) against fracture, erosion, and impact-damage.1,12?14 Since the failure of EBCs will result in the catastrophic degradation of the CMCs, EBCs need to be prime-reliant. EBCs also need to be readily manufacturable using commercially available scalable methods such as those based on thermal spray. Research on EBCs began in earnest in the late 1990s, soon after the discovery of active high-temperature oxidation of SiC in steam environment.12,13 Since typical refractory oxide structural ceramics such as Al2O3 and ZrO2 have a high CTE (~10 ? 10?6 ?C?1), they are not suitable for EBC application. Initially, (1-x)BaO?xSrO?Al2O3?2SiO2 (0 < x < 1; BSAS) ceramics with lower CTE (~4.3 ? 10?6 ?C?1, celsian phase) were considered, where a typical EBC structure (Fig. 1a) comprised three layers: Si bond-coat, BSAS/ mullite inter-layer, and BSAS top-coat.12,15 All these layers are typically deposited using thermal-spray methods. Since oxide coatings typically do not bond well to the non-oxide CMC, the Si bond-co (...truncated)