Thermal-barrier coatings for more efficient gas-turbine engines
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oduction Thermal-barrier coatings (TBCs) are refractory-oxide ceramic coatings applied to the surfaces of metallic parts in the hottest part of gas-turbine engines (Figures 1 and 2), enabling modern engines to operate at significantly higher gas temperatures than their predecessors (see recent reviews1–6). Gas-turbine engines, used to propel aircraft and to generate electricity, are Carnot engines where their efficiency and core power are directly related to the gas temperature entering the turbine section.7,8 Further increases in the energy efficiency of gasturbine engines, both to increase the electricity output and, for jet engines, the thrust-to-weight ratio and durability, will rely on further improvements in TBCs. At the same time, as gas temperatures are increased in the pursuit of higher engine efficiency, there are new challenges to existing TBCs. To place this in context, gas-turbine engines are a $42 billion industry worldwide (2010), with ∼65% of the sales accounting for jet engines and the remainder land-based engines for electricity generation.9 The latter, fueled by natural gas or liquid fuels, produce ∼25% of all electricity in the United States and ∼20% worldwide (2010).10 With the anticipated worldwide growth of electricity demand and the recent discovery of vast shale gas resources, the number of gas-turbine engines in service will inevitably grow in the coming decades.9 Similarly, airline traffic is expected to double in the next 20 years,11 while at the same time, there is a need to reduce high-altitude NOx
pollution produced by jet engine exhausts.12 Together, these developments will require continued innovation in gas turbine technology and high-temperature engine materials, including TBCs and associated technologies. Many engineering design factors influence the overall efficiency of gas-turbine engines, but a major step in increasing engine temperature and engine efficiency was the introduction of TBCs. Typically made of ∼7 wt% Y2O3-stabilized ZrO2 (7YSZ) ceramics, TBCs provide thermal insulation to the metallic/superalloy engine parts. These parts include the combustor (Figures 1 and 2); stationary guide vanes, rotating blades (Figure 1), blade outer air-seals, and shrouds in the high-pressure section behind the combustor; and afterburners in the tail section of jet engines. As illustrated in Figure 3, the gas-temperature increase facilitated by the use of TBCs, in conjunction with innovative air-cooling approaches, has been much greater than that enabled by earlier materials development, including the development of single-crystal Ni-based superalloys. Originally, TBCs were introduced to extend the useful life of stationary engine parts such as the combustor, but in the late 1980s, TBCs were first used on rotating blades.13 However, TBCs were not “prime reliant”; in other words, the ceramic coating was not considered in the design of the temperature capability of the underlying metal parts. Today, TBCs are critical components in gas-turbine engines, and because the gas temperatures are typically hi
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