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REVIEW: MATERIALS SCIENCE
Hundreds of different types of coatings are used to protect a variety of structural engineering materials from corrosion, wear, and erosion, and to provide lubrication and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most complex structure and must operate in the most demanding high-temperature environment of aircraft and industrial gas-turbine engines. TBCs, which comprise metal and ceramic multilayers, insulate turbine and combustor engine components from the hot gas stream, and improve the durability and energy efficiency of these engines. Improvements in TBCs will require a better understanding of the complex changes in their structure and properties that occur under operating conditions that lead to their failure. The structure, properties, and failure mechanisms of TBCs are herein reviewed, together with a discussion of current limitations and future opportunities.
The great difference in the thermomechanical properties of metals andceramics would seem to preclude their use in composite structures subjected to huge changes in temperature and thermal stresses. Nonetheless, thermal barrier coatings (TBCs) made of low-thermal conductivity ceramics are now being used to provide thermal insulation to metallic components from the hot gas stream in gas-turbine engines used for aircraft propulsion, power generation, and marine propulsion (1-4). The use of TBCs (100 to 500 (mu)m in thickness), along with internal cooling of the underlying superalloy component, provide major reductions in the surface temperature (100 deg to 300 deg C) of the superalloy. This has enabled modern gas-turbine engines to operate at gas temperatures well above the melting temperature of the superalloy (~ 1300 deg C), thereby improving engine efficiency and performance. Alternatively, at somewhat lower operating temperatures, TBCs help reduce metal temperature, making engine components more durable. TBCs are also being used, to some extent, in diesel engines, where higher operating temperatures translate into increased fuel economy and cleaner exhaust (3).
The structure of a thermal barrier-coated component consists of four layers-two ceramic and two metallic-with each layer having markedly different physical, thermal, and mechanical properties, making it inherently more complex than the individual components that are all metallic or all ceramic. The thermal barrier-coated component must withstand the most extreme temperature, temperature cycling, and stress conditions, and it is expected to last thousands of take-offs and landings in...