Thermal barrier coating

Thermal barrier coatings (TBCs) are advanced materials systems usually applied to metallic surfaces operating at elevated temperatures, such as gas turbine or aero-engine parts, as a form of exhaust heat management. These 100 μm to 2 mm thick coatings of thermally insulating materials serve to insulate components from large and prolonged heat loads and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface. In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue. In conjunction with active film cooling, TBCs permit working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications. Due to increasing demand for more efficient engines running at higher temperatures with better durability/lifetime and thinner coatings to reduce parasitic mass for rotating/moving components, there is significant motivation to develop new and advanced TBCs. The material requirements of TBCs are similar to those of heat shields, although in the latter application emissivity tends to be of greater importance. Thermal barrier coatings (TBCs) are advanced materials systems usually applied to metallic surfaces operating at elevated temperatures, such as gas turbine or aero-engine parts, as a form of exhaust heat management. These 100 μm to 2 mm thick coatings of thermally insulating materials serve to insulate components from large and prolonged heat loads and can sustain an appreciable temperature difference between the load-bearing alloys and the coating surface. In doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending part life by reducing oxidation and thermal fatigue. In conjunction with active film cooling, TBCs permit working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications. Due to increasing demand for more efficient engines running at higher temperatures with better durability/lifetime and thinner coatings to reduce parasitic mass for rotating/moving components, there is significant motivation to develop new and advanced TBCs. The material requirements of TBCs are similar to those of heat shields, although in the latter application emissivity tends to be of greater importance. An effective TBC needs to meet certain requirements to perform well in aggressive thermo-mechanical environments. To deal with thermal expansion stresses during heating and cooling, adequate porosity is needed, as well as appropriate matching of thermal expansion coefficients with the metal surface that the TBC is coating. Phase stability is required to prevent significant volume changes (which occur during phase changes), which would cause the coating to crack or spall. In air-breathing engines, oxidation resistance is necessary, as well as decent mechanical properties for rotating/moving parts or parts in contact. Therefore, general requirements for an effective TBC can be summarize as needing: 1) a high melting point. 2) no phase transformation between room temperature and operating temperature. 3) low thermal conductivity. 4) chemical inertness. 5) similar thermal expansion match with the metallic substrate. 6) good adherence to the substrate. 7) low sintering rate for a porous microstructure. These requirements severely limit the number of materials that can be used, with ceramic materials usually being able to satisfy the required properties. Thermal barrier coatings typically consist of four layers: the metal substrate, metallic bond coat, thermally-grown oxide (TGO), and ceramic topcoat. The ceramic topcoat is typically composed of yttria-stabilized zirconia (YSZ) which is desirable for having very low conductivity while remaining stable at nominal operating temperatures typically seen in applications. This ceramic layer creates the largest thermal gradient of the TBC and keeps the lower layers at a lower temperature than the surface. However, above 1200 °C, YSZ suffers from unfavorable phase transformations, going from t'-tetragonal to tetragonal to cubic to monoclinic. Such phase transformations lead to crack formation within the top coating. Recent advancements in finding an alternative for YSZ ceramic topcoat identified many novel ceramics (rare earth zirconates) having superior performance at temperatures above 1200 °C, however with inferior fracture toughness compared to that of YSZ. In addition, such zirconates may have a high concentration of oxygen ion vacancies, which may facilitate oxygen transport and exacerbate the formation of the TGO. With a large enough TGO, spalling of the coating may occur, which is a catastrophic mode of failure for TBCs. The use of such coatings would require addition coatings that are more oxidation resistant, such as alumina or mullite.