Thermal barrier coating (TBC) is an advanced material system 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.
Thermodynamic Requirements of Thermal Barrier Coating
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 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: a high melting point, no phase transformation between room temperature and operating temperature, low thermal conductivity, chemical inertness, similar thermal expansion match with the metallic substrate, good adherence to the substrate, 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.
The Metallic Oxidation Requirement
The bond-coat is an oxidation-resistant metallic layer which is deposited directly on top of the metal substrate. It is typically 75-150 μm thick and made of alloy, though other bond coats made of Ni and Pt aluminides also exist. The primary purpose of the bond-coat is to protect the metal substrate from oxidation and corrosion, particularly from oxygen and corrosive elements that pass through the porous ceramic top-coat. At peak operating conditions found in gas-turbine engines with temperatures in excess of 700 °C, oxidation of the bond-coat leads to the formation of a thermally-grown oxide (TGO) layer. Formation of the TGO layer is inevitable for many high-temperature applications, so thermal barrier coatings are often designed so that the TGO layer grows slowly and uniformly.