Thermal barrier coating
Thermal barrier coatings (TBC) are highly advanced material systems usually applied to metallic surfaces, such as gas turbine or aero-engine parts, operating at elevated temperatures, as a form of exhaust heat management. These coatings serve to insulate components from large and prolonged heat loads by utilizing thermally insulating materials which 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.
|Thermally grown oxide|
|Metallic bond coat|
Thermal barrier coatings typically consist of four layers: the metal substrate, metallic bond coat, thermally grown oxide, 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. 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. This ceramic layer creates the largest thermal gradient of the TBC and keeps the lower layers at a lower temperature than the surface.
TBCs fail through various degradation modes that include mechanical rumpling of bond coat during thermal cyclic exposure, especially, coatings in aircraft engines; accelerated oxidation, hot corrosion, molten deposit degradation. There are issues with oxidation (areas of the TBC getting stripped off) of the TBC also, which reduces the life of the metal drastically, which leads to thermal fatigue.
The TBC can also be locally modified at the interface between the bondcoat and the thermally grown oxide so that it acts as a thermographic phosphor, which allows for remote temperature measurement.
Thermal barrier ceramic-coatings are becoming more common in automotive applications. They are specifically designed to reduce heat loss from engine exhaust system components including exhaust manifolds, turbocharger casings, exhaust headers, downpipes and tailpipes. This process is also known as "exhaust heat management". When used under-bonnet, these have the positive effect of reducing engine bay temperatures, therefore reducing the intake air temperature.
Although most ceramic-coatings are applied to metallic parts directly related to the engine exhaust system, technological advances now allow thermal barrier coatings to be applied via plasma spray onto composite materials. It is now commonplace to find ceramic-coated components in modern engines and in high-performance components in race series such as Formula 1. As well as providing thermal protection, these coatings are also used to prevent physical degradation of the composite material due to friction. This is possible because the ceramic material bonds with the composite (instead of merely sticking on the surface with paint), therefore forming a tough coating that doesn't chip or flake easily.
Although thermal barrier coatings have been applied to the inside of exhaust systems, this has encountered problems due to the inability to prepare the internal surface prior to coating.
In industrial applications, where space is at a premium, thermal barrier coatings are commonly used to protect from heat loss (or gain). Especially in gas turbines and jet engines the turbine blades are covered with TBCs. Such turbine blades are made of superalloys and the TBC avoids melting, which allows to reach high gas inlet temperatures and enables a high energy conversion efficiency.
In industry, thermal barrier coatings are produced in a number of ways:
- Electron Beam Physical Vapor Deposition: EBPVD
- Air Plasma Spray: APS
- High Velocity Oxygen Fuel: HVOF
- Electrostatic Spray Assisted Vapour Deposition: ESAVD
- Direct Vapor Deposition
Additionally, the development of advanced coatings and processing methods is a field of active research. One such example is the Solution precursor plasma spray process which has been used to create TBCs with some of the lowest reported thermal conductivities while not sacrificing thermal cyclic durability.
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