Thermal barrier coating
Anatomy
Ceramic topcoat
Thermally grown oxide
Metallic bond coat
Superalloy substrate
Thermal barrier coatings 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.
TBCs with dual functionality: protection and sensing. Knowing the temperature of the surface of the TBC and at the interface between the bondcoat and the thermally grown oxide under realistic conditions is highly desirable. As the major life-controlling factors for TBC systems are thermally activated, therefore linked with temperature, this would provide useful data for a better understanding of these phenomena and to assess the remaining life-time of the TBC. The integration of an on-line temperature detection system would enable the full potential of TBCs to be realised due to improved precision in temperature measurement and early warning of degradation. The TBC is locally modified so it acts as a thermographic phosphor(Phosphor thermometry). Phosphors are an innovative way of remotely measuring temperatures and also other physical properties at different depths in the coating using photo stimulated phosphorescence.
Uses
Thermal barrier coating applied onto carbon composite
Automotive
“Composite coating” redirects here. For other uses, see Composite coating (disambiguation).
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 lessening the intake temperature.
Although most ceramic-coatings are applied to metallic parts directly related to the engine exhaust system, some new technology has been introduced that allows thermal barrier coatings to applied via plasma spray onto composite materials. This is now commonplace to find on high-performance automobiles and in various race series such as in Formula 1. As well as providing thermal protection, these coatings are also used to prevent physical degradation of the composite due to frictional processes. This is possible due to the fact that when applied, 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.
Industrial
In industrial applications, where space is at a premium, thermal barrier coatings are commonly used to protect from heat loss (or gain).
Processing
In industry, thermal barrier coatings are produced in a number of ways:
Electron Beam Physical Vapor Deposition: EBPVD
Air Plasma Spray: APS
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.
References
^ F.Yu and T.D.Bennett (2005). “A nondestructive technique for determining thermal properties of thermal barrier coatings”. J. Appl. Phys. 97: 013520. doi:10.1063/1.1826217.
^ J. P. Feist and A. L. Heyes (2000). “Europium-doped yttria-stabilized zirconia for high-temperature phosphor thermometry”. Proceedings of the Institution of Mechanical Engineers 214, Part L: 711.
^ X. Chen, Z. Mutasim, J. Price, J. P. Feist, A. L. Heyes and S. Seefeldt (2005). “Industrial sensor TBCs: Studies on temperature detection and durability”. International Journal of Applied Ceramic Technology 2 (5): 414421. doi:10.1111/j.1744-7402.2005.02042.x.
^ J. P. Feist, A. L. Heyes and J. R. Nicholls (2001). “Phosphor thermometry in an electron beam physical vapour deposition produced thermal barrier coating doped with dysprosium”. Proceedings of Institution of Mechanical Engineers 215 Part G: 333340.
Categories: Materials science | Thin film deposition | Thermal protection
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