A thermal history coating (THC) is a robust coating containing various non-toxic
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
s whose
crystal structure
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pattern ...
s irreversibly change at high temperatures. This allows for temperature measurements and thermal analysis to be performed on intricate and inaccessible components, which operate in harsh environments. Like
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 ...
s, THCs provide protection from intense heat to the surfaces on which they are applied. The temperature range that THCs provide accurate temperature measurements in is 900 °C to 1400 °C with an accuracy of ±10 °C.
Application of THCs
THCs are applied by atmospheric plasma spraying, which is a
thermal spraying
Thermal spraying techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame).
Thermal sprayi ...
technique. This ensures that the coatings are robust to allow long life-times in harsh environments, such as on
jet engine
A jet engine is a type of reaction engine discharging a fast-moving jet (fluid), jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include Rocket engine, rocket, Pump-jet, water jet, and ...
components, which experience temperatures in excess of 1000 °C
and angular velocities of up to 10,000rpm
(revolutions per minute).
Temperature Measurement
Phosphorescent Properties
THCs are composed of
phosphor
A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or v ...
materials, whose
luminescent
Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light".
It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a crystal ...
characteristics are temperature- and duration-dependent.
Phosphor thermometry Phosphor thermometry is an optical method for surface temperature measurement. The method exploits luminescence emitted by phosphor material. Phosphors are fine white or pastel-colored inorganic powders which may be stimulated by any of a variety of ...
is the measurement technique used for determining the past temperatures of THCs, whereby the luminescent characteristics of the coatings are exploited and matched to calibration tables.
Instrumentation
The phosphorescence of THCs is excited by use of an external light source such as a laser pen. An optical system then collects a reflected light signal, whose characteristics provide information on the crystal structure of the THC. Crystal structure properties are then converted into temperatures, which had previously been experience by the coatings. This allows for point measurements to be made across the coated surfaces of components and allows thermal analysis to be carried out.
Applications
R&D
THCs are used in high temperature applications where temperature knowledge is essential in
research and development
Research and development (R&D or R+D), known in Europe as research and technological development (RTD), is the set of innovative activities undertaken by corporations or governments in developing new services or products, and improving existi ...
programmes, for example in identifying hot spots, which could lead to structural damage of components.
Warranty
As the THCs provide historic temperature information, they can be used as warranty tools, where certain components, such as valves or particular engine or machinery components must not exceed certain temperatures.
Other High-Temperature Detection Technologies
*
Thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of th ...
* Thermocrystal
*
Pyrometer
A pyrometer is a type of remote-sensing thermometer used to measure the temperature of distant objects. Various forms of pyrometers have historically existed. In the modern usage, it is a device that from a distance determines the temperature o ...
References
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Materials science
Thermal protection