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From aircraft engines to car exhaust systems: What are the amazing applications of thermal barrier coatings?
Thermal barrier coatings (TBCs) are advanced material systems commonly applied to metal surfaces that operate under high temperature conditions, such as the combustion chambers and turbines of gas turbine engines, and exhaust thermal management systems in automobiles. These coatings of thermally insulating material, with thicknesses ranging from 100 microns to 2 millimeters, effectively insulate against heat, allowing components to maintain operating efficiency and durability despite severe thermal loads.
Thermal barrier coatings can extend the service life of components and reduce oxidation and thermal fatigue.
With the increasing demand for high-efficiency engines, which need to operate at higher operating temperatures and have better durability, there is increasing momentum for the development of new advanced thermal barrier coatings. . The material requirements for thermal barrier coatings are similar to those for heat shields, but in the latter application, the rate of heat generation is usually more important.
Structure and requirements of thermal barrier coating
An effective thermal barrier coating needs to meet certain requirements to work well in harsh thermomechanical environments. To cope with the thermal expansion stresses during heating and cooling, appropriate porosity is necessary, and the thermal expansion coefficient should match that of the metal surface being coated. Furthermore, phase stability needs to be maintained in order to prevent significant volume changes (such as occur during phase changes). Thermal barrier coatings usually consist of four layers: metal substrate, metal bonding layer, thermally grown oxide layer (TGO) and ceramic top layer.
For a thermal barrier coating to last, the coefficients of thermal expansion between all layers should be well matched.
Failure mechanism
The failure mechanisms of thermal barrier coatings are complex and may vary depending on the thermal cycling environment. Although there are many failure mechanisms that are not fully understood, the growth of thermally grown oxide (TGO), thermal shock, and sintering of the top layer are the three most important factors leading to thermal barrier coating failure.
The growth of the TGO layer is one of the most important reasons for TBC shedding and failure. When TGO is formed with heating, it will cause compressive growth stress related to volume expansion; when cooling, lattice mismatch strain will be generated due to different thermal expansion coefficients. This series of stresses will eventually lead to cracking and peeling of the thermal barrier coating. .
Thermal shock is a primary failure mechanism because the stresses induced by such drastic temperature changes can cause cracks in the thermal barrier coating.
In addition, sintering increases the density of the top layer, causing cracks to form. It is reported that silicon nitride-based ceramic composite materials also show superior performance than traditional zirconium nitride materials in thermal barrier coating applications.
Types of thermal barrier coatings
Different thermal barrier coating materials have different characteristics. Including commonly used zirconia (YSZ), earth metal zirconate, % nitrogen aluminum oxide, etc. YSZ is the most famous and is widely used in fuel engines because of its good thermal stability and low thermal conductivity. However, YSZ encounters phase changes at high temperatures, resulting in performance degradation.
Rare earth oxides (such as CeO2) and metal-glass composites have shown potential as alternative materials.
Application fields
The application of thermal barrier coatings is becoming more and more common in modern vehicles, especially to reduce heat loss in exhaust system components, such as components such as exhaust manifolds and turbocharger housings. In addition, in the aviation field, the use of such coatings is extremely important, often used to protect nickel-based superalloys and enable them to operate above the melting point to enhance engine performance.
However, with the demand for fuel and the advancement of green technology, how to continuously improve the performance of thermal barrier coatings and enable them to operate stably at higher temperatures is a challenge that the industry will attach great importance to in the future.
Thermal barrier coating technology has shown broad application potential in many industries. How will this technology be further developed in the future to meet changing needs?
