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Materials that resist temperatures up to 1500°C: What is the secret ingredient of thermal barrier coatings?
With the advancement of aerospace and energy technologies, thermal barrier coatings (TBCs) have become an important field in high-temperature materials science. These material systems are often applied to metal surfaces, especially components that operate in high-temperature environments, such as combustors and turbines in gas turbines. Their thickness can range from 100 microns to 2 millimeters, and as good thermal insulation materials, they effectively extend the life of components and their heat resistance.
The primary function of a thermal barrier coating is to insulate the metal substrate, enabling it to operate under extreme thermal loads, thereby maximizing operating temperatures without damaging the underlying structure.
Thermal barrier coatings are able to maintain a significant temperature difference between the component and the coated surface, which allows them to operate in environments with increasing operating temperatures without causing excessive thermal exposure to structural components. This reduces the effects of oxidation and thermal fatigue, thereby extending component life. As the demand for more efficient engines that can operate at higher temperatures increases, the material requirements for TBCs are also moving towards higher melting points, lower thermal conductivity and better oxidation resistance.
Structure and material properties of thermal barrier coatings
Thermal barrier coatings based on ceramic materials usually consist of four layers: metal substrate, metal bond coat, thermally grown oxide layer (TGO) and ceramic top layer. Currently, stabilized zirconia (YSZ) is widely used as a ceramic surface layer, which has very low thermal conductivity but undergoes phase changes above 1200°C, inducing cracks. In the development of YSZ-based materials, novel rare earth zirconates have been explored as alternatives, showing good performance above 1200°C, but their crack resistance is poor.
Risks and opportunities coexist. In the past, many new ceramic materials that can operate at extremely high temperatures have been developed, which paves the way for the development of more high-efficiency thermal barrier coatings in the future.
Failure mechanisms of thermal barrier coatings
The failure mechanism of TBC involves many factors, but there are three key mechanisms: the growth of thermally grown oxide (TGO), thermal shock and sintering of the surface layer. The formation of TGO induces compressive stress, which mismatches the thermal expansion of the substrate and leads to crack initiation. When subjected to multiple heating and cooling cycles, these cracks may propagate throughout the coating, leading to eventual failure. Especially in the operation of aircraft engines, thermal shock caused by frequent startup and shutdown operations is also one of the main reasons.
The design of thermal barrier coatings needs to consider the matching of thermal expansion coefficients between layers to extend service life and reduce crack formation.
Applications of Thermal Barrier Coatings
Thermal barrier coatings are widely used in the automotive and aviation fields. In automobiles, they are used to reduce heat loss from engine exhaust system components and to reduce noise and heat in the engine compartment. In the aerospace industry, TBCs are used to protect nickel-based superalloys and enhance their performance in high-temperature environments. The development of new technologies has enabled ceramic coatings to be applied to composite materials, which not only protects the materials but also improves wear resistance.
As studied by scientists and engineers, the materials and design of thermal barrier coatings represent a current frontier in materials science. As technology advances, a host of future improvements are expected to realize their potential in more extreme environments. However, can we find the perfect thermal barrier coating to adapt to the ever-increasing technological demands and challenges?
