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Materials for Thermal Barrier Coating Applications

Key Takeaways

  • To protect heat-affected systems from degradation, efficient thermal resistive coatings called thermal barrier coatings are applied on a surface.  

  • The thermal resistance of thermal barrier coatings is high and prevents the easy conduction of heat. 

  • Thermal barrier coatings are sprayed on commercial aero engines, flame tubes of gas turbines, injectors of rocket engines, turbine blades in power plants, and semiconductor substrates. 

 Gas turbines

Thermal barrier coatings are sprayed on flame tubes of gas turbines

Heat flux causes detrimental effects on the metal surfaces exposed to high temperatures. Heat flux is capable of reducing the lifespan as well as the performance of metal-enclosed devices and components. To prevent the degradation of systems due to heat, thermal barrier coating applications are employed in various industries such as power, aviation, and electronics.

Thermal barrier coatings slow down the degradation rate of components caused by heat flux and decrease the surface temperature of metal structures. These coatings improve the operating temperature range, performance, and efficiency of systems. 

Thermal Barrier Coatings 

Improving the thermo-mechanical performance of various systems working under harsh conditions such as high temperatures and corrosive environments is critical. If not removed on time, the heat generated in equipment shortens the service life of the system. To protect heat-affected systems from degradation, efficient thermal resistive coatings, called thermal barrier coatings (TBC), are applied on surfaces.

Thermal barrier coatings are protective coatings that guard surfaces coated from heat, corrosion, and any sort of degradation. Thermal barrier coatings are applied on most electrical and electronic systems to ensure performance is minimally impacted by heat flux. The thermal barrier coating on equipment decreases the metal surface degradation rate caused by heat. A thermal gradient is maintained between the metal surface and coating surface, thereby enhancing the operating temperature range of the system.    

A thermal barrier coating increases the efficiency of the system where it is applied and improves performance under extreme temperatures. It showcases characteristics such as thermal insulation and thermal expansion. Some other characteristics of thermal barrier coatings are:

  • Low thermal conductivity

  • High thermal shock resistance

  • High coefficient of thermal expansion

  • High surface emissivity

  • High melting point

  • Low density

  • Resistance to corrosion

  • Mechanical erosion

  • Oxidation  

Thermal Barrier Coating Structure

Thermal barrier coating consists of a layered structure. Thermal barrier coating layers on a metal surface area are in the order as follows:

  1. Ceramic thermal barrier - The ceramic layer is the immediate layer on the metal substrate. It provides thermal protection and prevents the erosion and corrosion of metal due to high temperatures. 

  2. Bond coat - The bond coat of a thermal barrier coating prevents oxidation of the metal, the thermal mismatch between the metal substrate and the thermal barrier coating outer layer, and interdiffusion between metals and layers.

  3. Thermally grown oxide layer - A thermally oxide layer is grown in between the bonded coat and outer layer of the top coat of the thermal barrier coating to prevent oxidation and corrosion. 

  4. Topcoat - This is the outer layer that comes in direct contact with the ambient, which aids in maximizing the thermal drop across the thickness of the thermal barrier coating. 

Materials for Thermal Barrier Coating Applications

The industrial applications of thermal barrier coatings started with using enamel coating for military engines in the 1950s. Thermal barrier coatings are sprayed on commercial aero engines, flame tubes of gas turbines, injectors of rocket engines, turbine blades in power plants, and semiconductor substrates. There are a variety of materials used as thermal barrier coatings in engineering systems.  Some of them are:

  1. Yttria stabilized zirconia (YSZ) - YSZ is one of the most widely used thermal barrier coating materials. It is best suited for high-temperature applications such as diesel engines or gas turbines. Properties such as high oxygen permeability, strain tolerance, low thermal conductivity, and a high coefficient of thermal expansion make YSZ suitable for numerous thermal barrier coating applications. 

  2. Aluminum oxide - Aluminum oxide is hard and chemically inert. It exhibits high thermal conductivity and a low thermal expansion coefficient compared to YSZ. Alumina coating on YSZ increases the oxidation resistance and is suitable for harsh conditions. 

  3. Mullite - Mullite is a ceramic thermal barrier coating material formed as a compound of  SiO2 and Al2O3, with composition 3Al2O3.2SiO2. It offers properties such as low density, low thermal conductivity, high chemical stability, and high thermal stability. Mullite is excellent for applications in diesel engines where temperatures vary largely across the thermal barrier coating.

  4. Rare earth oxides - Rare earth oxides are popular for their low thermal diffusivity and high thermal expansion coefficient. Rare earth oxides such as La2O3, CeO2, Pr2O3, and Nb2O5 are readily available and cheap and are used in systems operating at elevated temperatures.

Pyrochlores, hexaaluminate, perovskites, zirconates, silicates, and metal-glass composites are a few other material options for thermal barrier coating applications. Cadence’s design and analysis tools can help you evaluate the thermal performance of electrical systems covered with thermal barrier coatings.

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