Thermal barrier coatings are used in aviation, automotive, power generation, and marine industries to provide thermal insulation to any metallic part that gets heated. The coating is useful in reducing the temperature, and is a part of heat management in engineering systems.
A thermal barrier coating is a layered coating put on thermally loaded metal substrates. It consists of four layers: a ceramic thermal barrier, a bond coat, a thermally grown oxide layer, and a topcoat.
The characteristics of a thermal barrier coating is that it is corrosion free and has a high melting point, low thermal conductivity, high surface emissivity, high thermal resistance, high coefficient of thermal expansion, high thermal shock resistance, and low density.
Thermal barrier coating on a metal substrate
Sunscreen protects our skin from the harmful rays of the sun—when applied, it forms a coating that insulates from UVA and UVB rays. Similar to sunscreen, thermal barrier coatings (TBCs) are used to protect metal structures exposed to high temperatures. TBCs extend the life of metal structures exposed to high temperatures by decreasing their surface temperature.
A TBC is scientifically proven to be one of the factors that improves the overall performance of power generating units. TBCs accomplish this by slowing the component’s rate of degradation caused by heat flux. Coatings help maintain a thermal gradient between the metal surface and coating surface, enhancing the working temperature range and efficiency of the power generation system.
The Structure of Thermal Barrier Coatings
TBCs are employed in aviation, automotive, power generation, and marine industries to provide thermal insulation to any metallic parts that get heated. They are useful in reducing temperature and are a critical part of heat management in engineering systems.
The TBC forms a barrier to the flow of heat. When the working system gets heated above the melting temperature of the metals, there is a chance of failure. Coating the metal substrates with a TBC supports the operation of the system above the melting point, increasing efficiency and performance. As the TBC coating minimizes the metal surface temperature, it insulates the electronic circuits and other related components in the system encased by the metal covers and improves the durability of the components.
A TBC is a layered coating on thermally loaded metal substrates. It consists of four layers as shown in the figure below. The layers can be described as follows:
Schematic of a thermal barrier coating
Ceramic thermal barrier- This layer comes immediately after the metal substrate. It provides thermal protection and prevents the erosion and corrosion of metal due to high temperatures.
Bond coat- The oxidation of the metal, the thermal mismatch between the metal substrate and the TBC outer layer, interdiffusion between metals, and layers can be prevented by the bond coat. One example of a bond coat compound is low sulfur platinum aluminide.
Thermally grown oxide layer- To prevent oxidation and corrosion, a thermally grown oxide layer comes between the bonded coat and outer layer (topcoat). The adhesion of this layer to the metal surface is essential to avoid premature failures of the TBC system. This layer should be stress-free and stable.
Topcoat- This is the outer layer that comes in direct contact with the ambient. It is used to maximize the thermal drop across the thickness of the TBC. Yttria stabilized zirconia (YSZ) layer is a common topcoat used, due to its low thermal conductivity.
When selecting the TBC layers, there are certain material characteristics that need to be evaluated. Once these characteristics are satisfied, we can say the TBC is good enough to block heat flow.
Characteristics of TBC
High melting point- The melting point of the TBC should be high in order to operate the system at elevated temperatures without getting melted.
Low thermal conductivity- Low thermal conductivity of the coating is preferred in order to allow thermal drop across the TBC.
Low density- The TBC should be of lightweight and low density to reduce the load on the metal where it is coated.
High thermal shock resistance- The materials used in TBC possess high thermal resistance to prevent the easy conduction of heat.
Free from corrosion, mechanical erosion, and oxidation- The coating materials should not corrode, erode, or get oxidized easily. This will affect the performance of the TBC.
High surface emissivity- The high surface emissivity property of the TBC allows the heat incident of it to reflect away.
High coefficient of thermal expansion- The cracking of layers is a serious issue in TBC and this problem can be solved by using coating materials having a higher coefficient of thermal expansion than the metal.
Methods of TBC Deposition
The methods of layer deposition are of vital importance when it comes to the properties of TBC. There are several methods of TBC deposition such as Air Plasma Spray (APS), Electron Beam Physical Vapour Deposition (EB PVD), High-Velocity Oxygen Fuel (HVOF), Electrostatic Spray Assisted Vapour Deposition (ESAVD), and Direct Vapour Deposition (DVD).
Air Plasma Spray (APS)
Plasma spray coatings are the most preferred method of TBC development, as it is capable of economically producing durable coating. In plasma sprayed coatings, the TBC ceramics are fed into a high-velocity, high-temperature arc plasma. The coating materials are melted and sprayed towards the metal surface. The tool used in plasma sprayed coating is a plasma torch. Several factors affect the reproducibility of the plasma sprayed coatings—arc power, arc gas flow rate, powder feed rate, material size distribution, and shape.
Electron Beam Physical Vapour Deposition (EB PVD)
The EB PVD process is usually used for depositing the ceramic thermal barrier coatings. A high energy electron beam is required in this process for heating the TBC ceramics. The TBC ceramic is vaporized and travels along the length of the metal substrate, where it condenses atom by atom. When deposited using the EB PVD process, the interface between the bond coat and ceramic thermal layer is smooth compared to the rough surface from the APS process. The growth of the dense ceramic layer exhibits a columnar structure, starting from a thin initial deposition region. The advantage of columnar growth is the reduction of the elastic modulus of the surface where it is coated. When coated in cylindrical substrates, EB PVD has shown an outstanding extension of lifetime.
If you are an engineer working in industries such as aviation, automobile, or power generation, you know high temperature affects the efficiency and performance of engineering systems. A TBC is one of the best and most economical approaches to preventing the degradation of metal-based structures from high temperatures. Next time your team is worried about the elevated operating temperatures of a gas turbine or generator, consider TBCs.