TBCs fail through several degradation modes including mechanical wrinkling of the bond layer during cyclic thermal exposure (especially coatings in aircraft engines), accelerated oxidation, hot corrosion, or degradation of molten deposits. There are also problems with oxidation (areas of the TBC begin to peel off) of the TBC, which dramatically reduces the life of the metal component, leading to thermal fatigue.

A key characteristic of all TBC components is the need for well-matched thermal expansion coefficients between all layers. Thermal barrier coatings expand and contract at different rates as the environment heats and cools, so if the materials in the different layers have poorly adjusted coefficients of thermal expansion, deformation is introduced that can lead to cracking and ultimately failure of the coating.

Cracking in the thermal grown oxide (TGO) layer between the top layer and the bond layer is the most common failure mode for gas turbine blade coatings. The growth of TGO produces a stress associated with volume expansion that persists at all temperatures. When the system cools, even more mismatch is introduced from the mismatch in thermal expansion coefficients. The result is very high stresses (2-6 GPa) that occur at low temperature and can lead to cracking and ultimately spalling of the barrier coating. The formation of TGO also results in the depletion of Al in the bond layer. This can lead to the formation of undesirable phases that contribute to mismatch stresses. All of these processes are accelerated by the thermal cycling that many thermal barrier coatings undergo in practice.[5]​.

YSZ

YSZ is the most studied and used TBC because it provides excellent performance in applications such as diesel engines and gas turbines. In addition, it was one of the few refractory oxides that could be deposited as thick films using the then known technology of plasma spraying. In terms of properties, it has low thermal conductivity, high coefficient of thermal expansion and low resistance to thermal shock. However, it has a fairly low operating limit of 1200 °C due to phase instability and can corrode due to its oxygen transparency.

Mullita

Mullite is a compound of alumina and silica, with the formula 3Al2O3-2SiO2. It has a low density, along with good mechanical properties, high thermal stability, low thermal conductivity and is resistant to corrosion and oxidation. However, it suffers from crystallization and volume contraction above 800 °C, leading to cracking and delamination. Therefore, this material is suitable as a zirconia alternative for applications such as diesel engines, where surface temperatures are relatively low and temperature variations across the coating can be large.

Alumina

Only Al2O3 in the α phase is stable among aluminum oxides. With high hardness and chemical inertness, but high thermal conductivity and low coefficient of thermal expansion, alumina is often used as an addition to an existing TBC coating. By incorporating alumina into YSZ TBC, oxidation and corrosion resistance as well as hardness and bond strength can be improved without significant changes in elastic modulus or toughness. One challenge with alumina is the application of the coating by plasma spraying, which tends to create a variety of unstable phases, such as γ-alumina. When these phases finally transform into the stable α phase through thermal cycling, a significant volume change of ~15% (γ to α) follows, which can lead to the formation of microcracks in the coating.

CeO2 + YSZ

CeO2 (Ceria) has a higher thermal expansion coefficient and lower thermal conductivity than YSZ. Adding ceria to a YSZ coating can significantly improve TBC performance, especially in thermal shock resistance. This is most likely due to lower bond layer stress due to better insulation and better coefficient of net thermal expansion. Some negative effects of the addition of ceria include decreased hardness and accelerated sintering rate of the coating (less porous).

rare earth zirconates

LaZrO, also known as LZ, is an example of a rare earth zirconate that shows potential for use as a TBC. This material is phase stable up to its melting point and can largely tolerate vacancies in any of its sublattices. Together with the ability to replace the site with other elements, this means that the thermal properties can potentially be tailored. Although it has very low thermal conductivity compared to YSZ, it also has a low thermal expansion coefficient and low toughness.

rare earth oxides

Mixture of rare earth oxides is available, inexpensive, and may hold promise as effective TBCs. Rare earth oxide coatings (e.g., La2O3, Nb2O5, Pr2O3, CeO2 as main phases) have lower thermal conductivity and higher thermal expansion coefficients compared to YSZ. The main challenge to overcome is the polymorphic nature of most rare earth oxides at elevated temperatures, as phase instability tends to negatively impact thermal shock resistance.

Metal and glass composites

A powdered mixture of metal and normal glass can be sprayed with vacuum plasma, with a suitable composition resulting in a TBC comparable to YSZ. Additionally, metal-glass composites have superior bond layer adhesion, higher thermal expansion coefficients, and no open porosity, which prevents bond layer oxidation.

Automotive

Ceramic thermal barrier coatings are increasingly common in automotive applications. They are specifically designed to reduce heat loss from the exhaust system components "Exhaust System (Engine)") of the engine, including the exhaust manifolds, turbocharger housings, exhaust manifolds, downpipes and tailpipes. This process is also known as "exhaust heat management." When used under the hood, these have the positive effect of reducing engine bay temperatures and therefore reducing intake air temperatures.

Although most ceramic coatings are applied to metal parts directly related to the engine exhaust system, technological advances now allow thermal barrier coatings to be applied by plasma spraying onto composite materials. It is now common to find ceramic-coated components in modern engines and in high-performance components in racing series such as Formula 1. In addition to providing thermal protection, these coatings are also used to prevent physical degradation of the composite material due to friction. This is possible because the ceramic material adheres to the composite (rather than simply adhering to the surface with paint), thus forming a durable coating that does not chip or peel easily.

Although thermal barrier coatings have been applied to the interior of exhaust system components, problems have been encountered due to the difficulty of preparing the internal surface prior to coating.

Aviation

Interest in increasing the efficiency of gas turbine engines for aviation applications has driven research into higher combustion temperatures. Turbine efficiency is strongly correlated with combustion temperature. Combustion at higher temperatures improves the thermodynamic efficiency of the machine, giving a more favorable ratio of work generated relative to waste heat.[6] Thermal barrier coatings are commonly used to protect nickel-based superalloys from both melting and thermal cycling in aviation turbines. Combined with cold air flow, TBCs increase the allowable gas temperature above the melting point of the superalloy.[7]​.

To avoid the difficulties associated with the melting point of superalloys, many researchers are investigating ceramic matrix composites (CMCs) as high-temperature alternatives. Generally, these are made of fiber-reinforced SiC. Rotating parts are especially good candidates for material change due to the enormous fatigue they endure. CMCs not only have better thermal properties, but are also lighter, meaning less fuel would be needed to produce the same thrust for lighter aircraft.[8]​ However, the material change is not without consequences. At high temperatures, these CMCs are reactive with water and form gaseous silicon hydroxide compounds that corrode the CMC.

SiOH + HO = SiO(OH).

SiOH + 2HO = Si(OH).

2SiOH + 3HO = SiO(OH)[9]​.

Thermodynamic data from these reactions have been determined experimentally for many years to determine that Si(OH) is generally the dominant vapor species.[10] Even more advanced environmental barrier coatings are required to protect these CMCs from water vapor and other environmental degraders. For example, as the gas temperature increases towards 1400K-1500K, the sand particles begin to melt and react with the coatings. Molten sand is generally a mixture of calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide (commonly known as CMAS). Many research groups are investigating the harmful effects of CMAS on turbine coatings and how to prevent damage. CMAS is a major barrier to increasing the combustion temperature of gas turbine engines and will need to be resolved before the turbines experience a large increase in efficiency due to increased temperature.[11]​.

In industry, thermal barrier coatings are applied in several ways:.

Furthermore, the development of advanced coatings and processing methods is a field of active research. One such example is the solution precursor plasma spray process, which has been used to create TBCs with some of the lowest thermal conductivities reported without sacrificing thermal cyclic durability.

TBCs fail through several degradation modes including mechanical wrinkling of the bond layer during cyclic thermal exposure (especially coatings in aircraft engines), accelerated oxidation, hot corrosion, or degradation of molten deposits. There are also problems with oxidation (areas of the TBC begin to peel off) of the TBC, which dramatically reduces the life of the metal component, leading to thermal fatigue.

A key characteristic of all TBC components is the need for well-matched thermal expansion coefficients between all layers. Thermal barrier coatings expand and contract at different rates as the environment heats and cools, so if the materials in the different layers have poorly adjusted coefficients of thermal expansion, deformation is introduced that can lead to cracking and ultimately failure of the coating.

Cracking in the thermal grown oxide (TGO) layer between the top layer and the bond layer is the most common failure mode for gas turbine blade coatings. The growth of TGO produces a stress associated with volume expansion that persists at all temperatures. When the system cools, even more mismatch is introduced from the mismatch in thermal expansion coefficients. The result is very high stresses (2-6 GPa) that occur at low temperature and can lead to cracking and ultimately spalling of the barrier coating. The formation of TGO also results in the depletion of Al in the bond layer. This can lead to the formation of undesirable phases that contribute to mismatch stresses. All of these processes are accelerated by the thermal cycling that many thermal barrier coatings undergo in practice.[5]​.

YSZ

YSZ is the most studied and used TBC because it provides excellent performance in applications such as diesel engines and gas turbines. In addition, it was one of the few refractory oxides that could be deposited as thick films using the then known technology of plasma spraying. In terms of properties, it has low thermal conductivity, high coefficient of thermal expansion and low resistance to thermal shock. However, it has a fairly low operating limit of 1200 °C due to phase instability and can corrode due to its oxygen transparency.

Mullita

Mullite is a compound of alumina and silica, with the formula 3Al2O3-2SiO2. It has a low density, along with good mechanical properties, high thermal stability, low thermal conductivity and is resistant to corrosion and oxidation. However, it suffers from crystallization and volume contraction above 800 °C, leading to cracking and delamination. Therefore, this material is suitable as a zirconia alternative for applications such as diesel engines, where surface temperatures are relatively low and temperature variations across the coating can be large.

Alumina

Only Al2O3 in the α phase is stable among aluminum oxides. With high hardness and chemical inertness, but high thermal conductivity and low coefficient of thermal expansion, alumina is often used as an addition to an existing TBC coating. By incorporating alumina into YSZ TBC, oxidation and corrosion resistance as well as hardness and bond strength can be improved without significant changes in elastic modulus or toughness. One challenge with alumina is the application of the coating by plasma spraying, which tends to create a variety of unstable phases, such as γ-alumina. When these phases finally transform into the stable α phase through thermal cycling, a significant volume change of ~15% (γ to α) follows, which can lead to the formation of microcracks in the coating.

CeO2 + YSZ

CeO2 (Ceria) has a higher thermal expansion coefficient and lower thermal conductivity than YSZ. Adding ceria to a YSZ coating can significantly improve TBC performance, especially in thermal shock resistance. This is most likely due to lower bond layer stress due to better insulation and better coefficient of net thermal expansion. Some negative effects of the addition of ceria include decreased hardness and accelerated sintering rate of the coating (less porous).

rare earth zirconates

LaZrO, also known as LZ, is an example of a rare earth zirconate that shows potential for use as a TBC. This material is phase stable up to its melting point and can largely tolerate vacancies in any of its sublattices. Together with the ability to replace the site with other elements, this means that the thermal properties can potentially be tailored. Although it has very low thermal conductivity compared to YSZ, it also has a low thermal expansion coefficient and low toughness.

rare earth oxides

Mixture of rare earth oxides is available, inexpensive, and may hold promise as effective TBCs. Rare earth oxide coatings (e.g., La2O3, Nb2O5, Pr2O3, CeO2 as main phases) have lower thermal conductivity and higher thermal expansion coefficients compared to YSZ. The main challenge to overcome is the polymorphic nature of most rare earth oxides at elevated temperatures, as phase instability tends to negatively impact thermal shock resistance.

Metal and glass composites

A powdered mixture of metal and normal glass can be sprayed with vacuum plasma, with a suitable composition resulting in a TBC comparable to YSZ. Additionally, metal-glass composites have superior bond layer adhesion, higher thermal expansion coefficients, and no open porosity, which prevents bond layer oxidation.

Automotive

Ceramic thermal barrier coatings are increasingly common in automotive applications. They are specifically designed to reduce heat loss from the exhaust system components "Exhaust System (Engine)") of the engine, including the exhaust manifolds, turbocharger housings, exhaust manifolds, downpipes and tailpipes. This process is also known as "exhaust heat management." When used under the hood, these have the positive effect of reducing engine bay temperatures and therefore reducing intake air temperatures.

Although most ceramic coatings are applied to metal parts directly related to the engine exhaust system, technological advances now allow thermal barrier coatings to be applied by plasma spraying onto composite materials. It is now common to find ceramic-coated components in modern engines and in high-performance components in racing series such as Formula 1. In addition to providing thermal protection, these coatings are also used to prevent physical degradation of the composite material due to friction. This is possible because the ceramic material adheres to the composite (rather than simply adhering to the surface with paint), thus forming a durable coating that does not chip or peel easily.

Although thermal barrier coatings have been applied to the interior of exhaust system components, problems have been encountered due to the difficulty of preparing the internal surface prior to coating.

Aviation

Interest in increasing the efficiency of gas turbine engines for aviation applications has driven research into higher combustion temperatures. Turbine efficiency is strongly correlated with combustion temperature. Combustion at higher temperatures improves the thermodynamic efficiency of the machine, giving a more favorable ratio of work generated relative to waste heat.[6] Thermal barrier coatings are commonly used to protect nickel-based superalloys from both melting and thermal cycling in aviation turbines. Combined with cold air flow, TBCs increase the allowable gas temperature above the melting point of the superalloy.[7]​.

To avoid the difficulties associated with the melting point of superalloys, many researchers are investigating ceramic matrix composites (CMCs) as high-temperature alternatives. Generally, these are made of fiber-reinforced SiC. Rotating parts are especially good candidates for material change due to the enormous fatigue they endure. CMCs not only have better thermal properties, but are also lighter, meaning less fuel would be needed to produce the same thrust for lighter aircraft.[8]​ However, the material change is not without consequences. At high temperatures, these CMCs are reactive with water and form gaseous silicon hydroxide compounds that corrode the CMC.

SiOH + HO = SiO(OH).

SiOH + 2HO = Si(OH).

2SiOH + 3HO = SiO(OH)[9]​.

Thermodynamic data from these reactions have been determined experimentally for many years to determine that Si(OH) is generally the dominant vapor species.[10] Even more advanced environmental barrier coatings are required to protect these CMCs from water vapor and other environmental degraders. For example, as the gas temperature increases towards 1400K-1500K, the sand particles begin to melt and react with the coatings. Molten sand is generally a mixture of calcium oxide, magnesium oxide, aluminum oxide, and silicon oxide (commonly known as CMAS). Many research groups are investigating the harmful effects of CMAS on turbine coatings and how to prevent damage. CMAS is a major barrier to increasing the combustion temperature of gas turbine engines and will need to be resolved before the turbines experience a large increase in efficiency due to increased temperature.[11]​.

In industry, thermal barrier coatings are applied in several ways:.

Furthermore, the development of advanced coatings and processing methods is a field of active research. One such example is the solution precursor plasma spray process, which has been used to create TBCs with some of the lowest thermal conductivities reported without sacrificing thermal cyclic durability.