Thermal Barrier Coatings (TBC) are advanced material systems typically applied to metal surfaces that operate at elevated temperatures, such as gas turbines or aircraft engine parts, as a form of exhaust heat management. These coatings of 100 μm to 2 mm thick thermal insulating materials serve to insulate components from large and prolonged thermal loads and can withstand an appreciable temperature difference between the load-bearing alloys and the coating surface.[1] By doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending the life of the parts by reducing oxidation and thermal fatigue. Along with active film cooling, TBCs allow working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications. Due to the increasing demand for more efficient motors operating at higher temperatures with better durability/life and thinner coatings to reduce parasitic mass of rotating/moving components, there is significant motivation to develop new and advanced TBCs. The material requirements of TBCs are similar to those of heat shields, although in the latter application emissivity tends to be of greater importance.
Structure
An effective TBC must meet certain requirements to perform well in aggressive thermomechanical environments.[2] To cope with thermal expansion stresses during heating and cooling, adequate porosity is needed, as well as proper matching of thermal expansion coefficients to the surface of the metal that is coating the TBC. Phase stability is required to avoid significant volume changes (which occur during phase changes), which would cause the coating to crack or peel. In air breathing engines, oxidation resistance is necessary as well as decent mechanical properties for rotating/moving parts or contacting parts. Therefore, the general requirements for an effective TBC can be summarized as: 1) a high melting point. 2) no phase transformation between ambient temperature and operating temperature. 3) low thermal conductivity. 4) chemical inertness. 5) Similar thermal expansion match with metal substrate. 6) good adhesion to the support. 7) low sintering rate for a porous microstructure. These requirements severely limit the number of materials that can be used, and ceramic materials can generally satisfy the required properties.[3].
Thermal barrier coatings typically consist of four layers: the metal substrate, the metal bond layer, the thermal growth oxide (TGO), and the ceramic top layer. The top ceramic layer is typically composed of yttria-stabilized zirconia (YSZ), which has very low conductivity while remaining stable at nominal operating temperatures typically seen in TBC applications. This ceramic layer creates the largest thermal gradient of the TBC and keeps the lower layers at a lower temperature than the surface. However, above 1200 °C, YSZ undergoes unfavorable phase transformations, changing from t'-tetragonal to tetragonal to cubic to monoclinic. Such phase transformations lead to the formation of cracks within the top coating. Recent efforts to develop an alternative to the YSZ ceramic top layer have identified many new ceramics (e.g., rare earth zirconates) that exhibit superior performance at temperatures above 1200 °C, but with inferior fracture toughness compared to that of YSZ. Furthermore, such zirconates may have a high concentration of oxygen ion vacancies, which may facilitate oxygen transport and exacerbate TGO formation. With a sufficiently thick TGO, coating spalling can occur, which is a catastrophic failure mode for TBCs. The use of such coatings would require additional coatings that are more resistant to oxidation, such as alumina or mullite.[4].
Delamination of coatings
Introduction
Thermal Barrier Coatings (TBC) are advanced material systems typically applied to metal surfaces that operate at elevated temperatures, such as gas turbines or aircraft engine parts, as a form of exhaust heat management. These coatings of 100 μm to 2 mm thick thermal insulating materials serve to insulate components from large and prolonged thermal loads and can withstand an appreciable temperature difference between the load-bearing alloys and the coating surface.[1] By doing so, these coatings can allow for higher operating temperatures while limiting the thermal exposure of structural components, extending the life of the parts by reducing oxidation and thermal fatigue. Along with active film cooling, TBCs allow working fluid temperatures higher than the melting point of the metal airfoil in some turbine applications. Due to the increasing demand for more efficient motors operating at higher temperatures with better durability/life and thinner coatings to reduce parasitic mass of rotating/moving components, there is significant motivation to develop new and advanced TBCs. The material requirements of TBCs are similar to those of heat shields, although in the latter application emissivity tends to be of greater importance.
Structure
An effective TBC must meet certain requirements to perform well in aggressive thermomechanical environments.[2] To cope with thermal expansion stresses during heating and cooling, adequate porosity is needed, as well as proper matching of thermal expansion coefficients to the surface of the metal that is coating the TBC. Phase stability is required to avoid significant volume changes (which occur during phase changes), which would cause the coating to crack or peel. In air breathing engines, oxidation resistance is necessary as well as decent mechanical properties for rotating/moving parts or contacting parts. Therefore, the general requirements for an effective TBC can be summarized as: 1) a high melting point. 2) no phase transformation between ambient temperature and operating temperature. 3) low thermal conductivity. 4) chemical inertness. 5) Similar thermal expansion match with metal substrate. 6) good adhesion to the support. 7) low sintering rate for a porous microstructure. These requirements severely limit the number of materials that can be used, and ceramic materials can generally satisfy the required properties.[3].
The bonding layer is an oxidation-resistant metallic layer that is deposited directly on the metal substrate. It is typically 75-150 μm thick and made from a NiCrAlY or NiCoCrAlY alloy, although other adhesive layers made from Ni and Pt aluminides also exist. The primary purpose of the adhesive layer is to protect the metal substrate from oxidation and corrosion, particularly from oxygen and corrosive elements passing through the porous ceramic top layer.
At the peak operating conditions encountered in gas turbine engines with temperatures above 700°C, oxidation of the bond layer leads to the formation of a thermal growth oxide (TGO) layer. TGO layer formation is inevitable for many high-temperature applications, so thermal barrier coatings are often designed so that the TGO layer grows slowly and uniformly. Such TGO will have a structure that has low diffusivity for oxygen, so that further growth is controlled by metal diffusion from the bond layer rather than oxygen diffusion from the top layer.[5].
The TBC can also be modified locally at the interface between the bond layer and the thermally grown oxide to act as a thermographic phosphor, allowing remote temperature measurement.
Failure modes
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].
Types[3]
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.
Uses
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].
Application procedures
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.
References
[1] ↑ F.Yu and T.D.Bennett (2005). «A nondestructive technique for determining thermal properties of thermal barrier coatings». J. Appl. Phys. 97: 013520. doi:10.1063/1.1826217.: https://dx.doi.org/10.1063%2F1.1826217
[3] ↑ a b Cao , Vassen R., Stoever D. (2004). «Ceramic Materials for Thermal Barrier Coatings». Journal of the European Ceramic Society 24 (1): 1-10. doi:10.1016/s0955-2219(03)00129-8.: https://dx.doi.org/10.1016%2Fs0955-2219%2803%2900129-8
[4] ↑ Cao X.Q., Vassen R., Stoever D. (2004). «Ceramic Materials for Thermal Barrier Coatings». Journal of the European Ceramic Society 24 (1): 1-10. doi:10.1016/s0955-2219(03)00129-8.: https://dx.doi.org/10.1016%2Fs0955-2219%2803%2900129-8
[5] ↑ a b Padture Nitin P.; Gell Maurice; Jordan Eric H. (2002). «Thermal Barrier Coatings for Gas-Turbine Engine Applications». Science 296 (5566): 280-284. PMID 11951028. doi:10.1126/science.1068609.: https://es.wikipedia.org//www.ncbi.nlm.nih.gov/pubmed/11951028
[8] ↑ Evans A. G.; Clarke D. R.; Levi C. G. (2008). «The influence of oxides on the performance of advanced gas turbines». Journal of the European Ceramic Society 28 (7): 1405-1419. doi:10.1016/j.jeurceramsoc.2007.12.023.: https://dx.doi.org/10.1016%2Fj.jeurceramsoc.2007.12.023
[9] ↑ Padture N. P.; Gell M.; Jordan E. H. (2002). «Thermal Barrier Coatings for Gas-Turbine Engine Applications». Science 296 (5566): 280-284. PMID 11951028. doi:10.1126/science.1068609.: https://es.wikipedia.org//www.ncbi.nlm.nih.gov/pubmed/11951028
[10] ↑ Jacobson Nathan S.; Opila Elizabeth J.; Myers Dwight L.; Copland Evan H. (2005). «Thermodynamics of gas phase species in the Si–O–H system». The Journal of Chemical Thermodynamics 37 (10): 1130-1137. doi:10.1016/j.jct.2005.02.001.: https://ntrs.nasa.gov/search.jsp?R=20020025451
[11] ↑ Zhao H.; Levi C. G.; Wadley H. N. G. (2014). «Molten silicate interactions with thermal barrier coatings». Surface and Coatings Technology 251: 74-86. doi:10.1016/j.surfcoat.2014.04.007.: https://dx.doi.org/10.1016%2Fj.surfcoat.2014.04.007
Thermal barrier coatings typically consist of four layers: the metal substrate, the metal bond layer, the thermal growth oxide (TGO), and the ceramic top layer. The top ceramic layer is typically composed of yttria-stabilized zirconia (YSZ), which has very low conductivity while remaining stable at nominal operating temperatures typically seen in TBC applications. This ceramic layer creates the largest thermal gradient of the TBC and keeps the lower layers at a lower temperature than the surface. However, above 1200 °C, YSZ undergoes unfavorable phase transformations, changing from t'-tetragonal to tetragonal to cubic to monoclinic. Such phase transformations lead to the formation of cracks within the top coating. Recent efforts to develop an alternative to the YSZ ceramic top layer have identified many new ceramics (e.g., rare earth zirconates) that exhibit superior performance at temperatures above 1200 °C, but with inferior fracture toughness compared to that of YSZ. Furthermore, such zirconates may have a high concentration of oxygen ion vacancies, which may facilitate oxygen transport and exacerbate TGO formation. With a sufficiently thick TGO, coating spalling can occur, which is a catastrophic failure mode for TBCs. The use of such coatings would require additional coatings that are more resistant to oxidation, such as alumina or mullite.[4].
The bonding layer is an oxidation-resistant metallic layer that is deposited directly on the metal substrate. It is typically 75-150 μm thick and made from a NiCrAlY or NiCoCrAlY alloy, although other adhesive layers made from Ni and Pt aluminides also exist. The primary purpose of the adhesive layer is to protect the metal substrate from oxidation and corrosion, particularly from oxygen and corrosive elements passing through the porous ceramic top layer.
At the peak operating conditions encountered in gas turbine engines with temperatures above 700°C, oxidation of the bond layer leads to the formation of a thermal growth oxide (TGO) layer. TGO layer formation is inevitable for many high-temperature applications, so thermal barrier coatings are often designed so that the TGO layer grows slowly and uniformly. Such TGO will have a structure that has low diffusivity for oxygen, so that further growth is controlled by metal diffusion from the bond layer rather than oxygen diffusion from the top layer.[5].
The TBC can also be modified locally at the interface between the bond layer and the thermally grown oxide to act as a thermographic phosphor, allowing remote temperature measurement.
Failure modes
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].
Types[3]
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.
Uses
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].
Application procedures
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.
References
[1] ↑ F.Yu and T.D.Bennett (2005). «A nondestructive technique for determining thermal properties of thermal barrier coatings». J. Appl. Phys. 97: 013520. doi:10.1063/1.1826217.: https://dx.doi.org/10.1063%2F1.1826217
[3] ↑ a b Cao , Vassen R., Stoever D. (2004). «Ceramic Materials for Thermal Barrier Coatings». Journal of the European Ceramic Society 24 (1): 1-10. doi:10.1016/s0955-2219(03)00129-8.: https://dx.doi.org/10.1016%2Fs0955-2219%2803%2900129-8
[4] ↑ Cao X.Q., Vassen R., Stoever D. (2004). «Ceramic Materials for Thermal Barrier Coatings». Journal of the European Ceramic Society 24 (1): 1-10. doi:10.1016/s0955-2219(03)00129-8.: https://dx.doi.org/10.1016%2Fs0955-2219%2803%2900129-8
[5] ↑ a b Padture Nitin P.; Gell Maurice; Jordan Eric H. (2002). «Thermal Barrier Coatings for Gas-Turbine Engine Applications». Science 296 (5566): 280-284. PMID 11951028. doi:10.1126/science.1068609.: https://es.wikipedia.org//www.ncbi.nlm.nih.gov/pubmed/11951028
[8] ↑ Evans A. G.; Clarke D. R.; Levi C. G. (2008). «The influence of oxides on the performance of advanced gas turbines». Journal of the European Ceramic Society 28 (7): 1405-1419. doi:10.1016/j.jeurceramsoc.2007.12.023.: https://dx.doi.org/10.1016%2Fj.jeurceramsoc.2007.12.023
[9] ↑ Padture N. P.; Gell M.; Jordan E. H. (2002). «Thermal Barrier Coatings for Gas-Turbine Engine Applications». Science 296 (5566): 280-284. PMID 11951028. doi:10.1126/science.1068609.: https://es.wikipedia.org//www.ncbi.nlm.nih.gov/pubmed/11951028
[10] ↑ Jacobson Nathan S.; Opila Elizabeth J.; Myers Dwight L.; Copland Evan H. (2005). «Thermodynamics of gas phase species in the Si–O–H system». The Journal of Chemical Thermodynamics 37 (10): 1130-1137. doi:10.1016/j.jct.2005.02.001.: https://ntrs.nasa.gov/search.jsp?R=20020025451
[11] ↑ Zhao H.; Levi C. G.; Wadley H. N. G. (2014). «Molten silicate interactions with thermal barrier coatings». Surface and Coatings Technology 251: 74-86. doi:10.1016/j.surfcoat.2014.04.007.: https://dx.doi.org/10.1016%2Fj.surfcoat.2014.04.007