1. Introduction to Thermal Barrier Coatings

Thermal barrier coatings, also known as thermal or heat-insulating coatings (Thermal Barrier Coatings, abbreviated as TBC or TBCs in English), are coating systems designed to provide effective thermal insulation, oxidation resistance, and corrosion resistance for components. They create a significant temperature drop between the high-temperature combustion gases and the base metal of the component, thereby extending the service life of hot-end components, reducing cooling requirements, and improving the thermal efficiency of the engine. The basic principle behind these coatings is to spray a coating with extremely low thermal conductivity or thermal diffusivity onto the surface of the metallic substrate, ensuring that the coating can withstand substantial temperature gradients when operating in high-temperature thermal environments.
Research on thermal barrier coatings (TBCs) began in the 1940s. Starting in the late 1960s, TBCs were first applied to the combustion chambers of JT8D engines and later to JT9D engines. Ground durability tests conducted on the guide vanes and first- and second-stage turbine blades of the JT9D engine showed that the first-stage blades equipped with TBCs remained in excellent condition after 2,778 cycles, whereas blades without TBCs exhibited significant damage to their blade leading edges after only 1,500 cycles. The U.S. company GE has adopted an improved plasma-sprayed TBC, enabling the overall service life of combustion chambers to exceed 30,000 hours.
TBCs typically consist of a metallic bond coat and a ceramic top coat. The primary function of the metallic bond coat is to firmly bond the ceramic top coat to the substrate metal. The ceramic top coat, on the other hand, primarily serves as an insulator and provides corrosion resistance. It is required to have low vapor pressure, low thermal conductivity, low thermal emissivity, high thermal emissivity, as well as excellent resistance to thermal fatigue or thermal shock.
The calculation results show that by using a zirconia thermal barrier coating with a thickness of 0.25 mm, the temperature of the substrate metal can be reduced by approximately 170℃. This reduction is greater than the cumulative increase in the temperature resistance capability of blade alloys achieved over the 20-year period from 1965 to 1985 thanks to continuous human efforts.
The application of TBC has already achieved remarkably significant results. Not only has it reduced manufacturing costs and specific fuel consumption, lowered the demand for cooling air, but it has also enhanced the durability of turbine blades. According to reports, applying a ceramic thermal barrier coating with a thickness of 0.25 mm to the first-stage turbine blades of an aviation gas-turbine engine can reduce the cooling-air requirement by 6%, improve specific fuel consumption by 13%, and extend blade life by a factor of four. As a result, TBC technology has been widely adopted in numerous industrial sectors to enhance thermal efficiency—for instance, in various gas turbines and internal-combustion engines. In the United States, many aircraft engines and nearly all land-based and marine gas turbines—including hot-end components such as flame tubes, swirlers, afterburners, shroud plates, fuel nozzles, exhaust ducts, igniters, combustion-chamber liners, flame stabilizers, and turbine blades—have already incorporated TBC technology. Each year, approximately several hundred tons of zirconia material are used for TBC applications, and the scope of these applications continues to expand. According to research conducted by the Gorham Advanced Materials Institute in the U.S., in the future, the proportion of TBC applications in diesel engines will surpass that in the aerospace industry. Moreover, the use of TBC in automobiles and motorcycles is also steadily increasing. In Sweden, for example, a single branch of Volvo Airlines consumed nearly 10 tons of zirconia in 1997 alone—a doubling of its consumption compared to 1995.
With advances in science and technology, numerous fields—including aerospace, aviation, gas-fired power generation, chemical engineering, and metallurgy—have spurred the research and development of thermal barrier coatings (TBCs). Today, TBCs are widely used across a broad range of applications. In blast furnaces, tuyeres and slag outlets must withstand the erosive action of high-speed pulverized coal and the corrosive attack of molten iron at temperatures ranging from 1,100 to 1,450°C. By applying TBCs as heat-resistant protective coatings, the service life of these components can be significantly extended. Moreover, novel atomizing metal nozzles coated with TBCs exhibit outstanding corrosion resistance and thermal shock resistance, boasting long operational lifetimes and playing a crucial role in ensuring the quality of ultrafine powders. In the automotive industry, valve seats equipped with TBCs in engine intake and exhaust ports can reduce component wear and tear. TBCs are also extensively used for the tops and edges of piston cylinders made from lightweight aluminum alloy substrates. Some experts predict that, over the next decade, TBCs will find even broader applications across an ever-expanding array of industries.
Thermal barrier coatings undoubtedly hold great technological potential and promising development prospects. However, they also face certain challenges that require further improvement, primarily in the areas of controlling coating adhesion, studying coating failure mechanisms, and evaluating coating performance. Among these, controlling coating adhesion is the most critical issue. Coating adhesion—also referred to as bonding strength or cohesive strength—is a key quality indicator that directly affects the performance of the coating during service. Coating spalling is the primary mode of failure for coated components and represents the major factor hindering the wider adoption of thermal barrier coatings in gas turbine engines. The main causes of coating spalling are twofold: first, oxidation of the bond coat; and second, the significant mismatch in thermal expansion coefficients between the substrate metal and the ceramic coating, which leads to pronounced strain mismatch. The development of thermal barrier coatings has thus been a continuous process of addressing and improving these two fundamental issues.
 

2. Thermal Barrier Coating Design
The design of thermal barrier coatings involves selecting coating materials, designing the coating structure, and choosing the appropriate spraying method.

(1) Ingredient selection.  
1. Bonding primer.

A typical bond coat material is the MCrAlX alloy, where M represents the fundamental constituent element of the bond coat—typically a member of the iron group or a high-melting-point metallic element, or a combination of such elements—for example, Ni, Co, Fe, Ni-Co, or Ni-Fe. X denotes an active metal—an element added to enhance bonding strength and improve the coating’s oxidation resistance. These active metals include relatively reactive elements such as Y, Hf, Sc, Ce, La, and Th; among them, Y is the most commonly used.
By applying an aluminizing process to prepare an aluminum-rich layer on the surface of the bond coat, the oxidation rate of the bond coat can be reduced, thereby extending the service life of the TBC. Adding Re and Ta to CoNiCrAlY can significantly improve both the oxidation resistance and mechanical properties of the bond coat.
2. Ceramic surface layer.

Currently, the ceramic top layer in TBC coatings is predominantly composed of either fully stabilized or partially stabilized zirconia ceramics. Since pure zirconia crystals exhibit different crystal structures depending on temperature, when the temperature exceeds 1,000°C, the monoclinic crystal structure transforms into the tetragonal phase, accompanied by a volume change of approximately 7%. During subsequent cooling, although the monoclinic crystal structure can be restored, the volume does not return to its original state—meaning that the volume undergoes an irreversible transformation before and after heating and cooling. This phase transition and associated volume change give rise to significant thermal stresses within the coating under thermal cycling conditions, leading to early cracking of the coating and even eventual spalling failure. Therefore, it is necessary to add stabilizers to the pure ZrO2 crystals.
After adding a stabilizer to pure ZrO2 crystals and subjecting them to sintering or melting treatment, a solid solution is formed, yielding cubic-stabilized ZrO2 with extremely low thermal expansion coefficients that remains stable over the entire temperature range below its melting point. However, at high temperatures, although the expansion and contraction of fully stabilized cubic ZrO2 can be modeled, its linear thermal expansion and contraction are both substantial, which is detrimental to enhancing its thermal-shock resistance. Therefore, in practice, partially stabilized zirconia—composed of a mixed structure of monoclinic and cubic crystals—is commonly employed. In this crystal structure, at high temperatures, the monoclinic phase undergoes a volume-reducing phase transition, while the cubic phase experiences volume expansion as the temperature rises. These two opposing changes mutually counteract each other, thereby giving partially stabilized ZrO2 a lower average thermal expansion coefficient than fully stabilized ZrO2 and endowing it with superior thermal-shock resistance.

    Stabilizers added to zirconia include calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (Y2O3), and cerium oxide (CeO). Among these, the addition levels of CaO stabilizer range from 5%, 6%, 8%, 10%, 15%, to 30%. As the CaO content increases, the hardness of the coating also rises. Coatings with a CaO content as high as 30% exhibit exceptionally high hardness and demonstrate excellent resistance to high-temperature particle erosion. However, CaO-stabilized ZrO2 coatings, when exposed for extended periods or cyclically to temperatures above 1093°C, tend to have CaO diffusing beyond the stabilized ZrO2 crystal structure, thereby limiting the service temperature of the coating. Such coatings can be used continuously at temperatures above 845°C but below 1093°C; beyond 1093°C, they can only be used for short durations. When MgO is used as a stabilizer, typically at concentrations ranging from 20% to 30%, ZrO2 maintains its crystal structure stability across various temperatures, especially during high-temperature thermal cycling. At temperatures below 1400°C, MgO-stabilized ZrO2 exhibits an equilibrium microstructure consisting of either the tetragonal or monoclinic phase plus MgO. During thermal cycling, MgO may precipitate out of the solid solution, leading to an increase in the coating's thermal conductivity and a reduction in its thermal insulation performance, thus restricting its broader application. In contrast, Y2O3-partially stabilized ZrO2, when used continuously at temperatures up to 1650°C, does not exhibit the same tendency as CaO to diffuse outward from the crystal structure. It demonstrates superior chemical and thermal stability compared to both CaO- and MgO-stabilized ZrO2, making it an outstanding thermal barrier coating material capable of operating at the highest temperatures. The addition levels of Y2O3 are typically 6–8%, 13%, and 20%. The first two levels correspond to partially stabilized ZrO2, while the third level represents fully stabilized ZrO2. For thermal barrier coatings, partially stabilized zirconia offers better resistance to thermal shock. Consequently, 6–8% Y2O3-partially stabilized zirconia has become the preferred material for the ceramic top layer in thermal barrier coatings.
In recent years, studies on certain stabilizers (PSZ) such as Y2O3, Nd2O3, and Sc2O3 have revealed that under rapid cooling conditions, the ZrO2 ceramic layer can contain either partially or fully “non-transformed” tetragonal phase t′. Although still metastable, this phase does not decompose into the equilibrium tetragonal and cubic phases even under high-temperature cycling conditions ranging from 1100 to 1200℃. In contrast, 6–8% Y2O3-ZrO2 (YSZ) coatings exhibit no decomposition of the t′ phase at temperatures between 1100 and 1200℃. In CeO-Y2O3-ZrO2 systems, the t′ phase demonstrates superior stability compared to 8% YSZ; however, these coatings perform less effectively when exposed to corrosive gases containing elements such as V and S. On the other hand, Sc2O3-Y2O3-ZrO2 (SYSZ) coatings exhibit higher t′ phase stability and enhanced resistance to hot salt corrosion even at elevated temperatures (1400℃).  

(2) Coating structure design.

Thermal barrier coating structures are primarily categorized into three types: two-layer structures, multi-layer structures, and gradient structures.
The double-layer structure consists of a ceramic top layer—typically a ZrO2-based ceramic—sprayed onto a high-temperature alloy substrate, and an underlying bond coat—often of the MCrAlY type. The ceramic top layer primarily serves to provide thermal insulation and oxidation resistance; while the bond coat mainly enhances the adhesion between the ceramic top layer and the substrate, improves the tolerance for mismatch in thermal expansion coefficients, and further boosts oxidation resistance. Due to its simple structure and ease of fabrication, the double-layer thermal barrier coating is currently widely used in practical applications.
The multi-layer structure is primarily designed to reduce thermal expansion mismatch between the ceramic top layer and the metallic bond coat by introducing an intermediate layer between them. Alternatively, to further enhance the oxidation resistance of the thermal barrier coating, a thin Al2O3 layer can be added between the ceramic top layer and the metallic bond coat. However, the addition of this Al2O3 layer provides only marginal improvement in thermal shock resistance, and the process is relatively complex, resulting in slightly poorer coating repeatability and reliability.
A gradient-structure thermal barrier coating refers to a coating in which the chemical composition, microstructural features, and mechanical properties gradually and continuously vary along the thickness direction—from the metallic bond coat to the ceramic top layer. This structural design enhances both the bonding strength between the coating and the substrate and the cohesive strength within the coating itself, delivering high-temperature performance that is ideal for coating applications. Moreover, such coatings exhibit superior thermal-shock resistance compared to conventional bilayer coatings. However, in practical fabrication, what is typically obtained is a multilayer stepped structure, and the preparation technology remains complex, placing this approach still in the laboratory-design and research stage.

(3) Selection of spraying method.

Due to the high melting point (2760℃) and low thermal conductivity (approximately 1.0–2.0 W/mK) of ZrO2 ceramic materials, among the thermal spraying processes described in Chapter 2, only arc spraying, cold gas dynamic spraying, high-velocity flame spraying, oxy-acetylene flame remelting, medium-frequency induction remelting, and plasma spray welding cannot be used for preparing ZrO2 ceramic coatings; all other thermal spraying processes, however, are suitable for this purpose.
However, as the performance requirements for coatings continue to increase, plasma spraying has become the primary method for fabricating thermal barrier coatings. In practical applications, though, either due to limitations in available conditions or in order to reduce costs while maintaining performance, various thermal spray processes are often employed. Based on whether a single piece of equipment or multiple pieces of equipment is used in the preparation of thermal barrier coatings, the coating fabrication processes can be categorized into two types: single-process fabrication and composite-process fabrication.
The single-process preparation method refers to a technique in which both the bond coat and the ZrO2 ceramic top coat of a thermal barrier coating are fabricated using the same spraying method. This includes processes such as atmospheric plasma spraying, low-pressure plasma spraying, vacuum plasma spraying, detonation spraying, and high-velocity plasma spraying.
The composite preparation process refers to a technique in which the bond coat of the thermal barrier coating and the ZrO2 ceramic top coat are prepared separately using different spraying methods. This includes: ① a vacuum-plus-atmospheric plasma composite spraying process, in which the bond coat of the thermal barrier coating is prepared by vacuum plasma spraying, while the ZrO2 ceramic top coat is prepared by atmospheric plasma spraying; ② a high-velocity flame-plus-atmospheric plasma composite spraying process, in which the bond coat of the thermal barrier coating is prepared by high-velocity flame spraying, and the ZrO2 top coat is prepared by atmospheric plasma spraying; ③ a high-velocity flame-plus-high-velocity plasma composite spraying process, in which the bond coat is prepared by high-velocity flame spraying, and the ZrO2 top coat is prepared by high-velocity plasma spraying, and so forth.

To address the issues of high porosity and cracks in plasma-sprayed thermal barrier coatings (TBCs), which lead to reduced oxidation resistance and shorter coating lifetimes, both domestically and internationally, extensive research has been conducted on laser-based TBC preparation methods, focusing on two distinct areas: laser surface remelting and laser cladding. There are two main laser-based TBC preparation techniques: the single-pass laser cladding method and the double-pass laser cladding method. The single-pass laser cladding method for TBC preparation is a relatively new approach, with research reports emerging only within the past decade. This method primarily involves two approaches: the pre-placement method and the powder-feeding method. In the pre-placement method, a partially stabilized YPSZ powder mixed with Ni-based composite powder is pre-placed onto the substrate, followed by CO2 laser cladding to form a layered composite coating. The surface consists of a dense ZrO2 ceramic layer, beneath which lies a Ni-based alloy transition layer. The upper part of the ZrO2 ceramic layer exhibits equiaxed crystals, while the middle and lower parts feature columnar crystals predominantly composed of the t′ phase. In the powder-feeding method, a feeding device delivers a mixture of partially stabilized YPSZ and alloy composite powder into the laser irradiation zone, where the laser melts and deposits the material onto the substrate, forming an automatically layered ceramic coating characterized by a columnar microstructure and largely composed of the t′ phase. The double-pass laser cladding method refers to a process in which a ZrO2 ceramic layer is first applied to the substrate surface via plasma spraying, followed by laser cladding treatment. This approach enables the production of a ceramic cladding layer that is smooth, continuous, dense, and free from defects such as cracks and pores—issues that cannot be fully addressed by the powder-feeding laser cladding method. The microstructure of the ceramic cladding layer produced by this method consists of columnar crystals whose growth direction is perpendicular to the substrate. The double-pass laser cladding method offers a viable pathway for fabricating high-performance, low-cost TBCs; however, it remains at the preliminary research stage. High-temperature performance testing is still lacking, and further in-depth studies are needed to elucidate the effects of cladding process parameters, the layered microstructure of the coating, its chemical composition, morphology, internal and external quality, and high-temperature performance on the coating’s service life.

In recent years, there has been growing interest in preparing thermal barrier coatings (TBCs) with high thermal-shock resistance using the electron-beam physical vapor deposition (EB-PVD) technique. Research on EB-PVD thermal barrier coatings began in the 1970s, and a breakthrough was achieved in the 1980s by Pratt & Whitney in the United States. Subsequently, this technology has also been successfully applied in countries such as Germany. EB-PVD thermal barrier coatings are formed by heating and vaporizing ceramic sources with a high-energy electron beam to produce ceramic vapor, which is then deposited onto the substrate atom by atom. The resulting coating microstructure consists of columnar grains oriented perpendicularly to the substrate surface, with metallurgical bonding between the columns and the substrate, ensuring excellent stability. In high-temperature environments, the columns can separate from each other, thereby relieving thermal stresses caused by differences in thermal expansion coefficients and significantly enhancing the coating's resistance to thermal fatigue. Studies have shown that an Al2O3 zone forms between the ZrO2 ceramic layer and the bond coat, and the presence of this zone helps improve the oxidation resistance of the thermal barrier coating. Moreover, thermal barrier coatings prepared by EB-PVD exhibit a smooth surface that faithfully reproduces the roughness of the original substrate, eliminating the need for further machining. This reduces gas flow resistance, extends the coating’s service life, and makes the process parameters easier to control compared to plasma spraying. However, the EB-PVD process also has several drawbacks, including relatively low thermal insulation capability of the coating, uncontrollable coating thickness, complex surface cleaning requirements, sophisticated and expensive equipment, relatively low deposition rates, and a cumbersome process flow—all of which urgently require further research and improvement.

Given the significant differences between TBCs prepared by plasma spraying and those prepared by EB-PVD, some researchers refer to TBCs fabricated by plasma spraying as first-generation TBCs, while TBCs prepared by EB-PVD are called second-generation TBCs.
3. Preparation of Thermal Barrier Coatings
The most mature and widely used process for preparing TBC is plasma spraying. In addition to its high production efficiency and relatively low cost, the coating’s excellent thermal insulation performance is also a prominent advantage. Consequently, both domestically and internationally, continuous improvements and developments are being carried out on this method. This article provides a relatively detailed discussion solely on the preparation of TBC via atmospheric plasma spraying.
During the plasma-spraying process for preparing thermal barrier coatings, numerous process parameters influence coating quality—some even claim that there are more than 100 such parameters. Therefore, when preparing thermal barrier coatings, it is essential to strictly control each stage and optimize the relevant process parameters. After repeated optimization of the process and thorough evaluation of coating performance, the final coating preparation process will be determined based on actual application tests. Once the process has been finalized, it must be rigorously adhered to. During implementation, attention should be paid to the following key stages.
(1) Strictly control the quality of spray powder.  

Selecting high-quality spray powders is the foundation for preparing high-performance, thermally stable thermal barrier coatings. Therefore, before carrying out the spraying process, it is essential to rigorously inspect the powder quality, covering the following aspects: ① powder composition; ② preparation method; ③ powder morphology; ④ flowability; ⑤ bulk density; ⑥ particle size; ⑦ particle size distribution; ⑧ batch-to-batch uniformity and stability. Once the powder supplier has been determined, among these eight aspects, only five—namely, ③ through ⑦—need to be strictly controlled. The following example illustrates the critical importance of spray powders to the quality of thermal barrier coatings.
Wigren et al. used exactly the same spraying parameters to compare the microstructures of ZrO2 powder coatings—both with identical specifications but differing in powder morphology—in terms of hollow-sphere agglomerated sintered powders. The results showed significant differences in the microstructures of the coatings. At 1100℃, the thermal conductivities of these two coatings were measured as 0.6 W/mK and 0.4 W/mK, respectively, clearly demonstrating the influence of the sprayed powder on the microstructure and performance of the coatings. Wigren et al. also conducted spraying studies using five different parameter sets for two types of agglomerated sintered powders, one type of hollow-sphere agglomerated powder, one type of sintered crushed powder, and two types of sol-gel powders. The results of thermal shock tests indicated that the number of thermal shock cycles each coating could withstand varied significantly.
(2) Strictly control spraying parameters. 。

The ZrO2 material used as the top coat in thermal barrier coatings features a high melting point and low thermal conductivity. When plasma spraying, it is essential to input a relatively high power level to ensure that the powder reaches a molten state. However, all the parameters involved in plasma spraying must be appropriately matched. The following section introduces several key process parameters.
1. The electrical parameters must match the equipment.

The spray gun is one of the core pieces of equipment used to carry out the plasma spraying process. Different models of spray guns have their own unique characteristics and requirements; therefore, when setting electrical parameters, it’s essential to meet the specifications of the equipment being used. Relatively speaking, the Plexus SG100 spray gun operates in a high-current, low-voltage mode, whereas the Meiko 7M/9M spray gun operates in a low-current, low-voltage mode.
High current and high voltage are required. When spraying ZrO2-based powders, the SG100 spray gun should be set to a current of 800–900 A and a voltage of around 40 V; whereas the 7M/9M spray gun should be set to a current of 600 A and a voltage of about 80 V. If the voltage falls outside the above-mentioned ranges, it can be adjusted by regulating the auxiliary gas flow. In this case, care must be taken to ensure that the auxiliary gas flow does not exceed a certain limit. If, after adjusting the auxiliary gas flow to its maximum setting, the desired voltage still cannot be achieved, the spraying operation should be immediately halted. Thoroughly inspect the spray gun configuration to check whether the nozzle and electrode have been damaged and whether there are any leaks in the gas delivery lines between the flowmeter and the spray gun.
2) Selection of plasma gases 。 

The gases used in plasma spraying fall into two categories: one is called the primary gas, which has relatively high pressure and flow rate; it mainly consists of argon (Ar) and nitrogen (N2). The other is called the secondary gas, which has lower pressure and flow rate; it mainly comprises hydrogen (H2) and helium (He). Among these four plasma gases, the primary gas N2 and the secondary gas H2 are diatomic molecules. At high temperatures, they first undergo dissociation to become monatomic, with dissociation energies of 9.76 eV and 4.477 eV, respectively. Subsequently, they ionize to form positive ions and free electrons. The ionization energies of the four atoms—hydrogen (H), nitrogen (N), argon (Ar), and helium (He)—are 13.595 eV, 14.54 eV, 15.755 eV, and 24.58 eV, respectively. Consequently, the plasma generated by the primary gas N2 has a higher enthalpy and faster heat transfer, which facilitates efficient heating and melting of the powder. On the other hand, the plasma generated by the primary gas Ar has a relatively lower enthalpy, but it produces a stable plasma arc that is easy to ignite and features a shorter arc flame, making it particularly suitable for spraying small parts and thin-walled components. The plasma generated by the secondary gas H2 has a higher enthalpy than that produced by helium (He).
When actually spraying ZrO2 thermal barrier coatings, the choice of primary and secondary gas combinations also depends on the type of spray gun used. The Plasmax SG100 spray gun recommends using argon (Ar) as the primary gas and helium (He) as the secondary gas for plasma generation; whereas the Meiko 7M/9M spray guns typically use nitrogen (N2) as the primary gas and hydrogen (H2) as the secondary gas for plasma generation—but it’s important to ensure compatibility with the N2 gas distribution ring.
In addition, the flow rate of the plasma gas is also a crucial process parameter. It directly affects the enthalpy and velocity of the plasma jet, which in turn influences the spraying efficiency, coating porosity, and coating adhesion strength. At a given power level, there exists an optimal gas flow rate. If the gas flow is too high, the ion concentration decreases, and the gas atoms or molecules absorb heat from the plasma jet, thereby cooling it down and reducing both its enthalpy and temperature. This results in insufficient powder melting, lower spraying efficiency, a loose coating microstructure, and increased porosity. Conversely, if the gas flow is too low, the plasma jet becomes weak and ineffective, causing both the temperature and velocity of the powder particles to drop, which can easily lead to damage of the nozzle and cathode.
Therefore, it is essential to optimize the characteristics of ZrO2 powder and determine the plasma gas flow rate that meets the coating performance requirements.
3) Powder feeding parameters 。 

The powder-feeding parameters mainly include the powder-feeding position (internal feeding, external feeding, distance from the nozzle axis), the configuration of the powder-feed nozzle (nozzle number), the powder-feeding angle (vertical, inward tilt, outward tilt), the carrier gas flow rate, and the powder-feeding rate. Once the spray powder has been selected, the two primary parameters to adjust are the carrier gas flow rate and the powder-feeding rate.
The carrier gas flow rate should be matched to the working gas flow rate, typically ranging from about 10% to 20% of the working gas flow rate. The adjustment principle is to ensure that the ZrO2 powder can be effectively delivered into the flame core.
The powder feed rate and the position at which the powder enters the arc are critical parameters that influence both the coating microstructure and spraying efficiency. Only when the spray powder is delivered to the flame core—the region where the flame temperature and velocity are highest—can it achieve optimal heating and maximum velocity. The powder feed rate must be appropriately matched to the input power level. If the feed rate is too high, the powder may not melt completely, leading to incomplete fusion and poor bonding between layers. Conversely, if the feed rate is too low, spraying efficiency will decline and the substrate may overheat.
4) Spray distance and spray angle.

The spray distance refers to the vertical distance from the nozzle outlet along the axis of the flame jet to the surface of the workpiece. If the spray distance is too small, the substrate temperature may rise excessively, leading to significant internal stresses between the substrate and the thermal barrier coating, thereby compromising the coating’s performance. On the other hand, if the spray distance is too large, both the temperature and velocity of the ZrO2 particles upon impact with the substrate will decrease, resulting in reduced coating adhesion, lower deposition efficiency, and an increased porosity. Provided that the substrate temperature permits, the spray distance can be appropriately reduced.
The DPV-2000 thermal spray online detector was used to measure the temperature and velocity of zirconia particles as a function of spraying distance. The temperature of ZrO2 particles gradually decreased with increasing spraying distance. Immediately after leaving the nozzle—at a distance of 60 mm—the average particle temperature reached nearly 3,000°C, exceeding the melting point of ZrO2 (approximately 2,700°C), indicating that the particles had undergone some degree of melting. As the spraying distance increased, the temperature of the ZrO2 particles steadily declined, dropping to around 2,700°C when the spraying distance reached 80 mm. The flight velocity of the ZrO2 particles exhibited a Gaussian distribution pattern as a function of spraying distance, with the peak occurring at approximately 80 mm from the nozzle. Therefore, considering both the temperature and velocity of the ZrO2 particles, an optimal spraying distance of 80 mm is appropriate under the selected spraying process conditions.
The spray angle refers to the angle between the axis of the flame jet and the surface being sprayed. During spraying, the spray angle should be maintained at 90° as much as possible. When the spray angle is less than 45°, a "shadowing effect" occurs, leading to an increase in coating porosity and making the coating more porous.
5) The relative moving speed between the spray gun and the workpiece.

Under certain powder-feeding rate conditions, the relative moving speed between the spray gun and the workpiece—commonly referred to as the gun-moving speed—determines the area swept by the spray gun per unit time or the thickness of each sprayed coating layer. Therefore, controlling the gun-moving speed is essentially equivalent to controlling the thickness of each coating layer. The thickness of each layer, in turn, affects the residual stresses in the coating; as the layer thickness decreases, the residual stresses can be significantly reduced. Moreover, the gun-moving speed also influences the substrate temperature. For ZrO2 thermal barrier coatings, when spraying the workpiece, it is generally advisable to keep the thickness of each pass of the spray gun below 25 μm.
(3) Control the substrate temperature 。

The substrate temperature is one of the critical process parameters in plasma spraying. Studies have shown that when ZrO2 coatings are deposited onto substrates made of different materials, there exist distinct critical transition temperatures. When the substrate temperature is below this critical transition temperature, individual ZrO2 particles, after expanding and cooling with the substrate, exhibit a splashing morphology, as illustrated in the figure. As the substrate temperature rises, the degree of splashing diminishes. Once the substrate temperature exceeds the critical transition temperature, individual ZrO2 particles assume a disc-like deformation pattern, as shown in the figure. The varying deformation states of individual ZrO2 particles significantly influence the microstructure of the coating.

Although raising the substrate temperature is beneficial for enhancing the bonding between ZrO2 particles, when using atmospheric plasma spraying, excessively high substrate temperatures not only exacerbate substrate oxidation but also increase thermal stresses within the coating, thereby increasing the likelihood of cracking and spalling. Typically, the substrate preheating temperature ranges from 95 to 120°C. However, it’s important to note that for magnesium substrates, due to magnesium’s rapid oxidation, preheating should be avoided prior to spraying; otherwise, it may compromise the bond strength between the coating and the substrate. For aluminum substrates, preheating is permissible only within the temperature range of 65 to 95°C. To prevent oxidation, preheating can be carried out from the back or side of the substrate, and it’s best to avoid directly preheating the spray surface. If indirect preheating is not feasible, preheating can be omitted altogether. Furthermore, in-die preheating is not recommended.
To control the substrate temperature during the spraying process, compressed air is typically used to cool the sprayed surface. This method is highly effective in maintaining both the substrate and coating performance. However, in certain situations where higher cooling capacity is required, liquid argon or liquid CO2 can be employed instead. This approach enables the substrate temperature to be maintained at around 50℃.
The color of a properly sprayed ZrO2 thermal barrier coating should be close to white or pale yellow. If the color turns dark yellow or orange, it indicates that the coating has been overheated. Conversely, if the coating appears light gray, it suggests that the workpiece temperature was too low during spraying, resulting in relatively poor thermal shock resistance of the coating.
Some black spots can be observed on the surface of the ZrO2 thermal barrier coating. These spots do not affect the performance of the coating, and energy-dispersive spectroscopy analysis indicates that the composition of these black spots remains ZrO2.
4. Post-processing of thermal barrier coatings
Failure analysis of thermal barrier coatings indicates that there are three primary reasons for the early delamination failure of these coatings. First, the low bonding strength between coating layers plays a crucial role, including the bonding strength between the substrate and the bond coat, between the bond coat and the ZrO2 top coat, and between the deformation particles within the ZrO2 top coat itself. Second, the relatively high porosity of the ZrO2 top coat is another contributing factor. Although these pores can to some extent help relieve thermal stresses, during service, oxygen can penetrate through these pores into the bond coat and even the substrate, causing oxidation. As a result, an Al2O3 layer with a thickness of 5–10 μm forms between the bond coat and the ZrO2 top coat. This oxidation process has a highly detrimental impact on the service life of the thermal barrier coating. Third, mismatch in thermal expansion coefficients between the bond coat and the ZrO2 top coat leads to thermal stresses. Under repeated heating and cooling cycles, cracks develop within the coating, eventually resulting in delamination failure. Currently, to further extend the service life of thermal barrier coatings, research has been conducted on post-treatment processes aimed at enhancing the bonding strength of the coatings and reducing their porosity. This paper presents the findings from the following three aspects:
(1) Vacuum heat treatment.

In thermal barrier coatings, the bond coat and the substrate primarily rely on mechanical interlocking for adhesion. The relatively low bonding strength between these two layers is one of the main factors contributing to the thermal-shock failure of thermal barrier coatings. Some researchers have investigated the effects of vacuum heat treatment on the bonding condition between the NiCoCrAlY bond coat and the TC4 titanium alloy substrate, as well as on the thermal-shock resistance of the coating. The vacuum heat treatment process employed was as follows: with a vacuum level of 0.1 Pa, the samples were heated to 900℃ using a furnace-heating method, held at this temperature for 8 hours, and then cooled down to room temperature together with the furnace. The results showed that after vacuum heat treatment, metallurgical and chemical reactions occurred between the NiCoCrAlY bond coat and the TC4 titanium alloy substrate, forming an interfacial reaction layer containing compounds such as NiTi, NiTi2, and TiAl3. As a result, the thermal-shock resistance of the coating was significantly improved. Therefore, vacuum heat treatment represents an effective approach for enhancing the bonding between the thermal barrier coating and the substrate and improving the thermal-shock resistance of the thermal barrier coating.
(2) Hot isostatic pressing.

Thermal-Hot Isostatic Pressing (HIP) treatment of coatings involves placing the sprayed thermal-barrier coating samples into a HIP apparatus, where they are heated to a relatively high temperature—typically around 1200°C—in an Ar atmosphere. Subsequently, the samples are subjected to a high pressure—generally ranging from 50 to 200 MPa—and held at this pressure for several hours before being cooled down to room temperature. Research results indicate that after HIP treatment, both the bond coat and the ZrO2 layer become denser, and the number of cracks in the ZrO2 coating is reduced. In the ZrO2 coating, the tetragonal phase with higher Y2O3 content, which had not undergone phase transformation, experiences a certain degree of recrystallization, leading to the formation of a fine, equiaxed grain structure. Due to the sintering effect, a certain degree of metallurgical bonding develops between layers or between deformed particles, thereby enhancing the bonding strength of the coating and altering the fracture mode from interlayer cracking to through-layer cracking. A thin Al2O3 layer forms at the interface between the bond coat and the ZrO2 surface layer, which improves the oxidation resistance of the thermal-barrier coating. Moreover, at the interface between the bond coat and the substrate, significant mutual diffusion occurs between Ni in the bond coat and Fe in the substrate. All these structural changes induced by HIP treatment contribute positively to the enhancement of the thermal-barrier coating’s performance.
(3) Laser remelting treatment.

Laser remelting treatment refers to a post-processing method in which a laser beam with a specific energy density is directed onto the surface of a thermally sprayed thermal barrier coating sample, causing changes in the microstructure of the ZrO2 surface layer and thereby altering the performance of the thermal barrier coating. After laser remelting treatment, the primary region where the microstructural changes occur is confined to a depth of approximately 70 μm beneath the ZrO2 surface layer, with little impact on the microstructure of the deeper ZrO2 coating or on the interfacial bond between the bonding substrate and the ZrO2 layer. Following laser remelting, the ZrO2 surface layer develops a columnar dendritic microstructure, which differs significantly from the lamellar structure formed by plasma spraying and resembles the microstructure of electron-beam physical vapor deposition (EB-PVD)-grown thermal barrier coatings. This microstructure exhibits exceptional resistance to thermal shock. Studies have shown that the thermal shock resistance of laser-remelted thermal barrier coatings is four times greater than that of plasma-sprayed thermal barrier coatings. As mentioned earlier, currently, a few countries—including the United States and Japan—are actively pursuing research into hybrid plasma-laser spraying processes aimed at further enhancing the overall performance of thermal barrier coatings.
(4) Sealing treatment.

When operating in the combustion chamber, thermal barrier coatings are subjected to thermal cycling due to temperature fluctuations. Since ZrO2 ceramic coatings contain a certain number of pores and microcracks that are closely interconnected, corrosive media can penetrate through these pores and cracks into the interface between the bond coat and the ceramic layer, and even reach the substrate surface, thereby causing oxidation and corrosion. This is the primary cause of delamination failure in thermal barrier coatings. To mitigate oxidation and corrosion, densifying and sealing the coating surface represents an important approach for extending the service life of thermal barrier coatings. To this end, several types of thermally barrier coatings treated with sealing processes have been studied, along with their thermal shock resistance. The relationship between coating failure modes and delamination mechanisms has also been explored.
Three different sealing agents—silicone resin, NiCrBSi metal coating, and water glass—were used separately to seal the surfaces of thermal barrier coating specimens. Thermal shock tests were then conducted: The specimens were heated to 900℃, held at that temperature for 5 minutes, removed from the furnace, and immediately subjected to forced air cooling for 20 minutes. This cycle was repeated until the coating began to fail and peel off; the number of thermal cycles at which failure occurred was recorded. The results showed that after sealing, the thermal shock life of the thermal barrier coatings was significantly improved. Among the three sealing agents, silicone resin exhibited the best performance, followed by the NiCrBSi coating, while the effect of water glass was less pronounced.

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1. Introduction to Thermal Barrier Coatings

Thermal barrier coatings, also known as thermal or heat-insulating coatings (Thermal Barrier Coatings, abbreviated as TBC or TBCs in English), are coating systems designed to provide effective thermal insulation, oxidation resistance, and corrosion resistance for components. They create a significant temperature drop between the high-temperature combustion gases and the base metal of the component, thereby extending the service life of hot-end components, reducing cooling requirements, and improving the thermal efficiency of the engine. The basic principle behind these coatings is to spray a coating with extremely low thermal conductivity or thermal diffusivity onto the surface of the metallic substrate, ensuring that the coating can withstand substantial temperature gradients when operating in high-temperature thermal environments.
Research on thermal barrier coatings (TBCs) began in the 1940s. Starting in the late 1960s, TBCs were first applied to the combustion chambers of JT8D engines and later to JT9D engines. Ground durability tests conducted on the guide vanes and first- and second-stage turbine blades of the JT9D engine showed that the first-stage blades equipped with TBCs remained in excellent condition after 2,778 cycles, whereas blades without TBCs exhibited significant damage to their blade leading edges after only 1,500 cycles. The U.S. company GE has adopted an improved plasma-sprayed TBC, enabling the overall service life of combustion chambers to exceed 30,000 hours.
TBCs typically consist of a metallic bond coat and a ceramic top coat. The primary function of the metallic bond coat is to firmly bond the ceramic top coat to the substrate metal. The ceramic top coat, on the other hand, primarily serves as an insulator and provides corrosion resistance. It is required to have low vapor pressure, low thermal conductivity, low thermal emissivity, high thermal emissivity, as well as excellent resistance to thermal fatigue or thermal shock.
The calculation results show that by using a zirconia thermal barrier coating with a thickness of 0.25 mm, the temperature of the substrate metal can be reduced by approximately 170℃. This reduction is greater than the cumulative increase in the temperature resistance capability of blade alloys achieved over the 20-year period from 1965 to 1985 thanks to continuous human efforts.
The application of TBC has already achieved remarkably significant results. Not only has it reduced manufacturing costs and specific fuel consumption, lowered the demand for cooling air, but it has also enhanced the durability of turbine blades. According to reports, applying a ceramic thermal barrier coating with a thickness of 0.25 mm to the first-stage turbine blades of an aviation gas-turbine engine can reduce the cooling-air requirement by 6%, improve specific fuel consumption by 13%, and extend blade life by a factor of four. As a result, TBC technology has been widely adopted in numerous industrial sectors to enhance thermal efficiency—for instance, in various gas turbines and internal-combustion engines. In the United States, many aircraft engines and nearly all land-based and marine gas turbines—including hot-end components such as flame tubes, swirlers, afterburners, shroud plates, fuel nozzles, exhaust ducts, igniters, combustion-chamber liners, flame stabilizers, and turbine blades—have already incorporated TBC technology. Each year, approximately several hundred tons of zirconia material are used for TBC applications, and the scope of these applications continues to expand. According to research conducted by the Gorham Advanced Materials Institute in the U.S., in the future, the proportion of TBC applications in diesel engines will surpass that in the aerospace industry. Moreover, the use of TBC in automobiles and motorcycles is also steadily increasing. In Sweden, for example, a single branch of Volvo Airlines consumed nearly 10 tons of zirconia in 1997 alone—a doubling of its consumption compared to 1995.
With advances in science and technology, numerous fields—including aerospace, aviation, gas-fired power generation, chemical engineering, and metallurgy—have spurred the research and development of thermal barrier coatings (TBCs). Today, TBCs are widely used across a broad range of applications. In blast furnaces, tuyeres and slag outlets must withstand the erosive action of high-speed pulverized coal and the corrosive attack of molten iron at temperatures ranging from 1,100 to 1,450°C. By applying TBCs as heat-resistant protective coatings, the service life of these components can be significantly extended. Moreover, novel atomizing metal nozzles coated with TBCs exhibit outstanding corrosion resistance and thermal shock resistance, boasting long operational lifetimes and playing a crucial role in ensuring the quality of ultrafine powders. In the automotive industry, valve seats equipped with TBCs in engine intake and exhaust ports can reduce component wear and tear. TBCs are also extensively used for the tops and edges of piston cylinders made from lightweight aluminum alloy substrates. Some experts predict that, over the next decade, TBCs will find even broader applications across an ever-expanding array of industries.
Thermal barrier coatings undoubtedly hold great technological potential and promising development prospects. However, they also face certain challenges that require further improvement, primarily in the areas of controlling coating adhesion, studying coating failure mechanisms, and evaluating coating performance. Among these, controlling coating adhesion is the most critical issue. Coating adhesion—also referred to as bonding strength or cohesive strength—is a key quality indicator that directly affects the performance of the coating during service. Coating spalling is the primary mode of failure for coated components and represents the major factor hindering the wider adoption of thermal barrier coatings in gas turbine engines. The main causes of coating spalling are twofold: first, oxidation of the bond coat; and second, the significant mismatch in thermal expansion coefficients between the substrate metal and the ceramic coating, which leads to pronounced strain mismatch. The development of thermal barrier coatings has thus been a continuous process of addressing and improving these two fundamental issues.
 

2. Thermal Barrier Coating Design
The design of thermal barrier coatings involves selecting coating materials, designing the coating structure, and choosing the appropriate spraying method.

(1) Ingredient selection.  
1. Bonding primer.

A typical bond coat material is the MCrAlX alloy, where M represents the fundamental constituent element of the bond coat—typically a member of the iron group or a high-melting-point metallic element, or a combination of such elements—for example, Ni, Co, Fe, Ni-Co, or Ni-Fe. X denotes an active metal—an element added to enhance bonding strength and improve the coating’s oxidation resistance. These active metals include relatively reactive elements such as Y, Hf, Sc, Ce, La, and Th; among them, Y is the most commonly used.
By applying an aluminizing process to prepare an aluminum-rich layer on the surface of the bond coat, the oxidation rate of the bond coat can be reduced, thereby extending the service life of the TBC. Adding Re and Ta to CoNiCrAlY can significantly improve both the oxidation resistance and mechanical properties of the bond coat.
2. Ceramic surface layer.

Currently, the ceramic top layer in TBC coatings is predominantly composed of either fully stabilized or partially stabilized zirconia ceramics. Since pure zirconia crystals exhibit different crystal structures depending on temperature, when the temperature exceeds 1,000°C, the monoclinic crystal structure transforms into the tetragonal phase, accompanied by a volume change of approximately 7%. During subsequent cooling, although the monoclinic crystal structure can be restored, the volume does not return to its original state—meaning that the volume undergoes an irreversible transformation before and after heating and cooling. This phase transition and associated volume change give rise to significant thermal stresses within the coating under thermal cycling conditions, leading to early cracking of the coating and even eventual spalling failure. Therefore, it is necessary to add stabilizers to the pure ZrO2 crystals.
After adding a stabilizer to pure ZrO2 crystals and subjecting them to sintering or melting treatment, a solid solution is formed, yielding cubic-stabilized ZrO2 with extremely low thermal expansion coefficients that remains stable over the entire temperature range below its melting point. However, at high temperatures, although the expansion and contraction of fully stabilized cubic ZrO2 can be modeled, its linear thermal expansion and contraction are both substantial, which is detrimental to enhancing its thermal-shock resistance. Therefore, in practice, partially stabilized zirconia—composed of a mixed structure of monoclinic and cubic crystals—is commonly employed. In this crystal structure, at high temperatures, the monoclinic phase undergoes a volume-reducing phase transition, while the cubic phase experiences volume expansion as the temperature rises. These two opposing changes mutually counteract each other, thereby giving partially stabilized ZrO2 a lower average thermal expansion coefficient than fully stabilized ZrO2 and endowing it with superior thermal-shock resistance.

    Stabilizers added to zirconia include calcium oxide (CaO), magnesium oxide (MgO), yttrium oxide (Y2O3), and cerium oxide (CeO). Among these, the addition levels of CaO stabilizer range from 5%, 6%, 8%, 10%, 15%, to 30%. As the CaO content increases, the hardness of the coating also rises. Coatings with a CaO content as high as 30% exhibit exceptionally high hardness and demonstrate excellent resistance to high-temperature particle erosion. However, CaO-stabilized ZrO2 coatings, when exposed for extended periods or cyclically to temperatures above 1093°C, tend to have CaO diffusing beyond the stabilized ZrO2 crystal structure, thereby limiting the service temperature of the coating. Such coatings can be used continuously at temperatures above 845°C but below 1093°C; beyond 1093°C, they can only be used for short durations. When MgO is used as a stabilizer, typically at concentrations ranging from 20% to 30%, ZrO2 maintains its crystal structure stability across various temperatures, especially during high-temperature thermal cycling. At temperatures below 1400°C, MgO-stabilized ZrO2 exhibits an equilibrium microstructure consisting of either the tetragonal or monoclinic phase plus MgO. During thermal cycling, MgO may precipitate out of the solid solution, leading to an increase in the coating's thermal conductivity and a reduction in its thermal insulation performance, thus restricting its broader application. In contrast, Y2O3-partially stabilized ZrO2, when used continuously at temperatures up to 1650°C, does not exhibit the same tendency as CaO to diffuse outward from the crystal structure. It demonstrates superior chemical and thermal stability compared to both CaO- and MgO-stabilized ZrO2, making it an outstanding thermal barrier coating material capable of operating at the highest temperatures. The addition levels of Y2O3 are typically 6–8%, 13%, and 20%. The first two levels correspond to partially stabilized ZrO2, while the third level represents fully stabilized ZrO2. For thermal barrier coatings, partially stabilized zirconia offers better resistance to thermal shock. Consequently, 6–8% Y2O3-partially stabilized zirconia has become the preferred material for the ceramic top layer in thermal barrier coatings.
In recent years, studies on certain stabilizers (PSZ) such as Y2O3, Nd2O3, and Sc2O3 have revealed that under rapid cooling conditions, the ZrO2 ceramic layer can contain either partially or fully “non-transformed” tetragonal phase t′. Although still metastable, this phase does not decompose into the equilibrium tetragonal and cubic phases even under high-temperature cycling conditions ranging from 1100 to 1200℃. In contrast, 6–8% Y2O3-ZrO2 (YSZ) coatings exhibit no decomposition of the t′ phase at temperatures between 1100 and 1200℃. In CeO-Y2O3-ZrO2 systems, the t′ phase demonstrates superior stability compared to 8% YSZ; however, these coatings perform less effectively when exposed to corrosive gases containing elements such as V and S. On the other hand, Sc2O3-Y2O3-ZrO2 (SYSZ) coatings exhibit higher t′ phase stability and enhanced resistance to hot salt corrosion even at elevated temperatures (1400℃).  

(2) Coating structure design.

Thermal barrier coating structures are primarily categorized into three types: two-layer structures, multi-layer structures, and gradient structures.
The double-layer structure consists of a ceramic top layer—typically a ZrO2-based ceramic—sprayed onto a high-temperature alloy substrate, and an underlying bond coat—often of the MCrAlY type. The ceramic top layer primarily serves to provide thermal insulation and oxidation resistance; while the bond coat mainly enhances the adhesion between the ceramic top layer and the substrate, improves the tolerance for mismatch in thermal expansion coefficients, and further boosts oxidation resistance. Due to its simple structure and ease of fabrication, the double-layer thermal barrier coating is currently widely used in practical applications.
The multi-layer structure is primarily designed to reduce thermal expansion mismatch between the ceramic top layer and the metallic bond coat by introducing an intermediate layer between them. Alternatively, to further enhance the oxidation resistance of the thermal barrier coating, a thin Al2O3 layer can be added between the ceramic top layer and the metallic bond coat. However, the addition of this Al2O3 layer provides only marginal improvement in thermal shock resistance, and the process is relatively complex, resulting in slightly poorer coating repeatability and reliability.
A gradient-structure thermal barrier coating refers to a coating in which the chemical composition, microstructural features, and mechanical properties gradually and continuously vary along the thickness direction—from the metallic bond coat to the ceramic top layer. This structural design enhances both the bonding strength between the coating and the substrate and the cohesive strength within the coating itself, delivering high-temperature performance that is ideal for coating applications. Moreover, such coatings exhibit superior thermal-shock resistance compared to conventional bilayer coatings. However, in practical fabrication, what is typically obtained is a multilayer stepped structure, and the preparation technology remains complex, placing this approach still in the laboratory-design and research stage.

(3) Selection of spraying method.

Due to the high melting point (2760℃) and low thermal conductivity (approximately 1.0–2.0 W/mK) of ZrO2 ceramic materials, among the thermal spraying processes described in Chapter 2, only arc spraying, cold gas dynamic spraying, high-velocity flame spraying, oxy-acetylene flame remelting, medium-frequency induction remelting, and plasma spray welding cannot be used for preparing ZrO2 ceramic coatings; all other thermal spraying processes, however, are suitable for this purpose.
However, as the performance requirements for coatings continue to increase, plasma spraying has become the primary method for fabricating thermal barrier coatings. In practical applications, though, either due to limitations in available conditions or in order to reduce costs while maintaining performance, various thermal spray processes are often employed. Based on whether a single piece of equipment or multiple pieces of equipment is used in the preparation of thermal barrier coatings, the coating fabrication processes can be categorized into two types: single-process fabrication and composite-process fabrication.
The single-process preparation method refers to a technique in which both the bond coat and the ZrO2 ceramic top coat of a thermal barrier coating are fabricated using the same spraying method. This includes processes such as atmospheric plasma spraying, low-pressure plasma spraying, vacuum plasma spraying, detonation spraying, and high-velocity plasma spraying.
The composite preparation process refers to a technique in which the bond coat of the thermal barrier coating and the ZrO2 ceramic top coat are prepared separately using different spraying methods. This includes: ① a vacuum-plus-atmospheric plasma composite spraying process, in which the bond coat of the thermal barrier coating is prepared by vacuum plasma spraying, while the ZrO2 ceramic top coat is prepared by atmospheric plasma spraying; ② a high-velocity flame-plus-atmospheric plasma composite spraying process, in which the bond coat of the thermal barrier coating is prepared by high-velocity flame spraying, and the ZrO2 top coat is prepared by atmospheric plasma spraying; ③ a high-velocity flame-plus-high-velocity plasma composite spraying process, in which the bond coat is prepared by high-velocity flame spraying, and the ZrO2 top coat is prepared by high-velocity plasma spraying, and so forth.

To address the issues of high porosity and cracks in plasma-sprayed thermal barrier coatings (TBCs), which lead to reduced oxidation resistance and shorter coating lifetimes, both domestically and internationally, extensive research has been conducted on laser-based TBC preparation methods, focusing on two distinct areas: laser surface remelting and laser cladding. There are two main laser-based TBC preparation techniques: the single-pass laser cladding method and the double-pass laser cladding method. The single-pass laser cladding method for TBC preparation is a relatively new approach, with research reports emerging only within the past decade. This method primarily involves two approaches: the pre-placement method and the powder-feeding method. In the pre-placement method, a partially stabilized YPSZ powder mixed with Ni-based composite powder is pre-placed onto the substrate, followed by CO2 laser cladding to form a layered composite coating. The surface consists of a dense ZrO2 ceramic layer, beneath which lies a Ni-based alloy transition layer. The upper part of the ZrO2 ceramic layer exhibits equiaxed crystals, while the middle and lower parts feature columnar crystals predominantly composed of the t′ phase. In the powder-feeding method, a feeding device delivers a mixture of partially stabilized YPSZ and alloy composite powder into the laser irradiation zone, where the laser melts and deposits the material onto the substrate, forming an automatically layered ceramic coating characterized by a columnar microstructure and largely composed of the t′ phase. The double-pass laser cladding method refers to a process in which a ZrO2 ceramic layer is first applied to the substrate surface via plasma spraying, followed by laser cladding treatment. This approach enables the production of a ceramic cladding layer that is smooth, continuous, dense, and free from defects such as cracks and pores—issues that cannot be fully addressed by the powder-feeding laser cladding method. The microstructure of the ceramic cladding layer produced by this method consists of columnar crystals whose growth direction is perpendicular to the substrate. The double-pass laser cladding method offers a viable pathway for fabricating high-performance, low-cost TBCs; however, it remains at the preliminary research stage. High-temperature performance testing is still lacking, and further in-depth studies are needed to elucidate the effects of cladding process parameters, the layered microstructure of the coating, its chemical composition, morphology, internal and external quality, and high-temperature performance on the coating’s service life.

In recent years, there has been growing interest in preparing thermal barrier coatings (TBCs) with high thermal-shock resistance using the electron-beam physical vapor deposition (EB-PVD) technique. Research on EB-PVD thermal barrier coatings began in the 1970s, and a breakthrough was achieved in the 1980s by Pratt & Whitney in the United States. Subsequently, this technology has also been successfully applied in countries such as Germany. EB-PVD thermal barrier coatings are formed by heating and vaporizing ceramic sources with a high-energy electron beam to produce ceramic vapor, which is then deposited onto the substrate atom by atom. The resulting coating microstructure consists of columnar grains oriented perpendicularly to the substrate surface, with metallurgical bonding between the columns and the substrate, ensuring excellent stability. In high-temperature environments, the columns can separate from each other, thereby relieving thermal stresses caused by differences in thermal expansion coefficients and significantly enhancing the coating's resistance to thermal fatigue. Studies have shown that an Al2O3 zone forms between the ZrO2 ceramic layer and the bond coat, and the presence of this zone helps improve the oxidation resistance of the thermal barrier coating. Moreover, thermal barrier coatings prepared by EB-PVD exhibit a smooth surface that faithfully reproduces the roughness of the original substrate, eliminating the need for further machining. This reduces gas flow resistance, extends the coating’s service life, and makes the process parameters easier to control compared to plasma spraying. However, the EB-PVD process also has several drawbacks, including relatively low thermal insulation capability of the coating, uncontrollable coating thickness, complex surface cleaning requirements, sophisticated and expensive equipment, relatively low deposition rates, and a cumbersome process flow—all of which urgently require further research and improvement.

Given the significant differences between TBCs prepared by plasma spraying and those prepared by EB-PVD, some researchers refer to TBCs fabricated by plasma spraying as first-generation TBCs, while TBCs prepared by EB-PVD are called second-generation TBCs.
3. Preparation of Thermal Barrier Coatings
The most mature and widely used process for preparing TBC is plasma spraying. In addition to its high production efficiency and relatively low cost, the coating’s excellent thermal insulation performance is also a prominent advantage. Consequently, both domestically and internationally, continuous improvements and developments are being carried out on this method. This article provides a relatively detailed discussion solely on the preparation of TBC via atmospheric plasma spraying.
During the plasma-spraying process for preparing thermal barrier coatings, numerous process parameters influence coating quality—some even claim that there are more than 100 such parameters. Therefore, when preparing thermal barrier coatings, it is essential to strictly control each stage and optimize the relevant process parameters. After repeated optimization of the process and thorough evaluation of coating performance, the final coating preparation process will be determined based on actual application tests. Once the process has been finalized, it must be rigorously adhered to. During implementation, attention should be paid to the following key stages.
(1) Strictly control the quality of spray powder.  

Selecting high-quality spray powders is the foundation for preparing high-performance, thermally stable thermal barrier coatings. Therefore, before carrying out the spraying process, it is essential to rigorously inspect the powder quality, covering the following aspects: ① powder composition; ② preparation method; ③ powder morphology; ④ flowability; ⑤ bulk density; ⑥ particle size; ⑦ particle size distribution; ⑧ batch-to-batch uniformity and stability. Once the powder supplier has been determined, among these eight aspects, only five—namely, ③ through ⑦—need to be strictly controlled. The following example illustrates the critical importance of spray powders to the quality of thermal barrier coatings.
Wigren et al. used exactly the same spraying parameters to compare the microstructures of ZrO2 powder coatings—both with identical specifications but differing in powder morphology—in terms of hollow-sphere agglomerated sintered powders. The results showed significant differences in the microstructures of the coatings. At 1100℃, the thermal conductivities of these two coatings were measured as 0.6 W/mK and 0.4 W/mK, respectively, clearly demonstrating the influence of the sprayed powder on the microstructure and performance of the coatings. Wigren et al. also conducted spraying studies using five different parameter sets for two types of agglomerated sintered powders, one type of hollow-sphere agglomerated powder, one type of sintered crushed powder, and two types of sol-gel powders. The results of thermal shock tests indicated that the number of thermal shock cycles each coating could withstand varied significantly.
(2) Strictly control spraying parameters. 。

The ZrO2 material used as the top coat in thermal barrier coatings features a high melting point and low thermal conductivity. When plasma spraying, it is essential to input a relatively high power level to ensure that the powder reaches a molten state. However, all the parameters involved in plasma spraying must be appropriately matched. The following section introduces several key process parameters.
1. The electrical parameters must match the equipment.

The spray gun is one of the core pieces of equipment used to carry out the plasma spraying process. Different models of spray guns have their own unique characteristics and requirements; therefore, when setting electrical parameters, it’s essential to meet the specifications of the equipment being used. Relatively speaking, the Plexus SG100 spray gun operates in a high-current, low-voltage mode, whereas the Meiko 7M/9M spray gun operates in a low-current, low-voltage mode.
High current and high voltage are required. When spraying ZrO2-based powders, the SG100 spray gun should be set to a current of 800–900 A and a voltage of around 40 V; whereas the 7M/9M spray gun should be set to a current of 600 A and a voltage of about 80 V. If the voltage falls outside the above-mentioned ranges, it can be adjusted by regulating the auxiliary gas flow. In this case, care must be taken to ensure that the auxiliary gas flow does not exceed a certain limit. If, after adjusting the auxiliary gas flow to its maximum setting, the desired voltage still cannot be achieved, the spraying operation should be immediately halted. Thoroughly inspect the spray gun configuration to check whether the nozzle and electrode have been damaged and whether there are any leaks in the gas delivery lines between the flowmeter and the spray gun.
2) Selection of plasma gases 。 

The gases used in plasma spraying fall into two categories: one is called the primary gas, which has relatively high pressure and flow rate; it mainly consists of argon (Ar) and nitrogen (N2). The other is called the secondary gas, which has lower pressure and flow rate; it mainly comprises hydrogen (H2) and helium (He). Among these four plasma gases, the primary gas N2 and the secondary gas H2 are diatomic molecules. At high temperatures, they first undergo dissociation to become monatomic, with dissociation energies of 9.76 eV and 4.477 eV, respectively. Subsequently, they ionize to form positive ions and free electrons. The ionization energies of the four atoms—hydrogen (H), nitrogen (N), argon (Ar), and helium (He)—are 13.595 eV, 14.54 eV, 15.755 eV, and 24.58 eV, respectively. Consequently, the plasma generated by the primary gas N2 has a higher enthalpy and faster heat transfer, which facilitates efficient heating and melting of the powder. On the other hand, the plasma generated by the primary gas Ar has a relatively lower enthalpy, but it produces a stable plasma arc that is easy to ignite and features a shorter arc flame, making it particularly suitable for spraying small parts and thin-walled components. The plasma generated by the secondary gas H2 has a higher enthalpy than that produced by helium (He).
When actually spraying ZrO2 thermal barrier coatings, the choice of primary and secondary gas combinations also depends on the type of spray gun used. The Plasmax SG100 spray gun recommends using argon (Ar) as the primary gas and helium (He) as the secondary gas for plasma generation; whereas the Meiko 7M/9M spray guns typically use nitrogen (N2) as the primary gas and hydrogen (H2) as the secondary gas for plasma generation—but it’s important to ensure compatibility with the N2 gas distribution ring.
In addition, the flow rate of the plasma gas is also a crucial process parameter. It directly affects the enthalpy and velocity of the plasma jet, which in turn influences the spraying efficiency, coating porosity, and coating adhesion strength. At a given power level, there exists an optimal gas flow rate. If the gas flow is too high, the ion concentration decreases, and the gas atoms or molecules absorb heat from the plasma jet, thereby cooling it down and reducing both its enthalpy and temperature. This results in insufficient powder melting, lower spraying efficiency, a loose coating microstructure, and increased porosity. Conversely, if the gas flow is too low, the plasma jet becomes weak and ineffective, causing both the temperature and velocity of the powder particles to drop, which can easily lead to damage of the nozzle and cathode.
Therefore, it is essential to optimize the characteristics of ZrO2 powder and determine the plasma gas flow rate that meets the coating performance requirements.
3) Powder feeding parameters 。 

The powder-feeding parameters mainly include the powder-feeding position (internal feeding, external feeding, distance from the nozzle axis), the configuration of the powder-feed nozzle (nozzle number), the powder-feeding angle (vertical, inward tilt, outward tilt), the carrier gas flow rate, and the powder-feeding rate. Once the spray powder has been selected, the two primary parameters to adjust are the carrier gas flow rate and the powder-feeding rate.
The carrier gas flow rate should be matched to the working gas flow rate, typically ranging from about 10% to 20% of the working gas flow rate. The adjustment principle is to ensure that the ZrO2 powder can be effectively delivered into the flame core.
The powder feed rate and the position at which the powder enters the arc are critical parameters that influence both the coating microstructure and spraying efficiency. Only when the spray powder is delivered to the flame core—the region where the flame temperature and velocity are highest—can it achieve optimal heating and maximum velocity. The powder feed rate must be appropriately matched to the input power level. If the feed rate is too high, the powder may not melt completely, leading to incomplete fusion and poor bonding between layers. Conversely, if the feed rate is too low, spraying efficiency will decline and the substrate may overheat.
4) Spray distance and spray angle.

The spray distance refers to the vertical distance from the nozzle outlet along the axis of the flame jet to the surface of the workpiece. If the spray distance is too small, the substrate temperature may rise excessively, leading to significant internal stresses between the substrate and the thermal barrier coating, thereby compromising the coating’s performance. On the other hand, if the spray distance is too large, both the temperature and velocity of the ZrO2 particles upon impact with the substrate will decrease, resulting in reduced coating adhesion, lower deposition efficiency, and an increased porosity. Provided that the substrate temperature permits, the spray distance can be appropriately reduced.
The DPV-2000 thermal spray online detector was used to measure the temperature and velocity of zirconia particles as a function of spraying distance. The temperature of ZrO2 particles gradually decreased with increasing spraying distance. Immediately after leaving the nozzle—at a distance of 60 mm—the average particle temperature reached nearly 3,000°C, exceeding the melting point of ZrO2 (approximately 2,700°C), indicating that the particles had undergone some degree of melting. As the spraying distance increased, the temperature of the ZrO2 particles steadily declined, dropping to around 2,700°C when the spraying distance reached 80 mm. The flight velocity of the ZrO2 particles exhibited a Gaussian distribution pattern as a function of spraying distance, with the peak occurring at approximately 80 mm from the nozzle. Therefore, considering both the temperature and velocity of the ZrO2 particles, an optimal spraying distance of 80 mm is appropriate under the selected spraying process conditions.
The spray angle refers to the angle between the axis of the flame jet and the surface being sprayed. During spraying, the spray angle should be maintained at 90° as much as possible. When the spray angle is less than 45°, a "shadowing effect" occurs, leading to an increase in coating porosity and making the coating more porous.
5) The relative moving speed between the spray gun and the workpiece.

Under certain powder-feeding rate conditions, the relative moving speed between the spray gun and the workpiece—commonly referred to as the gun-moving speed—determines the area swept by the spray gun per unit time or the thickness of each sprayed coating layer. Therefore, controlling the gun-moving speed is essentially equivalent to controlling the thickness of each coating layer. The thickness of each layer, in turn, affects the residual stresses in the coating; as the layer thickness decreases, the residual stresses can be significantly reduced. Moreover, the gun-moving speed also influences the substrate temperature. For ZrO2 thermal barrier coatings, when spraying the workpiece, it is generally advisable to keep the thickness of each pass of the spray gun below 25 μm.
(3) Control the substrate temperature 。

The substrate temperature is one of the critical process parameters in plasma spraying. Studies have shown that when ZrO2 coatings are deposited onto substrates made of different materials, there exist distinct critical transition temperatures. When the substrate temperature is below this critical transition temperature, individual ZrO2 particles, after expanding and cooling with the substrate, exhibit a splashing morphology, as illustrated in the figure. As the substrate temperature rises, the degree of splashing diminishes. Once the substrate temperature exceeds the critical transition temperature, individual ZrO2 particles assume a disc-like deformation pattern, as shown in the figure. The varying deformation states of individual ZrO2 particles significantly influence the microstructure of the coating.

Although raising the substrate temperature is beneficial for enhancing the bonding between ZrO2 particles, when using atmospheric plasma spraying, excessively high substrate temperatures not only exacerbate substrate oxidation but also increase thermal stresses within the coating, thereby increasing the likelihood of cracking and spalling. Typically, the substrate preheating temperature ranges from 95 to 120°C. However, it’s important to note that for magnesium substrates, due to magnesium’s rapid oxidation, preheating should be avoided prior to spraying; otherwise, it may compromise the bond strength between the coating and the substrate. For aluminum substrates, preheating is permissible only within the temperature range of 65 to 95°C. To prevent oxidation, preheating can be carried out from the back or side of the substrate, and it’s best to avoid directly preheating the spray surface. If indirect preheating is not feasible, preheating can be omitted altogether. Furthermore, in-die preheating is not recommended.
To control the substrate temperature during the spraying process, compressed air is typically used to cool the sprayed surface. This method is highly effective in maintaining both the substrate and coating performance. However, in certain situations where higher cooling capacity is required, liquid argon or liquid CO2 can be employed instead. This approach enables the substrate temperature to be maintained at around 50℃.
The color of a properly sprayed ZrO2 thermal barrier coating should be close to white or pale yellow. If the color turns dark yellow or orange, it indicates that the coating has been overheated. Conversely, if the coating appears light gray, it suggests that the workpiece temperature was too low during spraying, resulting in relatively poor thermal shock resistance of the coating.
Some black spots can be observed on the surface of the ZrO2 thermal barrier coating. These spots do not affect the performance of the coating, and energy-dispersive spectroscopy analysis indicates that the composition of these black spots remains ZrO2.
4. Post-processing of thermal barrier coatings
Failure analysis of thermal barrier coatings indicates that there are three primary reasons for the early delamination failure of these coatings. First, the low bonding strength between coating layers plays a crucial role, including the bonding strength between the substrate and the bond coat, between the bond coat and the ZrO2 top coat, and between the deformation particles within the ZrO2 top coat itself. Second, the relatively high porosity of the ZrO2 top coat is another contributing factor. Although these pores can to some extent help relieve thermal stresses, during service, oxygen can penetrate through these pores into the bond coat and even the substrate, causing oxidation. As a result, an Al2O3 layer with a thickness of 5–10 μm forms between the bond coat and the ZrO2 top coat. This oxidation process has a highly detrimental impact on the service life of the thermal barrier coating. Third, mismatch in thermal expansion coefficients between the bond coat and the ZrO2 top coat leads to thermal stresses. Under repeated heating and cooling cycles, cracks develop within the coating, eventually resulting in delamination failure. Currently, to further extend the service life of thermal barrier coatings, research has been conducted on post-treatment processes aimed at enhancing the bonding strength of the coatings and reducing their porosity. This paper presents the findings from the following three aspects:
(1) Vacuum heat treatment.

In thermal barrier coatings, the bond coat and the substrate primarily rely on mechanical interlocking for adhesion. The relatively low bonding strength between these two layers is one of the main factors contributing to the thermal-shock failure of thermal barrier coatings. Some researchers have investigated the effects of vacuum heat treatment on the bonding condition between the NiCoCrAlY bond coat and the TC4 titanium alloy substrate, as well as on the thermal-shock resistance of the coating. The vacuum heat treatment process employed was as follows: with a vacuum level of 0.1 Pa, the samples were heated to 900℃ using a furnace-heating method, held at this temperature for 8 hours, and then cooled down to room temperature together with the furnace. The results showed that after vacuum heat treatment, metallurgical and chemical reactions occurred between the NiCoCrAlY bond coat and the TC4 titanium alloy substrate, forming an interfacial reaction layer containing compounds such as NiTi, NiTi2, and TiAl3. As a result, the thermal-shock resistance of the coating was significantly improved. Therefore, vacuum heat treatment represents an effective approach for enhancing the bonding between the thermal barrier coating and the substrate and improving the thermal-shock resistance of the thermal barrier coating.
(2) Hot isostatic pressing.

Thermal-Hot Isostatic Pressing (HIP) treatment of coatings involves placing the sprayed thermal-barrier coating samples into a HIP apparatus, where they are heated to a relatively high temperature—typically around 1200°C—in an Ar atmosphere. Subsequently, the samples are subjected to a high pressure—generally ranging from 50 to 200 MPa—and held at this pressure for several hours before being cooled down to room temperature. Research results indicate that after HIP treatment, both the bond coat and the ZrO2 layer become denser, and the number of cracks in the ZrO2 coating is reduced. In the ZrO2 coating, the tetragonal phase with higher Y2O3 content, which had not undergone phase transformation, experiences a certain degree of recrystallization, leading to the formation of a fine, equiaxed grain structure. Due to the sintering effect, a certain degree of metallurgical bonding develops between layers or between deformed particles, thereby enhancing the bonding strength of the coating and altering the fracture mode from interlayer cracking to through-layer cracking. A thin Al2O3 layer forms at the interface between the bond coat and the ZrO2 surface layer, which improves the oxidation resistance of the thermal-barrier coating. Moreover, at the interface between the bond coat and the substrate, significant mutual diffusion occurs between Ni in the bond coat and Fe in the substrate. All these structural changes induced by HIP treatment contribute positively to the enhancement of the thermal-barrier coating’s performance.
(3) Laser remelting treatment.

Laser remelting treatment refers to a post-processing method in which a laser beam with a specific energy density is directed onto the surface of a thermally sprayed thermal barrier coating sample, causing changes in the microstructure of the ZrO2 surface layer and thereby altering the performance of the thermal barrier coating. After laser remelting treatment, the primary region where the microstructural changes occur is confined to a depth of approximately 70 μm beneath the ZrO2 surface layer, with little impact on the microstructure of the deeper ZrO2 coating or on the interfacial bond between the bonding substrate and the ZrO2 layer. Following laser remelting, the ZrO2 surface layer develops a columnar dendritic microstructure, which differs significantly from the lamellar structure formed by plasma spraying and resembles the microstructure of electron-beam physical vapor deposition (EB-PVD)-grown thermal barrier coatings. This microstructure exhibits exceptional resistance to thermal shock. Studies have shown that the thermal shock resistance of laser-remelted thermal barrier coatings is four times greater than that of plasma-sprayed thermal barrier coatings. As mentioned earlier, currently, a few countries—including the United States and Japan—are actively pursuing research into hybrid plasma-laser spraying processes aimed at further enhancing the overall performance of thermal barrier coatings.
(4) Sealing treatment.

When operating in the combustion chamber, thermal barrier coatings are subjected to thermal cycling due to temperature fluctuations. Since ZrO2 ceramic coatings contain a certain number of pores and microcracks that are closely interconnected, corrosive media can penetrate through these pores and cracks into the interface between the bond coat and the ceramic layer, and even reach the substrate surface, thereby causing oxidation and corrosion. This is the primary cause of delamination failure in thermal barrier coatings. To mitigate oxidation and corrosion, densifying and sealing the coating surface represents an important approach for extending the service life of thermal barrier coatings. To this end, several types of thermally barrier coatings treated with sealing processes have been studied, along with their thermal shock resistance. The relationship between coating failure modes and delamination mechanisms has also been explored.
Three different sealing agents—silicone resin, NiCrBSi metal coating, and water glass—were used separately to seal the surfaces of thermal barrier coating specimens. Thermal shock tests were then conducted: The specimens were heated to 900℃, held at that temperature for 5 minutes, removed from the furnace, and immediately subjected to forced air cooling for 20 minutes. This cycle was repeated until the coating began to fail and peel off; the number of thermal cycles at which failure occurred was recorded. The results showed that after sealing, the thermal shock life of the thermal barrier coatings was significantly improved. Among the three sealing agents, silicone resin exhibited the best performance, followed by the NiCrBSi coating, while the effect of water glass was less pronounced.

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