Research progress of high-temperature protective coatings

　　1. Introduction

　　High-temperature protective coatings provide effective oxidation and corrosion protection for metallic materials used at high temperatures, and have been widely used in aerospace, energy, petrochemical and other fields. A representative application is in various gas turbine engines used in aircraft, ships, and ground-based power generation. The development of high-temperature protective coatings has gone through three stages: first-generation diffusion coatings, second-generation M (M = Fe, Ni, or Co) CrAlY overlay coatings; and third-generation thermal barrier coatings.

　　In order to further improve the efficiency of turbine engines and achieve energy saving and emission reduction, the inlet temperature of the engine must be increased. Therefore, scientists are constantly working to develop more advanced materials, coating systems, and preparation technologies. For example, the fourth-generation nickel-based single-crystal superalloy has a temperature-bearing capacity of up to 1180℃. Correspondingly, higher requirements have been put forward for high-temperature protective coatings, and various new high-temperature protective coatings with unique design concepts have emerged. This article describes the structure, preparation methods, and application characteristics of commonly used high-temperature protective coatings, introduces several characteristic high-temperature protective coatings, and reviews the latest progress in research on high-temperature coatings at home and abroad. The development trend of high-temperature protective coatings is also prospected.

 

　　2. Commonly Used High-Temperature Protective Coatings

　　2.1 Diffusion Coatings

　　Diffusion coatings are formed when some oxidation-resistant elements, such as Al, Cr, and Si, come into contact with the metal substrate and enter the substrate surface. During the formation of the diffusion coating, the substrate participates in the formation of the coating, and the elements in the substrate enter the coating, forming a diffusion layer below the coating. Diffusion coatings include aluminide coatings, chromide coatings, silicide coatings, and improved aluminide coatings, with representative examples being aluminide coatings and improved aluminide coatings.

　　2.1.1 Aluminide Coatings

　　Aluminide coatings were first described in a US patent by Van Aller in 1911, prepared using the pack cementation method. Later, hot-dip aluminizing, slurry aluminizing, and non-contact "substrate above the pack" aluminizing and chemical vapor deposition (CVD) aluminizing methods appeared. In the 1950s, pack cementation aluminide coatings began to be used for cobalt-based guide vanes. By the 1970s, most nickel-based and cobalt-based turbine and guide vanes used pack cementation aluminide coatings and non-contact aluminide coatings.

　　In the pack cementation aluminizing method, the sample is embedded in a pack powder. The pack consists of an aluminum source powder, a halide activator, and a filler. The aluminum source powder can be metallic Al or a suitable alloy powder, and the filler is usually inert Al2O3. The pack generally contains 2% to 5% activator, such as ammonium chloride, 25% aluminum source, and the rest is filler. During heating, the activator volatilizes in the pack and reacts with the aluminum source to form volatile compounds of the coating metal. The volatile substances diffuse to the surface of the substrate and undergo a deposition reaction there. Protective gases such as argon must be introduced during aluminizing to prevent oxidation of the aluminum source and the metal substrate.

　　The structure and deposition rate of the aluminide coating depend on factors such as the activity of Al in the pack, the aluminizing temperature, the substrate composition, and the post-treatment process. Taking the aluminide coating on a nickel-based superalloy as an example, in a relatively low temperature range, such as 700-800℃, the activity of Al is higher than that of Ni at this time. During the aluminizing process, the growth of the coating mainly relies on the inward diffusion of Al through the initially formed Ni2Al3 layer to form an inward diffusion coating, also known as high-activity aluminizing (HALT). This coating needs to undergo a secondary annealing treatment to form the NiAl phase. In a relatively high temperature range, such as 980-1090℃, the activity of Al is lower than that of Ni at this time. The growth of the coating is mainly due to the outward diffusion of Ni and the combination with the surface-deposited Al to form the NiAl phase, forming an outward diffusion coating, also known as low-activity aluminizing (LAHT).

　　Pack cementation aluminizing has several advantages compared to other methods for preparing diffusion coatings. First, the pack supports the material being aluminized, preventing large components from bending. Commercially used aluminizing processes can produce aluminum-rich coatings for pipes several meters long. Second, the pack is in contact with the substrate, resulting in a more uniform coating composition and a faster deposition rate. However, there are also disadvantages; materials from the pack can be incorporated into the coating. Non-contact aluminizing processes, such as "substrate above the pack" aluminizing and chemical vapor deposition aluminizing, can avoid this. In the former, the workpiece is fixed above the pack, and the coating reaction gas is generated from the pack and flows upward to the substrate surface. In the latter, the coating reaction gas is generated externally during the deposition process and then introduced into a vacuum container containing the material to be aluminized. Therefore, the composition of the reaction gas is highly adjustable, and the reaction gas can be delivered to the inner cavity, such as the internal cooling holes of gas turbine blades.

　　Simple aluminide coatings have good oxidation resistance, simple processes, stable performance, and low cost. However, they also have many disadvantages, such as poor hot corrosion resistance, especially type II hot corrosion, high coating brittleness, and fast degradation rate. In the 1970s, improved aluminide coatings were developed.

　　2.1.2 Improved Aluminide Coatings

　　Adding a small amount of elements such as Si, Cr, and Pt to simple aluminide coatings can significantly improve the coating's performance. The main types of improved aluminide coatings are:

　　(1) Cr-modified aluminide coatings

　　Adding Cr to the coating can significantly improve its resistance to hot corrosion and slow down the degradation caused by mutual diffusion between the coating and the substrate. The coating can be prepared using a one-step or two-step method. Due to the huge difference in the thermal stability of Al and Cr halides, it is difficult to achieve Al and Cr co-infiltration through the method of embedding and infiltrating pure metal powder. Therefore, the two-step method was usually used in the early stage to prepare chromium-modified aluminide coatings, that is, a chromium layer was pre-deposited on the metal substrate, which can be achieved by embedding, slurry, and electroplating methods, and then aluminum was infiltrated by embedding and heat diffusion. By using a Cr-Al binary alloy, the vapor pressure of the relatively high Al halide is reduced, and Al and Cr co-infiltration on nickel-based alloys is achieved. The coating is mainly composed of the NiAl phase, with Cr dissolved in the NiAl phase or precipitated as an α-Cr phase.

　　(2) Si-modified aluminide coating

　　Adding an appropriate amount of Si to the coating can slow down the degradation caused by mutual diffusion between the coating and the substrate, and can also improve its hot corrosion resistance. It also has better high-temperature oxidation resistance compared to Al-Cr co-infiltration, but the content of Si should not be too high, because Si and Ni will form harmful low-melting-point phases at high temperatures, making the coating brittle and easy to peel off during oxidation. The most commonly used method for preparing Al-Si coatings is the slurry method. The coating is mainly composed of the β-NiAl phase, and Si is distributed in the coating as Si-rich second-phase particles.

　　(3) Pt-modified aluminide coating

　　Among the improved aluminide coatings, the modification effect of the Pt-Al coating is the most significant. Pt improves the spalling and self-healing ability of the α-Al2O3 film, increases the organizational stability of the aluminide coating, and reduces the mutual diffusion between the coating and the substrate. Pt-Al coatings are usually prepared using a two-step method. First, a layer of Pt is electroplated on the alloy substrate, and then annealed. After annealing, powder embedding and aluminum infiltration are carried out. The coating usually has a double-layer structure. The outer layer is a two-phase structure of PtAl2 and NiAl or a Pt-rich (Pt, Ni) Al single-phase layer, and the inner layer is a single-phase layer of NiAl.

　　The above-mentioned improved aluminide coatings have been widely used in hot-end components of gas turbines, etc.

　　2.2 Overlay Coatings

　　Overlay coatings refer to coatings formed by directly depositing coating materials on the surface of an alloy using physical or chemical methods. The significant difference between overlay coatings and diffusion coatings is that when the coating is deposited, it only interacts with the substrate to improve the bonding strength of the coating, and the substrate does not participate in the formation of the coating, so the choice of coating composition is more diverse. Overlay coatings can be metal coatings, ceramic coatings, etc., the most typical of which is the MCrAlY overlay coating.

　　MCrAlY overlay coatings were developed in the 1970s and have now developed into a series of coating systems, where M is Fe, Co, Ni, or a combination thereof; Al is used to form a protective Al2O3 film; Cr is used to promote the formation of an oxide film and improve hot corrosion resistance; Y is used to improve the adhesion of the oxide film; and one or more elements such as Hf, Si, Ta, Re, Zr, and Nb can also be added to the coating to meet some specific application needs. These coatings are mainly composed of a β phase (NiAl or CoAl) and a γ solid solution of Ni or Co. Nickel-based Ni-Cr-Al-Y coatings have excellent oxidation resistance, cobalt-based Co-Cr-Al-Y coatings are more resistant to hot corrosion, and Ni-Co-Cr-Al-Y coatings combine both. The literature compares the oxidation and hot corrosion resistance of aluminide coatings, improved aluminide coatings, and MCrAlY overlay coatings. By adjusting the composition of the MCrAlY coating, oxidation-resistant coatings and hot corrosion-resistant coatings can be prepared to meet the needs of different working environments and different substrate alloys. Common preparation methods for MCrAlY coatings include physical vapor deposition, including electron beam physical vapor deposition (EB-PVD), sputtering, ion plating, and spraying techniques, including low-pressure plasma spraying, argon-shielded plasma spraying, supersonic flame spraying, etc.

　　2.3 Thermal Barrier Coatings

　　The main function of thermal barrier coatings (TBCs) is heat insulation. They consist of a ceramic topcoat with a low thermal conductivity and a metallic bond coat. Early thermal barrier coatings were Al2O3 and ZrO2 (MgO or CaO stabilized) ceramic insulation layers directly sprayed onto the alloy surface. In the mid-1970s, Y2O3-stabilized ZrO2 topcoats were prepared using NiCrAlY as a bond coat and plasma spraying technology, and the development of EB-PVD technology for depositing ceramic topcoats in the early 1980s was a significant advance in the history of thermal barrier coating development. Current thermal barrier coatings often use MCrAlY and Pt-modified aluminide coatings as bond coats. The main function of the metallic bond coat is to increase the bonding strength between the ceramic coating and the substrate, improve the mismatch of the thermal expansion coefficient between the two, and also improve the oxidation resistance of the substrate. 8% Y2O3 partially stabilized ZrO2 (Y-PSZ) has a high melting point, high-temperature stability, low thermal conductivity, and a thermal expansion rate closest to that of the substrate material, making it the preferred material for ceramic insulation layers. At high temperatures, the Al in the bond coat reacts with oxygen diffusing from the ceramic layer to form a thermally grown oxide layer (TGO) at the bond coat/ceramic layer interface, the main component of which is α-Al2O3, effectively preventing the oxidation of the substrate.

　　There are various methods for preparing ceramic insulation layers, including thermal spraying, physical vapor deposition, chemical vapor deposition, sol-gel method, etc. The commonly used techniques are plasma spraying and EB-PVD. The Y-PSZ prepared by plasma spraying has a lamellar structure and often contains 15% to 25% porosity, so it has low thermal conductivity and a certain strain tolerance. It is usually used in components with lower requirements in aeroengines, such as combustion chambers, combustion evaporators, and stator blades. The Y-PSZ layer prepared by EB-PVD has a columnar crystal structure and has high strain tolerance during temperature changes, so it has a longer lifespan than plasma-sprayed coatings, but the equipment is expensive and the cost is high, so it is used in components with more demanding conditions in engines, such as aero gas turbine blades. The heat insulation effect of TBCs can reach 175℃.

　　Currently, the main challenge facing TBC applications is the durability of the coating, especially its resistance to spalling. Numerous factors influence this, such as the stress state in the ZrO2 layer, the microstructure of the bond coat, the thickness and stress state of the TGO layer, and the fracture resistance of various interfaces between the bond coat and TGO. It is currently accepted that the oxidation of the bond coat is a key factor determining the life of EB-PVD TBCs.

 

　　3. Special High-Temperature Protective Coatings

　　3.1 New Concept Coatings

　　These coatings introduce some basic theories from materials science, physical chemistry, solid diffusion, and high-temperature oxidation into coating design, forming a unique high-temperature coating system.

　　3.1.1 High-Temperature Microcrystalline Coatings

　　Lou Hanyi and Wang Fuhu, etc., developed a novel protective coating for high-temperature alloys—high-temperature alloy microcrystalline coatings. Unlike traditional high-temperature protective coatings, the microcrystalline coating has the same composition as the substrate alloy, thus avoiding the decrease in mechanical properties caused by mutual diffusion between the coating and the substrate at high temperatures. Meanwhile, the coating grain size is 20-100 nm, which not only promotes the selective oxidation of Al but also improves the adhesion of the oxide film. Studies on the oxidation behavior of Co-30Cr-5Al alloy and its sputtered microcrystalline coating at 1100℃ in air show that a protective Al2O3 film is formed on the surface of the alloy in the first 25h of oxidation. After 25h, due to the cracking and spalling of the Al2O3 film and the formation of the Cr2O3 film, the alloy weight increases rapidly. In comparison, the sputtered microcrystalline coating shows excellent protection, and the oxide film is still a uniform and dense Al2O3 film after 100h of oxidation, without separation and cracking of the oxide film from the substrate.

　　3.1.2 EQ Coatings

　　When traditional high-temperature protective coatings, such as aluminized coatings (β-NiAl) and MCrAlY coatings, are used for nickel-based single-crystal superalloys, due to the significantly increased content of refractory metals in single-crystal alloys compared to traditional superalloys, mutual diffusion between the coating and the substrate leads to the formation of a harmful SRZ (secondary reaction zone) at the coating/substrate interface, significantly reducing the creep rupture life of nickel-based single-crystal superalloys. Kawagishi et al. and Sato et al. proposed the preparation of EQ coatings (equilibrium coating) to inhibit the formation of SRZ. Nickel-based superalloys are composed of γ and γ′ phases, and these two phases maintain an equilibrium state, so the chemical potentials of the elements in the two phases are equal. Using the γ′ phase in the alloy as the EQ coating material results in a zero chemical potential difference for the elements that mutually diffuse between the coating and the substrate, thus inhibiting the mutual diffusion between the coating and the substrate. However, the oxidation resistance of this coating is limited, and it is prone to degradation into γ and γ′ phases after prolonged oxidation.

　　3.1.3 Functionally Graded Coatings

　　Functionally graded coatings are the application of the design concept of functionally graded materials (FGM) in coating/substrate systems. The basic idea of functionally graded materials is to prepare composite materials with a gradient distribution of composition (or/and structure) in a certain direction from two or more different materials, so that the material has functions that cannot be achieved by non-gradient structures. The application of the FGM concept in coating/substrate systems is to solve interface problems. In coating/substrate systems, when the coating and substrate and/or the materials constituting the coating are different, due to the sudden change in material properties (thermal expansion coefficient, elastic modulus, etc.) at the interface, severe mismatch occurs near the interface, increasing the driving force for structural spalling. In order to alleviate the mismatch, a functionally graded layer can be introduced between the two materials, in which the composition (and/or structure) of the two materials changes continuously along the thickness to reduce and overcome the performance mismatch at the bonding site and alleviate the stress field. The most studied functionally graded coatings in high-temperature protective coatings are functionally graded thermal barrier coatings. As mentioned earlier, thermal barrier coatings consist of an 8%Y2O3-ZrO2 ceramic top layer and an MCrAlY metallic bond coat. The mismatch of material properties between ceramics and metals leads to spalling of the ceramic layer during thermal cycling. By preparing gradient coatings between the ceramic top layer and the metallic bond coat using plasma spraying and other methods, the ceramic and metal components in the layer change gradually along the thickness direction to alleviate the mismatch at the ceramic/metal interface. Although the current results are not very satisfactory, exploration in this area is still ongoing.

　　3.1.4 Smart Coatings

　　Smart coatings proposed by Nicholls et al. are a compositionally graded coating system that can optimally respond to corrosion and erosion over a wide temperature range and in complex corrosive environments, providing corrosion protection for high-temperature components of industrial and marine gas turbines. The coating has both high-temperature oxidation resistance and low-temperature hot corrosion resistance. The coating is based on an MCrAlY coating, with an outer Al-rich layer that can quickly form an Al2O3 film at high temperatures above 900℃ and under type I hot corrosion conditions above 800℃ to provide protection; the middle layer is a Cr-rich layer, which can act as a diffusion barrier at high temperatures to prevent the diffusion of Al from the Al-rich layer to the substrate and the diffusion of elements from the substrate to the Al-rich layer, and can quickly form Cr2O3 at lower temperatures (600-800℃ under type II hot corrosion conditions) to reduce the corrosion rate. At this time, the outer Al-rich layer fails due to the inability to quickly form an Al2O3 film in this environment. This coating should have good application prospects in industrial and marine gas turbines.

　　3.2 Glass-Based Coatings

　　3.2.1 Enamel and Glass-Ceramic Coatings

　　Wang Fuhu and Zhu Shenglong, etc., developed enamel coatings for high-temperature alloys and titanium alloys. Enameling is the process of applying one or more layers of non-metallic inorganic materials to the surface of a metal. During high-temperature firing, the metal and inorganic materials undergo appropriate physical and chemical reactions at high temperatures, forming chemical bonds at the interface, so that the coating and the substrate material can be firmly bonded into a whole. Enamel coatings have adjustable thermal expansion coefficients, high thermochemical stability, dense structure, and excellent corrosion resistance; at the same time, the coating preparation process is simple and the cost is low; and as an inert high-temperature corrosion-resistant coating, there are no problems such as the consumption of anti-oxidation components in traditional high-temperature coatings; therefore, it has good application prospects as a long-life corrosion-resistant coating. To address the drawback of the inherent brittleness of enamel, Chen Minghui et al. modified the enamel by adding NiCrAlY metal powder to prepare a new type of metal composite enamel with excellent thermal shock resistance, and conducted in-depth research on its toughening mechanism.

　　Das et al., Datta et al., and Sarkar et al. prepared high-temperature MgO-Al2O3-TiO2, ZnO-Al2O3-SiO2, and BaO-MgO-SiO2 glass-ceramic coatings. These coatings were also prepared using high-temperature firing and are suitable for gas turbine nickel-based superalloy components and can also be used for the protection of γ-TiAl alloys. The coatings exhibit excellent oxidation resistance and good thermal shock resistance below 1050℃. They also attempted to prepare the above coatings using microwave heating.

　　3.2.2 Metal-Glass Based Composite Thermal Barrier Coatings

　　The literature proposes a novel metal-glass based composite thermal barrier coating (MGC) composed of glass and NiCoCrAlY alloy. The MGC coating is prepared by vacuum plasma spraying. The thermal expansion coefficient of the coating can be adjusted by the ratio of metal to glass to match the substrate. In addition, the coating is airtight and can protect the substrate and bond coat from corrosive atmospheres. Constant and cyclic oxidation tests in air at 1000 and 1200℃ show that the life of the MGC coating is significantly higher than that of the traditional YSZ thermal barrier coating.

　　Glass-based coatings have the advantages of high thermal stability and good corrosion resistance, but their disadvantages, such as softening at high temperatures and high brittleness at lower temperatures, limit their use. By adjusting and optimizing the coating composition, its softening point can be improved and its toughness can be enhanced. Therefore, this type of coating still has great application potential.

 

　　4. Recent Research Progress on High-Temperature Protective Coatings

　　4.1 Pt-modified aluminide coatings and MCrAlY coatings

　　Pt-modified aluminide coatings and MCrAlY coatings have been widely used as high-temperature protective coatings or bond coats for thermal barrier coatings because they possess both oxidation and hot corrosion resistance. Recent research on these two coatings mainly includes: modifying the coatings by adding new elements to improve the oxidation resistance of the coatings, for example, adding Ir to Pt-modified aluminide coatings and Re to NiCrAlY coatings; preparing gradient composition coatings to improve the oxidation resistance of the coatings, for example, aluminizing the surface of MCrAlY coatings or using PVD to deposit Al-rich coatings (pure Al or AlSiY, etc.) followed by heat diffusion treatment to obtain composite coatings with a gradient distribution of Al content in the coating.

　　At the same time, improvements and new explorations have been made in coating preparation techniques. MCrAlY coatings are mostly prepared by PVD. Due to the "line-of-sight" effect, the coating thickness is uneven on the surface of components with complex shapes. Lu Jintao developed a new thermal diffusion process to prepare MCrAlY coatings: a Y-Cr co-infiltration + secondary aluminizing method was used to prepare NiCrAlY co-infiltration coatings. First, a Y slurry with a thickness of 0.8-1.0 mm was sprayed onto the cleaned sample (K417G and K438 substrates); then, the sample after spraying the slurry was buried in Cr-containing powder, and thermal diffusion was carried out in an argon protective atmosphere to obtain a Y-Cr coating; finally, the sample with the Y-Cr coating was buried in FeAl-containing powder, and thermal diffusion was carried out in an argon protective atmosphere to obtain a NiCrAlY co-infiltration coating. The coating showed excellent high-temperature oxidation and hot corrosion resistance.

　　Composite plating is a process in which metals (such as Ni, Co, Cr, Cu, etc.) and solid particles (such as Al2O3, Cr2O3, SiO2, etc.) are co-deposited by electroplating to obtain the desired coating. In the mid-1980s, Foster et al. and Honey et al. proposed to develop an oxidation-resistant Ni-Cr-Al type coating by electrodepositing Ni and pre-alloyed, micro-sized, high-Cr, Al-containing particles simultaneously. Foster et al. [63] advocated placing the cathode surface horizontally and rotating it, which not only increases the inclusion rate of particles but also effectively prevents the formation of excess precipitates. However, at that time, only the feasibility of co-deposition and the factors affecting co-deposition were analyzed theoretically.

　　Yang et al. [65, 66] first developed binary Ni-Cr and Ni-Al nanocomposite coatings by electroplating, and then prepared ternary Ni-6Cr-7Al nanocomposite coatings. The hot corrosion behavior of the coatings in 900℃ (0.9Na, 0.1K) 2SO4 molten salt was evaluated. The results show that compared with traditional arc-melted Ni-6Cr-7Al, the electroplated composite coating has significantly improved hot corrosion resistance in molten salt. The effects of different contents and sizes of Cr and Al nanoparticles on the electroplated Ni-Cr-Al composite coating were studied, and the oxidation mechanism was discussed, especially the selective oxidation of Cr and Al.

　　Praxair Surface Technologies, Inc. uses electroplating to co-deposit CrAlY powder with Ni and (or) Co, followed by subsequent heat treatment (usually 1100℃ for 2h) to obtain an MCrAlY alloy coating.

　　M+CrAlY→MCrAlY

　　A patent was applied for this method of manufacturing MCrAlY coatings, and it was named Tribomet MCrAlY. This method has many advantages, such as: no line-of-sight effect, thickness can reach 101μm, no thermal stress on the substrate material, environmentally friendly, low cost, etc. It can be used as a coating for turbine blades, guide vanes, and bond coats for thermal barrier coatings.

　　4.2 γ′ based coatings

　　Coating systems designed for single-crystal superalloys aim to inhibit the formation of SRZ. These coatings include the EQ coatings mentioned above, and Pt-modified γ′-Ni3Al coatings, the latter exhibiting excellent high-temperature oxidation resistance. The mechanism of Pt is related to its promotion of uphill diffusion of Al.

　　4.3 Novel Thermal Barrier Coating Systems

　　At 1170℃, pure zirconia undergoes a monoclinic to tetragonal phase transformation. The volume change accompanying the phase transformation causes cracks in the coating. The addition of stabilizers such as Y2O3 inhibits the phase transformation. Therefore, the most commonly used material for the top layer of thermal barrier coatings is 8% Y2O3 partially stabilized ZrO2, which can be used for a long time at temperatures below 1200℃. However, at higher temperatures, ZrO2 undergoes a transformation from the t' phase to the monoclinic and cubic phases, and the sintering of the coating leads to a decrease in the strain tolerance of the coating, thereby accelerating crack generation and subsequent spalling and failure in the coating. In order to meet the development direction of aero-gas turbines towards higher pressure ratios, thrust-to-weight ratios, and inlet temperatures, researchers are constantly working on the research and development of new thermal barrier coating materials and preparation technologies, structural optimization of thermal barrier coatings, and exploration of coatings and technologies resistant to CMAS corrosion.

　　4.3.1 New Ceramic Thermal Insulation Materials

　　New ceramic thermal insulation materials are required to have a variety of advantages, including high-temperature sintering resistance, phase stability from room temperature to service temperature, the lowest possible thermal conductivity, controllable coating composition and structure, and a long service life. Recently, Vasen et al. summarized and classified the newly studied thermal barrier coating ceramic materials in recent years.

　　The first category has an A2B2O7 structure, including Ln2Zr2O7 (Ln is La, Gd, Sm, Nd, Eu, Yb or a combination thereof) based on ZrO2 (B=Zr), La2Hf2O7 and Gd2Hf2O7 based on HfO2 (B=Hf), La2Ce2O7 and La2(Zr0.7Ce0.3)2O7 based on CeO2 (B=Ce). These materials have attracted attention due to their lower thermal conductivity than YSZ. The most promising is La2Zr2O7 (LZ). Compared with YSZ, La2Zr2O7 has higher thermal stability (below 2000℃), lower thermal conductivity, and better sintering resistance. However, due to its lower thermal expansion coefficient than YSZ, the thermal stress caused by thermal expansion mismatch is greater, resulting in a very short service life of the single-layer La2Zr2O7 coating. The preparation of a La2Zr2O7/YSZ double-layer ceramic layer seems to solve this problem, so it has received widespread attention and research. Existing research results show that compared with the YSZ single-layer structure, the LZ/YSZ double-layer structure coating has a longer life, better CMAS erosion resistance, and can overcome the reaction between LZ and α-Al2O3 at high temperatures, which leads to the low thermal cycle life of the single-layer LZ thermal barrier coating. Moreover, LZ and YSZ do not react chemically below 1250℃, making the double-layer system have good chemical stability.

　　The second category is called defect cluster thermal barrier coatings (DefectclusterTBCs), which are ZrO2 modified with rare earth element cations, such as ZrO2 stabilized with 5.5mol% Y2O3-2.25mol% Gd2O3-2.25mol% Yb2O3. After modification, the thermal conductivity of the coating is significantly reduced.

　　The nominal composition of the third category can be written as (La,Nd)MAl11O19, where M is Mg, MnZn, Cr, Sm. These compounds have high melting points, large thermal expansion coefficients, low thermal conductivity, strong sintering resistance, and stable structure (below 1800℃), suitable for thermal barrier coating materials. However, the recrystallization phenomenon during the plasma spraying deposition process of this type of coating is the main drawback that restricts its application.

　　The fourth category has an ABO3 structure, which is divided into zirconate, such as BaZrO3, SrZrO3, CaZrO3, and composite, such as Ba(Mg1/3Ta2/3)O3, La(Al1/4Mg1/2Ta1/4)O3. The common characteristics of these compounds are high temperature stability, and they each have their own advantages and disadvantages compared to YSZ.

　　4.3.2 New Ceramic Coating Preparation Technology

　　In addition to the development of new ceramic thermal insulation materials, explorations have also been carried out in new coating preparation technologies. The hollow cathode physical vapor deposition technology and thin-film/low-pressure plasma spraying technology (TF-LPPS) have both prepared columnar crystal structure thermal barrier coatings. Compared with EB-PVD, the cost is reduced, the diffraction performance is better, it is suitable for coating complex-shaped parts, and the deposition rate is higher. Suspension plasma spraying technology [70] has been used to prepare low thermal conductivity ceramic thermal insulation layers with higher porosity and microcrack density.

　　4.3.3 Optimization of Coating Structure

　　Using existing coating preparation technologies, including EBPVD and APS, by changing the deposition process and controlling the growth direction of the columnar crystals and the type of pores in the coating, ceramic layers with different microstructures can be obtained, thereby optimizing the thermal conductivity and mechanical properties of the ceramic thermal insulation layer. For example, by tilting the rotating axis or sample, a bent (Zig-Zag) columnar crystal structure is obtained, which reduces the thermal conductivity of the coating by 40% [71-73].

　　4.3.4 Research on CMAS Corrosion Resistant Coatings and Technologies

　　CMAS mainly refers to Ca-Mg-Al-Si oxides. Depending on the environment, trace oxides of Ni, Fe, Ti, and Cr may also be mixed in. CMAS has a relatively low melting point (1190~1260℃), which is even lower in the presence of impurities such as sulfur. When CMAS deposits on the surface of a thermal barrier coating, it can wet the outer YSZ layer and penetrate into the pores inside the thermal barrier coating, reducing or even eliminating the pores and columnar grain boundaries, thereby reducing the strain tolerance of the coating and leading to spalling of the thermal barrier coating. Thermal barrier coatings eroded by CMAS are divided into three types: penetration-resistant, sacrificial, and non-wetting. Penetration-resistant coatings are those that can prevent liquid CMAS from penetrating into the pores of the YSZ layer. They can be metals, oxides, and non-oxides, such as 80%Pd-20%Ag, Pd, Pt, SiC, SiO2, Ta2O5, CaZrO3, MgAlO4, Si-OC, etc. Sacrificial coatings are those that react with CMAS at high temperatures, increasing its melting point or viscosity. Examples include Al2O3, MgO, CaO, Sc2O3, SiO2, MgAlO4, etc. Non-wetting coatings are those coated on the surface of TBC to reduce the contact area between molten CMAS and TBC and are non-wetting with TBC, such as 80%Pd-20%Ag, Pd, Pt, AlN, BN, SiC, MoSi2, SiO2, Zr-SiO4, SiOC, etc. However, existing research results show that the above coatings cannot completely prevent CMAS erosion. Another approach is to use post-treatment techniques or change the coating structure during the coating deposition process to prevent CMAS from eroding TBC, such as using electron beam glazing or laser glazing, or changing the deposition process to densify the surface of the thermal barrier coating. Preliminary research results show that the effect is good, but further research is needed.

　　4.3.5 Advanced Bond Coat

　　In the thermal barrier coating system, the properties of the TGO grown between the ceramic thermal insulation layer and the bond coat during high-temperature oxidation play a crucial role in the durability of the entire thermal barrier coating. Reducing the growth rate of TGO can significantly improve the life of the thermal barrier coating, and the growth rate of TGO is determined by the composition of the bond coat, etc. Therefore, research on advanced bond coats focuses on optimizing the composition of MCrAlY and Pt-modified aluminide coatings, preparing composition gradient bond coats, etc., to reduce the growth rate of TGO. This work is closely related to the research on new oxidation and corrosion-resistant coatings and is inseparable. It will not be elaborated here.

　　4.4 Diffusion Barrier Layer

　　Mutual diffusion between the high-temperature protective coating and the alloy substrate can lead to coating degradation and the formation of brittle phases in the substrate, resulting in a decrease in the high-temperature oxidation resistance and mechanical properties of the coating/alloy system. When traditional high-temperature protective coatings such as aluminized coatings (β-NiAl) and MCrAlY coatings are used for nickel-based single-crystal superalloys, mutual diffusion between the coating and the substrate leads to the formation of a harmful SRZ (secondary reaction zone) at the coating/substrate interface. The SRZ consists of a γ′ matrix phase and needle-like or lamellar γ and TCPs distributed on it. Its formation causes coarsening of the γ/γ′ structure of the single crystal alloy, significantly reducing the creep rupture life of the nickel-based single crystal alloy. Applying a diffusion barrier layer between the alloy and the coating is an effective solution. Research on diffusion barrier layers has been conducted early on and is gradually becoming a research hotspot. Diffusion barrier layer materials are mainly divided into two types: metal barrier layers and ceramic barrier layers. Metal barrier layers include Ta, Ir-Ta-Al, Hf-Ni, Ni-W, Re(NiCr), etc., while ceramic barrier layers include TiN, Al-O-N, Al2O3, Cr-Al-O-N, and Cr-O-N, etc. Metal diffusion barrier layers can only prevent the diffusion of some alloy elements, and they themselves will also diffuse and oxidize, forming brittle phases at the interface. In comparison, ceramic diffusion barrier layers have stronger diffusion barrier capabilities and can completely prevent mutual diffusion of all elements in the coating and substrate, but the bonding strength between the coating and the substrate needs to be improved.

 

　　5. Development Trends of High-Temperature Protective Coatings

　　In summary, the problems to be solved in high-temperature oxidation and corrosion-resistant coatings are still how to improve the oxidation and corrosion resistance of the coatings while suppressing the mutual diffusion between the coating and the substrate material (especially single-crystal superalloys). Preparing composition gradient coatings may solve this problem and become a future development direction.

　　Thermal barrier coatings will remain a research hotspot in the field of high-temperature protective coatings in the future. Research and development of new ceramic layer materials with advantages such as high-temperature anti-sintering, phase stability in the range from room temperature to service temperature, low thermal conductivity, and long service life; optimization of the composition and structure of the bond coat to form a TGO with a slow growth rate and good adhesion; development of new coating preparation technologies with good diffraction, high deposition rate, and deposited coatings with good thermal insulation performance and high strain tolerance may be future development trends.