The high temperature protective coating can provide effective anti-oxidation and corrosion protection for metal materials used at high temperatures, and has been widely used in aerospace, energy, petrochemical and other fields. The representative applications are in various gas turbine engines for aircraft, ship and ground power generation. The development of high temperature protective coatings has gone through three stages: first generation thermal diffusion coating, second generation M （M=Fe, Ni or Co） CrAlY coating; third generation thermal barrier coating.
In order to further improve the working efficiency of the turbine engine and achieve the goal of energy saving and emission reduction, it is necessary to increase the inlet temperature of the engine. Therefore, scientists are constantly working on the development of more advanced materials, coating systems and preparation techniques, such as The temperature-bearing capacity of the fourth-generation nickel-based single crystal superalloy has reached 1180 °C. Correspondingly, higher requirements have been placed on high temperature protective coatings, and a variety of new high temperature protective coatings with unique design concepts have emerged. This paper describes the structure, preparation method and application characteristics of common high temperature protective coatings, introduces several high temperature protective coatings, and summarizes the latest developments in high temperature coating research at home and abroad. The development trend of high temperature protective coatings is prospected.
2. Common high temperature protective coating
2.1 diffusion coating
Some of the antioxidant elements, such as Al, Cr, Si, etc., are in contact with the metal substrate, and the coating formed on the surface of the substrate is a diffusion coating. During the formation of the diffusion coating, the matrix participates in the formation of the coating, the elements in the matrix enter the coating, and a diffusion layer is formed in the substrate below the coating. The diffusion coating has an aluminized coating, a chromized coating, a siliconized coating, and an improved aluminized coating, such as an aluminized coating and an improved aluminized coating.
2.1.1 Aluminized coating
Aluminide coatings were first described by VanAller in U.S. Patent in 1911. They were prepared by powder embedding, followed by hot-dip coating and slurry coating. And non-contact ”substrate on the permeation agent“ aborting-the-packaluminizing and chemical vapor deposition （CVD） aluminizing and other preparation methods. Powder-embedded aluminized coatings began to be used in cobalt-based guide vanes in the 1950s. By the 1970s, most nickel-based and cobalt-based turbines and guide vanes used powder-embedded aluminized coatings and non-contact infiltration. Aluminum coating.
In the powder-embedded aluminizing method, the sample is embedded in the infiltrant powder, the infiltrant is composed of an aluminum source powder, a halide activator and a filler, and the aluminum source powder may be metal Al or a suitable alloy powder, and the filler is usually inert. Al2O3. The osmotic agent generally contains from 2% to 5% of an activator, such as ammonium chloride, 25% of the aluminum source, and the remainder is a filler. The activator volatilizes in the osmotic agent upon heating and reacts with the aluminum source to form a volatile coating metal compound. Volatile substances diffuse to the surface of the substrate and a deposition reaction takes place there. When aluminizing, a protective gas such as argon gas must be introduced to prevent the aluminum source and the metal substrate from being oxidized.
The structure and deposition rate of the aluminized coating depend on factors such as the activity of Al in the infiltrant, the aluminizing temperature, the composition of the substrate, and the post-treatment process. Taking a nickel-based superalloy coating as an example, in a relatively low temperature range, such as 700-800 ° C, the activity of Al is higher than Ni, and the growth of the coating during the aluminizing process mainly depends on Al. The initially formed Ni2Al3 surface layer diffuses inward to form an internal diffusion coating, also known as high activity aluminizing （HALT）, which is subjected to a secondary annealing treatment to form a NiAl phase. In a relatively high temperature range, such as 980~1090 °C, the activity of Al is lower than that of Ni. The growth of the coating mainly depends on the outward diffusion of Ni and the deposition of Al on the surface to form NiAl phase. Forming an outer diffusion coating, also known as low activity aluminizing （LAHT）.
Powder-embedded aluminizing has several advantages over other methods for preparing diffusion-type coatings. One is that the infiltrant has the function of supporting the infiltrated material, which can prevent large devices from bending down. The commercially used aluminizing process can be counted. The long pipe is made of aluminum-rich coating; the second is that the infiltrant is in contact with the substrate, so that the composition of the layer is relatively uniform, the deposition rate is fast, but there is also a deficiency, and the material in the infiltration agent is wrapped in the coating. This is avoided by aluminizing and chemical vapor deposition aluminizing of the non-contact aluminizing process ”substrate above the infiltrant“. The former is to fix the workpiece on top of the infiltrant, and the coating reaction gas is infiltrated. Produced and flowed up to the surface of the substrate. The latter reacted with the coating reaction gas from the outside during the deposition process, and then filled into the vacuum vessel containing the material to be infiltrated, so that the composition of the reaction gas is adjustable, and the reaction gas can be transported. Into the inner cavity, for example in the inner cooling hole of the gas turbine blade.
The simple aluminized coating has good oxidation resistance, simple process, stable performance and low cost. However, there are also many disadvantages, such as hot corrosion resistance, especially type II hot corrosion performance, large coating brittleness, fast degradation, etc. In the 1970s, improved aluminide coatings were developed.
2.1.2 Improved aluminide coating
Adding a small amount of Si, Cr, Pt and other elements to the simple aluminized coating can significantly improve the performance of the coating. The improved aluminide coatings are mainly the following:
（1） Cr modified aluminide coating
The addition of Cr to the coating can significantly improve the hot corrosion resistance of the coating and slow the degradation caused by the interdiffusion of the coating and the substrate. The coating can be prepared in a one-step and two-step process. Since the thermal stability of the halides of Al and Cr differs greatly, it is difficult to realize the co-infiltration of Al and Cr by the method of embedding and infiltrating pure metal powder. Therefore, the chromium-modified aluminide coating is usually prepared by a two-step method in the early stage, that is, in advance The chromium layer is deposited on the metal substrate, and the method can be embedded, slurryed, and plated, and then thermally diffused and aluminized. 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 the nickel-based alloy is achieved. The coating is mainly composed of a NiAl phase, and Cr is 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 the interdiffusion of the coating and the substrate, and also improve its resistance to hot corrosion, and has better resistance to high temperature oxidation than Al-Cr co-infiltration, but Si The content should not be too high, because at high temperature, Si and Ni will form a harmful low melting point phase, which makes the coating brittle and easily peels off during oxidation. The most common method of preparing Al-Si coatings is the slurry method. The coating is dominated by a β-NiAl phase, and Si is distributed in the coating with Si-rich second phase particles.
（3） Pt modified aluminide coating
In the improved aluminide coating, the modification effect of the Pt-Al coating is most obvious. Pt improves the anti-flaking and self-healing ability of α-Al2O3 film, increases the structural stability of the aluminide coating, and reduces the interdiffusion between the coating and the substrate. The Pt-Al coating is usually prepared by a two-step process. First, a layer of Pt is electroplated on the alloy substrate, then annealed, and annealed and then subjected to powder-embedded aluminizing. The coating is usually a two-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 improved aluminide coating has been widely used in hot end parts of gas turbines and the like.
Coated coating refers to a coating formed by direct deposition of a coating material on the surface of an alloy by physical or chemical means. The difference between the coating and the diffusion coating is that the deposition of the coating only affects the adhesion of the coating to the substrate. The substrate does not participate in the formation of the coating, so the selection of the coating composition is more diverse. . The coating layer may be a metal coating, a ceramic coating or the like, and the most typical one is an MCrAlY coating.
MCrAlY coatings were developed in the 1970s and have been 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, and Cr is used to promote The formation of an oxide film and the improvement of the resistance to hot corrosion, Y is used to improve the adhesion of the oxide film, and the coating may also be added by one or more of Hf, Si, Ta, Re, Zr, Nb and the like. Meet some specific application needs. Such coatings consist essentially of a beta phase （NiAl or CoAl） and a gamma solid solution of Ni or Co. The nickel-based Ni-Cr-Al-Y coating has excellent oxidation resistance, the cobalt-based Co-Cr-Al-Y coating is more resistant to hot corrosion, and the Ni-Co-Cr-Al-Y coating is both. The literature compares the oxidation and hot corrosion resistance of aluminized coatings, modified aluminide coatings and MCrAlY coatings. Anti-oxidation coatings and hot corrosion resistant coatings can be prepared by adjusting the MCrAlY coating composition to meet the needs of different working environments and different matrix alloys. Common preparation methods for MCrAlY coatings include physical vapor deposition, including electron beam physical vapor deposition （EB-PVD）, sputtering, ion plating, and spray coating techniques, including low pressure plasma spraying, argon plasma spraying, supersonic flame spraying, etc. .
2.3 Thermal barrier coating
The main function of thermal barrier coatings （TBCs） is thermal insulation, consisting of a ceramic surface layer with a low thermal conductivity and a metal bonding layer. The early thermal barrier coatings were sprayed directly onto the surface of the alloy by Al2O3 and ZrO2 （MgO or CaO stabilized） ceramic insulation. In the mid-1970s, the Y2O3 stabilized ZrO2 surface layer was prepared using NiCrAlY as the bonding layer and plasma spray technique. The EB-PVD technology developed in the early 1980s deposited ceramic surface is an important advance in the development of thermal barrier coatings. The bonding layer of the current thermal barrier coating is mostly MCrAlY and Pt modified aluminide coating. The main function of the metal bonding layer is to increase the bonding force between the ceramic coating and the substrate, and improve the thermal expansion coefficient mismatch between the two. It also increases the oxidation resistance of the matrix. 8% Y2O3 partially stabilized ZrO2 （Y-PSZ） has high melting point, high temperature stability, low thermal conductivity and the closest thermal expansion rate to the matrix material, making it the material of choice for ceramic insulation. At high temperatures, Al in the bonding layer reacts with oxygen diffused from the ceramic layer to form a thermally grown oxide layer TGO （thermally grown oxides） between the bonding layer/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 thermal 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. Y-PSZ prepared by plasma spraying is a lamellar structure, often containing 15% to 25% porosity, so it has low thermal conductivity and has a certain strain tolerance. It is usually applied to components requiring lower requirements in aeroengines. For example, a combustion chamber, a combustion evaporator, a stator blade, and the like. The Y-PSZ layer prepared by EB-PVD is a columnar crystal structure with high strain tolerance during temperature change, so it has a longer life than plasma spray coating, but it is expensive, high cost, and is used in engine conditions. More demanding components such as aviation gas turbine blades. TBCs have an insulation effect of up to 175 °C.
At present, the main challenge for TBCs applications is the durability of the coating, especially the ability of the coating to resist flaking, which has many influencing factors, such as the stress state in the ZrO2 layer, the microstructure of the bonding layer, the thickness of the TGO layer, and Stress state and fracture resistance of various interfaces between the bonding layer and TGO. It is now recognized that the oxidation of the bonding layer is a key factor in determining the lifetime of EB-PVDTBCs.
3. Features high temperature protective coating
3.1 new concept coating
This type of coating introduces some basic theories of materials science, physical chemistry, solid diffusion, high temperature oxidation, etc. into the coating design, forming a unique high temperature coating system.
3.1.1 high temperature microcrystalline coating
Lou Hanyi and Wang Fuhui have developed a new high-temperature alloy protective coating - high-temperature alloy microcrystalline coating. Unlike conventional high temperature protective coatings, the microcrystalline coating is identical in composition to the base alloy, thus avoiding the degradation of mechanical properties caused by the interdiffusion of the coating and the substrate at high temperatures, while at the same time, the coating grain size is 20 ~100nm can not only promote the selective oxidation of A1, but also improve the adhesion of the oxide film. The oxidation behavior of Co-30Cr-5Al alloy and its sputtered microcrystalline coating in air at 1100 °C showed that the alloy formed a protective Al2O3 film on the surface of the first 25 h of oxidation, and the crack of Al2O3 film after 25 h. Exfoliation and the formation of Cr2O3 film lead to a rapid increase in the weight of the alloy. In contrast, the sputtered microcrystalline coating shows excellent protection. After 100 h of oxidation, the oxide film is still a uniform and dense Al2O3 film, and no oxide film appears. Separation from the substrate and cracking and flaking.
3.1.2 EQ coating
When the traditional high-temperature protective coating aluminized coating （β-NiAl） and MCrAlY coating are used in nickel-based single crystal superalloys, the content of refractory metals in single crystal alloys is greatly improved compared with conventional superalloys, coatings and substrates. The interdiffusion results in a harmful SRZ zone at the coating/substrate interface, which significantly reduces the creep rupture life of the nickel-based single crystal alloy. Kawagishi et al. and Sato et al. proposed the preparation of EQ coating to inhibit the formation of SRZ. The nickel-based superalloy consists of γ and γ‘ phases. The two phases maintain an equilibrium state, so the chemical potentials of the elements in the two phases are equal. The γ' phase as the EQ coating material causes the elemental chemical potential difference between the coating and the substrate to be zero, so that the interdiffusion of the coating and the substrate can be suppressed. However, this coating has limited oxidation resistance and is easily degraded into γ and γ' phases when the oxidation time is long.
3.1.3 Functional Gradient Coating
Functionally graded coatings are the application of functionally graded materials （FGM） in coating/matrix systems. The basic idea of ??a functionally graded material is to prepare two or more different materials into a composite material with a gradient distribution of components （or/and structures） in a certain direction, so that the material has a function that cannot be achieved by a non-gradient structure. The application of the FGM concept in coating/substrate systems addresses interface issues. In coating/substrate systems, when the coating is different from the substrate and/or the material that makes up the coating, the interface of the different materials is severe near the interface due to sudden changes in material properties （thermal expansion coefficient, modulus of elasticity, etc.）. The mismatch increases the driving force for the structure to peel off. 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 is continuously varied along the thickness to reduce and overcome the performance mismatch of the binding site and slow down Stress field. Most of the high temperature protective coatings are functionally graded thermal barrier coatings. As mentioned earlier, the thermal barrier coating is bonded by 8% Y2O3-ZrO2 ceramic top layer and MCrAlY metal.