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Research Progress of High Temperature Protective Coating

  • Time of issue:2018-10-15 18:46

(Summary description)High-temperature protective coatings can provide effective anti-oxidation and corrosion protection for metal materials used at high temperatures, and have been widely used in aerospace, energy, petrochemical and other fields.

Research Progress of High Temperature Protective Coating

(Summary description)High-temperature protective coatings can provide effective anti-oxidation and corrosion protection for metal materials used at high temperatures, and have been widely used in aerospace, energy, petrochemical and other fields.

  • Categories:Industry News
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  • Origin:Northeastern University Materials
  • Time of issue:2018-10-15 18:46
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  1 Introduction

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

  In order to further improve the working efficiency of the turbine engine and achieve the purpose of energy saving and emission reduction, it is necessary to increase the engine's inlet temperature. Therefore, scientists continue to devote themselves to the research and development of more advanced materials, coating systems and preparation technologies, such as the development of The fourth-generation nickel-based single crystal superalloy has a temperature bearing capacity of 1180℃. Correspondingly, higher requirements are put forward for high-temperature protective coatings, and a variety of 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 developments in high-temperature coating research at home and abroad. The development trend of high-temperature protective coatings is prospected.

 

  2. Commonly used high temperature protective coating

  2.1 Diffusion coating

  The coating formed by bringing some anti-oxidation elements, such as Al, Cr, Si, etc., into contact with the metal substrate and entering the surface of the substrate is a diffusion coating. In the formation process of the diffusion coating, the substrate participates in the formation of the coating, the elements in the substrate enter the coating, and the diffusion layer is formed in the substrate under the coating. Diffusion coatings include aluminized coatings, chromized coatings, siliconized coatings, and improved aluminized coatings. Representative aluminized coatings and improved aluminized coatings.

  2.1.1 Aluminized coating

  Aluminide coatings (aluminide coatings) were first described by VanAller in a US patent in 1911. They were prepared by powder embedding method (packcementation). Later, hot-dip coatings and slurry coatings appeared. ), and non-contact "above-the-packaluminizing" and chemical vapor deposition (CVD) aluminizing methods. In the 1950s, powder embedded aluminizing coatings began to be used for cobalt-based guide blades. By the 1970s, most nickel-based and cobalt-based turbines and guide blades used powder embedded aluminizing coatings and non-contact infiltration. Aluminum coating.

  In the powder embedding aluminizing method, the sample is embedded in the infiltrating agent powder. The infiltrating agent is composed of aluminum source powder, halide activator and filler. The aluminum source powder can be metal Al or suitable alloy powder. The filler is usually inert. Al2O3. Penetrating agent generally contains 2%~5% activator, such as ammonium chloride, 25% aluminum source, and the rest is filler. When heated, the activator volatilizes in the penetrating agent and reacts with the aluminum source to generate a volatile coating metal compound. Volatile substances diffuse to the surface of the substrate, where a deposition reaction occurs. When aluminizing, a protective gas such as argon 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 infiltrating agent, the aluminizing temperature, the composition of the substrate, and the post-treatment process. Take an aluminized 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. The growth of the coating during the aluminizing process mainly depends on Al passing through. The initially formed Ni2Al3 surface layer diffuses inward to form an internal diffusion coating, also known as high activity aluminizing (HALT). This coating needs to undergo 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. The growth of the coating mainly depends on the outward diffusion of Ni and the combination of Al deposited on the surface to form NiAl phase , Forming an external diffusion coating, also known as low activity aluminizing (LAHT).

  Powder embedded aluminizing has several advantages compared with other methods of preparing diffusion coatings. One is that the infiltrating agent can support the material to be infiltrated and can prevent large devices from bending down. Commercially used aluminizing processes can be several The aluminum-rich coating is prepared for a meter-long pipe; the second is the contact between the penetrating agent and the substrate, so that the composition of the penetrating layer is more uniform and the deposition rate is faster, but there are also shortcomings. Aluminizing and chemical vapor deposition aluminizing in the non-contact aluminizing process "substrate on top of the infiltrating agent" can avoid this situation. The reaction gas is generated from the outside during the deposition process and flows upward to the surface of the substrate. The latter is generated from the outside during the deposition process, and then filled into the vacuum container containing the infiltrated material. Therefore, the composition of the reaction gas is adjustable and the reaction gas can be transported. Into the inner cavity, such as the inner cooling hole of a gas turbine blade.

  Simple aluminized coating has good oxidation resistance, simple process, stable performance and low cost. However, there are also many disadvantages, such as poor thermal corrosion resistance, especially type II thermal corrosion, high brittleness of the coating, and rapid degradation. 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. There are mainly the following types of improved aluminide coatings:

  (1) Cr modified aluminide coating

  Adding Cr to the coating can significantly improve the thermal corrosion resistance of the coating and slow down the degradation caused by the mutual diffusion of the coating and the substrate. The preparation of the coating can adopt a one-step method and a two-step method. Because the thermal stability of Al and Cr halides is very different, it is difficult to achieve Al and Cr co-infiltration by pure metal powder embedding infiltration method. Therefore, in the early days, a two-step method was usually used to prepare chromium-modified aluminide coatings, that is, in advance. The chromium layer is deposited on the metal substrate by embedding, slurry, electroplating and other methods, and then embedding thermal diffusion aluminizing. By using the Cr-Al binary alloy, the vapor pressure of the relatively high Al halide can be reduced, and the co-infiltration of Al and Cr on the nickel-based alloy is realized. The coating is mainly NiAl phase, and Cr is dissolved in NiAl phase or precipitated as 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 mutual diffusion of the coating and the substrate, and can also improve its thermal corrosion resistance. Compared with Al-Cr co-infiltration, it has better resistance to high temperature oxidation, but Si The content of Si should not be too high, because Si and Ni will form harmful low-melting phases at high temperatures to make the coating brittle and easy to peel off during the oxidation process. The most common method for preparing Al-Si coatings is the slurry method. The coating is mainly β-NiAl phase, and Si is distributed in the coating with Si-rich second phase particles.

  (3) Pt modified aluminide coating

  Among the improved aluminide coatings, the Pt-Al coating has the most obvious modification effect. Pt improves the anti-stripping and self-healing ability of the α-Al2O3 film, increases the structural stability of the aluminide coating, and reduces the mutual diffusion between the coating and the substrate. The preparation of Pt-Al coating usually adopts a two-step method. First, a layer of Pt is electroplated on the alloy substrate, then annealed, and then powder embedded aluminizing is performed after annealing. The coating is usually a double-layer structure, the outer layer is a dual-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 coating has been widely used in the hot end parts of gas turbines.

  2.2 Coating

  "Cover coating" refers to the coating formed by directly depositing the coating material on the surface of the alloy by physical or chemical means. The obvious difference between the cladding coating and the diffusion coating is that the coating only interacts with the substrate when it is deposited, which can improve the bonding force of the coating, and the substrate does not participate in the formation of the coating, so the choice of coating composition is more diverse . The cladding coating can be a metal coating, a ceramic coating, etc., and the most typical one is the MCrAlY cladding coating.

  The MCrAlY cladding coating was developed in the 1970s and has now developed into a series of coating systems, where M is Fe, Co, Ni or their combination, Al is used to form a protective Al2O3 film, and Cr is used to promote The formation of oxide film and improve the thermal corrosion resistance, Y is used to improve the adhesion of the oxide film, the coating can also be added by adding one or more of Hf, Si, Ta, Re, Zr, Nb and other elements to Meet some specific application requirements. This type of coating is mainly composed of β phase (NiAl or CoAl) and γ 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 thermal corrosion, and the Ni-Co-Cr-Al-Y coating takes care of both. The literature compares the oxidation resistance and thermal corrosion resistance of aluminized coatings, improved aluminide coatings and MCrAlY coatings. By adjusting the composition of the MCrAlY coating, an oxidation-resistant coating and a thermal corrosion-resistant coating can be prepared to meet the needs of different working environments and different base 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 gas hood plasma spraying, supersonic flame spraying, etc. .

  2.3 Thermal Barrier Coating

  The main function of    Thermal Barrier Coatings (TBCs) is heat insulation, which consists of a ceramic surface layer with low thermal conductivity and a metal bonding layer. The early thermal barrier coating was Al2O3 and ZrO2 (MgO or CaO stabilized) ceramic heat insulation layer sprayed directly on the alloy surface. In the mid-1970s, NiCrAlY was used as the bonding layer and plasma spraying technology to prepare the Y2O3 stabilized ZrO2 surface layer and the The EB-PVD technology developed in the early 1980s to deposit ceramic surface layer is an important development in the history of the development of thermal barrier coatings. The bonding layer of current thermal barrier coatings is mostly MCrAlY and Pt modified aluminide coatings. The main function of the metal bonding layer is to increase the bonding force between the ceramic coating and the substrate and improve the mismatch of the thermal expansion coefficient between the two. It also improves the oxidation resistance of the substrate. 8% Y2O3 partially stabilized ZrO2 (Y-PSZ) has high melting point, high temperature stability, low thermal conductivity, and thermal expansion coefficient closest to the base material, making it the material of choice for ceramic insulation. At high temperatures, the 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 prevent the oxidation of the substrate.

  There are many methods for preparing ceramic heat insulation layer, 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, often containing 15% to 25% porosity, so it has low thermal conductivity and a certain strain tolerance. It is usually used in aero-engines with lower requirements. , Such as combustion chamber, combustion evaporator, stator blades, etc. The Y-PSZ layer prepared by EB-PVD has a columnar crystal structure and has a high strain tolerance during temperature changes, so it has a longer life than plasma sprayed coatings, but the equipment is expensive and the cost is high. It is used in engine conditions. In more demanding components, such as aviation gas turbine blades. The thermal insulation effect of TBCs can reach 175°C.

  At present, the main challenge facing the application of TBCs is the durability of the coating, especially the ability of the coating to resist peeling. There are many influencing factors, such as the stress state in the ZrO2 layer, the microstructure of the bonding layer, and the thickness of the TGO layer. The stress state and the 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 life of EB-PVDTBCs.

 

  3. Featured 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 and other disciplines 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—superalloy microcrystalline coating. Different from the traditional high-temperature protective coating, the microcrystalline coating and the base alloy composition are exactly the same, thus avoiding the drop in mechanical properties caused by the mutual diffusion of the coating and the base at high temperature. At the same time, the coating grain size is 20 ~100nm, not only can promote the selective oxidation of A1, but also improve the adhesion of the oxide film. The study on the oxidation behavior of Co-30Cr-5Al alloy and its sputtered microcrystalline coating in air at 1100°C shows that the alloy forms a protective Al2O3 film on the surface of the alloy during the first 25 hours of oxidation, and the Al2O3 film cracks after 25 hours. Exfoliation and the formation of Cr2O3 film resulted in a rapid increase in the weight of the alloy. In contrast, the sputtered microcrystalline coating showed its excellent protection. After 100h of oxidation, the oxide film is still a uniform and dense Al2O3 film, and there is no oxide film. Separation from the substrate and cracking and peeling.

  3.1.2EQ 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 the single crystal alloy is significantly higher than that of traditional superalloys, and the coating and substrate The inter-diffusion caused the formation of a harmful SRZ zone (secondary reaction zone) at the coating/substrate interface, which significantly reduced the creep rupture life of the nickel-based single crystal alloy. Kawagishi et al. and Sato et al. proposed to prepare EQ coating (equilibriumcoating) to inhibit the formation of SRZ. Nickel-based superalloys are composed of γ and γ′ phases. The two phases maintain an equilibrium state. Therefore, the chemical potentials of the elements in the two phases are equal. The γ'phase in the EQ coating material causes zero chemical potential difference of the elements interdiffusion between the coating and the substrate, so the inter-diffusion of the coating and the substrate can be inhibited. However, the oxidation resistance of this coating is limited, and it is easy to degrade into γ and γ'phases when the oxidation time is long.

  3.1.3 Functionally graded coating

  Functionally graded coating is the application of functionally graded material (FGM) design concept in coating/substrate system. The basic idea of ​​functionally graded materials is to prepare two or more different materials into composite materials with gradient distribution of composition (or/and structure) in a certain direction, so that the material has functions that cannot be achieved by non-gradient structures. The application of the FGM concept in the coating/substrate system is to solve the interface problem. In the coating/substrate system, when the coating and the substrate and/or the materials constituting the coating are different, the interface of different materials will cause serious changes near the interface due to the sudden change of the material properties (thermal expansion coefficient, elastic modulus, 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, and the composition (and/or structure) of the two materials in the layer changes continuously along the thickness to reduce and overcome the performance mismatch of the bonding site and slow down Stress field. The most researched high-temperature protective coatings are functionally graded thermal barrier coatings. As mentioned above, thermal barrier coatings are composed of 8% Y2O3-ZrO2 ceramic top layer and MCrAlY metal bonding layer. The mismatch of the properties of ceramic and metal materials causes heat The ceramic layer peels off during the cycle, and a gradient coating is prepared between the ceramic top layer and the metal bonding layer by plasma spraying, etc., so that the ceramic and metal components in the layer change gradually along the thickness direction to alleviate the mismatch of the ceramic/metal interface. Although the current results are not very satisfactory, the exploration in this area has been continuing.

  3.1.4 Smart Coating

  The smart coating (smartcoatings) proposed by Nicholls et al. is a composition gradient coating system that can respond best to corrosion in a wide temperature range and complex corrosive environment. The components provide corrosion protection. The coating has the function of resisting both high temperature oxidation and low temperature thermal corrosion. The coating is based on MCrAlY coating, and the outer layer is an aluminum-rich layer. In a high temperature oxidation environment above 900℃ and type I thermal corrosion conditions above 800℃ It can quickly form Al2O3 film and play a protective role; the middle layer is a chromium-rich layer, which can be used as a diffusion barrier under high temperature conditions to prevent the Al in the aluminum-rich layer from diffusing to the substrate and the elements in the substrate from diffusing to the aluminum-rich layer. It can quickly form Cr2O3 under the condition of type II hot corrosion at 600~800℃ to reduce the corrosion rate when the temperature is low. At this time, the outer aluminum-rich layer can not quickly form Al2O3 film in this environment and corrosion failure. This kind of coating should have good application prospects on industrial and marine gas turbines.

  3.2 Glass-based coating

  3.2.1 Enamel and glass-ceramic coating

  Wang Fuhui and Zhu Shenglong have developed enamel coatings for high-temperature alloys and titanium alloys. Enamel is to coat one or more layers of non-metallic inorganic materials on the metal surface. During high-temperature enameling, the metal and inorganic materials undergo appropriate physical and chemical reactions at high temperatures to form chemical bonds at the interface, so that the coating and the base material can be firm. Combine into a whole. The enamel coating has adjustable thermal expansion coefficient, high thermochemical stability, compact structure, and excellent corrosion resistance; at the same time, the coating preparation process is simple and low in cost; and as an inert high temperature corrosion resistant coating, there is no traditional high temperature coating The anti-oxidation component consumption and other problems; therefore, it has a good application prospect as a long-life corrosion-resistant coating. Aiming at the shortcomings of the brittleness of the enamel itself, Chen Minghui and others added NiCrAlY metal powder to the enamel to modify the enamel, prepared 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 MgO-Al2O3-TiO2, ZnO-Al2O3-SiO2 and BaO-MgO-SiO2 high-temperature resistant glass-ceramic coatings. This coating is also prepared by high-temperature enamelling, which is suitable for Nickel-based superalloy parts for gas turbines can also be used for the protection of γ-TiAl alloys. The coating has excellent oxidation resistance and good thermal shock resistance at 1050°C and below. They also tried to prepare the above-mentioned coating by microwave heating.

  3.2.2 Metal-glass-based composite thermal barrier coating

  The literature proposed a new type of metal-glass-based composite thermal barrier coating (MGC for short) composed of glass and NiCoCrAlY alloy. The MGC coating is prepared by vacuum plasma spraying, and 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 the bonding layer from corrosive atmospheres. The experimental results of constant temperature and cyclic oxidation in air at 1000 and 1200°C show that the life of MGC coating is significantly longer than that of traditional YSZ thermal barrier coating.

  Glass-based coating has the advantages of high thermal stability and good corrosion resistance, but it is easy to soften at high temperature, and its use is limited by shortcomings such as greater brittleness at lower temperature. By adjusting and optimizing the coating composition, its softening point can be increased and strengthened. Because of its toughness, this type of coating has great application potential.

 

  4. The latest research progress of high temperature protective coatings

  4.1Pt modified aluminide coating and MCrAlY coating

  Pt modified aluminide coating and MCrAlY coating have both anti-oxidation and thermal corrosion properties at the same time, and they have been widely used as high temperature protective coatings or bonding primers for thermal barrier coatings. In recent years, the research on these two kinds of coatings mainly includes: modifying the coating by adding new elements to improve the oxidation resistance of the coating, for example, adding Ir to the Pt modified aluminide coating and adding to the NiCrAlY coating Re; Prepare composition gradient coating to improve the oxidation resistance of the coating, for example, aluminizing on the surface of the MCrAlY coating or depositing an Al-rich coating (pure Al or AlSiY, etc.) by PVD and then thermal diffusion treatment to obtain the Al content in the coating Composite coating with gradient distribution in the layer, etc.

  At the same time, improvements and new explorations have also been made in the coating preparation technology. MCrAlY coatings are mostly prepared by PVD method. Due to the "line of sight" effect, the thickness of the surface coatings on components with complex shapes is uneven. Lu Jintao developed a new type of thermal diffusion process to prepare MCrAlY coatings: using Y-Cr co-infiltration + secondary aluminizing method to prepare NiCrAlY co-infiltration coatings. First, spray a layer of Y slurry with a thickness of 0.8~1.0mm on the surface of the clean sample (K417G and K438 substrate); then embed the sample after spraying the coating slurry in the Cr-containing powder, and thermally diffuse it in an argon protective atmosphere. Y-Cr coating: Finally, the sample with the obtained Y-Cr coating is buried in FeAl-containing powder, and thermally diffused in an argon atmosphere to obtain a NiCrAlY co-permeated coating. The coating exhibits excellent resistance to high temperature oxidation and thermal corrosion.

  Composite plating (compositeplating) 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 to obtain the desired coating by electroplating. In the mid-1980s, Foster et al. and Honey et al. proposed to simultaneously deposit Ni and pre-alloyed, micro-sized, high-Cr, and Al-content particles through electrodeposition to develop an oxidation-resistant Ni-Cr-Al type package. Coating. 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 sediment. But at that time, only theoretically analyzed the feasibility of co-deposition and the factors affecting co-deposition.

  Yang et al. [65,66] first developed binary Ni-Cr and Ni-Al nano-composite coatings by electroplating, and then prepared ternary Ni-6Cr-7Al nano-composite coatings. The thermal corrosion behavior of 0.9Na, 0.1K) 2SO4 molten salt was evaluated, and the results showed that compared with traditional arc smelting Ni-6Cr-7Al, the thermal corrosion resistance of electroplated composite coating in molten salt was greatly improved. The effects of different content and size of Cr and Al nanoparticles on electroplated Ni-Cr-Al composite coatings were studied and the oxidation mechanism was discussed, especially the selective oxidation of Cr and Al.

  Praxair Surface Technology Co., Ltd. uses electroplating to co-deposit CrAlY powder with Ni and/or Co, and then undergoes subsequent heat treatment (usually 2h at 1100°C) to obtain MCrAlY alloy coating.

  M+CrAlY→MCrAlY

  applied for a patent for the MCrAlY coating produced by this method and named it TribometMCrAlY. This method has many advantages, such as: no line-of-sight effect, a thickness of 101 μm, no thermal pressure on the base material, environmental friendliness, low cost, and so on. It can be used as a bonding layer for turbine blades, guide blade coatings and thermal barrier coatings.

  4.2γ'based coating

  The coating system designed for single crystal superalloys aims to suppress the formation of SRZ. Such coatings include the aforementioned EQ coatings, and Pt modified γ'-Ni3Al coatings, etc. The latter exhibits excellent high temperature oxidation resistance. The mechanism of Pt is related to its promotion of the uphill diffusion of Al. .

  4.3 New thermal barrier coating system

  Pure zirconia undergoes a monoclinic to tetragonal phase transition at 1170°C. The volume change accompanying the phase transition will cause cracks in the coating. The addition of stabilizers such as Y2O3 will inhibit the phase transition. Therefore, the current thermal barrier coating The most commonly used material for the top layer is 8% Y2O3 partially stabilized ZrO2, which can be used for a long time when the temperature is lower than 1200°C. However, at higher temperatures, ZrO2 will undergo a transition from t'phase to monoclinic and cubic phases, and the sintering of the coating will reduce the strain tolerance of the coating, thereby accelerating the generation of cracks in the coating and the subsequent Peeling and failure. In order to meet the development of aviation gas turbines towards higher pressure gas ratio, thrust-to-weight ratio and inlet temperature, researchers have been committed to the research and development of new thermal barrier coating materials and preparation technologies, structural optimization of thermal barrier coatings, and CMAX corrosion resistant coatings. Layer and technology exploration, etc.

  4.3.1 New ceramic insulation material

  New ceramic heat insulation layer materials require sintering resistance at high temperatures, phase stability from room temperature to service temperature, low thermal conductivity, controllable coating composition and structure, and long service life. Recently, Vasen et al. summarized and classified the new thermal barrier coating ceramic materials that have been studied in recent years.

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

  The second type is called DefectclusterTBCs, which is ZrO2 modified with rare earth element cations, such as 5.5mol%Y2O3-2.25mol%Gd2O3-2.25mol%Yb2O3 stabilized ZrO2, the modified coating The thermal conductivity is significantly reduced.

  The nominal composition of the third category can be written as (La, Nd) MAl11O19, M is Mg, MnZn, Cr, Sm, this type of compound has high melting point, large thermal expansion coefficient, low thermal conductivity, strong anti-sintering ability, and stable structure (1800℃ Below), suitable for thermal barrier coating materials. However, the recrystallization phenomenon of this kind of coating in the plasma spray deposition preparation process is the main disadvantage that restricts its application.

  The fourth type has the structure of ABO3, which is divided into zirconium acid types, such as BaZrO3, SrZrO3, CaZrO3, and composite types, such as Ba(Mg1/3Ta2/3)O3, La(Al1/4Mg1/2Ta1/4)O3. The common feature of these compounds is high temperature stability, and each has its own advantages and disadvantages compared to YSZ.

  4.3.2 New ceramic coating preparation technology

  In addition to the research and development of new ceramic insulation materials, it also explored new coating preparation technologies. Both hollow cathode physical vapor deposition technology and thin-film/low-pressure plasmaspraying (TF-LPPS) have been used to prepare thermal barrier coatings with columnar crystal structure. Compared with EB-PVD, the cost is reduced, and the cost is reduced. It has good shooting performance and is suitable for coating complex-shaped parts with a high deposition rate. The use of suspension plasma spraying technology (suspensionplasmaspraying) [70] prepared a ceramic insulation layer with higher porosity and microcrack density and low thermal conductivity.

  4.3.3 Optimize the coating structure

  Using existing coating preparation technologies, including EBPVD and APS, by changing the deposition process, controlling the growth direction of columnar crystals and the pore type of the coating, ceramic layers with different microstructures can be obtained, so as to improve the thermal conductivity of the ceramic insulation layer. The rate and mechanical properties are optimized, for example, by tilting the axis of rotation or the sample, a zig-zag columnar crystal structure is obtained, which reduces the thermal conductivity of the coating by 40% [71~73].

  4.3.4 Anti-CMAS corrosion coating and technology research

  CMAS mainly refers to the oxides of Ca-Mg-Al-Si. 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°C). The melting point will be lower in the presence of impurity sulfur. When CMAS is deposited on the surface of the thermal barrier coating, it can wet the outer layer YSZ and penetrate into the pores inside the thermal barrier coating, reducing or even losing the pores and columnar grain boundaries, thereby reducing the strain tolerance of the coating and causing heat Peeling of barrier coating. Protective thermal barrier coatings The coatings that are eroded by CMAS are divided into three types, anti-permeation type, sacrificial type and non-wetting type. Anti-penetration coating refers to a coating that can prevent liquid CMAS from penetrating into the pores of the YSZ layer. It can be metal, oxide and non-oxide, for example, 80%Pd-20%Ag, Pd, Pt, SiC, SiO2 , Ta2O5, CaZrO3, MgAlO4, Si-OC, etc.; sacrificial coating refers to a coating that reacts with CMAS at high temperature to increase its melting point or increase its viscosity, such as Al2O3, MgO, CaO, Sc2O3, SiO2, MgAlO4, etc.; non-wetting coating refers to a coating that is applied to the surface of the TBC in order to reduce the contact area between the molten CMAS and the TBC and does not wet the 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-mentioned coatings cannot completely prevent CMAS corrosion. There is also the use of post-processing technology or changing the coating structure during the coating deposition process to prevent the erosion of TBC by CMAS, such as 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 bonding layer

  In the thermal barrier coating system, the properties of the TGO grown between the ceramic insulation layer and the bonding layer during the high temperature oxidation process play a vital role in the durability of the entire thermal barrier coating. Reducing the growth rate of TGO can significantly increase the life of the thermal barrier coating, and the growth rate of TGO is in turn determined by the composition of the bonding layer. Therefore, the research of advanced bonding layer focuses on optimizing the composition of MCrAlY and Pt modified aluminide coatings, preparing composition gradient bonding layer, etc. to reduce the growth rate of TGO. This work is related to the research of new anti-oxidation and corrosion coatings. It is closely related and indivisible, so I won't go into details here.

  4.4 Diffusion barrier

  The interdiffusion between the high temperature protective coating and the alloy substrate can lead to coating degradation and the production of brittle phases in the substrate, resulting in the degradation of the high temperature oxidation resistance and mechanical properties of the coating/alloy system. The traditional high-temperature protective coating aluminized coating (β-NiAl) and MCrAlY coating are used for nickel-based single crystal superalloys. The inter-diffusion of the coating and the substrate leads to the formation of a harmful SRZ zone at the coating/substrate interface (Secondaryreactionzone). SRZ is composed of γ'parent phase and needle-like or flake-like γ and TCPs distributed on it. Its formation makes the γ/γ' structure of single crystal alloys coarser and significantly reduces the creep fracture of nickel-based single crystal alloys. life. Applying a diffusion barrier between the alloy and the coating is an effective solution. The research work of diffusion barrier has been in the early stage, and now it has gradually become a research hotspot. Diffusion barrier 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., ceramic barrier layers include TiN, Al-ON, Al2O3, Cr-Al-ON and Cr-ON, etc. Metal The diffusion barrier layer can only prevent the diffusion of some alloying elements, and it will also diffuse and oxidize itself, and form a brittle phase at the interface. In contrast, the ceramic barrier layer has a stronger diffusion barrier ability and can completely prevent the coating and the substrate. The mutual diffusion of all elements in the material, but the bonding force between the coating and the substrate needs to be improved.

 

  5. The development trend of high temperature protective coatings

  In summary, the problem to be solved for high-temperature anti-oxidation and corrosion coatings is still how to inhibit the inter-diffusion of the coating and the base material (especially single crystal superalloys) while improving the oxidation and corrosion resistance of the coating, and prepare composition gradient coatings. The layer may solve this problem and become the direction of future development.

  Thermal barrier coatings will still be a research hotspot in the field of high-temperature protective coatings in the future. New ceramic coatings with multiple advantages such as anti-sintering at high temperatures, phase stability from room temperature to service temperature, low thermal conductivity, and long service life are developed. Material; optimize the composition and structure of the bonding layer to form a TGO with a slow growth rate and good adhesion; develop a structure with good diffraction, high deposition rate, and deposited coating with good heat insulation performance and high strain tolerance The characteristic new coating preparation technology may be the trend of future development.

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