Modern aircraft, such as spaceships, artificial satellites, rockets, missiles and supersonic aircraft, are developing towards the direction of high thrust, high speed and long distance, which puts forward higher requirements for the high temperature resistance of materials. For example, the nose cone of rocket and the leading edge of supersonic aircraft wing need to work in a neutral or oxidizing environment of 2000-2400 ℃. This makes the development of ultra-high temperature materials more and more urgent. The traditional ultra-high temperature materials mainly include the following three kinds: refractory metal materials and C / C composites represented by niobium, tantalum, tungsten and molybdenum, and ultra-high temperature ceramic materials (UHTCs) represented by transition metal borides, carbides and nitrides. Refractory metal materials have good high-temperature mechanical properties and are easy to process and shape, but they are easy to oxidize at high temperature. For example, niobium alloy will undergo catastrophic oxidation and spalling in air above 400 ℃. C / C composites have high strength, high modulus, good fracture toughness and wear resistance, but they are very prone to oxidation in the air environment above 370 ℃.

The application of the above two ultra-high temperature materials in extreme aviation environment requires the development of corresponding high-temperature oxidation resistant coatings. The melting point of ultra-high temperature ceramic materials is very high, such as HfB2, which can reach 3250 ℃, with high hardness, good chemical stability and excellent high-temperature oxidation resistance [3]. This series of advantages makes ultra-high temperature ceramic materials not only can be used independently as ultra-high temperature structural materials, It can also be used as the anti-oxidation coating of the first two high-temperature structural materials. Among many ultra-high temperature ceramic materials, boride ultra-high temperature ceramic materials TiB2, HfB2 and ZrB2 are considered to be the ultra-high temperature ceramic materials with the best oxidation resistance and become a research hotspot.

I Physical properties of boride ultrahigh temperature ceramics

Boride ultra-high temperature ceramic materials have high melting point and good chemical stability. Compared with other ultra-high temperature ceramic materials, boride ultra-high temperature ceramic materials have high conductivity, high thermal conductivity and good corrosion resistance. Under high temperature oxidation environment, it undergoes oxidation reaction MB₂+O₂→MO₂+B₂O₃, and the products are glass phase B2O3 and metal oxide Mo2. Some of their physical properties have been summarized in Table 1.

II Thermodynamic analysis of oxidation behavior of boride ultrahigh temperature ceramics

Taking the ultra-high temperature ceramic TiB Ω as an example, under the condition of high-temperature oxidation, TiB Ω may react with O Ω in the following ways. The standard Gibbs free energy of each reaction equation can be calculated by referring to the thermodynamic data table Δ Relationship between go and temperature T:

It can be seen from Fig. 1-10 that the reaction can occur under the standard oxidation temperature (k) of less than 17000. The oxidation kinetic conditions of TiB З materials in the same environment are similar. When the reaction temperature is between 500K and 1450k, the standard Gibbs free energy of reaction (1) is the smallest and the oxidation trend according to reaction (1) is the largest. When the temperature exceeds 1450k, the standard Gibbs free energy of reaction (6) is the smallest and the oxidation trend according to reaction (6) is the largest. Reaction (1) and reaction (6) are combined to obtain reaction (11):

When the temperature T = 1450 K, the standard Gibbs free energy of reaction (11) is less than 0, and reaction (11) begins. However, when oxidized in actual air, Pb З o З is much lower than the standard atmospheric pressure. According to van tehoff isothermal formula:

Rtln (Pb З o ₃ / P) < 0, make Δ GR = 0, t = 402548.58 / (277.68 - RLN (Pb ν o ₃ / P)), T < 1450 K. therefore, in the atmosphere of air oxidation, when the temperature is lower than 1450 K, the reaction formula (11) begins to occur. From the above inference, we can know that the following two reactions mainly occur when TiB2 ceramics are oxidized in high temperature air:

When the temperature is low, the reaction formula is: TiB З (s) + 5 / 2O З (g) = TiO З (s) + B З o З (L); When the temperature is high, the reaction formula is: TiB З (s) + 5 / 2O З (g) = TiO З (s) + B З o З (g).

III High temperature protection mechanism of boride ultra high temperature ceramics

The oxidation behavior of boride ultra-high temperature ceramics is different in different temperature ranges. We usually divide the oxidation temperature range into three temperature ranges. Take ZrB З as an example, respectively (1) (2) (3)

(1) T<1000℃；

(2) T=1000-1800℃；

(3) T>1800℃。

In the low temperature range of T < 1000 ℃, the oxide layer is mainly composed of porous refractory metal oxide skeleton ZrO ₃ and glass phase b2o ₃. The glass phase B ₃ o ₃ is filled in the refractory metal oxide skeleton, and the surface of the oxide layer is covered with a layer of glass phase B ₃ o ₃ with good fluidity. Oxygen is directly dissolved in the glass phase and diffused to the interface between the matrix and the oxide layer for oxidation reaction; In the medium temperature range of T = 1000 ℃ - 1800 ℃, the oxide layer is also mainly composed of porous refractory metal oxide skeleton ZrO З and glass phase B З o З, which is filled in the refractory metal oxide skeleton. However, at this time, the surface of the oxide layer is a bare refractory metal oxide skeleton, and oxygen first reaches the glass phase through the refractory metal oxide skeleton on the surface of the oxide layer, Then it dissolves into the glass phase and diffuses to the interface between the matrix and the oxide layer to produce oxidation reaction; In the high temperature range of T > 1800 ℃, oxygen directly reaches the interface between the matrix and the oxide layer through the connected holes in the refractory metal oxide framework.

The pores of the refractory metal oxide framework are filled with the well fluidity of B З O3 glass phase, both of which have low oxygen permeability, which hinders the diffusion of oxygen. In addition, the flowing B З O3 glass phase can timely make up for the thermal shock or prevent the further oxidation of cracks and holes in the oxidation process. At the same time, this "reinforced concrete" structure of "refractory metal oxide skeleton + flowing B З O3 glass phase" makes boride ultra-high temperature ceramics excellent in thermal shock resistance. In addition, compared with other ultra-high temperature ceramics (carbide and nitride ultra-high temperature ceramics), boride ultra-high temperature ceramics form a destructive B З o З vapor pressure at the interface at a higher temperature (1950 ℃), breaking through the protective layer; The temperature of destructive vapor pressure is 1700 ℃; Nitride ceramics produce destructive vapor pressure at a lower temperature.

Nevertheless, the effect of high-temperature oxidation resistance of single-phase boride ultra-high temperature ceramics is not ideal, because at the temperature above 1200 ℃, the evaporation rate of glass phase B ν o ₃ is greater than its generation rate, and oxygen directly oxidizes the matrix through the gap of refractory metal oxide. In order to improve its protection temperature range, scientists all over the world began to dope and modify it to make boride based ultra-high temperature ceramics. By doping 20% SiC into HfB2 ceramic materials, the U.S. Air Force Test Department has obtained boride based ultra-high temperature ceramics with good high-temperature oxidation resistance. The U.S. carborundum company has developed ZrB З + 10% MoSi З composite with better oxidation resistance, which is named "border-z" material. This material shows excellent oxidation resistance in the oxidation environment of 1950 ℃.

IV Sintering Densification of boride based ultra high temperature ceramics

The Sintering Densification methods of boride based ultra-high temperature ceramic materials mainly include pressureless sintering, hot pressing sintering (HP), reaction sintering and discharge plasma sintering (SPS).

1. Hot pressing sintering method

Because boride ultra-high temperature ceramics have strong covalent bonds and low self diffusivity, they must be sintered and densified under high temperature and high pressure. In early studies, it was believed that pure boride ultra-high temperature ceramics could be sintered and densified only under high temperature and high pressure of more than 2000 ℃ and 20-30Mpa. For example, the original average particle size is 10 μ HfB2 of M was sintered by hot pressing at 2160 ℃ and 27.3mpa for 180min, and the density was less than 95%. Later, it was found that the sintering temperature and pressure can also be reduced to a certain extent by reducing the particle size of raw materials. It is reported that when the average particle size of ZrB2 is reduced to 2 μ M, sintering at 1900 ℃ and 32 MPa for 45 min can obtain completely dense boride ceramics. However, too small average particle size will also hinder sintering, because the particle size is too small, the raw material powder is easy to oxidize and form oxide, which hinders the diffusion and migration of materials in the sintering process.

In the current research literature, there is little about the hot pressing sintering of pure boride, because the sintering temperature can be greatly reduced and the sintering compactness can be improved by adding sintering additives. There are mainly two kinds of additives in the sintering process of boride ultra-high temperature ceramics. One is metal additives, such as Al, Cr, Ni, etc., and the other is ceramic additives dominated by SiC. It is reported that [14] after adding Ni, ZrB2 can achieve Sintering Densification at 1600 ℃ and 20-50 MPa. In China, Han Wenbo [15] of Harbin Institute of Technology prepared b4c-zrb2-sic composites by hot pressing sintering method with B4C as matrix and ZrB2 SiC as additive at sintering temperature of 1900 ℃ and sintering pressure of 30 MPa; Xie Zhipeng [16] of Tsinghua University prepared (SiC, CNTs) / ZrB2 composite ceramics with excellent properties by hot pressing sintering method with silicon, activated carbon and CNTs as additives under the conditions of 1900 ℃ and 30 MPa argon.

2. Pressureless sintering

Pressureless sintering is more efficient and economical than hot pressing sintering. Both of them can promote Sintering Densification by adding sintering additives and refining the particle size of raw materials. Previous studies suggested that single-phase pure boride could not be densified in the environment of pressureless sintering, but Baumgartner sintered submicron TiB2 powder into TiB2 ceramics with a density greater than 99% by pressureless sintering at 2000 ℃ - 2100 ℃. Compared with the method of improving sintering compactness by refining the particle size of raw materials, the method of adding sintering additives is more simple and effective.

In recent studies, under the condition of pressureless sintering, the relative density of ZrB2 ceramics prepared by Kida and Segawa reached more than 95%. However, this Sintering Densification must be completed by adding sintering additives such as BN (5wt.%), AlN (15wt.%) and SiC (5wt.%). ZrB2 SiC ceramics were prepared by Pressureless Sintering in Shanghai Silicate Research Institute. Boron powder was used as sintering additive and sintered at 2100 ℃ for 3h. The density of ZrB2 SiC ceramics was 100%; Zhou Changling [20] and others prepared zirconium boride based ZrB2 SiC multiphase ceramics by Pressureless Sintering by adding YAG as sintering additive. The obtained ceramics were uniform and compact with good mechanical properties.

3. Reactive sintering 

The principle of reactive sintering is to use the chemical reaction between raw materials to generate a new phase with thermodynamic stability and complete Sintering Densification at the same time. This can greatly improve the production efficiency and save the cost, but at the same time, it has the disadvantage that the reaction process is not easy to control and the grain is relatively coarse. Some foreign scholars have compared the particle size of ZrB2 ceramics obtained by reactive hot pressing sintering and ordinary hot pressing sintering. The average particle size of ZrB2 ceramics is 12 with submicron raw powder under the condition of reactive hot pressing sintering at 2100 ℃ μ The average particle size of ZrB З ceramics is 6.5% under the condition of hot pressing sintering at 1900 ℃ μ M ZrB З ceramics. Reaction sintering has the advantages of in-situ synthesis and Sintering Densification at the same time. It is used for the sintering preparation of boride based ultra-high temperature composite ceramics ZrB З - SiC and HFB З - SiC. The reaction formula is as follows: 2Zr + Si + B4C → 2zrb З + SiC (12) (2 + X) HF + (1 - x) Si + B4C → 2hfb З + (1 - x) sic + xhfc (13)

It is worth mentioning that the SiC produced by in-situ reaction sintering can not only reduce the sintering temperature to a great extent, but also affect the microstructure. The reaction sintering temperature is 1650 ℃, lower than the ordinary reaction sintering temperature of 2100 ℃, and the average grain diameter is 2 μ m. It is much smaller than the particle size of ordinary reaction sintering 12 μ m。 Wang Yujin of Harbin Institute of Technology prepared BN ZrB З - ZrO З composites by reaction sintering process. The raw materials such as BN powder, ZrO З powder, B4C powder, C powder, SiO З powder and sintering additives were designed according to the composition of the composites, and the compactness of the composites reached more than 93%; In addition, Zhai Yanxia et al. [25] obtained a bulk density of 2.92g/cm by reactive sintering at 1560 ℃ for 2h in the proportion of B4C / SiC = 0.6 ³ B4C / SiC composite ceramics.

4. Spark plasma sintering (SPS)

Compared with the previous Sintering Densification methods, spark plasma sintering appeared later, but now it has been widely used in the Sintering Densification of various ultra-high temperature ceramic materials. Monteverde et al. Obtained completely dense HfB2 + 30 vol.% under the heating rate of 30 MPa and 100 ℃ / min and holding at 2100 ℃ for 2 hours SiC composite ceramics. Medri et al. Prepared zrb2-zrc-sic composites by HP and SPS at the same time. Without adding sintering additives, HP can only sinter samples with the highest density of 90% at 1870 ℃, while SPS can obtain fully dense boride composite ceramics at 2100 ℃ for less than 60min.

Zhao Yuan [28] of the Institute of silicate, Chinese Academy of Sciences and others used SPS technology to prepare ZrB2 SiC composites with relative density of 98.5% by using Zr, B4C and Si powder as starting materials at 1450 ℃ and 30MPa; Huang Anqi of Beijing University of technology and others prepared sic-tib2 multiphase ceramic materials with different components under the condition of 1700 ℃, 50 MPa by SPS process, using SiC as matrix, TiB2 as second phase and YAG as sintering additive.

V Preparation method of boride based ultra-high temperature ceramic coating

The main preparation methods of boride based ultra-high temperature ceramic coatings are embedding method, slurry method, vapor deposition method and thermal spraying method.

1. Embedding method

The ceramic coating prepared by embedding method is earlier and the technology is more mature. The process is that the matrix sample is placed in the mixed solid powder. Under the condition of high temperature, the matrix sample and the solid powder diffuse each other, and then complex physical and chemical reactions occur, so as to form a coating on the surface of the matrix. The embedding mixture consists of four parts: matrix, powder containing coating elements, halide (NaCl, NaF, etc.), active agent (al ν o ₃, B ν o ₃, etc.). The process of preparing boride ceramic matrix coating by embedding method is simple, the resulting coating is relatively dense and firmly bonded with the matrix, but the thickness of the prepared coating is difficult to control, and the coating is prone to uneven phenomenon.

Pwang et al. Prepared a layer of ZrB2 SiC / SiC coating on the surface of graphite by embedding method to improve its surface wear resistance and greatly reduce the abrasion rate of graphite surface. J pourasad [et al. Prepared a layer of sic-zrb2 coating on the surface of SiC modified graphite by embedding method and studied its high-temperature oxidation resistance. The research shows that the oxidation weight gain rate is only 1.1% after oxidation for 10 h at 1773 K.

Li Hejun and others prepared ZrB2 modified silicon-based coating on the surface of carbon / carbon (C / C) composites by embedding method. The prepared coating has compact structure and good oxidation resistance at 1773 K, 1873 K and 1953 K. In addition, the embedding method is often used to prepare multi-layer multiphase ceramic coatings together with other methods. In order to improve the oxidation resistance of C / C composites, Zhang armed [34] and others used the embedding method to prepare the coating SiC transition layer, the thermal spraying method to prepare the zrb2-mosi2 outer layer, and the zrb2-mosi2 / SiC double-layer multiphase ceramic coating was prepared on the C / C composite matrix, which was oxidized for 30h and 10h at 1273K and 1773K respectively, The mass loss of zrb2-mosi2 / SiC coating samples is 5.3% and 3.0% respectively.

2. Slurry method

Boride based ultra-high temperature ceramic coating is prepared by slurry method. Firstly, boride powder and binder (varnish, PVB glue, etc.) are mixed into slurry and coated on the surface of the substrate. The coating is formed on the surface of the substrate by solid-phase or liquid-phase sintering in the environment of inert gas or vacuum. Zhang Xiang [and others of Central South University prepared ZrB З based ceramic coatings on the surfaces of C / C and C / C-SiC respectively by slurry method; Wu Dingxing and others combined slurry method and chemical vapor deposition method to prepare SiC (ZrB З - SiC / SiC) multi-layer composite anti-oxidation coating. The coating was oxidized at 1500 ℃ for 25h, and the weight of the coating increased by only 2.5%, showing good anti-oxidation property.

3. Vapor deposition method 

Vapor deposition method is mainly divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD). Both deposition methods can deposit a layer of dense boride ceramic coating with firm combination with the substrate and controllable thickness on the surface of the substrate. PVD method is to melt and evaporate the ceramic blank with an electron gun under vacuum, and vapor deposition to the substrate surface to form a coating [38]; The CVD method is to vaporize the raw materials for synthesizing boride ceramics and make them react chemically on the surface of the base material, so as to deposit boride ceramic films. Boride based ceramic coating prepared by PVD method is generally used on the surface of various metal cutting tools. Zhang Shushen used high power pulsed magnetron sputtering deposition technology (hipims) to deposit CRB З coating on the surface of cemented carbide tools. The coating showed (101) preferred orientation. The phase structure composition was mainly CrB2 and a small amount of Cr. the atomic ratio of B / Cr in the coating was 1.76, and the hardness and elastic modulus were 26.9 ± 1.0GPa and 306.7 ± 6.0 GPA respectively; SDS Cruz deposited a layer of TiB2 / DLC multiphase coating on the surface of AISI 1095 steel by PVD method, which not only overcomes the disadvantage of brittleness of single-phase TiB2 coating; It also overcomes the disadvantage of insufficient adhesion between DLC coating and matrix. M Berger used hybrid PVD technology to combine electron beam evaporation Ti and magnetron sputtering TiB2 to prepare a high hardness coating with certain ductility. Although the ceramic coating prepared by PVD method is uniform and dense and has good adhesion with the substrate, compared with other coating preparation methods, its deposition efficiency is too low to prepare thicker coatings.

CVD method directly makes the original material into coating, which has higher production efficiency than PVD method. Sun Caiyun used CVD technology and ticl4-bcl 3-h2 as reaction system to prepare TiB2 wear-resistant coatings on the surface of low carbon steel and graphite respectively; Y Xiang combined CVD method and slurry method to prepare ZrB2 SiC ultra-high temperature oxidation resistant coating on the surface of C-SiC composites. The prepared coating has excellent oxidation resistance at different oxidation temperatures; A polycrystalline ZrB2 based ceramic coating was deposited on the surface of graphite by CVD at 1200 ℃ with zrcl4-bcl 3-h2 as the reaction system.

4. Thermal spraying method 

Thermal spraying method is a promising surface modification method, which has the unique advantages of fast deposition speed, accurate and controllable coating thickness and so on. In recent years, it has developed rapidly in the preparation of boride ceramic coatings such as ZrB2, TiB2 and CrB2. Thermal spraying can be divided into many categories according to different heat sources. At present, explosive spraying, plasma spraying and laser spraying are commonly used to prepare boride ceramic coatings. Explosive spraying is to detonate after mixing oxygen and acetylene in a certain proportion. The energy released at the moment of explosion melts the material powder and strikes the substrate surface at high speed to form a coating. Cheng Xiangyu prepared zr-o-b ceramic coating by electrothermal explosion spraying. The reaction raw materials were Zr and B2O3 powder. The main components of the coating were zro2-zrb2 and a zirconium compound; SX Hou prepared Mo Si Al coating by electrothermal explosion spraying method. The coating has uniform and dense structure and high hardness.

The technology of preparing boride based ceramic coating by plasma spraying is relatively mature and stable, and its reports emerge one after another at home and abroad. Cheng Hanchi [48] sprayed and deposited Al2O3-TiB2 composite powder with three cathode axial powder feeding plasma spraying system (axial - III) to obtain TiB2 / Al2O3 coating; Wang Haijun prepared mo-30% NiCrBSi coating on al-10si alloy substrate by supersonic plasma spraying. The coating has high hardness and good wear resistance; Cagri tekmen prepared tib2-al2o3 coating by in-situ reactive plasma spraying with Al-12Si, B2O3 and TiO2 raw powders; Iozdemir [51] prepared a layer of Al-12Si / TiB2 / h-BN composite coating on the surface of aluminum by atmospheric plasma spraying, and the coating has good wear resistance.

Laser spraying, also known as laser cladding, rapidly melts the coating powder and melts the micro area on the surface of the substrate at the same time. The coating and the substrate form a solid metallurgical combination. Laser spraying is not limited to the coating material. It is an ideal preparation method of boride ultra-high temperature ceramic coating. Chun g prepared a layer of ZrB2 Reinforced Ni based composite coating on the surface of pure titanium by laser cladding. As a result, the surface wear resistance and hardness of pure titanium were significantly improved; T Simsek prepared a layer of ZrB2 coating on the surface of low carbon steel by CO2 laser cladding. The coating was uniform and compact without cracks and holes. However, at present, the laser cladding technology is not mature. Due to the extremely fast heating and cooling speed, the temperature gradient and thermal expansion coefficient of coating and matrix materials are different, which may lead to the formation of microcracks and holes in the coating process and affect the coating quality.

Vi Prospect

As an ultra-high temperature antioxidant material with high melting point, boride ultra-high temperature ceramics have broad application prospects in the field of aerospace. However, it is still a long way from large-scale industrial production and application. As a structural material, boride ultra-high temperature ceramics have problems such as brittleness and difficult sintering and densification. As a coating material, boride ultra-high temperature ceramics have problems such as mismatch of thermal expansion coefficient with C / C composite matrix and refractory metal matrix, and cracks are easy to occur in the process of coating use. The future research directions of boride ultra-high temperature ceramics are:

(1) The Sintering Densification Technology was optimized by doping single-phase boride ultra-high temperature ceramics.

(2) Develop new coating preparation processes and methods, and prepare boride based ultra-high temperature ceramic coatings with good adhesion, uniformity, continuity, compactness and excellent microstructure and properties.