Boride and carbide ultra-high temperature materials play an important role in the space vehicle, which is an indispensable part of the aircraft in long-term flight, transatmosphere or re-entry flight, and plays a crucial role in the thermal protection system of the aircraft. In recent years, the latest research results of refractory metals and their alloys, carbon-carbon composites, ultra-high temperature ceramics and other ultra-high temperature materials are summarized and summarized, the advantages and disadvantages of ultra-high temperature materials are analyzed, the main problems are put forward, and the main research objectives and key development direction in the future are discussed.

Ultra high temperature ceramics

Ultra high temperature ceramic refers to a special material that can maintain physical and chemical stability in high temperature environment (2000℃) and reaction atmosphere (such as atomic oxygen environment). It is a ceramic matrix composite material with excellent high temperature mechanical properties, high temperature oxidation resistance and thermal shock resistance. Uht ceramics are mainly composed of high melting point borides and carbides, mainly including hafnium boride (HfB2), zirconium boride (ZrB2), hafnium carbide (HfC), zirconium carbide (ZrC) and so on. The melting point of boride and carbide ultra high temperature ceramics is more than 3000℃, which has excellent thermochemical stability and excellent physical properties, including high elastic modulus, high hardness, low saturated steam pressure, moderate thermal expansion rate and good thermal shock resistance, and can maintain high strength at high temperature. Ultra-high temperature ceramics can adapt to ultra-sonic long-duration flight, atmospheric re-entry, transatmospheric flight and rocket propulsion system and other extreme environments, can be applied to aircraft nose cone, wing leading edge, engine hot end and other key components. As an important material used in the aerospace vehicle, ultra-high temperature ceramic materials have been paid great attention by many countries.

Research progress of ultra-high temperature ceramics

Foreign research on ultra-high temperature ceramic materials began in the early 1960s. With the strong support of the United States Department of Defense, Manlab began to research on ultra-high temperature ceramic materials, mainly focusing on ZrB2 and HfB2 and their composites. The 80vol % HFB2-20Vol %SiC composite developed by the company can basically meet the requirements of continuous use in high temperature oxidation environment, which provides a great help for the analysis and design of sharp leading edge aircraft and its thermal protection system. In the 1990s, NASA Ames Laboratory began research on ultrahigh temperature ceramic materials. Ames and its partners conducted a series of research work on system thermal analysis, material development, and arc heater testing, and conducted two flight experiments (Sharp-B1 and Sharp-B2). Among them, the SHARP wing leading edge in sharp-B2 flight experiment is divided into three parts due to different thermal environment, and ZrB2/ SiC/C, ZrB2/ SiC and HfB2/SiC materials are used respectively. The experimental results show that the hafnium diboride (HfB2) and zirconium diboride (ZrB2) ultra-high temperature ceramic materials can be used as thermal protection system materials for high supersonic aircraft in the atmosphere, and the application prospect is immeasurable. In early February 2003, the Us space shuttle "Columbia" had a shocking explosion tragedy. In order to improve the future of the space shuttle flight safety, make similar Columbia tragedy is not repeated, explosion accident in Columbia, after the us space agency (NASA) quickly start the related research projects, including a focus on research and development of a new generation of melting point higher than 3000 ℃, ultra high temperature ceramics as the shuttle's thermal resistance materials in the future.

The research of ultra-high temperature ceramic materials is also emphasized in China. At the 2014 International New Materials Development Trend Forum, Academician Li Zhongping emphasized that the research and development of SiC precursors and SiC fibers with high performance and low cost should be accelerated, and the basic research and application of carbide ultra-high temperature ceramics should be accelerated. Professor Cheng Laifei from Northwestern Polytechnical University introduced the research progress of SiCw/SiC layered ceramics. Academician Zhang Litong's research group prepared Cf/SiC ceramic matrix composites by CVI, PIP and RMI processes, proposed the concept of interface region, established the physical model of matrix crack and interface region interaction in Cf/SiC, and systematically evaluated its service performance. Shanghai silicate research institute, Chinese Academy of Sciences, professor s. introduces the in situ reaction process, carbide and nitride ceramic matrix composites is tried by PIP process, in Cf/SiC, SiCf/SiC composites preparation process of adding boron, aluminum and other additives, to shorten the PIP densification time, improve the oxidation resistance, and mechanical properties. At present, domestic ultra-high temperature ceramic materials are gradually applied in China's aerospace field.

Boride ultra-high temperature ceramics

Ultra-high temperature borides mainly include hafnium boride (HfB2), zirconium boride (ZrB2), tantalum boride (TaB2) and titanium boride (TiB2), etc. At present, the research on zirconium boride (ZrB2) and hafnium boride (HfB2) is the most concentrated. Boride ultra-high temperature ceramics (UHTCs) are composed of strong covalent bonds, which have the characteristics of high melting point, high hardness, high strength, low evaporation rate, high thermal conductivity and electric conductivity. However, the strong covalent bonds lead to the disadvantages of difficult sintering and densification. In order to improve the sintering performance and increase the density, it can be solved by increasing the surface energy of reactants, reducing the grain boundary energy of products, increasing the volume diffusion rate of materials, speeding up the material transfer rate and improving the mass transfer kinetics.

The single-phase zirconium boride (ZrB2) and hafnium boride (HfB2) have good oxidation resistance below 1200℃, which is due to the formation of liquid boron oxide (B2O3) glass phase on the surface and play a good oxidation protection. Such as zirconium boride (ZrB2) oxidation process, zirconium boride (ZrB2) oxidation into zirconium oxide (ZrO2) and boron oxide (B2O3), formed an oxidation protection layer, prevent the oxidation of zirconium boride (ZrB2), when the temperature exceeds the melting point of boron oxide (B2O3) (450℃), boron oxide (B2O3) slowly evaporation, the higher the temperature, The higher the evaporation rate of boron oxide (B2O3), the lower its role as an oxygen diffusion barrier layer, resulting in the decline of the oxidation resistance of borides. For the oxidation of zirconium (ZrB2), hafnium (HfB2) and titanium (TiB2) at 1000 ~ 1800℃, Parthasarathy et al. pointed out that below 1400℃, the oxidation kinetic process of boride conforms to parabolic law, and the oxides of metal atoms form the skeleton. The resulting liquid boron oxide fills the skeleton and coats the boride surface. At this point, the oxidation rate is controlled by the diffusion of oxygen through liquid boron oxide (B2O3). The oxidation rate is controlled by the diffusion of oxygen vacancy through the oxide lattice at high temperature.

Zrb2-sic composites prepared by adding silicon carbide (SiC) have better comprehensive properties, such as higher binary eutectic temperature, good oxidation resistance and so on. Clougherty et al introduced silicon carbide (SiC) into zirconium boride (ZrB2) and hafnium boride (HfB2) in the 1960s. The initial purpose was to refine grains and improve strength. After the addition of silicon carbide (SiC) at high temperature, the outermost layer of boride surface is mainly composed of glass layer rich in silicon dioxide (SiO2), and the inner layer is oxide layer (ZrO2, HfO2). The glass layer can prevent the diffusion of oxygen, so zirconium boride (ZrB2) still has a high oxidation resistance at 2000℃ after adding 20 ~ 30% SiC. Sun et al. studied the effect of toughening zirconia (ZrO2) fiber on ZRB2-sic composites. The elastic strength and fracture toughness of ZRB2-sic-ZRO2F ceramics prepared by hot pressing at 1850 ℃ were 1086 ± 79 MPa and 6.9 ± 0.4 MPa•m1/2, respectively. At high temperature, the surface layer of ZRB2-sic composites forms a protective layer of borosilicate, which can maintain its parabolic oxidation law to more than 1600℃. Other additives, such as molybdenum silicate (MoSi2), zirconium silicate (ZrSi2), tantalum silicate (TaSi2), and tantalum boride (TaB2), are also used to improve the oxidation resistance of zirconium boride (ZrB2) and hafnium boride (HfB2). The addition of the second phase makes the surface of the material form a glass phase with high melting point at high temperature, which prevents the diffusion of oxygen into the interior of the material and improves the oxidation resistance of the material at high temperature.

Carbides ultra high temperature ceramics

Carbide uHT ceramics have high melting point, high strength, high hardness and good chemical stability, and are widely used as uHT ceramic materials. Currently, the commonly used carbide UHT ceramics mainly include silicon carbide (SiC), zirconium carbide (ZrC), and hafnium carbide (HfC). The melting point of hafnium carbide (HfC) and zirconium carbide (ZrC) is much higher than that of their oxides, and they do not undergo any solid phase transformation. They have better thermal shock resistance and still have high strength at high temperature. However, such carbide uHT ceramics have relatively low fracture toughness and oxidation resistance, and are usually reinforced and toughened by fibers.

Oxidation of ultra-high temperature carbides is a combination of internal diffusion of oxygen or outward diffusion of metal ions, and outward escape of gaseous or liquid by-products (at relatively low temperatures) through oxide layers. The oxidation resistance of uHT carbides is mainly affected by the formation and escape of gaseous by-products such as CO and CO2 during the oxidation process. Zirconium carbide (ZrC) is a promising ultra-high temperature material with relatively low price and high melting point and hardness. The oxidation resistance of single phase zirconium carbide (ZrC) at high temperature is poor. When heated to 800℃ in air, serious oxidation begins, forming zirconia (ZrO2) and carbon (C); As the temperature rises to 1100℃, carbon (C) continues to react with oxygen (O2) to form carbon monoxide (CO) or carbon dioxide (CO2). The results show that the oxidation zone of hafnium carbide (HfC), zirconium carbide (ZrC) and tantalum carbide (TaC) consists of at least two layers after absorbing a large amount of oxygen into the lattice at high temperature. One is an inner oxide layer with few voids, and the other is a porous outer oxide layer that does not prevent oxygen diffusion. Therefore, the oxidation resistance of single-phase zirconium carbide (ZrC) is poor, so zirconium carbide (ZrC) is generally used in combination with other materials, such as ZRC-Mo-Si2, ZRC-ZRB2, ZRC-sic, ZRC-ZRO2 and ZRC-Mo. Savino et al. added 5% molybdenum silicide (MoSi2) to hafnium carbide (HfC), and found that molybdenum silicide (MoSi2) promoted sintering, and the density of sintered body reached 98% of the theoretical density, with few gaps. The surface layer is a multi-layer structure with cracks, but it is firmly combined with the unreacted hafnium carbide (HfC) at the bottom. The outermost layer is still porous hafnium oxide (HfO2), and no continuous glass phase is found. The addition of the second phase can not only improve the oxidation resistance and sintering performance of zirconium carbide (ZrC) and hafnium carbide (HfC), but also effectively inhibit the growth of matrix grain, introduce residual stress, and improve the strength and toughness of materials. In addition, Al and Cr can be oxidized into dense Al2O3 and Cr2O3 films at high temperature. Liu Dongliang used the first principle to compare the formation energy of hafnium carbide (HfC) mixed with Al and Cr. He found that the stability of hafnium carbide (HfC) doped with Cr was better than that of Al.

The sinter and density of carbon oxides have great influence on the diffusion of oxygen. Compared with metallic carbon oxides, borosilicate glass is relatively compact and has a better inhibition on the diffusion of oxygen. This is also one of the reasons why silicate boride doped ultra-high temperature ceramics have been widely studied so far.

Conclusion

At present, great breakthroughs have been made in the research of ultra-high temperature materials in China, but there are still many problems in the research of ultra-high temperature materials. Future research on ultra-high temperature materials should focus on the following aspects:

(1) Strengthen the research on the modification of C/C composite matrix. At present, most of the research on matrix modification of C/C composites is carried out in micro samples. Therefore, the research object should be changed from micro samples to applied components for specific application components. Efforts should be made on how to improve the stability of preparation process, the portability of matrix modification measures and the coordination of comprehensive properties of components.

(2) Study of atomic oxygen using material calculation method. This method can avoid the oxidation caused by the contact between materials and atomic oxygen in conventional experiments. Using fluid dynamics method to simulate the phenomenon of fluid flow around the material, the oxidation mechanism of ultra-high temperature ceramic materials is explored from these aspects.

(3) Research on the surface of ultra-high temperature ceramic materials. How molecular oxygen and atomic oxygen combine and diffuse with the surface of these ceramic materials, and explore how to prevent the combination of ultra-high temperature ceramic surface with oxygen and oxygen diffusion.

(4) Explore measures to improve the toughness of ultra-high temperature ceramic materials. For example, whether nanowire, nanoribbon and nanorods can be introduced into carbides, borides and their composite ceramics to explore whether and how they can improve the toughness of uHT ceramics.

(5) To solve the defect control problem of ultra-high temperature ceramic materials. Defects can not be avoided in uHT ceramic materials, and at the same time, defects have a great influence on the performance of UHT ceramic materials. Therefore, it is one of the directions of future research to explore the formation causes of defects and their detection, characterization and control techniques.