Zirconium boride plays an important role in aerospace vehicles. It is an integral part of long-term flight, cross atmosphere or re-entry flight, as well as in the thermal protection system of 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. The advantages and disadvantages of ultra-high temperature materials are analyzed, and the main existing problems are proposed. The main research objectives and key development directions in the future are discussed.
Ultra high temperature ceramic is a kind of special material which can maintain physical and chemical stability in high temperature environment (2000 ℃) and reaction atmosphere (such as atomic oxygen environment). It is a kind of ceramic matrix composite material with excellent high temperature mechanical properties, high temperature oxidation resistance and thermal shock resistance. Ultra high temperature ceramics are mainly composed of high melting point borides and carbides, including zirconium boride(ZrB2), hafnium boride (HfB2), zirconium carbide (ZrC), hafnium carbide (HFC), etc. The melting point of boride and carbide ultra-high temperature ceramics is over 3000 ℃, which has excellent thermochemical stability and excellent physical properties, including high elastic modulus, high hardness, low saturated vapor 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 high speed long-term flight, atmospheric re-entry, cross atmosphere flight， rocket propulsion system and other extreme environments, it can be used in various key components such as nose cone, wing leading edge, engine hot end and so on. As an important material used in aerospace vehicles, ultra-high temperature ceramic materials have been highly concerned by many countries.
The research on ultra-high temperature ceramic materials began in the early 1960s. With the strong support of the U.S. Department of defense, manlab began to study ultra-high temperature ceramic materials. The main research objects are ZrB2, HfB2 and their composites. 80 vol% hfb2-20 was developed。 The results show that the vol% SiC composite can basically meet the requirements of continuous use in high temperature oxidation environment, which provides a great help for the analysis and design of the sharp leading edge aircraft and its thermal protection system. In the 1990s, NASA Ames laboratory began to carry out relevant research on ultra-high temperature ceramic materials. Ames laboratory and relevant partners carried out a series of research work on system thermal analysis, material development and arc heater test, and carried out two flight experiments (sharp-b1 and sharp-b2). The leading edge of sharp wing in sharp-b2 flight experiment is divided into three parts due to the different thermal environment. ZrB2 / SiC / C, ZrB2 / SiC and HfB2 / SiC materials are used respectively. The experimental results show that zirconium diboride (ZrB2) and hafnium diboride (HfB2) can be used as thermal protection system materials for hypersonic vehicles in the atmosphere, and the application prospect is immeasurable. In early February 2003, the U.S. space shuttle Columbia experienced a shocking explosion. In order to improve the flight safety of future space shuttles and make the similar "Columbia" explosion tragedy no longer repeat, after the "Columbia" crash, NASA quickly launched relevant research projects, including the key research and development of a new generation of ultra-high temperature ceramics with melting point higher than 3000 ℃ as heat insulation materials for future space shuttle.
The research of ultra-high temperature ceramic materials also attract attention in China. At the 2014 international new material development trend forum, Academician Li Zhongping stressed that it be necessary to accelerate the research and development of SiC precursors and SiC fibers with high performance and low cost, accelerate the basic research and application basic research of carbide ultra-high temperature ceramics. Professor Cheng Laifei of Northwestern Polytechnic University introduced the research progress of SiCw / SiC laminated structure ceramics. The research group of academician Zhang Litong prepared CF / SiC Ceramic Matrix Composites by CVI, PIP and RMI. At the same time, the concept of interface zone was proposed, and the physical model of interaction between matrix crack and interface zone in CF / SiC was established, and its service performance was systematically evaluated. In 2018, the high-purity zirconium diboride ultra-high temperature ceramics independently developed and produced by Jinzhou Haixin metal materials Co., Ltd. were successfully applied to the thermal insulation layer of the rocket tail by docking with military enterprises. At present, the ultra-high temperature ceramic materials of Jinzhou Haixin metal materials Co., Ltd. are gradually applied in the field of aerospace in China.
Boride ultrahigh temperature ceramics
Ultra high temperature borides mainly include hafnium boride (HfB2), zirconium boride (ZrB2), tantalum boride (TAB2) and titanium boride (TiB2). 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 electrical conductivity. However, the strong covalent bonds lead to the defects of difficult sintering and densification. In order to improve the sintering performance and density, we can improve the surface energy of reactants, reduce the grain boundary energy of products, improve the bulk diffusion rate of materials, accelerate the mass transfer rate and improve the mass transfer kinetics.
Single phase zirconium boride(ZrB2) and hafnium boride (HfB2) have good oxidation resistance below 1200 ℃, that is due to the formation of liquid boron oxide (B2O3) glass phase on the surface, which plays a good anti-oxidation protection role. For example, in the oxidation process of ZrB2, zirconium boride(ZrB2) is oxidized to form zirconia (ZrO2) and boron oxide (B2O3), forming an anti-oxidation protective layer, which prevents the oxidation of ZrB2. When the temperature exceeds the melting point of boron oxide (B2O3) (450 ℃), boron oxide (B2O3) slowly evaporates, The higher the temperature, the greater the evaporation rate of boron oxide (B2O3), and the lower its role as an oxygen diffusion barrier layer The oxidation resistance of borides decreased. Parthasarathy et al. Focused on the oxidation of zirconium boride (ZrB2), hafnium boride (HfB2) and titanium boride (TiB2) at 1000 ~ 1800 ℃, it was pointed out that the oxidation kinetics of borides followed the parabolic law below 1400 ℃, and the oxides of metal atoms formed the skeleton, and the liquid boron oxide produced was filled into the skeleton and coated on the boride surface. The oxidation rate is controlled by the diffusion of oxygen through liquid boron oxide (B2O3). At high temperature, the diffusion of oxygen vacancies through the oxide lattice restricts the oxidation rate.
ZrB2 SiC composites prepared by adding silicon carbide (SIC) have better comprehensive properties, such as higher eutectic temperature and better oxidation resistance. Clougherty et al. Introduced silicon carbide (SIC) into zirconium boride (ZrB2) and hafnium boride (HfB2) in the 1960s. The original purpose was to refine grains and improve strength. After adding silicon carbide (SIC), the outer layer of boride surface is mainly composed of silica rich glass layer, and the inner layer is oxide layer (ZrO2, HfO2). The glass layer can prevent the diffusion of oxygen, so zirconium boride (ZrB2) still has high oxidation resistance at 2000 ℃ after adding 20 ~ 30% volume ratio of silicon carbide (SIC). Sun et al. Studied the effect of zirconia fiber Toughening 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, borosilicate protective layer can be formed on the surface of ZrB2 SiC composite, which can maintain its parabolic oxidation law to over 1600 ℃. Other additives, such as molybdenum silicide (MoSi2), zirconium silicide (zrsi2), tantalum silicide (TaSi2), tantalum boride (TAB2), are also used to improve the oxidation resistance of zirconium boride (ZrB2) and hafnium boride (HfB2). With the addition of the second phase, the high melting point glass phase was formed on the surface of the material at high temperature, which prevented the diffusion of oxygen into the material, and improved the high temperature oxidation resistance of the material.
Carbide ultrahigh temperature ceramics
Carbide ultra-high temperature ceramics have high melting point, high strength, high hardness and good chemical stability, which are widely used in ultra-high temperature ceramic materials. At present, the commonly used carbide ultra-high temperature ceramics mainly include silicon carbide (SIC), zirconium carbide (ZrC), and hafnium carbide (HFC). Hafnium carbide (HFC) and zirconium carbide (ZrC) have much higher melting point than their oxides and do not undergo any solid phase transition. They have good thermal shock resistance and high strength at high temperature. However, the fracture toughness and oxidation resistance of this kind of carbide ultra-high temperature ceramics are relatively low, and fibers are usually used for reinforcement and toughening.