1.Ultra-high temperature ceramics
Ultra-high temperature ceramics refers to a special material that can maintain physical and chemical stability in a high temperature environment (2000℃) and in a reaction atmosphere (such as in an atomic oxygen environment), and is a 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, mainly including hafnium boride (HfB2), zirconium boride (ZrB2), hafnium carbide (HfC), zirconium carbide (ZrC), tantalum carbide (TaC) 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 vapor pressure, moderate thermal expansion rate and good thermal shock resistance, and can maintain high strength at high temperatures. Ultra-high temperature ceramics can adapt to extreme environments such as hypersonic long-duration flight, atmospheric re-entry, trans-atmospheric flight and rocket propulsion systems, and can be applied to various key components such as aircraft nose cones, wing leading edges, and engine hot ends. As an important material used in aerospace vehicles, ultra-high temperature ceramic materials have been highly concerned by various countries.
1.1Research progress of ultra-high temperature ceramics
Foreign research on ultra-high temperature ceramic materials began in the early 1960s, under the strong support of the United States Department of Defense, Manlab began to study ultra-high temperature ceramic materials, the main research objects are ZrB2 and HfB2 and their composites. The 80 vol% HfB2-20 vol%SiC composite material developed by the company can basically meet the requirements of continuous use in high temperature oxidation environment, and provides a great help for the analysis and design of sharp front aircraft and its thermal protection system. In the 1990s, NASA Ames Laboratory began to conduct research on ultra-high temperature ceramic materials, and Ames Laboratory and related partners carried out 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 the SharP-B2 flight test is divided into three parts due to the different thermal environment, respectively using ZrB2/ SiC/C, ZrB2/ SiC and HfB2/SiC materials. The experimental results show that the ultra-high temperature ceramic materials with hafnium diboride (HfB2) and zirconium diboride (ZrB2) as the main body can be used as the thermal protection system materials of high supersonic aircraft in the atmosphere, and the application prospect is inestimate. In early February 2003, the United States space shuttle "Columbia" suffered a shocking explosion tragedy. In order to improve the flight safety of the future space shuttle, so that similar to the "Columbia" explosion tragedy will not be repeated, after the "Columbia" accident, NASA quickly launched relevant research programs, including focusing on the research and development of a new generation of ultra-high temperature ceramics with a melting point higher than 3000 ° C, as a thermal resistance material for the future space shuttle.
Domestic research on ultra-high temperature ceramic materials is also attached importance. At the 2014 International New Materials Development Trend Forum, Academician Li Zhongping stressed that it is necessary to accelerate the research and development of high-performance, low-cost SiC precursors and SiC fibers, and accelerate the basic research and application of carbide ultra-high temperature ceramics. Professor Cheng Laifei from Northwestern Polytechnical University introduced the research progress of SiCw/SiC layered structure ceramics. The research group of Academician Zhang Litong prepared Cf/SiC ceramic matrix composites by CVI, PIP and RMI processes. At the same time, the concept of interfacial zone was proposed, the physical model of the interaction between matrix cracks and interfacial zone in Cf/SiC was established, and the service performance of CF/sic was systematically evaluated. Professor Dong Shaoming of Shanghai Institute of Silicate, Chinese Academy of Sciences introduced the preparation of carbide and nitride ceramic matrix composites by in-situ reaction method. Through PIP process, additives such as boron and aluminum were added in the preparation process of Cf/SiC and SiCf/SiC composites to shorten the PIP densification time and improve the antioxidant capacity and mechanical properties. At present, domestic ultra-high temperature ceramic materials are gradually applied to the aerospace field in China.
1.2Boride ultra-high temperature ceramics
The main ultra-high temperature borides are 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 electrical conductivity, etc. However, the strong covalent bonds lead to the shortcomings 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 the reactant, reducing the grain boundary energy of the product, increasing the bulk diffusivity of the material, accelerating the material transport rate and improving the mass transfer kinetics.
Single-phase zirconium boride (ZrB2) and hafnium boride (HfB2) have good oxidation resistance below 1200℃, which is because the liquid boron oxide (B2O3) glass phase is generated on the surface, which plays a good antioxidant protection role. Such as zirconium boride (ZrB2) oxidation process, zirconium boride (ZrB2) oxidation to zirconium oxide (ZrO2) and boron oxide (B2O3), forming an antioxidant protective layer, prevent zirconium boride (ZrB2) oxidation, 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 antioxidant properties of borides. Aiming at the oxidation of zirconium boride (ZrB2), hafnium boride (HfB2) and titanium boride (TiB2) at 1000 ~ 1800℃, Parthasarathy et al. pointed out that below 1400℃, the oxidation kinetic process of borides conforms to the parabolic law, and the oxides of metal atoms form the skeleton. The resulting liquid boron oxide fills the skeleton and is coated on the boride surface. At this point, the oxidation rate is controlled by the diffusion of oxygen through liquid boron oxide (B2O3). In the high temperature stage, the diffusion process of oxygen vacancy through the oxide lattice restricts the oxidation rate.
The ZrB2-SiC composites prepared by adding silicon carbide (SiC) have better comprehensive properties, such as higher binary eutectic temperature and good oxidation resistance. Clougherty et al. introduced silicon carbide (SiC) into zirconium boride (ZrB2) and hafnium boride (HfB2) in the 1960s, with the initial purpose of refining grains and improving strength. After the addition of silicon carbide (SiC), the outermost layer of the boride surface at high temperature is mainly composed of a glass layer rich in silicon dioxide (SiO2), and the interior is an oxide (ZrO2, HfO2) layer. 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% volume ratio of silicon carbide (SiC). Sun et al. studied the influence of zirconia (ZrO2) 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 temperatures, the surface layer of the ZrB2-SiC composite forms a protective layer of borosilicate, which is able to maintain its parabolic oxidation rule to more than 1600 ° C. Other additives, such as molybdenum silicide (MoSi2), zirconium silicide (ZrSi2), tantalum silicide (TaSi2), tantalum boride (TaB2), etc. 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 high melting point glass phase at high temperature, preventing the diffusion of oxygen to the inside of the material, and improving the high temperature oxidation resistance of the material.
1.3Carbide ultra-high temperature ceramics
Carbide ultra-high temperature ceramics have high melting point, high strength, high hardness and good chemical stability, is widely used in ultra-high temperature ceramic materials, the current commonly used carbide ultra-high temperature ceramics mainly include silicon carbide (SiC), zirconium carbide (ZrC), tantalum carbide (TaC) and hafnium carbide (HfC). Hafnium carbide (HfC), zirconium carbide (ZrC) and tantalum carbide (TaC) have much higher melting points than their oxides, do not undergo any solid phase transition, have good thermal shock resistance, and still have high strength at high temperatures. However, the fracture toughness and oxidation resistance of such carbide ultra-high temperature ceramics are relatively low, and fiber is usually used to strengthen and toughen them.
The oxidation of ultra-high temperature carbides is a combined process of inward diffusion of oxygen or outward diffusion of metal ions, and the outward escape of gaseous or liquid (at relatively low temperatures) by-products through the oxide layer. The oxidation resistance of ultra-high temperature carbides is mainly affected by the formation and escape of gaseous byproducts during oxidation, such as CO and CO2. In carbide ultra-high temperature ceramics, zirconium carbide (ZrC) is relatively cheap and has high melting point, high hardness and other properties, so it is a very promising ultra-high temperature material. The oxidation resistance of single-phase zirconium carbide (ZrC) is poor at high temperature. When heated to 800℃ in the air, serious oxidation begins to form zirconia (ZrO2) and carbon (C); When the temperature rises to 1100 ° C, carbon (C) continues to react with oxygen (O2) to form carbon monoxide (CO) or carbon dioxide (CO2). The results show that after hafnium carbide (HfC), zirconium carbide (ZrC) and tantalum carbide (TaC) absorb a lot of oxygen into the lattice, the oxidation zone formed under high temperature environment includes at least 2 layers. One layer is an inner oxide layer with very few voids, and the other is a porous outer oxide layer that does not prevent the diffusion of oxygen. 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 molybdenum silicide (MoSi2) with a volume fraction of 5% to hafnium carbide (HfC), and found that molybdenum silicide (MoSi2) promoted sintering, and the sintered body density reached 98% of the theoretical density, and there were few voidage. The surface layer is a multilayer 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 has been found. The second phase additive can not only improve the oxidation resistance and sintering properties of zirconium carbide (ZrC) and hafnium carbide (HfC), but also effectively inhibit the growth of matrix grains, introduce residual stress, and improve the strength and toughness of the material. In addition, Al and Cr can be oxidized into dense alumina (Al2O3) and chromium oxide (Cr2O3) films at high temperatures. Using first principles, Liu Dongliang compared the formation energy of Al and Cr in Hafnium carbide (HfC). He found that the stability of mixing Cr in hafnium carbide (HfC) was better than that of mixing Al.
The sintering property and densification of carbon oxide have great influence on oxygen diffusion. Borosilicate glass is relatively dense compared with metallic carbon oxide and has a better inhibition effect on oxygen diffusion. This is also one of the reasons why silicon-boride ultra-high temperature ceramics have been widely studied so far.
2.Epilogue
At present, China's research in the field of ultra-high temperature materials has made great breakthroughs, but there are still many problems unresolved 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 studies on matrix modification of C/C composite materials are carried out in micro samples, and the research object should be changed from micro samples to applied components for specific application components, and efforts should be made on how to improve the stability of the preparation process, the portability of matrix modification measures and the coordination of comprehensive properties of components.
(2) Atomic oxygen is studied using material calculation methods. This method can avoid the oxidation caused by the contact between the material and atomic oxygen in conventional experiments. The oxidation mechanism of ultra-high temperature ceramic materials was explored from these aspects by using fluid dynamics method to simulate the phenomenon of fluid flow around the material.
(3) Research on the surface of ultra-high temperature ceramic materials. How molecular oxygen and atomic oxygen bind and diffuse with the surface of these ceramic materials, explore how to prevent the combination of ultra-high temperature ceramic surfaces with oxygen and oxygen diffusion.
(4) Explore measures to improve the toughness of ultra-high temperature ceramic materials. For example, whether nanowires, nanoribbons and nanorods can be introduced into carbides, borides and their composite ceramics to explore whether and how they can improve the toughness of ultra-high temperature ceramics.
(5) To solve the problem of defect control of ultra-high temperature ceramic materials. Defects are unavoidable in ultra-high temperature ceramic materials, and at the same time, defects have a great impact on the performance of ultra-high temperature ceramic materials. Therefore, it is one of the directions of future research to explore the causes of defects and the techniques and means of detection, characterization and control.
