Ultra-high-temperature ceramic matrix composite material refers to a high-temperature structural material that can maintain physical and chemical properties stable under a high-temperature environment above 2000°C and uses a ceramic phase as a matrix. It has low density, wear resistance, excellent high-temperature physical properties, and thermochemical stability. It has outstanding advantages such as good performance and good thermal shock resistance. Ultra-high temperature ceramic matrix composites are considered to be the most promising materials for the manufacture of spacecraft thermal protection components.
Types of ultra-high temperature ceramic matrix composites
Carbide ceramic matrix composites
Ultra-high temperature carbide ceramics mainly include zirconium carbide (ZrC), hafnium carbide (HfC), tantalum carbide (TaC), titanium carbide (TiC), etc., with high melting point, high temperature strength, excellent electrical and thermal conductivity, and good thermal shock resistance And other advantages; however, due to its high brittleness and insufficient oxidation resistance, silicon carbide (SiC), zirconium boride (ZrB2), zirconium oxide (ZrO2), molybdenum silicide (MoSi2), molybdenum (Mo) metal, graphite are often added (Cg) and other particles enhance their mechanical properties, improve their oxidation resistance and sintering performance. Common carbide ceramic matrix composite materials are ZrC-SiC, ZrC-ZrB2, ZrC-ZrO2, ZrC-MoSi2, ZrC-Mo, ZrC-SiC-Cg, HfC-SiC, TaC-SiC, TiC-SiC and so on.
Boride ceramic matrix composites
Ultra-high temperature boride ceramics mainly include zirconium boride (ZrB2), hafnium boride (HfB2), tantalum boride (TaB2) and titanium boride (TiB2). Ultra-high temperature boride ceramics have strong covalent bonds, which have the advantages of high melting point, high strength, large thermal conductivity and electrical conductivity, and small evaporation rate. However, the strong characteristics of covalent bonds make it difficult to densify and sinter the material. The high temperature oxidation resistance of the material also needs to be improved. ZrB2 and HfB2 are the most widely studied ultra-high temperature boride ceramics. By adding SiC to prepare ZrB2-SiC, HfB2-SiC composite materials, a higher binary eutectic temperature can be obtained, improving the mechanical and oxidation resistance of the material.
Continuous fiber toughened ceramic matrix composites
Continuous fiber toughened ceramic matrix composites refer to ultra-high temperature ceramics or multiphase ceramics as the matrix, such as ZrC, ZrB2, HfC, HfB2, TaC, ZrC-SiC, ZrB2-SiC, HfB2-SiC, ZrB2-ZrC-SiC, etc. ，Using high temperature resistant fiber as reinforcement, such as carbon fiber (Cf), silicon carbide fiber (SiCf), etc., formed an ultra-high temperature composite material with high strength, high toughness, small density, high temperature resistance and other excellent properties.
Preparation of ultra-high temperature ceramic matrix composites
At present, the commonly used preparation methods of carbide and boride ultra-high temperature ceramic matrix composite materials include pressureless sintering method, hot press sintering method, reaction hot press sintering and spark plasma sintering method.
Pressureless sintering (PS): Pressureless sintering is the heating of raw materials under normal pressure. It is the simplest sintering method. It is suitable for the preparation of components of different shapes and sizes. The temperature is easy to control, but the resulting material is dense The particle size of the raw material and the sintering aid have a great influence on the density of the material.
Hot-press sintering: Hot-press sintering is a sintering method that fills the raw material powder into the mold and simultaneously pressurizes and heats from the uniaxial direction. This method has a high process cost and the size of the prepared material is limited, so its application is subject to Certain restrictions.
Reaction hot-press sintering: Reaction hot-press sintering is a sintering process formed by using the chemical reaction between raw materials and combining with the hot-press sintering process. This method has a lower sintering temperature and a high density of materials, without powder preparation, and costs relatively low.
Spark plasma sintering: Spark plasma sintering is to pass a high-energy pulse current to a mold containing powder to generate plasma discharge between powder particles for heating and sintering. It is a kind of low sintering temperature, fast speed and high density. Sintering process, but in practice, the uneven distribution of sintering temperature for large-size samples restricts the application of this technology.
At present, the preparation methods of fiber-reinforced ceramic matrix composite materials are: precursor impregnation cracking method (PIP), reactive melt infiltration method (RMI), chemical vapor infiltration method (CVI), slurry method (SI) and so on.
Precursor impregnation cracking method (PIP): precursor impregnation cracking method, also known as polymer impregnation cracking method or precursor conversion method, the general process is: using fiber preforms as the skeleton, impregnating polymer as precursor, in inert gas Under the protection, it is cross-linked and cured, and then pyrolysis in a certain atmosphere to obtain a ceramic matrix composite material. The advantages of this method are: the precursor molecules can be designed to realize the control of the composition, structure and performance of the final composite ceramic matrix; the preparation temperature is low and the equipment requirements are simple; large-scale and complex-shaped structural parts can be prepared to achieve net molding. But there are also shortcomings such as high porosity and long preparation cycle.
Reactive melt impregnation method (RMI): The reactive melt impregnation method is to infiltrate molten metal into a porous preform (generally a C/C preform) under high temperature conditions and then react with C in the preform to form a ceramic matrix. The general process is as follows: the fiber preform is partially densified, and finally the molten metal is used for infiltration. After the molten metal reacts with the matrix carbon, a high-density carbide matrix is obtained. This method has a short preparation period, low cost, high material density, and the matrix composition can be adjusted; however, due to the high impregnation temperature, the fiber is easily damaged.
Chemical vapor infiltration method (CVI): The chemical vapor infiltration method is to place the fiber preform into a special furnace, the gas phase precursor diffuses around the preform with the pressure difference and diffuses into the interior through the pores, and the products formed in the pores are deposited. Methods. The method has the following advantages: it can be used to prepare ceramic matrix composites with higher melting points at lower temperatures; it can be used to prepare ceramic matrix composites with larger sizes and complex structures; the pressure is low during the preparation process and the fibers are mechanically damaged Smaller; can prepare all kinds of ceramic substrates, with a wide range of applications.
Slurry method (SI): The slurry method is to make the required ceramic powder into a slurry, and then introduce it into the fiber preform, and then sinter at a high temperature to obtain a continuous fiber-reinforced ceramic matrix composite material, which can be divided into slurry according to the introduction method of the slurry Material dipping method and slurry brushing method. The slurry impregnation method can promote the dispersion of ceramic powder and improve the comprehensive performance of the composite material, but the powder distribution in the mud is difficult to be uniform, resulting in uneven mechanical properties of the material, poor oxidation resistance, and phase separation.