Silicon carbide is resistant to high temperature, does not react with strong acid or strong alkali, has good electrical and thermal conductivity, and has strong radiation resistance. Due to its excellent physical and chemical properties, silicon carbide is widely used in petroleum, chemical, microelectronics, automobile, aerospace, laser, atomic energy, machinery, and metallurgical industries. Such as grinding wheels, nozzles, bearings, seals, gas turbine moving and stationary blades, reflective screen substrates, engine parts, refractory materials, etc.
Tank and armored vehicle composite plates are made of silicon carbide composite materials. This composite plate is 30%-50% lighter than ordinary tank steel plates, and the impact resistance can be increased by 1-3 times. It is an excellent composite material. High-tech ceramics with special functions (electricity, magnetism, sound, light, heat, chemistry, mechanics, biology, etc.) are new materials that have developed rapidly in the past 20 years and are called the third largest material after metal materials and polymer materials.
SiC is a compound with strong covalent bonds, and its diffusion rate during sintering is quite low. According to the research results of J. D. Hon et al., even at a high temperature of 2100℃, the self-diffusion coefficients of C and Si are very small. Therefore, SiC is difficult to sinter, and it must be densified with the help of additives or external pressure or siliconization reaction.
The main methods for preparing high-density SiC ceramics include hot pressing sintering, pressureless sintering and reaction sintering.
(1) Hot pressing sintering of pure silicon carbide powder can approach the theoretical density, but it requires high temperature (greater than 2000℃) and high pressure (350MPa). The use of additives can strongly promote the densification rate and obtain silicon carbide materials with a density close to the theoretical density. Commonly used additives: Al2O3, AIN, BN, B, etc.; the maximum amount of B added is 0.36%. Mechanism: The presence of free carbon reacts with B to form B4C, which then forms a solid solution with SiC. The liquid phase sintering process plays an important role in material migration.
(2) Pressureless sintering (normal pressure sintering) Sintering mechanism: diffusion sintering; the difficulty of diffusion sintering is related to the ratio between grain boundary energy and surface energy. When promoting sintering: pure SiC cannot be sintered. When boron is added, boron is on the SiC grain boundary and partially forms a solid solution with SiC, reducing the grain boundary energy of SiC; in addition, the addition of C helps to reduce and remove the SiO2 film on the SiC surface, thereby increasing the surface energy so that the rg/rs ultrafine powder can provide the mechanical driving force required for densification, shorten the diffusion distance, and enter the initial sintering.
(3) Reaction sintering Reaction sintered SiC, also known as self-bonded SiC, is made by mixing a-SiC powder and graphite powder in a certain ratio and pressing them into a green body, then heating them to about 1650℃, and at the same time infiltrating Si or infiltrating Si into the green body through gas phase Si, so that it reacts with graphite to form β-SiC, combining the existing a-SiC particles. Features: If full Si infiltration is allowed, a material with zero porosity and no geometric size change can be obtained during the whole process. In actual production, the green body must have excessive pores to prevent the formation of an airtight SiC layer due to the Si infiltration process first being carried out on the surface, thereby preventing the reaction sintering from continuing. During the reaction sintering process, the excess pores are filled with excess Si, thereby obtaining a non-porous dense product.
Comparison of three common sintering methods:
1. Hot pressing sintering: only simple-shaped silicon carbide parts can be prepared, with low production efficiency, which is not conducive to large-scale commercial production.
2. Pressureless sintering (normal pressure sintering): can produce complex-shaped and large-sized silicon carbide parts, and is currently the most widely recognized sintering method.
3. Reaction sintering: can produce complex-shaped silicon carbide parts, with low sintering temperature, but the product has poor high-temperature performance.
SiC ceramics using pressureless sintering, hot pressing sintering, hot isostatic pressing sintering and reaction sintering have different performance characteristics. For example, in terms of sintering density and flexural strength, hot pressing sintering and hot isostatic pressing sintering SiC ceramics are relatively more, while reaction sintering SiC is relatively lower. On the other hand, the mechanical properties of SiC ceramics also vary with different sintering additives.
Pressureless sintering, hot pressing sintering and reaction sintering SiC ceramics have good resistance to strong acids and strong alkalis, but reaction sintering SiC ceramics have poor corrosion resistance to superacids such as HF. In terms of high temperature resistance, when the temperature is lower than 900℃, the strength of almost all SiC ceramics is improved; when the temperature exceeds 1400℃, the flexural strength of reaction sintered SiC ceramics drops sharply. (This is because the sintered body contains a certain amount of free Si, and the flexural strength drops sharply when the temperature exceeds a certain temperature) For pressureless sintering and hot isostatic pressing sintering SiC ceramics, their high temperature resistance is mainly affected by the type of additives.
