In recent years, multiphase ceramics have become an important direction in the field of advanced ceramic research. As ceramic materials with excellent and similar properties in all aspects, silicon nitride combined with silicon carbide ceramic materials have become the research focus.
Adding SiC to Si3N4 ceramics for particle dispersion strengthening can not only obtain better mechanical properties than single ceramics, but also further improve the high-temperature performance of Si3N4 ceramics. In recent years, scientific researchers have conducted a lot of research on silicon nitride/silicon carbide composite ceramics, and the research results show that composite ceramics are expected to become a material system with better performance.
SiC particles or whiskers have various strengthening and toughening effects on the Si3N4 matrix material. The performance of the prepared composite material is more outstanding than that of single-phase Si3N4 or single-phase SiC. Its fracture toughness is much higher than that of single-phase SiC, and its high temperature resistance , Creep resistance and high temperature oxidation resistance are much higher than single-phase Si3N4.
What are the outstanding performance
1. Silicon nitride combined with silicon carbide ceramics are hard, with a Mohs hardness of about 9, which is an ultra-high hardness material in non-metallic materials.
2. Silicon nitride combined with silicon carbide ceramic not only has high strength at room temperature, but also maintains almost the same strength and hardness as room temperature at a high temperature of 1200-1400°C. With the use of different atmosphere, the highest safe use temperature can reach 1650 ~ 1750 ℃.
3. The thermal expansion coefficient of silicon nitride combined with silicon carbide material is small, compared with silicon carbide and other products, the thermal conductivity is high, thermal stress is not easily generated, it has good thermal shock stability and long service life. High temperature creep resistance, corrosion resistance, extreme cold and extreme heat resistance, oxidation resistance, easy to make products with high dimensional accuracy and meeting requirements.
4. Silicon nitride combined with silicon carbide material does not infiltrate with non-ferrous metals, and has good insulation properties, so it is widely used in the production process of non-ferrous metals such as aluminum, copper and zinc, especially in the production of side walls of electrolytic cells Ideal material for lining bricks.
Powder preparation method
Obtaining uniformly dispersed composite powder is the key to the preparation of high-performance composite ceramics. In recent years, related preparation technologies have been extensively studied, and a series of effective preparation methods have been developed. The main preparation methods of silicon nitride combined with silicon carbide composite powder include mechanical mixing method, gas phase pyrolysis method, polymer precursor method, carbothermic reduction method and sol-gel method.
1. Mechanical mixing method
The mechanical mixing method, also known as the high-energy ball milling method, is the most commonly used method for preparing composite powders. The process is simple and the cost is low, the equipment requirements are not high, and there are no special requirements for personnel. The mechanical mixing process is to mix Si3N4 raw material powder with SiC submicron or nanopowder and sintering aid, ball mill in ethanol or other solvent medium for a certain period of time, and dry the slurry to obtain a uniform mixed powder.
2. Gas phase pyrolysis
The gas-phase pyrolysis process is similar to the preparation of pure Si3N4 powder, and mainly uses the gas-phase thermal reaction or gas-phase pyrolysis reaction of organic gases containing Si, C, and N at high temperatures to form uniformly dispersed Si3N4 and SiC nanoparticles.
3. Polymer precursor method
The solubility and melting properties of the polymer can be used to realize the dispersion of the polymer at the molecular level in the Si3N4 particles. Therefore, polymer precursors containing Si-C (such as polycarbosilane) are often coated on the surface to form uniformly dispersed SiC nanopowders after pyrolysis, while polymer precursors containing Si-CN (such as polysilazane) are used to produce at the same time Si3N4 and SiC nano powder. The preparation process of using polycarbosilane as the precursor is: mixing α-Si3N4 powder, sintering aid and polymer precursor, ball milling, drying, cold pressing, pyrolyzing at about 1000°C, and finally sintering.
4. Carbothermal reduction method
The carbothermic reduction method is an early material preparation method with a long history. This method is derived from the famous Acheson method. The main methods used are:
3SiO2+6C+2N2=Si3N4+6CO
SiO2+3C=SiC+2CO
This method can also realize the in-situ combination of Si3N4 and SiC, becoming an important method for cheaply preparing silicon nitride bonded silicon carbide materials, and the product has a high content of α-Si3N4 phase, but the carbothermic reduction method is a complex reaction system. Moreover, due to the excessive amount of reactants, it is easy to introduce impurities, leading to many factors affecting the reaction, and therefore the product quality is not easy to accurately control.
5. Sol-gel method
Sol-gel is an effective way to prepare structural ceramic composite powders. It has been reported that a silicon nitride silicon carbide composite powder with a sintering aid has been prepared. First, a silica sol or a mixture of silica sol and yttrium nitrate is mixed with a urea solution. The water bath is heated to 80°C, and ammonia gas is introduced and continuously stirred. , Until solidified. Then, the precursor formed by ammonolysis is mixed with carbon black in a certain proportion, using anhydrous ethanol as the medium, ball milling and mixing for several hours, drying to obtain a gel sample, placing it in a graphite crucible, and performing it in a high-temperature furnace under nitrogen atmosphere Carbothermal reduction reaction, the obtained powder is decarburized in the air to obtain a composite powder.
Composite ceramic preparation method
1. Reaction sintering method
The reaction sintering method is a commonly used sintering method, and the reaction raw materials are SiC and Si powder with a certain particle size, or SiC, Si3N4 and Si powder with a certain particle size. Before sintering, the powders are uniformly mixed and then pressed into the required shape, and then sintered in a nitrogen atmosphere to obtain the corresponding multiphase ceramics.
2. Hot pressing sintering method
Hot-pressing sintering is to densify the sintered components by means of mechanical pressure. Hot-pressing sintering is improved on the basis of the reaction sintering method. Hot-pressing sintering can overcome the high porosity of the reaction sintering product. Due to the shortcoming of low strength caused by insufficient densification, high-strength ceramics close to the theoretical density are prepared. The process of the hot pressing method is basically the same as the reaction sintering method. The difference is that the hot pressing method requires a sintering aid and must be fully mixed before adding the reaction materials, otherwise the product will be difficult to achieve dense sintering.
3. Air pressure sintering method
Air pressure sintering is a popular sintering method now. It has simple process and low cost. A relatively dense sintered body can be obtained with certain sintering aids. Air pressure sintering can choose a variety of process conditions, which can effectively control the structure and properties of the material.
4. Self-propagating high-temperature synthesis method
The self-propagating high-temperature synthesis method is an extension of reaction sintering, also known as combustion synthesis method, which uses the heat released by the combustion reaction to cause a violent chemical reaction of the compact containing Si, C, and N substances, and the combustion surface moves forward at a certain speed Advance, continue to spread the method of generating ceramic compounds automatically. The combination of self-propagating synthesis and in-situ synthesis can produce high-performance ceramics at low cost. However, self-propagating synthesis also has obvious shortcomings. Due to the fast combustion synthesis speed, the temperature changes rapidly during the synthesis process, which makes it impossible to accurately control the reaction process. Therefore, the final ceramic has high porosity and low density, resulting in the strength of the ceramic. Low, cannot get the application it should be.
Application
Silicon nitride combined with silicon carbide material has high high temperature strength, good thermal conductivity, good oxidation resistance, and is not easy to be corroded. Many excellent properties determine that it is suitable for use as a refractory material. In the ceramic industry, it can be used as high temperature resistant kiln furniture, which can effectively increase the life of kiln furniture, provide a stable firing environment and ensure product quality.
Silicon nitride combined with silicon carbide material does not infiltrate with non-ferrous metals and has good insulation properties. Therefore, it is widely used in the production process of non-ferrous metals such as aluminum, copper and zinc, especially in the production of electrolytic cell side wall lining bricks The ideal material.
At present, in the environmental protection industry, most of the nozzles in desulfurization and denitrification projects are made of silicon nitride combined with silicon carbide materials. The nozzles formed by using this material have precise dimensions, corrosion resistance, good erosion resistance, and fog Excellent chemical performance and long service life. At present, the consumption of desulfurization and denitration nozzles at home and abroad is increasing year by year.
In aerospace and aviation, other high-temperature structural parts such as radomes can be manufactured by using its thermal shock resistance, oxidation resistance, and high temperature corrosion resistance.
In the military industry, it can be used to make missile nozzles and throat liners. In addition, silicon nitride combined with silicon carbide materials are also used in the support and isolation of reactors in the nuclear industry, medical industry and daily life.