One article understanding: silicon nitride combined with silicon carbide products

Silicon nitride/silicon carbide composite ceramic material is a special type of silicon carbide product, which was widely used in the abrasive and electric ceramic industries in the 1970s. In the 1980s, China introduced this material.

The density of silicon nitride and silicon carbide is similar. When columnar silicon nitride penetrates between silicon carbide particles and undergoes sintering, the toughening and strengthening effects produced are far superior to the performance of a single material. Silicon nitride ceramics have greater brittleness and can be combined with silicon carbide materials to improve brittleness and enhance fracture toughness; The insufficient properties of silicon carbide materials, such as mechanical properties, oxidation resistance, wear resistance, etc., can also be improved after being combined with silicon nitride.

The Mohs hardness of silicon nitride/silicon carbide composite ceramic material is about 9, second only to diamond; The material has high strength at room temperature and can maintain the same strength and hardness as at room temperature between 1200 and 1400 ℃. Due to the lack of glass phase in the sintering process of composite materials, excessive high-temperature deformation will not occur during actual use. This makes silicon nitride/silicon carbide composite ceramic materials widely used in high-temperature fields such as large-scale ironmaking furnaces, aluminum electrolysis tanks, ceramic kiln furniture, garbage incinerators, and Luqi liquid slag discharge furnaces.

Silicon nitride bonded silicon carbide products

In industry, silicon nitride/silicon carbide composite ceramic materials are mainly composed of SiC and Si, formed by adding additives, and then placed in a nitriding furnace with high-purity nitrogen gas, and sintered by nitriding at a certain temperature.

The production process flow of silicon nitride bonded silicon carbide products (Image source: Ren Yun et al., "Factors affecting the quality of silicon nitride bonded silicon carbide products and their control measures")

This method allows Si powder to be directly nitrided in situ to generate silicon nitride. During the growth process, Si3N4 crystals form a strong mechanical bond with SiC, forming large SiC particles surrounded by rod-shaped Si3N4 and partially granular SiC matrix. Due to the inability of direct nitriding method to accurately control crystal growth, its product is α- Si3N4 and β- Si3N4 two-phase mixture, α- The content of Si3N4 crystal phase directly determines the quality of composite materials.

raw material

The main production materials involved in silicon nitride/silicon carbide composite ceramic materials include silicon carbide, silicon powder, nitrogen, additives, etc.

Silicon carbide, silicon powder, nitrogen

The purity of silicon carbide should be above 98.5%, the purity of silicon powder should be above 99%, and the purity of nitrogen should be above 99.9%.

In addition to strict control of the purity of raw materials, the density of nitrided products depends on the density of the body as the body does not shrink during the subsequent nitriding process. The particle composition of the body has a significant impact on the density of the body. During the production and processing process, strict control of the particle size and size distribution of the raw materials is required, and coarse, medium, and fine SiC sand and Si powder are weighed and mixed according to the formula.

additive

Catalysts containing Fe, Co, Ni, or Cr can promote the nitriding of silicon powder and the growth of silicon nitride whiskers, and improve the mechanical or corrosion resistance of the material.

Sintering aids can promote the sintering densification of silicon nitride bonded silicon carbide materials and improve their mechanical properties. Oxide sintering aids are the most studied type of sintering aids in ceramic sintering, commonly including Al2O3, MgO, ZrO2, SiO2, RE2O3 (RE=La, Nd, Gd, Y, Yb, Sc), etc; Currently, YF3, YbF3, MgSiN2, and other non oxide sintering additives have been extensively studied.

Moderate reinforcement phases can improve the mechanical and antioxidant properties of materials. Common reinforcement phases include Al2O3, ZrSiO4, B4C, nano Si3N4, Ti Si Fe alloy powder, graphene or carbon nanotubes, etc.

binder

The addition of temporary binders has two main effects. Firstly, it can help achieve homogenization between raw materials, improve the dispersion of raw material particle surfaces, and create favorable conditions for embryo formation; The second is that silicon nitride bonded silicon carbide products face a heating process during the drying and firing process. Under high temperature conditions, the temporary binder in silicon nitride bonded silicon carbide products will decompose, leaving a large number of network shaped pore channels for the volatilization of gaseous substances in the silicon nitride bonded silicon carbide products. This not only facilitates the filling of nitrogen, but also improves the reaction efficiency between silicon powder and nitrogen, And it can also be more conducive to the stability of the final product.

Temporary binders mainly include organic dextrin, calcium lignosulfonate, and German Sima Chemical dispersant. Currently, the amount and mass percentage of temporary binders added in the industry are usually within 5%.

molding

At present, the forming process of silicon nitride/silicon carbide composite ceramic materials mainly includes two categories: semi dry forming and injection molding.

The application of semi dry molding is more common. The advantage of this molding method is its high efficiency, which is suitable for large-scale production. The disadvantage is that it can only manufacture products with simple and coarse shapes, and cannot produce thin plates and products with complex shapes.

Grouting molding is widely used in China, with a simple process and low cost. But it is required that the performance of the slurry must be good, and there are many factors that determine the quality of the slurry, among which the surface treatment of silicon carbide micro powder plays a very important role; In addition, this method has low strength for forming the green body and the water absorption process of the gypsum mold can easily cause a gradient distribution of the density of the green body.

Gel casting is a new ceramic forming technology developed in recent years. it can form complex and uniform green bodies, and has become an ideal forming technology for ceramic materials. The molding method is to first disperse the ceramic powder in a solution containing organic monomers and crosslinking agents, prepare a suspension slurry with high solid volume fraction, and then inject it into a mold of a certain shape. Under certain conditions, the organic monomers react with the crosslinking agent, and then the slurry solidifies in place.

Schematic diagram of gel injection molding process (picture source: Shanghai Institute of Silicon, Chinese Academy of Sciences)

Sintering

The sintering methods of silicon nitride/silicon carbide composite ceramic materials include reaction sintering, hot pressing sintering, gas pressure sintering, hot isostatic pressing sintering, oscillating pressure sintering, etc. Regardless of the sintering method, the sintering system of the material affects the nitriding rate of silicon powder. Choosing an appropriate heating rate, sintering temperature, and holding time will promote the nitriding of silicon powder and improve the density and strength of the material. When the sintering temperature is low, the nitriding of silicon powder is incomplete; When the sintering temperature is too high, silicon nitride will decompose, causing a decrease in material density and damaging its mechanical properties.

During the reaction between silicon powder and nitrogen, there are roughly two temperature stages: first, the heating stage, and then the nitriding reaction stage of the raw materials. The temperature inside the device during the heating stage increases from the initial temperature to around 1100 ℃, while the temperature during the raw material nitriding reaction stage is between 1100-1350 ℃.

epilogue

Silicon nitride/silicon carbide composite ceramic materials have characteristics such as low expansion, high thermal conductivity, insulation, and long service life. At the same time, this material has strong corrosion resistance, oxidation resistance, corrosion resistance, and good insulation properties, and has a certain bearing capacity for general inorganic acid and alkali solutions as well as molten non-ferrous metals.

These unique high-temperature characteristics are steadily increasing the market demand for silicon nitride/silicon carbide composite ceramic materials. With the development of industrial technology, the requirements for the size and accuracy of material products are becoming increasingly high, especially in complex shaped irregular parts. The market demand for ceramics and refractory materials is not only for simple shaped bricks and large plates, but also for some irregular parts, small parts, and thin plates, which are increasing year by year. To meet market requirements, it is also necessary to research and develop new production process routes for silicon nitride/silicon carbide composite ceramic materials, adding diverse shapes and sizes of silicon nitride/silicon carbide composite ceramic products to the market.