1. silicon nitride ceramics

1.1 basic characteristics of silicon nitride

Aα-Si3N4: low temperature type, is needle-like crystals. β-Si3N4: high temperature type, is a granular crystal. The high purity silicon is nitrided at 1200-1300 ° C to obtain white or off-white α-Si3N4, and β-Si3N4 can be obtained when nitriding at about 1450 ° C.

A? -Si3N4 is heated at 1400 to 1600C, and is converted into? -Si3N4.1900C. Si3N4 strength and toughness is better than SiC, but the oxidation resistance and high temperature strength less than SiC.

1.2 Preparation of silicon nitride

(A) Synthesis of silicon nitride powder:

Table 1 Synthesis of Si3N4 powder

(B) sintering aids: MgO, Al2O3, Y2O3

(C) Sintering method: atmospheric sintering, pressure sintering, hot pressing sintering, reactive sintering, hot isostatic pressing sintering

1.3 Properties and Application of Silicon Nitride Ceramics

(1) Si3N4 ceramic material with high temperature wear resistance, in the ceramic engine for the preparation of gas turbine rotor, stator and scroll tube; anhydrous cold ceramic engine, with hot pressing Si3N4 to do the piston cap; with reactive sintering Si3N4 The burner, which can also be used as a spark plug for a diesel engine, a piston cover, a cylinder liner, a sub-combustion chamber, and a piston-turbine modular aero engine.

(2) Si3N4 ceramic material has the characteristics of good wear resistance, high strength and low friction coefficient. The bending strength is high and the hardness is also high. At the same time, the friction coefficient is small, with self-lubricating property and small friction coefficient (only 0.1). So it can be widely used in machinery industry, such as manufacturing bearings, ball, roller, ball seat, high temperature bolts, molds, plunger pumps, sealing materials.

(3) Si3N4 ceramic materials are chemically stable, corrosion of all inorganic acids and certain alkaloids other than hydrofluoric acid, and are not protected by lead, aluminum, tin, silver, brass, nickel and other molten metal alloys. Infiltration and corrosion. So it can be widely used in the chemical industry to prepare corrosion-resistant wear-resistant parts, such as ball valves, sealing rings, filters, heat exchanger parts, pipes, catalyst carriers and so on.

(4) Si3N4 ceramic material high temperature resistivity is relatively high (1013-1014Ω · cm), on the one hand can be used as a better insulating material; also can be used for electronics, military and nuclear industry, such as switch circuit substrate, film Capacitors, high temperature insulators, radar radomes, missile nozzles, barrel lining, nuclear reactor support, and so on.

In addition, Si3N4 ceramics can also be used as heat engine materials, cutting tools, advanced refractory materials.

2. Boron nitride ceramics

Boron nitride (BN) has two crystal forms: cubic BN and hexagonal BN, in high temperature and high pressure hexagonal BN can be transformed into cubic BN.

Cubic boron nitride (C-BN): its structure and performance and diamond similar to its hardness after the diamond, the chemical inertness than the diamond and hard alloy. The main uses of C-BN are: cutting tools, abrasives and abrasives, which can be used to cut all metal and nonmetallic materials including ferrous metals and can be used for finishing.

Hexagonal boron nitride (H-BN): its structure and performance and graphite similar, also known as white graphite, is a good high-temperature lubricating materials and high-temperature electrical insulation materials. H-BN does not react with most metals and is not wettable and can be used as a crucible or pipe for melting and transporting metal. Can produce high-temperature structural components, such as rocket combustion chamber lining, the spacecraft's thermal shielding, magnetic fluid generator, such as corrosion-resistant pieces. But also as a high temperature, high frequency, high voltage insulation parts, semiconductor package cooling floor, nuclear reactor neutron absorption and shielding materials.

3. Aluminum nitride ceramics

The basic properties and uses of aluminum nitride ceramics: AlN is hexagonal crystal, density 3.26g / cm3, atmospheric pressure 2450 ℃ sublimation decomposition, Mohs hardness of 7 to 9. The biggest feature is the high thermal conductivity, up to 200W / m · K or more, thermal expansion coefficient is small, high strength, good electrical insulation properties, is the ideal substrate material for high temperature structural parts and heat exchangers.

Alumina ceramic sintering process: in the nitrogen at 1800 ~ 1900 ℃ firing, sintering aids for the SiO2, Y2O3, CaO, ceramic strength up to 200 ~ 300MPa.

4. Sialon (sialon) ceramics

Ceramics are the solid solution of Si3N4 and spinel AlN · Al2O3. The crystal structure is the same as β-Si3N4. The mechanical properties are similar to that of β-Si3N4. The hardness, strength and thermal conductivity are slightly lower than Si3N4, but the toughness is better than Si3N4.

The structural formula of β-Sialon is: Si6-xAlxOxN8-x, at 1700 ° C, X ≤ 4.2 and 1400 ° C, and X ≤ 2. Ceron Ceramics is similar to Si3N4, but is more suitable for use as superhard tool materials.