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1. Silicon nitride ceramics
1.1 Basic characteristics of silicon nitride
aα-Si3N4: Low temperature type, it is needle crystal. β-Si3N4: High temperature type, granular crystal. Nitriding high-purity silicon at 1200-1300°C can obtain white or off-white α-Si3N4, and when nitriding at about 1450°C, β-Si3N4 can be obtained.
When aα-Si3N4 is heated at 1400-1600°C, it will transform into β-Si3N4. It will decompose at 1900°C. The strength and toughness of Si3N4 are better than SiC, but the oxidation resistance and high temperature strength are not as good as SiC.
1.2 Preparation technology of silicon nitride
(a) Synthesis of silicon nitride powder:
Table 1 Synthesis method of Si3N4 powder
(b) Sintering aids: MgO, Al2O3, Y2O3
(c) Sintering method: normal pressure sintering, air pressure sintering, hot pressing sintering, reaction sintering, hot isostatic pressing sintering
1.3 Properties and applications of silicon nitride ceramics
(1) Si3N4 ceramic material has high temperature and wear resistance, and is used in the preparation of rotors, stators and scroll tubes of gas turbines in ceramic engines; in non-water-cooled ceramic engines, hot-pressed Si3N4 is used as the piston top cover; reaction sintered Si3N4 can be used As a burner, it can also be used as spark plugs, piston cover tops, cylinder liners, auxiliary combustion chambers, and parts of piston-turbine combined aeroengines for diesel engines.
(2) Si3N4 ceramic material has the characteristics of good wear resistance, high strength, and small friction coefficient. It has relatively high bending strength and high hardness. At the same time, it has low friction coefficient, self-lubrication, and small friction coefficient (only 0.1). Therefore, it can be widely used in the machinery industry, such as manufacturing bearings, balls, rollers, ball races, high-temperature bolts, tools and molds, plunger pumps, sealing materials, etc.
(3) The chemical stability of Si3N4 ceramic material is very good, it is resistant to the corrosion of all inorganic acids other than hydrofluoric acid and some lye, and is not affected by molten metal alloys such as lead, aluminum, tin, silver, brass, nickel, etc. Infiltration and corrosion. Therefore, it can be widely used in the preparation of corrosion-resistant and wear-resistant parts in the chemical industry, such as ball valves, sealing rings, filters, heat exchanger components, pipes, catalyst carriers, etc.
(4) The high temperature resistivity of Si3N4 ceramic material is relatively high (1013-1014Ω cm), on the one hand, it can be used as a better insulating material; at the same time, it can also be used in electronics, military and nuclear industries, such as switch circuit substrates, thin films Capacitors, high-temperature insulators, radar radomes, missile nozzles, barrel linings, nuclear reactor supports, etc.
In addition, Si3N4 ceramics can also be used as heat engine materials, cutting tools, advanced refractory materials, etc.

2. Boron nitride ceramics
Boron nitride (BN) has two crystal forms: cubic BN and hexagonal BN, and hexagonal BN can be transformed into cubic BN under high temperature and pressure.
Cubic boron nitride (C-BN): Its structure and performance are similar to diamond, its hardness is second only to diamond, and its chemical inertness is better than diamond and cemented carbide. The main uses of C-BN are: cutting tools, grinding tools and abrasives, which can be used to cut all metals and non-metals including ferrous metals, and can be used for finishing.
Hexagonal boron nitride (H-BN): Its structure and performance are similar to graphite, also known as white graphite, which is a good high-temperature lubricating material and high-temperature electrical insulation material. H-BN is non-reactive and non-wetting to most metals, and can be used as a crucible or pipe for melting and transporting metals. It can manufacture high-temperature structural components, such as rocket combustion chamber lining, spacecraft heat shield, corrosion-resistant parts of magnetic fluid generator, etc. It can also be used as heat dissipation parts for high temperature, high frequency and high voltage insulation, heat dissipation bottom plate for semiconductor packaging, and neutron absorbing and shielding materials in nuclear reactors.

3. Aluminum nitride ceramics
Basic characteristics and uses of aluminum nitride ceramics: AlN is a hexagonal crystal, with a density of 3.26g/cm3, sublimated and decomposed at 2450°C under normal pressure, and a Mohs hardness of 7~9. The biggest feature is high thermal conductivity, which can reach more than 200W/m·K, small thermal expansion coefficient, high strength, and good electrical insulation performance. It is an ideal substrate material for high-temperature structural parts and heat exchangers.
The sintering process of aluminum nitride ceramics: firing in nitrogen at 1800~1900℃, sintering aids are SiO2, Y2O3, CaO, the strength of ceramics can reach 200~300MPa.

4. Sialon ceramics
Sialon ceramics is a solid solution of Si3N4 and spinel AlN Al2O3. Its crystal structure is the same as that of β-Si3N4, and its mechanical properties are also similar to those of β-Si3N4. Its hardness, strength and thermal conductivity are slightly lower than those of Si3N4, but its toughness is better than that of Si3N4.
The general structural formula of β-Sialon is: Si6-xAlxOxN8-x, at 1700°C, X≤4.2, at 1400°C, X≤2. The application of Sialon ceramics is similar to that of Si3N4, but it is more suitable as a superhard tool material.