Si3N4 is a covalently bonded compound. Si3N4 ceramics are polycrystalline materials with hexagonal crystal structure. They are generally divided into α and β crystal directions and are composed of [SiN4] 4-tetrahedron, among which β -Si3N4 has high symmetry and small molar volume, and is thermo-stable phase at temperature. At high temperature (1400℃ ~ 1800℃), α phase transition occurs and becomes β type. This phase transition is irreversible, so α phase is beneficial to sintering.

Performance

Density and specific gravity

The theoretical density of Si3N4 is 3100±10kg/m3, and the true specific gravity of α -Si3N4 and β -Si3N4 are 3184 kg/m3. The volume density of silicon nitride ceramics varies greatly due to the process, generally more than 80% of the theoretical density, about 2200 ~ 3200 kg/m3, the level of porosity is the main reason for different density, reaction sintered silicon nitride porosity is generally about 20%, the density is 2200 ~ 2600 kg/m3, The porosity of hot-pressed silicon nitride is less than 5%, and the density is 3000 ~ 3200 kg/m3. Compared with other materials of similar use, the density is not only lower than all the superalloys, but also lower in the density of high temperature structural ceramics.

Electrical insulation

Silicon nitride ceramics can be used as high temperature insulating materials, and its performance indicators mainly depend on the synthesis method and purity. The free silicon in the material that has not been nitrided, and the alkali metal, alkali earth metal, iron, titanium, nickel and other materials introduced in the preparation can deteriorate the electrical properties of silicon nitride ceramics. General silicon nitride ceramics at room temperature, in the dry medium specific resistance is 1015 ~ 1016 ohm, dielectric constant is 9.4 ~ 9.5, at high temperature, silicon nitride ceramics still maintain a high specific resistance value, with the improvement of technological conditions, silicon nitride can enter the ranks of common dielectric.

The thermal properties of

Sintered silicon nitride has a low thermal expansion coefficient of 2.53 × 10-6 /℃ and a thermal conductivity of 18.42 W/ m ·K, so it has excellent thermal shock resistance, second only to quartz and glass ceramics. Experimental reports show that the reaction-sintered silicon nitride sample with a density of 2500 kg/m3 is composed of 1200 ℃ cooling to 20℃ thermal cycle thousands of times, still does not crack, silicon nitride ceramic thermal stability is good, can be used in high temperature for a long time. It can be used up to 1400℃ in oxidizing atmosphere and up to 1850℃ in neutral or reducing atmosphere.

The mechanical properties

Silicon nitride has high mechanical strength, the flexural strength of general hot-pressed products is 500 ~ 700MPa, and the high flexural strength can reach 1000 ~ 1200MPa; After reaction sintering, the flexural strength is 200MPa, and the high can be 300 ~ 400MPa. Although the strength of the reaction sintered product is not high at room temperature, it still does not decrease at 1200 ~ 1350℃. The creep of si3n4 decreases at high temperature. For example, the reaction-sintered si3n4 has a load of 24MPa at 1200℃, and its deformation is 0.5% after 1000h.

Coefficient of friction and self-lubricity

Silicon nitride ceramic friction coefficient is small, under the condition of high temperature and high speed, increase the friction coefficient is small, so can ensure the effective operation of the institutions, it is that it is a significant advantage of a silicon nitride ceramic began to grind the sliding friction coefficient is 1.0 to 1.5, after precision in the friction coefficient is greatly reduced, remain below 0.5, So silicon nitride ceramics are considered to be self-lubricating materials. The main reason for this self-lubricity, different from graphite, boron nitride, talc, etc., lies in the laminated scale structure of the material organization. It is under the action of pressure, friction surface trace decomposition formed thin gas film, so that the sliding resistance between the friction surface is reduced, friction surface smoothness increased. The more friction, the smaller the resistance, the amount of wear is particularly small, and most materials in constant friction, because of surface wear or temperature softening, friction coefficient often increases gradually.

Machinability

Silicon nitride ceramics can be machined to achieve the desired shape, precision and surface finish.

Chemical stability

Silicon nitride has excellent chemical properties and can resist corrosion of all inorganic acids except hydrofluoric acid and sodium hydroxide solution up to 25%. Its oxidation resistance temperature up to 1400℃, in the reducing atmosphere can be used up to 1870℃, metal (especially liquid AL), especially non-metal wetting.

From the above physical and chemical properties of si3N4 ceramics, it can be seen that the excellent performance of si3N4 ceramics has special use value in the working environment of high temperature, high speed and strong corrosive medium often encountered by modern technology. Its outstanding advantages are:

Has the following advantages:

(1) High mechanical strength, hardness close to corundum. The bending strength of hot-pressed silicon nitride at room temperature can be as high as 780 -- 980MPa, and some are even higher than that of alloy steel, and the strength can be maintained to 1200 ℃ without decreasing.

(2) mechanical self-lubrication, surface friction coefficient is small, wear resistance, elastic modulus, high temperature resistance.

(3) Small thermal expansion coefficient, large thermal conductivity, good thermal shock resistance.

(4) Low density, small specific gravity.

(5) Corrosion resistance, oxidation resistance.

(6) Good electrical insulation.

Product use

(1) In the metallurgical industry to make crucible, muffle furnace furnace, combustion nozzle, heating fixture, mold, aluminum conduit, thermocouple protection casing, aluminum electrolytic cell lining and other thermal equipment components.

(2) Manufacture high speed turning tools, bearings, supporting parts for heat treatment of metal parts, rotor engine blades, gas turbine guide blades and turbine blades in the mechanical manufacturing industry.

(3) in the chemical industry as ball valve, pump body, sealing ring, filter, heat exchanger parts and immobilized catalyst carrier, combustion boat, evaporation dish.

(4) in semiconductor, aviation, atomic energy and other industries used to manufacture switching circuit substrate, film capacitor, bearing high temperature or temperature changes of electrical insulation, radar wire cover, missile tail nozzle, atomic reactor supporting parts and isolation parts, the carrier of nuclear fission material.

(5) Artificial joints can be made in the pharmaceutical industry.