1) Composition and structure

 

The molecular formula of silicon nitride is Si3N4, which belongs to covalent bonding compound. Silicon nitride ceramics belong to polycrystalline material, and the crystal structure belongs to hexagonal system, which is generally divided into α、β Both crystal orientations are composed of [SiN4] 4-tetrahedron, in which β- Si3N4 has high symmetry and small molar volume. It is a thermodynamic stable phase at temperature, but α- Si3N4 is easy to form dynamically. At high temperature (1400 ℃ ~ 1800 ℃), α Phase transition occurs and becomes β This phase transition is irreversible, so α The phase is conducive to sintering.

 

2) Appearance

 

The appearance of silicon nitride obtained from different crystalline phases is different, α- Si3N4 is white or grayish white, loose wool or needle, β- Si3N4 is dark in color and appears as dense granular polyhedron or short prism. Silicon nitride whiskers are transparent or translucent. The appearance of silicon nitride ceramics is gray, blue gray to gray black, which varies with different density and phase ratio, as well as other colors due to additives. The silicon nitride ceramic surface has metallic luster after polishing.

 

3) Density and specific gravity

 

The theoretical density of silicon nitride is 3100 ± 10kg / m3, which is actually measured α- The true specific gravity of Si3N4 is 3184 kg / m3, β- The true specific gravity of Si3N4 is 3187 kg / m3. The bulk 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 porosity is the main reason for the different density. The porosity of reaction sintered silicon nitride is generally about 20%, the density is 2200 ~ 2600 kg / M3, while the porosity of hot pressed silicon nitride is less than 5%, and the density is 3000 ~ 3200 kg / m3, Compared with other materials with similar uses, the density is not only lower than all superalloys, but also lower in high temperature structural ceramics.

 

4) Electrical insulation

 

Silicon nitride ceramics can be used as high-temperature insulating materials, and their performance indexes mainly depend on the synthesis method and purity. The free silicon not nitrided in the material and the magazines such as alkali metal, alkaline earth metal, iron, titanium and nickel brought in during the preparation can deteriorate the electrical properties of silicon nitride ceramics. Generally, the specific resistance of silicon nitride ceramics at room temperature and in dry medium is 1015 ~ 1016 ohms, and the dielectric constant is 9.4 ~ 9.5. At high temperature, silicon nitride ceramics still maintain a high specific resistance. With the improvement of process conditions, silicon nitride can enter the ranks of common dielectrics.

 

5) Thermal properties

 

The coefficient of thermal expansion of sintered silicon nitride is low, which is 2 fifty-three × 10 - 6 / ℃ and the thermal conductivity is 18.42 w / m · K. therefore, 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 cooled from 1200 ℃ to 20 ℃ for thousands of thermal cycles and still does not crack. The silicon nitride ceramic has good thermal stability and can be used for a long time at high temperature. It can be used up to 1400 ℃ in oxidizing atmosphere and up to 1850 ℃ in neutral or reducing atmosphere.

 

6) Mechanical properties

 

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

 

7) Friction coefficient and self lubrication

 

The friction coefficient of silicon nitride ceramics is small. Under the condition of high temperature and high speed, the increase of friction coefficient is also small, so it can ensure the normal operation of the mechanism. This is its outstanding advantage. When silicon nitride ceramics start to grind, the sliding friction coefficient reaches 1.0 to 1.5. After precision running in, the friction coefficient decreases greatly and remains below 0.5, Therefore, silicon nitride ceramics are considered to be self-lubricating materials. The main reason for this self-lubricating property, which is different from graphite, boron nitride and talc, lies in the scale layered structure of the material structure. Under the action of pressure, the friction surface decomposes slightly to form a thin gas film, so as to reduce the sliding resistance between the friction surfaces and increase the smoothness of the friction surface. In this way, the greater the friction, the smaller the resistance, and the wear amount is particularly small. After continuous friction, most materials tend to gradually increase the friction coefficient due to surface wear or softening due to temperature rise.

 

8) Machinability

 

Silicon nitride ceramics can achieve the required shape, accuracy and surface finish by machining.

 

9) Chemical stability

 

Silicon nitride has excellent chemical properties and can resist the corrosion of all inorganic acids except hydrofluoric acid and less than 25% sodium hydroxide solution. Its oxidation resistance temperature can reach 1400 ℃, and it can be used up to 1870 ℃ in reducing atmosphere. It is not wet to metals (especially al liquid), especially non-metals.

 

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

 

It has the following advantages:

 

(1) The mechanical strength is high and the hardness is close to corundum. The room temperature bending strength of hot pressed silicon nitride can be as high as 780-980mpa, some even higher, which can be compared with alloy steel, and the strength can be maintained until 1200 ℃.

 

(2) Mechanical self lubrication, small surface friction coefficient, wear resistance, large elastic modulus and high temperature resistance.

 

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

 

(4) Low density and small specific gravity.

 

(5) Corrosion resistance and oxidation resistance.

 

(6) Good electrical insulation.

 

Shenzhen Fangtai New Material Technology Co., Ltd. specializes in customized production of silicon nitride, alumina, silicon carbide and other ceramic products. Welcome to consult and order. The company's hotline is 0755-27826396, with a radius of degrees and an ruotaishan.

 

2. Product application of silicon nitride ceramics

 

(1) In the metallurgical industry, we make crucible, muffle furnace, burner, heater fixture, mold, aluminum conduit, thermocouple protective sleeve, aluminum electrolytic cell lining and other thermal equipment components.

 

(2) In the mechanical manufacturing industry, we manufacture high-speed turning tools, bearings, metal parts, heat treatment supports, rotor engine blades, guide blades and turbine blades of gas turbines.

 

(3) In the chemical industry, it is used as ball valve, pump body, sealing ring, filter, heat exchanger components, fixed catalyst carrier, combustion boat and evaporation dish.

 

(4) In semiconductor, aviation, atomic energy and other industries, it is used to manufacture switching circuit substrates, thin film capacitors, electrical insulators bearing high temperature or temperature changes, radar wire covers, missile tail nozzles, supports and isolators in atomic reactors, and carriers of nuclear fission materials.

 

(5) It can be used as artificial joint in pharmaceutical industry.