Silicon nitride is a compound of nitrogen and silicon that was discovered more than a hundred years ago. It was synthesized in Germany and only began to be used in the 1950s. As an engineering material, it attracted attention in the 1960s. Silicon nitride is a synthetic substance, and no naturally occurring silicon nitride has been found in nature.
As early as the 1850s, people had directly synthesized silicon nitride in the laboratory using elemental silicon and ammonia or nitrogen: Si3N4 was also obtained by thermal decomposition of ammonia silicon (Si(NH2)4): 3Si(NH2)4→ Si3N4+8NH3. But at that time, silicon nitride did not attract attention. Heat resistance. Under normal pressure, Si3N4 has no melting point and decomposes directly at around 1870°C. It can withstand oxidation up to 1400°C and can reach 1200°C in actual use (mechanical strength will decrease above 1200°C). The thermal expansion coefficient is small (2.8-3.2) × 10-6/℃, the thermal conductivity is high, and it is thermal shock resistant. It will not crack under thermal shock from room temperature to 1000℃. The friction coefficient is small (0.1) and has self-lubricating properties (the friction coefficient of the refueled metal surface is 0.1-0.2).
Stable chemical properties, corrosion resistance, does not react with other inorganic acids except hydrofluoric acid, does not react with oxygen in a dry atmosphere of 800°C, exceeds 800°C, begins to form a silicon oxide film on the surface, and as the temperature increases, silicon oxide The film gradually becomes stable, and can form a dense silicon oxide film with oxygen at around 1000°C. It can remain basically stable up to 1400℃. Silicon nitride has high hardness and wear resistance. Its Mohs hardness is second only to diamond, cubic boron nitride, boron carbide and silicon carbide, and it is resistant to mechanical impact. Silicon nitride is a covalently bonded compound and is difficult to densify. Sometimes external additives are needed, and the density is about 3.4 (different molding methods have different densities. Hot pressing molding has a higher density, the density of steel is about 7.85, and the density of titanium alloy is about 7.85. The density is about 4.5, and the units are g/cm³).
