Silicon nitride ceramic is an inorganic material that does not shrink during sintering. It is made of silicon powder that is used as raw material. First, it is made into the required shape by the usual molding method. Then it is preliminarily nitrided in nitrogen gas at the high temperature of 1200℃, so that part of the silicon powder reacts with nitrogen to generate silicon nitride. At this time, the whole body has a certain strength. Then, the second nitridation is carried out in the high temperature furnace at 1350 ℃~1450 ℃ to form silicon nitride. By hot pressing sintering, silicon nitride of 99% purity can be obtained . Silicon nitride has a high strength, especially hot pressed silicon nitride, which is one of the hardest materials in the world. It is extremely resistant to high temperature. Its strength can be maintained even when the temperature reaches 1200 ℃. It will not melt after being heated until 1900 ℃. It has an amazing chemical corrosion resistance. It can withstand almost all inorganic acids and caustic soda solution less than 30%, as well as many organic acids. It is also a high performance electrical insulating material. Silicon nitride ceramics can be used as combustion chamber of gas turbine, mechanical seal ring, pipeline and valve of electromagnetic pump for aluminum liquid delivery, permanent mold, liquid steel separation ring, etc. The friction coefficient of silicon nitride is low, so it is particularly suitable for making high-temperature bearings. Its working temperature can reach 1200 ℃, 2.5 times higher than that of ordinary alloy bearings, and its working speed is 10 times higher than that of ordinary bearings. With good electrical insulation and rapid cooling and heating resistance, silicon nitride ceramics can be used as glow plugs, which can greatly shorten the starting time of the engine and quickly start the vehicle in cold weather. Silicon nitride ceramics also have good microwave transmission performance, dielectric property and high temperature strength. It can be used for the radome of missile and aircraft even at the flight speed of 6 Mach or even 7 Mach.