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Is Si3N4 Ceramic Harder Than Diamond?

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Update time : 2019-12-19 10:01:16
What is the hardest substance in the world? Everyone usually thinks of diamonds. But you never believe that ceramics may also be the hardest substance in the world. Here we introduce to you silicon nitride ceramics (Si3N4 ceramics), which have high strength, especially hot-pressed silicon nitride ceramics, which is one of the hardest substances in the world. It has high strength, low density, and high-temperature resistance.
 
Si3N4 ceramic is a covalent bond compound. The basic structural unit is silicon nitride tetrahedron. The silicon atom located in the middle of the tetrahedron. There're four nitrogen atoms around it, which are located at the four vertices of the tetrahedron. The tetrahedrons share the form of an atom, forming a continuous and stable network structure in three dimensions.
 
Many properties of silicon nitride are due to this structure. Pure Si3N4 is 3119 and has two crystal structures, α, and β, both of which are hexagonal crystals. The decomposition temperature is 1800 °C in air and 1850 °C in 110MPa nitrogen. Si3N4 has the low thermal expansion coefficient and high thermal conductivity, so its thermal shock resistance is excellent. The hot-pressed sintered silicon nitride will not crack even after being heated to 1000 °C. At a not too high temperature, Si3N4 has higher strength and impact resistance.
 

Still, it will break with the increase of the service time above 1200 , which will reduce its power. It will be more prone to fatigue damage above 1450 , so Si3N4 The operating temperature generally does not exceed 1300 ° C. Because the theoretical density of Si3N4 ceramics is low, it is much lighter than steel and engineering super heat-resistant alloy steels. Therefore, it is appropriate to replace Si3N4 ceramics with alloy steels in those places that require materials with high strength, low density, and high-temperature resistance. But that's it.
 
As an excellent high-temperature engineering material, Si3N4 ceramic material has the most advantages in its application in high-temperature fields. It is incredibly resistant to high temperatures, and its strength can be maintained up to 1200 °C without decreasing. It will not melt into melt after being heated. It will not decompose until 1900 °C. It has extreme chemical resistance and can withstand almost all Inorganic acid and caustic soda solution below 30% can also resist the corrosion of many organic acids; it is a high-performance electrical insulation material as well.
 
The future development direction of Si3N4 ceramics is:
1. Make full use of the excellent characteristics of Si3N4 itself;
2. Research and control the best composition of existing flux when Si3N4 powder is sintered;
3. Improve the milling, forming and sintering processes;
4. Develop composites of materials such as Si3N4 and SiC in order to make more high-performance composite materials.
 
Silicon nitride ceramics uses silicon powder as a raw material. It is first formed into the desired shape by ordinary molding methods. Initial nitriding is performed in nitrogen at a high temperature of 1200 ° C, and a part of the silicon powder reacts with nitrogen to form silicon nitride. At this time, the entire body has a particular strength. Then a second nitriding is performed in a high-temperature furnace at 1350 ° C to 1450 ° C to react to form silicon nitride. Hot-pressing sintering can be used to produce silicon nitride with a theoretical density of 99%.

 
 
Si3N4 ceramic has the characteristics of lightweight and robust hardness, which can be used to make ball bearings.  And it has higher accuracy than metal bearings, generates less heat, and can operate in higher temperatures and corrosive media. It has higher efficiency than metal bearings, generates less heat, and can work in higher temperatures and corrosive media.
 
The steam nozzle made of Si3N4 ceramic is with the characteristics of wear resistance and heat resistance, and it has no visible damage after being used in a 650 boiler for several months, while other heat-resistant and corrosion-resistant alloy steel nozzles can only be used for 1-2 months under the same conditions.
 
The Si3N4 ceramic glow plug developed by Chinese scientists solves the problem of cold starting of engines, and is suitable for direct injection or non-direct injection engines. This glow plug is the most advanced and ideal ignition device for engines today.
 
Japanese researchers have successfully developed a new rough pump with a rotor in the pump casing consisting of 11 Si3N4 ceramic rotors. Because the shoe uses a Si3N4 ceramic rotor with a small thermal expansion coefficient and precision air bearing, it can operate normally without lubrication and cooling media. If this pump is combined with an ultra-vacuum pump such as a turbo molecular pump, a vacuum system suitable for use in a nuclear fusion reactor or semiconductor processing equipment can be formed.
 
The above are just a few application examples of Si3N4 ceramics as structural materials. It's believed that with the improvement of Si3N4 powder production, molding, sintering, and processing technology, its performance and reliability will continue to improve, and silicon nitride ceramics will be more widely used.
 

Due to the improvement of the purity of Si3N4 ceramic raw materials, the rapid development of molding technology and the sintering technology of Si3N4 ceramic powder, and the continuous expansion of the application field, Si3N4 is taking an increasingly important position in the industry as a structural engineering ceramic.
 
Si3N4 ceramic has excellent comprehensive properties and abundant resources, is an ideal high-temperature structural material, has a wide application field and market, and countries around the world are racing to research and development. Si3N4 ceramic materials have the characteristics of wear resistance, corrosion resistance, high-temperature resistance, oxidation resistance, and low specific gravity, which are incomparable for general metal materials.

Silicon nitride ceramic has become the critical primary material to support the pillar industry of the 21st century after metal materials and polymer materials and has become one of the most active research fields. Today, countries around the world attach great importance to their research and development.
 
As an essential member of the high-temperature structural ceramic family, Si3N4 ceramics have better mechanical properties, thermal properties, and chemical stability than other high-temperature structural ceramics such as oxide ceramics and carbide ceramics. Therefore, it is considered to be the most promising material in high-temperature structural ceramics.
 
It's predicted that with the continuous progress of basic research and new technology development of ceramics, especially the increasingly sophisticated technology for the preparation of complicated and broad parts, Si3N4 ceramic will be more widely used as engineering materials with excellent performance.