What are the structure and properties of silicon nitride?

Overview of Silicon Nitride

Silicon nitride is an important structural ceramic material with high hardness, inherent lubricity, and wear resistance. It is an atomic crystal; it is resistant to oxidation at high temperatures. Silicon nitride is a compound of silicon and nitrogen. Si3N4 is the most thermodynamically stable among silicon nitride. Therefore, when referring to the term "silicon nitride", Si3N4 is the most commercially important among silicon nitrides. It is a white high melting point solid, relatively chemically inert, and is corroded by dilute HF and hot H2SO4. This is very difficult (Moss level is 8.5). Has high thermal stability. Silicon nitride is difficult to produce as a bulk material-due to its dissociation with silicon and nitrogen; it cannot be heated above 1850°C, which is well below its melting point. Therefore, there are problems in the application of traditional hot-press sintering technology. The bonding of silicon nitride powder can be achieved at lower temperatures by adding additional materials (sintering aids or "binders") that usually cause a certain degree of liquid phase sintering. A cleaner alternative is to use spark plasma sintering, in which a current pulse is passed through the compacted powder, thereby heating up very quickly (a few seconds). Through this technology, dense silicon nitride compacts can be obtained at a temperature of 1500-1700°C.

Due to the chemical combination of silicon and nitrogen, the weight of the silicon sample gradually increased. If there is no iron catalyst, after a few hours (about 7 hours), if no further weight increase due to nitrogen absorption (per gram of silicon) is detected, the reaction is complete. Several other silicon nitride phases have been reported in the literature (the chemical formulas correspond to different degrees of nitride/Si oxidation state). For example, gaseous silicon dioxide mononitride (Si2N); mono silicon nitride (SiN) and silicon sesquinitride (Si2N3), are all stoichiometric phases.

Applications of Silicon Nitride

Silicon nitride (Si3N4) was developed in the 1960s and '70s in a search for fully dense, high strength and high toughness materials. A prime driver for its development was to replace metals with ceramics in advanced turbine and reciprocating engines to give higher operating temperatures and efficiencies. Although the ultimate goal of a ceramic engine has never been achieved, silicon nitride has been used in a number of industrial applications, such as engine components, bearings and cutting tools.

Automobile industry

One of the main applications of silicon nitride sintering is as a material for engine parts in the automotive industry. These include glow plugs in diesel engines to speed up the starting speed; pre-combustion chambers (vortex chambers), which can reduce emissions, speed up starting speeds and reduce noise; turbochargers can reduce engine lag and emissions. In spark-ignition engines, silicon nitride is used to reduce wear on the rocker bushes, turbocharger turbines to reduce inertia and engine lag, and exhaust control valves to increase acceleration. As an example of production level, it is estimated that more than 300,000 sintered silicon nitride turbochargers are produced annually.

Bearings

Silicon nitride bearings can be either full ceramic bearings or ceramic hybrid bearings, including ceramic balls and steel rings. Compared with other ceramics, silicon nitride ceramics have good impact resistance. Therefore, ball bearings made of silicon nitride ceramics are used as performance bearings. A typical example is the use of silicon nitride bearings in the mainframe of the NASA space shuttle. Since silicon nitride ball bearings are harder than metal, contact with the bearing track is reduced. Compared with traditional metal bearings, this can reduce friction by 80%, extend service life by 3 to 10 times, increase speed by 80%, reduce weight by 60%, and operate under insufficient lubrication. Capacity, higher corrosion resistance and higher working temperature.

Metal working and cutting tools

The first major application of Si3N4 was grinding tools and cutting tools. Bulk monolithic silicon nitride is used as a material for cutting tools due to its hardness, thermal stability and wear resistance. It is especially recommended for high-speed machining of cast iron. Thermal hardness, fracture toughness and thermal shock resistance mean that silicon nitride sintering can cut cast iron, hard steel and nickel-based alloys, and its surface speed is 25 times faster than traditional materials such as tungsten carbide.

Silicon Nitride Price

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