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Silicon Nitride is a high-melting-point ceramic material that is extremely hard and relatively chemically inert. The material is prepared by heating powdered silicon between 1300 °C and 1400 °C in an atmosphere of nitrogen. Then the powder of silicon nitride can be sintered to the designed shape.
As it is one of the most thermodynamically stable technical ceramic materials with high hardness as well, silicon nitride ceramic is ideal for bearing parts, especially for those required to work at high speed and high temperature. Si3N4 bearing balls have already been applied in automobiles, rocket high-speed engines, equipment working in a corrosive environment, and some performance model cars. Compared with steel, high-performance ceramic balls have better mechanical properties under high temperatures and could survive most of the corrosive chemicals.
Silicon nitride has also been used in high-temperature applications, such as rocket engines. It was considered one of the few monolithic ceramic materials that could survive the severe thermal shock and thermal gradients generated in hydrogen-oxygen rocket engines.
Silicon nitride ceramics are available in several types, each produced using different manufacturing methods, resulting in variations in their properties and applications. The main types include:
Reaction-Bonded Silicon Nitride (RBSN):
Made by nitriding a silicon compact at around 1450°C.
Si₃N₄ grows in the porosity of the compact, resulting in minimal dimensional change.
Allows for near-net shape production, reducing the need for expensive grinding.
Contains porosity, leading to lower mechanical properties compared to other types.
Relatively inexpensive to produce.
Applications: kiln furniture.
Hot-Pressed Silicon Nitride (HPSN):
Applications as cutting tools.
Simple shapes due to the need for diamond grinding.
More expensive and largely superseded by SSN.
Sintered Reaction-Bonded Silicon Nitride (SRBSN):
An attempt to reduce the final porosity of RBSN, improving its mechanical properties.
More recent developments have shown improvements in toughness and impact resistance due to the growth of crack-deflecting silicon nitride grains.
Gas Pressure Sintered Silicon Nitride (GPSN):
Obtained at 1~10MPA pressure and around 2000℃.
The high nitrogen pressure inhibits the pyrolysis of silicon nitride and promotes the growth of Si3N4 grains so that high-toughness ceramics with a density of >99% can be obtained.
GPSN has high toughness, high strength, and excellent wear resistance; it has good processing performance and can be directly processed into the target shape, greatly reducing production costs and processing costs.
Sintered Silicon Nitride (SSN):
Densified by pressureless sintering in a nitrogen atmosphere at around 1750°C.
Uses sintering additives such as yttrium oxide, magnesium oxide, and aluminum oxide to aid densification.
Offers the best mechanical properties for silicon nitride.
Most widely used in industrial applications.
Applications: molten metal handling, industrial wear, metal forming, the oil and gas industries, and the chemical and process industries.
Silicon nitride's unique properties make it suitable for a wide range of demanding applications:
High-purity silicon nitride powder is produced using the imide-decomposition process, known for its purity, controlled morphology, and crystal phase. Different grades of powder are available with varying particle sizes and distributions to meet specific application requirements.
Silicon nitride exists in three main crystallographic structures: α, β, and γ phases. The α and β phases are the most common and can be produced under normal pressure conditions, while the γ phase is synthesized under high pressures and temperatures. The α- and β-Si₃N₄ have trigonal and hexagonal structures, respectively, built from corner-sharing SiN₄ tetrahedra.
A variety of silicon nitride products are available, including powders, components, and custom-engineered parts. These products are tailored to meet the specific requirements of diverse applications across various industries.
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