What are the applications of silicon nitride ceramics?

  Silicon Nitride is an inorganic compound with the chemical formula Si₃N₄. It is insoluble in water, with a molecular weight of 140.283 and appears gray or grayish-white. It belongs to a class of high-temperature insoluble compounds, exhibiting no melting point and strong resistance to high-temperature creep. Its solubility is limited to hydrofluoric acid and hot phosphoric acid, as it does not react with other inorganic acids, demonstrating high corrosion resistance. Silicon nitride (Si3N4) exists in three crystallographic forms: α, β, and γ phases. The α and β phases are the most common forms of Si3N4 and can be synthesized at atmospheric pressure. The γ phase can only be obtained under conditions of high pressure and high temperature, with a hardness that can reach 35 GPa.

1.Silicon Nitride Ceramics

• Production Methods of Silicon Nitride Ceramics:

  There are two methods for the production of silicon nitride ceramic products: the reactive sintering method and the hot-pressing sintering method. The reactive sintering method involves shaping a mixture of silicon powder or silicon powder combined with Silicon Nitride Powder according to general ceramic production methods. It is then pre-nitrided in a nitriding furnace at temperatures ranging from 1150 to 1200°C. After achieving a certain strength, it can be mechanically processed on machine tools, followed by further nitriding at temperatures between 1350 to 1450°C for 18 to 36 hours until it is fully converted to silicon nitride. Products produced in this manner exhibit precise dimensions and stable volume.The hot-pressing sintering method involves pressing silicon nitride powder with a small amount of additives (such as MgO, Al₂O₃, MgF₂, AlF₃, or Fe₂O₃) under conditions of pressure exceeding 19.6 MPa and at temperatures between 1600 and 1700°C. Generally, products obtained through the hot-pressing sintering method possess higher density and better performance compared to those made by reactive sintering.

• The advantages of silicon nitride ceramics：

  a. Heat resistance, under normal pressure, Si₃N₄ has no melting point, directly decomposes at about 1870°C, can resist oxidation to 1400°C, and actual use up to 1200°C (mechanical strength will decrease after 1200°C).
  b. High mechanical strength, hardness close to corundum, self-lubricating wear resistance. The flexural strength at room temperature can be as high as 980MPa, which can be compared with alloy steel, and the strength can be maintained until 1200° without decreasing.
  c. High thermal stability, small thermal expansion coefficient, high thermal conductivity, good thermal shock resistance, from room temperature to 1000°C thermal shock will not crack.
 d. Stable chemical properties, corrosion resistance, except hydrofluoric acid does not react with other inorganic acids, the concentration of caustic soda (NaOH) solution below 30% can also resist the erosion of many organic substances, do not wett a variety of non-ferrous metal melts (especially aluminum liquid), can withstand strong radiation irradiation. It does not react with oxygen in a dry atmosphere of 800°C, and begins to form a silicon oxide film on the surface when it exceeds 800°C. It can be maintained up to 1400°C and is basically stable.
  e. Low density, small specific gravity, only 2/5 of steel, good electrical insulation.
  f. Silicon nitride ceramics also have good microwave transmission properties, dielectric properties and high temperature strength, and can be used as radar radomes for aircraft.

2.The application of silicon nitride ceramics

• Aerospace field：Silicon nitride ceramic is heat-resistant and maintains high strength and stiffness even at 1400°C. Reinforced ceramics with continuous fiber reinforcement can be applied to turbine components, especially in ceramic blades of small engines, turbine outer rings, and air bearings. Additionally, silicon nitride ceramics have a lower density, only 41% that of steel bearings, which effectively reduces the weight of aircraft engines and decreases fuel consumption.

• Mechanical Engineering：Silicon nitride ceramics have a small friction coefficient, self-lubricating, high strength, small thermal expansion coefficient, small volume change by temperature, effectively prevent the ball or sealing ring from jamming, can withstand the harsh working environment, and can be made into ball valves, pump bodies, sealing rings, filters, heat exchanger parts, immobilized catalyst carriers, etc. The working life of the bearing ball is also higher than that of ordinary bearings, but the production cost is also relatively high. Silicon nitride has a small friction coefficient and is used in high-temperature bearings, and its working temperature can reach 1200°C, which is 2.5 times higher than that of ordinary alloy bearings, and the working speed is 10 times that of ordinary bearings. At present, silicon nitride ceramic bearings are mainly used in four aspects: (1) high-speed bearings, (2) high-temperature bearings, (3) vacuum bearings, and (4) corrosion bearings.
• Ultrafine Grinding：Silicon nitride has high hardness, second only to diamond and cubic boron nitride. Because of its very low consumption, it reduces the wear of the grinding medium and the contamination of the abrasive material, which is conducive to the acquisition of higher purity ultrafine powder. Silicon nitride ceramic balls are precision ceramic products sintered at high temperature in a non-oxidizing environment, with acid and alkali resistance, corrosion resistance and other characteristics, not only can be used in seawater for a long time, but also has excellent insulation and self-lubrication, so it can be used in an environment without lubrication media and high pollution. At 800°C, the strength and hardness of silicon nitride ceramic balls are almost unchanged, and the weight is almost 1/3 of that of bearing steel. As a result, silicon nitride ceramic balls can not only become the preferred ball beads for ceramic bearings and hybrid ceramic bearings, but also play an important role in the field of ultra-fine grinding.
• Materials in internal combustion engines for automobiles：At present, the manufacturing material of the heat-resistant parts of the internal combustion engine is nickel-based heat-resistant materials, and the working temperature is about 1000°C. If ceramic materials are used, the operating temperature can be increased to 1300°C, increasing engine efficiency by about 30%. Ceramics have high high-temperature strength and thermal conductivity, which can extend the service life of the engine. Using silicon nitride ceramics with good electrical insulation and rapid cold and heat resistance can be used to make glow plugs, silicon nitride ceramic glow plugs heat up to 1000 °C faster than metal glow plugs, high working temperature, using it for car ignition can greatly shorten the engine start time, and can quickly start the car in cold weather.