General Introduction of Silicon Nitride and Silicon Nitride Ceramics

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The development of modern science and technology continues to put forward higher requirements for new materials research and applications, so conventional metallic materials are increasingly difficult to meet this growing requirement, and it is urgent to develop new materials. Over the years, researchers have made unremitting efforts and achieved great progress in preparation techniques and performance of the materials.

Production principles of silicon nitride

Silicon nitride ceramic is an inorganic material without the shrinkage in the course of the sintering. It is used silicon powder as raw materials to form the desired shape as usual, and put it in the initial nitriding under nitrogen and at high temperature of 1,200°C; So part of the silicon powder reacts with nitrogen to generate silicon nitride, and the green body has a certain strength at this time. After that, the second nitride in a furnace at temperatures within 1,350°C~1,450°C generates a silicon nitride. Silicon nitride of theoretical density up to 99% can be obtained by hot pressing sintering.

The basic properties of silicon nitride ceramics

Silicon nitride ceramic is a kind of covalent compounds, with the basic structural unit as [SiN4] tetrahedron. Silicon atom is at the center of a tetrahedron, of which four nitrogen atoms are at the vertex, and then every three tetrahedra share a common atom to form a continuous and solid network structure in three-dimensional space. Many properties of silicon nitride are attributed to this structure. Pure Si3N4 has a specific gravity of 3,119, but also has α and β these two crystal structures as hexagonal crystal form. In addition, it will decompose at the temperature of 1,800°C in air, while at the temperature of 1,850°C in nitrogen of 011MPa. With low thermal expansion coefficient and high thermal conductivity, Si3N4 has excellent thermal shock resistance. So silicon nitride through the hot pressing sintering does not rupture when is heated to l000°C and then put into cold water. At not too high temperatures, Si3N4 has high strength and impact resistance, but its strength will decrease for the damage over time above 1,200°C and for fatigue damage over time above 1,450°C, so Si3N4 is generally used without exceeding 1,300°C. Since Si3N4 has low theoretical density as well as it is much lighter than steel and ultra heat-resistant alloy steel, the Si3N4 ceramic is very appropriate to replace alloy steel for those applications requiring materials with high strength, low density and high temperature.

Performance of silicon nitride

Silicon nitride has high strength, especially hot-pressing silicon nitride, which is one of the world’s hardest substances. It is highly resistant to heat; its strength does not decrease at the high temperature of 1,200°C; it will not melt by heated and will decompose until 1,900°C. And it has amazing chemical resistance to almost all inorganic acids and 30% caustic soda solution, but also much organic corrosion. It is also a high-performance electrical insulating material. Silicon nitride ceramics can be used as combustor of gas turbine, mechanical seal rings, pipes and valves of electromagnetic pump for the transmission of liquid aluminum, permanent mold, and liquid steel separating ring, etc.

Silicon nitride ceramic materials are characteristics of high thermal stability, oxidation resistance and product size accuracy, etc. Since silicon nitride is a covalent compound with high bond strength as well as it can form a oxide film in air, it has superior chemical stability, without oxidized below 1,200°C, forming a protective film to prevent further oxidation within 1,200~1,600°C. Meanwhile, it cannot be infiltrated and corroded by aluminum, lead, tin, silver, brass, nickel and other molten metals or alloys, but can be corroded by magnesium, nickel-chromium alloy and stainless steel.

Silicon nitride with low friction coefficient is particularly suitable for the production of high-temperature bearings, which its operating temperature is up to 1,200°C, 2.5 times than that of ordinary alloy bearings, while the operating speed is 10 times than that of ordinary bearings. With good electrical insulation and resistance to thermal shock, silicon nitride ceramics can be used to make glow plugs, which allow the engine start-up time significantly reduce and can rapidly start cars in cold weather. Silicon nitride ceramics have good microwave-transparent property, dielectric property and high-temperature strength; they can be used at the flight speed of Mach six or seven as radomes of missiles and aircraft.

Since silicon nitride can form strong bindings with silicon carbide, alumina, thoria, boron nitride and so on, it can be used as the bonding material to be modified in different ratios. In addition, silicon nitride powder can be applied to solar cells. After plating silicon nitride film by PECVD, it can be as an antireflection film to reduce reflection of incident light; and during the deposition of silicon nitride film, the reaction product hydrogen atom passes into the silicon nitride film to play a role in the passivation of defects. Nitrogen atoms and silicon atoms of silicon nitride are not in a strict ratio of 4:3, but rather fluctuating within a certain range depending on the process conditions. Different atomic ratio has different physical properties of corresponding films.