Silicon nitride is a compound synthesized under artificial conditions. Although silicon nitride was directly synthesized more than 140 years ago, it remained in people’s memory only as a stable “refractory” nitride. After the Second World War, the rapid development of science and technology, there is an urgent need for high temperature resistance, high hardness, high strength, corrosion resistant materials. After long-term efforts, it was not until 1955 that silicon nitride was paid attention to, and in the mid-1970s, high-quality, low-cost and widely used silicon nitride ceramic products were really produced.
Silicon nitride is used as a high-grade refractory, such as SI3N4-SIC refractory combined with sic for blast furnace body and other parts; If combined with BN as SI3N4-BN material, used for horizontal continuous casting separation rings. SI3N4-BN series horizontal continuous casting separation ring is a kind of fine-structure ceramic material with uniform structure and high mechanical strength. Good heat shock resistance, and will not be wet by liquid steel, in line with the process requirements of continuous casting.
Silicon nitride production method:
There are two kinds of production methods of silicon nitride ceramic products, namely reaction sintering method and hot pressing sintering method. The reaction sintering method is to form silicon powder or a mixture of silicon powder and silicon nitride powder according to the general production method of ceramic products. Then in the nitriding furnace, pre-nitriding at 1150~1200 ° C, after obtaining a certain strength, it can be machined on the machine tool, and then further nitriding at 1350~1450 ° C for 18~36h, until all become silicon nitride. In this way, the product size is accurate and the volume is stable.
Hot pressing sintering method is to press hot forming sintering of silicon nitride powder with a small amount of additives (such as MgO, Al2O3, MgF2, AlF3 or Fe2O3, etc.) at a pressure above 19.6MPa and under the condition of 1600~1700℃. Generally, the product obtained by hot pressing sintering method has higher density and better performance than that obtained by reaction sintering.
Silicon nitride application:
Silicon nitride ceramic material has high thermal stability, strong oxidation resistance and high dimensional accuracy. Since silicon nitride is a covalent compound with high bond strength and can form an oxide protective film in the air, it also has good chemical stability, is not oxidized below 1200 ° C, 1200~1600 ° C protective film can prevent further oxidation, and is not infiltrated or corroded by many kinds of molten metals or alloys such as aluminum, lead, tin, silver, brass, nickel, etc. However, it can be corroded by magnesium, nickel-chromium alloy, stainless steel and other molten fluids.
Silicon nitride ceramic materials can be used for high-temperature engineering components, high-grade refractory materials in metallurgical industry, corrosion resistance parts and sealing parts in chemical industry, cutting tools and cutting tools in machining industry.
Because silicon nitride can form a strong bond with silicon carbide, alumina, thorium dioxide, boron nitride, etc., it can be used as a bond material and modified with different ratios.
In addition, silicon nitride can also be used in solar cells. The silicon nitride film plated by PECVD can not only be used as an anti-reflection film to reduce the reflection of incident light, but also, during the deposition process of the silicon nitride film, the hydrogen atoms of the reaction product enter the silicon nitride film and silicon wafer, and play a role in passivating defects. The ratio of the number of silicon nitride atoms here is not strictly 4:3, but fluctuates within a certain range according to the different process conditions, and the physical properties of the films corresponding to different atomic ratios are different.
Silicon nitride can also be used in ultra-high temperature gas turbines, aircraft engines, electric furnaces and so on.
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