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Silicon nitride ceramic is an inorganic material ceramic that does not shrink when sintered. Silicon nitride is very strong, especially hot pressed silicon nitride, which is one of the hardest substances in the world. It has high strength, low density and high temperature resistance.
Â Â Silicon nitride has both ÃÂ± and ÃÂ² crystal structures, all of which are hexagonal crystals, and have a decomposition temperature of 1800 ÃÂ° C in air and 1850 ÃÂ° C in 110 MPa nitrogen. Si3N4 has a low thermal expansion coefficient and a high thermal conductivity, so its thermal shock resistance is excellent. The hot-pressed sintered silicon nitride is heated to l000 ÃÂ° C and then put into cold water without cracking. At less high temperatures, Si3N4 has higher strength and impact resistance, but it will break with the increase of use time above 1200 ÃÂ°C, which will reduce its strength and fatigue damage will occur more than 1450 ÃÂ°C, so Si3N4 The use temperature generally does not exceed 1300 ÃÂ° C. Due to the low theoretical density of Si3N4, it is much lighter than steel and engineering superalloy steel. Therefore, it is better to use Si3N4 ceramic instead of alloy steel in places where high strength, low density and high temperature resistance are required. It is.
Â Â It is extremely resistant to high temperatures, its strength can be maintained to a high temperature of 1200 ÃÂ°C without falling, it will not melt into a melt after heating, it will decompose until 1900 ÃÂ°C, and it has amazing chemical resistance and can withstand almost all inorganic acids. And less than 30% caustic soda solution can also resist the corrosion of many organic acids; at the same time it is a high-performance electrical insulation material.
Reaction sintering method (RS)
Â Â The general molding method is adopted, in which the silicon powder is first pressed into a green body of a desired shape, and placed in a nitriding furnace by pre-nitriding (partial nitriding) sintering treatment, and the green body after pre-nitriding has a certain strength, and Perform various mechanical processing (such as car, planing, milling, drilling). Finally, above the temperature of the melting point of silicon; once again, the green body is completely nitrided and sintered to obtain a product with a small dimensional change (ie, after the green body is sintered, The shrinkage is small and the line shrinkage is < 011%.) This product is generally used without grinding. The reaction sintering method is suitable for manufacturing parts with complicated shapes and precise dimensions, and the cost is low, but the nitriding time is long.
Hot press sintering (HPS)
Â Â The Si3N4 powder and a small amount of additives are hot-pressed and sintered at a pressure of 1916 MPa or more and a temperature of 1600 ÃÂ°C or