The preparation technology of Si3N4 ceramics has developed rapidly in the past few years. The preparation processes mainly focus on the types of reaction sintering method, hot pressing sintering method, normal pressure sintering method, and air pressure sintering method. Due to the different preparation processes, various types of silicon nitride ceramics have Different microstructures (such as porosity and pore morphology, grain morphology, intergranular morphology, and intercrystalline second phase content, etc.). Therefore, the performance varies greatly. To obtain Si3N4 ceramic materials with excellent performance, first of all, high-quality Si3N4 powders should be prepared. The quality of Si3N4 powders prepared by different methods is not completely the same, which leads to the difference in their uses. Many ceramic materials have failed to apply, often blamed Because developers do not understand the differences between various ceramic powders, they have insufficient knowledge of their properties. In general, high-quality Si3N4 powder should have the characteristics of high alpha phase content, uniform composition, less impurities and even distribution in ceramics, small particle size and narrow particle size distribution, and good dispersion. The α-phase of good Si3N4 powder should account for at least 90%. This is because during the sintering process of Si3N4, part of the α-phase will be transformed into β-phase, and without sufficient α-phase content, the strength of the ceramic material will be reduced.
Reaction Sintering (RS)
It is a general molding method, first pressing silicon powder into a green body of the desired shape, and putting it into a nitriding furnace for pre-nitriding (partial nitriding) sintering process. The pre-nitrided green body has a certain strength and can be Carry out various mechanical processes (such as turning, planing, milling, and drilling). Finally, at a temperature above the melting point of silicon; the green body is once again fully nitrided and sintered to obtain a product with a small change in size (that is, after the green body is sintered, Shrinkage rate is very small, linear shrinkage rate <011% ). This product can be used without grinding. The reaction sintering method is suitable for manufacturing parts with complex shapes and accurate dimensions, and the cost is low, but the nitriding time is very long.
Hot Press Sintering (HPS)
It is to sinter Si3N4 powder and a small amount of additives (such as MgO, Al2O3, MgF2, Fe2O3, etc.) at a pressure of more than 1916 MPa and a temperature of more than 1600 ℃. The hot-press sintered Si3N4 ceramics used by some companies in the United Kingdom and the United States have a strength of more than 981 MPa. Additives and phase composition during sintering have a great influence on product performance. Due to the strict control of the composition of the grain boundary phases and proper heat treatment after sintering of Si3N4 ceramics, it is possible to obtain Si3N4 ceramic materials that do not significantly decrease in strength (up to more than 490 MPa) even at temperatures up to 1300 ℃ and are resistant to creep Denaturation can be increased by three orders of magnitude. If the Si3N4 ceramic material is pre-oxidized at 1400-1500 ℃, a Si2N2O phase will be formed on the surface of the ceramic material, which can significantly improve the oxidation resistance and high temperature strength of the Si3N4 ceramic. The mechanical properties of Si3N4 ceramics produced by hot-press sintering are superior to those of reaction-sintered Si3N4, with high strength and high density. However, the manufacturing cost is high and the sintering equipment is complicated. Due to the large shrinkage of the sintered body, the dimensional accuracy of the product is limited. It is difficult to manufacture complex parts, and only parts with simple shapes can be manufactured. The machining of the workpiece is also difficult.
Atmospheric pressure sintering method (PLS)
In terms of increasing the pressure of the sintered nitrogen atmosphere, the nature of Si3N4 decomposition temperature increase (usually decomposed at 1800℃ under N2 = 1atm pressure) is used. After atmospheric sintering in the temperature range of 1700-1800℃, then Air pressure sintering is carried out in the temperature range of 1800-2000℃. The purpose of this method is to use air pressure to promote the densification of Si3N4 ceramic structure, thereby enhancing the strength of the ceramic. The performance of the resulting product is slightly lower than that of hot-press sintering. The disadvantage of this method is similar to hot press sintering.
Air pressure sintering method (GPS)
In recent years, people have conducted a lot of research on pneumatic sintering, and made great progress. Air pressure sintering silicon nitride is carried out at a pressure of about 1 to 10 MPa at a temperature of about 2000°C. The high nitrogen pressure suppresses the pyrolysis of silicon nitride. Due to the use of high temperature sintering, the addition of less sintering aids is also sufficient to promote the growth of Si3N4 grains, and to obtain high-toughness ceramics with in-situ growth of long columnar grains with a density of> 99%. It is also getting more and more attention in production. Air pressure sintered silicon nitride ceramics have high toughness, high strength and good wear resistance, and can directly produce various complex shaped products close to the final shape, which can greatly Reduce production costs and processing costs. And its production process is close to cemented carbide production process, suitable for large-scale production.
What is the sintering method of silicon nitride ceramics?
2020 06/29