The preparation of silicon nitride powder mainly includes three methods: solid phase, liquid phase and gas phase reaction. The direct nitriding method of silicon powder is a widely used method at present, but if high-purity and high-precision silicon nitride powder is to be produced, it needs to be synthesized by a gas phase method to avoid the interference of solid phase impurities. At present, the powders for ceramic balls mostly adopt the method of vapor phase synthesis. The specific preparation methods of silicon nitride ceramics are as follows:
Solid Phase Method to Prepare Silicon Nitride Ceramics
(1) Silicon Powder Direct Nitriding Method
High-purity silicon powder reacts with nitrogen at high temperature for nitriding. It is difficult to control the crystal phase, has low synthesis cost and is widely used in large-scale production, and the reaction product is easy to form a block.
(2) Carbothermal Reduction of Silica
Quartz powder and high-purity carbon powder react at high temperature under nitrogen or ammonia gas conditions. The generated micropowder has small particle size, many single crystal phases, fast speed and high efficiency, but there will be residual silicon dioxide.
(3) Self-Propagating Method
Ignite the powder to propel the reaction by itself. The method is simple in process and has a prospect of popularization.
Liquid Phase Reaction Method to Prepare Silicon Nitride Ceramics
(1) Thermal Decomposition
Silicon chloride generates imide silicon or amino silicon in hexane and ammonia, and then thermally decomposes to silicon nitride. The silicon nitride obtained by the method has high purity, fast reaction speed and high yield.
(2)Sol-Gel Method
Silica was first prepared using the sol-gel method, and then silicon nitride was prepared using the carbothermal reduction method. This method is similar to the carbothermal reduction method, and there is an incomplete reaction of silica.
Gas Phase Reaction Method to Prepare Silicon Nitride Ceramics
(1) High Temperature Gas Phase Reaction
Silicon chloride or silicon hydride reacts with ammonia gas at high temperature. The prepared silicon nitride powder has high purity, but low production efficiency.
(2) Laser Gas Phase Reaction
Silicon chloride or silicon hydride reacts with ammonia gas during laser excitation. The method can control the reaction process and the reaction area;
(3) Plasma Gas Phase Reaction
Silicon chloride or silicon hydride reacts with ammonia gas under plasma excitation. The reaction process can rapidly heat up and rapidly cool down.
Sintering of Silicon Nitride Ceramics
In various ceramic molding methods, hot isostatic pressing and gas pressure sintering are widely used because they can form dense and uniform ceramics with complex shapes. Taking silicon nitride ceramics as an example, several sintering processes are introduced as follows:
Reaction Sintering
The silicon powder or the mixture of silicon powder and silicon nitride powder is molded and pre-nitrided by nitrogen gas at about 1200°C to form a green body, which is then machined into required parts, and finally the final nitriding sintering is carried out at about 1400°C.
This method does not need to add sintering aids, and the strength of the material will not be significantly reduced at high temperatures. The reaction sintered silicon nitride has no shrinkage characteristics and can accurately prepare parts with complex shapes. However, due to the low density of the product (70% to 90%) , there are a large number of pores, and the mechanical properties are greatly affected.
Atmospheric Pressure Sintering
It is prepared by mixing high-purity, ultra-fine silicon nitride powder with high α-phase content and a small amount of sintering aid through molding, sintering and other processes.
Atmospheric pressure sintering can obtain ceramics with complex shapes and excellent performance. The disadvantage is that the sintering shrinkage rate is relatively large, generally 16% to 26%, which is easy to crack and deform the product.
Resintering Method
The reaction-sintered silicon nitride sintered body (precursor material) is placed in the silicon nitride powder in the presence of a sintering aid, and resintered at a high temperature to obtain a dense silicon nitride product.
The shrinkage during the re-sintering process is only 6% to 10%, so parts with complex shapes and excellent performance can be prepared.
Hot Pressing Sintering
The silicon nitride powder and sintering aid are placed in a graphite mold, and unidirectionally pressurized and sintered at high temperature.
This method can only produce products with simple shapes, and the processing costs for parts with complex shapes are high, and due to unidirectional pressure, there is a preferred orientation of the structure, which makes the performance different in the parallel and perpendicular directions to the hot-pressed surface.
Gas Pressure Sintering
The silicon nitride compact is sintered at 1800-2100°C in nitrogen gas of 5-12MPa.
This process is easier to densify materials than atmospheric pressure sintering, and can prepare ceramic parts with complex shapes.
Hot Isostatic Pressing
Using gas as the pressure medium, the material (powder, biscuit or sintered body) is subjected to isotropic and balanced pressure during the heating process, and the densification of the material is promoted by the combined action of high temperature and high pressure.
This method is beneficial to obtain ceramic materials with high density and uniform structure, and overcomes some defects in the pressureless sintering and hot-pressing sintering processes.
Plasma Sintering Method
Mix the silicon nitride powder with an appropriate amount of sintering aid and place it in a graphite mold. Under a pressure of 30 MPa, pulse electric energy is sent into the gap between the pressed powder particles, and the local high temperature field and discharge shock caused by the spark discharge at the initial stage of electrification are used. Direct thermal sintering is achieved by pressure, surface purification, Joule heating, and electric field diffusion effects.
This method is easy to obtain a microcrystalline structure, uniform heating, and easy to obtain a homogeneous sintered body. Compared with hot-press sintering, the operation is simple and the sintering speed is high, but it has the characteristics of expensive equipment and low output per furnace.