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Silicon Carbide Reaction Sintering Process
SiC mainly has two crystal forms, namely high-temperature stable hexagonal α-SiC and low-temperature stable cubic β-SiC. The density of α-SiC is 3.217g/cm^3, and the density of β-SiC is 3.215g/cm^3. β-SiC has good stability at 2100 ℃, and it begins to transform into α-SiC above 2100 ℃, especially when the temperature is 2300-2400 ℃, the transformation is rapid, and the β→α transformation becomes one-way irreversible. The SiC synthesized below 2000 ℃ is mainly β-SiC; the one synthesized above 2200 ℃ is mainly α-SiC.
The acquisition of silicon carbide must rely on vacuum sintering technology.
Silicon carbide reaction vacuum sintering (Reaction Bonded Silicon Carbide, RB)
Silicon carbide powder, carbon powder and organic binder are mixed, after molding, drying, degumming (degreasing), and finally siliconized to obtain the product. This method is mostly used in the manufacture of reactive vacuum sintered silicon carbide.
There is almost no shrinkage and dimensional change during the vacuum sintering process. It has the advantages of low vacuum sintering temperature, compact product structure, and low production cost. It is suitable for preparing large-scale and complex-shaped silicon carbide ceramic products.
The density of reactive vacuum sintered SiC is 3.05g/cm^3~3.15g/cm^3, the elastic modulus is 380GPa~430GPa, the bending strength is 350-500 MPa, and the fracture toughness is 4.0-5.0 MPa m^1/2. However, as the Si content increases, the strength and fracture toughness decrease linearly; when the Si content is constant, the strength increases as the SiC grain size decreases. The plasticity of free silicon increases at high temperature, so that the fracture toughness of the material increases with temperature, from 4 MPa·m^1/2 at room temperature to 12 MPa·m^1/2 at 1200 °C.
In order to ensure complete siliconization, the green body (α-SiC+C) should have sufficient porosity. Therefore, the green body density must be strictly controlled, and the appropriate green body density can be obtained by adjusting the content of α-SiC and C in the initial mixture, the particle size distribution of α-SiC, the shape and particle size of C, and the molding pressure. After the reaction is completed, the residual pores of SiC particles are filled with free Si, so that a dense vacuum sintered body can be obtained.
Using sub-micron SiC (0.4-0.5 μm) particles as raw materials, adjusting the mass ratio of C and SiC in the formula between 0.1-0.5, preparing green bodies by wet molding, and reacting vacuum sintering to prepare densities of 3.1 g/cm^ 3 RBSC ceramic material, due to the small size of free Si in the material, generally less than 100 nm, the distribution of the material structure is uniform, so the bending strength of the material can reach more than 1000 MPa.
The use temperature of reactive vacuum sintered SiC ceramic materials generally does not exceed 1400 °C. When the temperature is too high, free Si melts, resulting in a rapid decline in the strength of the material.
Note: High-performance reactive vacuum sintered silicon carbide can also be prepared by cracking high-molecular polymers to prepare a full-carbon porous body, and then high-temperature siliconizing, but the cost is very high.
Pressureless Sintering Silicon Carbide (PS)
Pressureless vacuum sintering, also known as normal pressure vacuum sintering, is relative to pressure vacuum sintering, and can be divided into solid-phase vacuum sintering and liquid-phase vacuum sintering.
3.1 Solid phase vacuum sintering
Add a small amount of B and C to α-SiC/β-SiC powder (sub-micron level) as a vacuum sintering aid, and vacuum sinter at 2020°C/2050°C under normal pressure/vacuum conditions to obtain dense silicon carbide. Usually the amount of B added is about 0.5wt.%, while the amount of C added depends on the oxygen content in SiC powder, and generally increases appropriately with the increase of oxygen content in SiC powder. Additives that have a solid phase vacuum sintering effect on SiC include B4C+C, BN+C, BP+C, AlB2+C, etc. Among them, B4C+C is also a commonly used solid phase vacuum sintering additive.
SiC ceramics sintered in solid phase vacuum, except for a small amount of residual C, there is no second phase or no glass phase in the grain boundary, the grain boundary is clean, and the high temperature performance is good. It can be used up to 1600 ℃ without basically changing the performance. However, SiC solid-state vacuum sintered cannot achieve complete densification, and usually there are a small amount of closed pores at the triangular grain boundaries of the grains, and the grains tend to grow at high temperature.
3.2 Liquid phase vacuum sintering
Liquid-phase vacuum sintering generally uses a certain amount of multi-element eutectic oxide as a vacuum sintering aid, and the material realizes the densification of SiC by liquid-phase vacuum sintering mechanism at a lower temperature. Commonly used liquid-phase vacuum sintering aids such as Y2O3 and Al2O3 can form YAG (Y3Al5O12) in situ.
Due to the low liquid-phase vacuum sintering temperature, the crystal grains are not easy to grow and are small and uniform equiaxed. At the same time, due to the introduction of the liquid phase at the grain boundary and the unique interface structure, the interface bonding is weakened, and the fracture of the material becomes complete. The intergranular fracture mode results in a significant increase in the strength and toughness of the material.
The pressureless vacuum sintering technology of SiC is very mature, and a variety of molding processes can be used to break through the limitations of product shape and size, and higher strength and toughness can be obtained under the action of appropriate additives. In addition, the pressureless vacuum sintering of SiC is easy to operate, moderate in cost, and suitable for mass production.
The density of pressureless vacuum sintered silicon carbide can reach 3.10 g/cm^3~3.18 g/cm^3, the elastic modulus is 410GPa~450GPa, and the bending strength is 400MPa~550MPa.
Silicon carbide hot-pressed vacuum sintering (Hot-pressed Silicon Carbide, HP)
Hot press vacuum sintering is to fill dry silicon carbide powder into a high-strength graphite mold, apply an axial pressure while raising the temperature, and realize the vacuum sintering of silicon carbide under the control of appropriate pressure-temperature-time process conditions. Generally, the vacuum sintering temperature is 1950°C and the pressure is tens of MPa.
Similar to pressureless vacuum sintering, the performance of the product can be improved by adding various vacuum sintering aids. Vacuum sintering aids include B4C, Al2O3, AlN, BN, Al, BeO, B+C, B.
The mechanism of various vacuum sintering aids in the process of vacuum sintering densification can be roughly divided into two categories: one is to form a liquid phase with impurities in SiC, and promote vacuum sintering through the liquid phase; the other is to form a solid solution with SiC, Reduces grain boundary energy and facilitates vacuum sintering.
Due to heating and pressing at the same time, the powder is in a thermoplastic state, which is conducive to the contact diffusion of particles and the process of flow and mass transfer. It can obtain fine grains and relatively Silicon carbide ceramic products with high density and good mechanical properties. The disadvantage of this process is that the equipment and process are complicated, the mold material requirements are high, and only simple-shaped parts can be prepared. The production efficiency is low and the production cost is high. It is suitable for the production of high-performance requirements and high value-added products.
The density of hot-pressed vacuum sintered silicon carbide can reach 3.17 g/cm^3~3.22 g/cm^3, the elastic modulus is 440GPa~450GPa, and the bending strength is 487MPa~770MPa.
The silicon carbide reaction sintering furnace produced by SIMUWU is of excellent quality, and the parameters are as follows:
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