Both SiC and Si3N4 have many good physical and chemical properties, such as high mechanical strength, good thermal conductivity, and chemical stability against acid and alkali corrosion. Silicon nitride combined with silicon carbide refractories combines the advantages of Si3N4 and SiC. It has the characteristics of high density, high strength, good thermal shock stability, high load softening temperature, good thermal conductivity, high resistance, and is not easily wetted by nonferrous metals. , And has good resistance to cryolite, aluminum fluoride, sodium fluoride, calcium fluoride and other melt erosion, is a very promising inorganic composite material.
(1) Physical performance
The sintering of silicon nitride combined with silicon carbide refractories belongs to solid-phase reaction sintering, and the amount of liquid phase is very small. This microstructure gives the material a higher high-temperature strength. In addition to good wear resistance, SiC itself in silicon nitride combined with silicon carbide refractories has high thermal conductivity and low coefficient of thermal expansion, so that Si3N4-SiC has excellent thermal shock resistance.
The literature uses the hot wire method to determine the thermal conductivity of clay-bonded SiC and Si3N4-SiC products with different Si3N4 contents, as shown in Table 1. The results show that the thermal conductivity of Si3N4-SiC products is significantly better than clay-bonded SiC products, and improves with the increase of Si3N4 content.
Thermal conductivity of different SiC products
Materials containing large aspect ratio β-Si3N4 can be regarded as fiber-toughened ceramics, and granular β-Si3N4 plays a role in increasing the strength of the material. It is possible to increase the strength and sinterability of the material by increasing the content of β-Si3N4 grains with a large aspect ratio in the material. In the study of hot-pressed Si3N4, the correlation between β-Si3N4 grain aspect ratio and material fracture toughness was proved. By controlling the nucleation density and grain size of β-Si3N4, a similar correlation was confirmed in the reaction sintered Si3N4 material. The material prepared with Si3N4 powder containing a large amount of α phase as the raw material has the highest strength. Materials with a large number of well-developed equiaxed β-Si3N4 grains have lower strength.
(2) Anti-oxidation performance
In the case of high oxygen partial pressure, Si3N4 and SiC are prone to passive oxidation, generating SiO2. In the case of low oxygen partial pressure, Si3N4 and SiC are susceptible to active oxidation, generating gaseous SiO. The oxidation resistance of Si3N4-SiC products is an important indicator for evaluating the performance of materials. If its oxidation resistance is poor, the surface of the product is gradually oxidized to form SiO2 or SiO during use, and its corrosion resistance is reduced. SiO2 will also make the thermal conductivity of the material worse, so the cooling efficiency of the furnace wall is reduced and it is difficult to form slag, which further accelerates the material erosion . Adding SiO2 to the material will cause SiO2 and Si3N4 to react to form Si2N2O, and affect the microstructure of the material, reducing the fracture strength of the material.
The thermal properties of Si3N4 and SiC are not stable due to oxidation under standard oxygen pressure. However, in a strong oxidizing environment, a layer of SiO2 will form on the surface, which prevents oxygen from further contacting the interior of the nitride and protecting the interior of the material from oxidation. Therefore, the material does not over-oxidize in a passive oxidation environment and can still withstand high temperature performance, which ultimately depends on the density and stability of the surface oxide film. Mainly consider the following two chemical reactions:
2Si3N4 (s) + 1.5O2 (g) = 3Si2N2O (s) + N2 (g) (1)
Si3N4 (S) + 3O2 (g) = 3SiO2 (s) + 2N2 (g) (2)
SiC (s) + 1.5O2 (g) = SiO2 (s) + CO (g) (3)
However, the oxidation behavior and oxidation resistance of silicon nitride combined with silicon carbide refractories have a great relationship with the service environment. The literature studies the oxidation behavior of SiC prepared by sintered SiC and chemical deposition in air and microwave-excited air. The results show that the total gas pressure, oxygen partial pressure, gas flow rate, and material properties in the environment are The effect of passive oxidation transition point is great. In addition to O2, Si3N4 and SiC can also be oxidized by CO, CO2, H2O.
The passive oxidation process of Si3N4 is generally considered to be achieved by the inward diffusion of oxygen through the surface SiO2 film, which is approximate to a parabolic dynamic curve. However, the oxidation rate of Si3N4 is very different from that of Si and SiC. Compared with the activation energy of Si material 112kJ / mol-1, the activation energy of Si3N4 is about 486kJ / mol-1. In the temperature range of 1000 ~ 1400 ℃, the oxidation rate of Si3N4 is much slower than that of Si. Detailed TEM, XPS, and AES analysis shows that this is because the oxide film on the Si surface is amorphous SiO2, and the Si3N4 surface is a nitrogen-containing double-layer film, including the outer layer of amorphous SiO2 and amorphous silicon oxynitride Si2N2O interface layer. Si2N2O was initially considered to have an approximately fixed ratio of SiO2: Si2N2O, but later detailed AES and RBS studies showed that Si2N2O was formed by the fusion of Si3N4 and SiO2.
The Si2N2O layer is obviously related to the oxidation resistance of Si3N4. The Si3N4 oxidation process includes two closely related steps, one is the infiltration of oxygen in the low-valent oxide layer (Si2N2O), and the other is that oxygen replaces the nitrogen in the nitride. Among them, the infiltration of oxygen in the low-valent oxide layer (Si2N2O) is the limiting factor for the oxidation rate of Si3N4. At present, the problem of O2 penetration in the Si2N2O layer is still to be studied. The thicker SiO2 layer on the Si3N4 surface tends to form crystals, which changes the oxidation kinetics. This process remains to be clarified. The above oxidation behavior is very sensitive to the purity of the system, and it is necessary to control the experimental environment precisely.
The oxidation of Si3N4 and SiC has obvious passive oxidation and active oxidation transition. Under low oxygen partial pressure (103Pa, 1000 ℃), silicon nitride combined with silicon carbide refractory can undergo active oxidation, generating unstable gaseous SiO, which cannot Form a protective layer.
Si3N4 (s) + 1.5O2 (g) = 3SiO (g) + 2N2 (g) (4)
There is still little research on active oxidation, but it has an important influence on the application performance of Si3N4 in low oxygen partial pressure or high temperature environment containing CO2 or H2O.
Some studies have used its oxidation in the air to heat silicon nitride combined with silicon carbide refractories to 1300 ° C to achieve self-healing of micro-cracks in the material. At the same time, the study found that the presence of gaseous state in the healing area has a great influence on the strength recovery of the material. Fully studying the oxidation behavior of Si3N4-SiC and using it reasonably can effectively improve its application performance.
(3) Chemical stability
The strength of the covalent bond in the Si3N4 crystal structure is very strong, with a tetrahedral space network structure, good chemical stability, strong corrosion resistance, does not react with all inorganic acids (except HF), but can react with certain alkalis or salts react. It does not react to the melts of most metals, especially non-ferrous metals. At low temperatures, SiC is relatively stable, does not react with general strong acids, and has excellent corrosion resistance. If SiC is doped with impurities, such as Si, it will react at about 1400 ° C to form a eutectic, and a small amount of liquid phase will be formed, which reduces the high-temperature performance of the material. Because SiC does not melt at high temperatures, it has significant advantages in resistance to high-temperature chemical attack.
The combination of silicon nitride and silicon carbide refractories combines the advantages of Si3N4 and SiC against chemical attack, and its tightly interwoven form provides good protection for the surface of the SiC particles. Silicon bonded silicon carbide refractories have good chemical stability.
Si3N4 and SiC have a certain difference in thermal stability. At high temperatures, Si3N4 is not as stable as SiC. Under the condition of high temperature and the presence of C or CO, Si3N4 can transform to SiC.
Si3N4 (s) + 3C (s) = 3SiC (s) + 2N2 (g) (5)
HARUEWADA, etc. conducted a thermodynamic analysis of the Si-C-N-O system and calculated that when the nitrogen pressure was 0.1 MPa, Si3N4 and SiC reached equilibrium at 1374 ° C. Above this temperature, SiC is more stable; below this temperature, Si3N4 is more stable. When the nitrogen pressure is 1.01MPa, the equilibrium temperature of Si3N4 and SiC rises to 1536 ℃. In addition, there are reactions in Table 2 in the Si-C-N-O system.
There is also a literature study on the phenomenon of high-pressure sintering of hot-pressed SiC in a nitrogen atmosphere. The results show that at 1850 ° C, only 14 μm nitride film is formed.
Si3N4 will not melt, but at 1880 ° C, when the nitrogen pressure is 1 atm, it will decompose into silicon and nitrogen.