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Influence of Reaction Sintering Process on Properties of Silicon Carbide Ceramics

In this paper, the effects of degumming temperature, silicon addition amount (that is, the mass ratio of silicon to silicon carbide green body), heating rate and refiring times on the bulk density, bending strength, fracture toughness and microstructure of reaction sintered silicon carbide products were studied. Realize the control method of reaction sintered silicon carbide products.

1 Experimental process

The main crystal phase of the silicon carbide powder selected in the experiment is 6H-SiC, the average particle size is about 9um, and the oxygen content is 0.48wt%. Carbon source: The carbon sources used are SO type graphite and FR6600 carbon black. Silicon source: silicon powder, purity 99%. Organic binder: N660 phenolic resin.

Silicon carbide, carbon black, graphite and phenolic resin were added to water according to the proportions in Table 1, ball milled for 24 h to obtain a well-dispersed slurry, and then spray-dried to obtain granulated powder with a bulk density of 0.86 g/cm3. Thousand press molding under MPa pressure. The formed green body was heated to 200 ℃, 500 ℃, 800 ℃ and 1100 ℃ at a rate of 2 ℃/min, respectively, and degummed for 2 h. Then, according to the mass ratio of silicon to green body (0.7~1.4): 1, the silicon powder was weighed to make the silicon powder and the green body in close contact. C/min, 1.2 ℃/min, 1.0 ℃/min, 0.8 ℃/min rate heating to 1700 ℃, holding for 2 h firing.

2 Analysis test

The metallographic structure of silicon carbide ceramics was observed by laboratory inverted metallographic microscope. The morphology of silicon carbide ceramics was observed by high-resolution scanning electron microscopy. The surface of the sample was electrochemically corroded, that is, under a DC voltage of 10V, 1.0wt% NaOH solution was used as the electrolyte for 180 s. The Vickers hardness of silicon carbide ceramics was tested by the indentation method, the loading load was 0.5kg, 5 test points were randomly selected on the surface of the sample and the average value was calculated. The bulk density of silicon carbide ceramics was measured by the Archimedes method.

3 Results and Discussion

3.1 The effect of degumming temperature on the porosity of the green body

The degumming process is an important process that affects the sintering process. In order to obtain a suitable degumming process, TG-DTA analysis was carried out on the reaction sintered silicon carbide ceramic body, as shown in Figure 1.

It can be seen from Figure 1 that the degumming process is mainly at 200-800 ℃, the most violent stage is 200-500 ℃, and the reaction is relatively slow at 500-800 ℃. The porosity of the china degummed at different temperatures is shown in Figure 2. When the green body is dried at 80 ℃, the porosity of the green body is 26.65%; with the increase of the degumming temperature, the porosity of the green body increases, and when the temperature rises to 800 ℃, the porosity of the green body is 30.23%, and it continues to rise. The increase of porosity at high temperature is not obvious. From the point of view of energy saving, the degumming temperature is determined to be 800 ℃, and the china obtained under this condition has high porosity and silicon infiltration channels, which is conducive to promoting the full reaction of silicon and carbon.

3.2 The effect of heating rate on the microstructure of silicon carbide ceramics

The heating rate (above 1400 ℃) directly affects the whole process of the product reaction. When the heating rate is low, it can be considered that the reaction process is divided into two stages, one is the infiltration of molten silicon, and the other is the reaction between carbon and molten silicon. When the heating rate is fast, the infiltration process of molten silicon and the reaction process of carbon and molten silicon are carried out at the same time, and the reaction between carbon and molten silicon produces volume expansion, which causes some channels to be blocked, so that there is some residual carbon in the green body, and carbonization The silicon is connected together in large areas, while the free silicon is a discontinuous filling.

Figure 3 shows the metallographic structure of silicon carbide ceramics prepared at different heating rates. It can be seen from Figure 3a that when the heating rate is 0.8 °C/min, the infiltration rate of molten silicon is greater than the reaction rate, free silicon preferentially fills all voids, and the distribution of silicon and silicon carbide is relatively uniform, but there is more free silicon in the product. When the heating rate was increased to 1.0 °C/min, the free silicon in the product was uniformly distributed and there was no residual carbon (Fig. 3b). With the increase of the heating rate, the reaction rate is relatively accelerated, and the newly formed silicon carbide isolates the free silicon into many “islands”, called silicon islands, and the area of the silicon islands increases (Fig. 3c). When the heating rate was 1.4 °C/min, the molten infiltration process of silicon and the reaction process of silicon and carbon proceeded simultaneously, and the silicon carbide was continuously distributed with a large area, but there was some residual carbon in the product (Fig. 3d).

3.3 Influence of the amount of silicon added on the bulk density and metallographic structure of silicon carbide ceramics

In order to investigate the effect of the amount of silicon added on the bulk density and metallographic structure of silicon carbide ceramics, the green body with a density of 1.95 g/cm3 was subjected to reaction sintering under different amounts of silicon, the sintering temperature was 1700 ℃, and the temperature was kept for 2 h. .

Figure 4 shows the effect of the amount of silicon added on the bulk density of the reaction sintered silicon carbide ceramics. It can be seen that with the increase of silicon content, the bulk density first increases and then decreases slightly. When the amount of silicon added is 0.9, the bulk density of the product reaches the maximum value of 3.09 g/cm3. During the experiment, it was found that when the amount of silicon added was less than 0.9, the surface of the product turned green, indicating that the silicon addition was insufficient. When the amount of silicon added is greater than 0.9, it is difficult to clean the excessive silicon particles adhered to the surface of the product, which means that the excessive silicon addition causes the capillary channels in the body to be completely filled, and the silicon addition amount continues to increase, and the product is no longer absorbed. Bulk density no longer changes.

It can be seen from Figure 5 that when the amount of silicon added is 0.7, there are residual carbon and pores in the product (Figure 5a); when the amount of silicon is 0.9, the carbon in the product is completely reacted, and the pores are completely filled with free silicon. At this time, the bulk density maximum (Fig. 5b); continue to increase the amount of silicon added, the content of free silicon in the product increases significantly, and the bulk density of the product decreases (Fig. 5c); when the amount of silicon added increases to 1.3, the free silicon content of the product reaches the maximum (Fig. 5c). 5d), continue to increase the amount of silicon, the free silicon content of the product is basically unchanged (Figure 5e).

After testing, when degumming at 800 ℃ for 2 hours, the heating rate is 1.0 ℃/min, and the mass ratio of silicon to silicon carbide green body is 0.9, the bulk density of the prepared silicon carbide ceramics is 3.09 g/cm3, showing good performance. Mechanical properties, its Vickers hardness, flexural strength and fracture toughness are 26.82 GPa, 388 MPa and 4.49 MPa·m1/2, respectively.

3.4 Influence of refiring on the properties and microstructure of silicon carbide ceramic products

In mass production, some products have the problem of insufficient silicon infiltration, and such products are called substandard products. The bulk density of unqualified products is low, and the surface is usually green, and these products are generally distinguished by visual inspection and bulk density testing. The samples were re-fired, and the heating system of the re-fire was the same as the first sintering. By observing the effect of reburning times on product performance, it can be found that one reburning is beneficial to improve the bulk density and mechanical properties of unqualified products. However, the hardness, flexural strength and fracture toughness of the samples decreased to varying degrees after the second refiring.

4 Conclusion

In this paper, the effects of sintering process parameters such as degumming temperature, silicon addition amount, heating rate and refiring times on the properties and microstructure of reaction sintered silicon carbide ceramics were studied. The conclusions are as follows:

(1) When the degumming temperature is 800 ℃, the silicon carbide blank has higher porosity (30.23%) and more silicon infiltration channels, which is conducive to promoting the full reaction of carbon and silicon;

(2) Too fast heating rate is not conducive to the complete reaction of sintered silicon carbide ceramics, and the optimal heating rate is 1.0 ℃/min;

(3) If the amount of silicon added is too small, the product will not react completely, and if the amount of silicon added is too large, too many silicon particles will adhere to the surface of the product, which is difficult to clean. high-performance products;

(4) Refiring the unqualified silicon carbide ceramic products with insufficient siliconization. One refiring can effectively improve the bulk density and mechanical properties of the product, but the second refiring will cause the grains of the sintered body to coarsen, resulting in Mechanical properties decreased.

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