The influence of SiC fine powder on the sintering performance of new SiC refractory materials

SiC ceramics have advantages such as high hardness, high strength, corrosion resistance, wear resistance, low expansion, and low density, and are applied in many fields [1]. This study introduces micrometer and submicron sized SiC fine powder into the matrix of SiC refractory materials, and sintering is carried out after batching under a certain gradation. The sintered material produces linear shrinkage, a denser structure, and a decrease in apparent porosity, resulting in a new type of SiC refractory material that falls between ceramics and traditional refractory materials. The focus was on analyzing the influence of micron and submicron SiC fine powder content on the pressureless sintering performance of new SiC refractory materials.

1 Experiment 1.1 Raw Materials

Experimental raw materials: The aggregate part is composed of Si C particles (particle size 1.43-0.5 mm, 0.5-0 mm); The matrix part is Shandong Fengyuan D 50=0.45 μ Submicron level fine powder of m and Shandong Xinyuan D 50=2.5 μ Micron level fine powder of m; The sintering additive is B4C fine powder [2], with a specification of W3.5 (Zhengxing boron carbide); The binder is alcohol soluble resin. The composition of the main raw materials is listed in Table 1.

1.2 Test methods

According to the experimental formulas in Tables 2 and 3, mix the ingredients under a certain gradation and use a high-speed mixer for mixing. First, add a binder to the aggregate and mix it evenly. Then, add micrometer and submicron fine powders mixed with the same proportion of sintering aids. After the mixture is uniform, machine press it into shape. After drying the formed sample, load it into the kiln and perform pressureless sintering at 2100 ℃ in an Ar gas protective atmosphere.

Table 1 Composition of Main Raw Materials

Table 1 Composition of Main Raw Materials

Table 2 Composition of Fine Powder Formula in the Sample and Volume Density after Sintering at 100 ℃

Table 2 Composition of Fine Powder Formula in the Sample and Volume Density after Sintering at 100 ℃

Table 2 B3 and μ B3 sample is a fully fine powder sample, and HB3-1, HB3-2, and HB3-3 are mixed with different proportions of micrometer and submicron SiC fine powder, respectively. The volume density of sintered B3 reached 3.02 g/cm 3, which means that the matrix is all 0.45 μ When the sub micron level fine powder is used, the sintering activity of the sample is the highest.

0.45 with the best sintering activity μ A new type of SiC refractory material sample was developed using sub micron sized fine powder as the matrix, and different contents were added to 0.5-0 mm SiC aggregates. The formula design is listed in Table 3.

Table 3 SiC fine powder variable experimental formula design

Table 3 SiC fine powder variable experimental formula design

Several groups of samples in Table 3 were pre treated at 1400 ℃ and sintered at 2100 ℃, and their volumetric density, apparent porosity, and linear shrinkage were compared to determine the changes in sintering performance.

2 Results and Discussion 2.1 Effect of Submicron SiC Fine Powder Content on Sintering Performance

Figure 1 shows the effect of sub micron SiC fine powder content on the volume density and apparent porosity of the above samples after pretreatment at 1400 ℃ and sintering at 2100 ℃ for 3 hours. It should be noted that the pre-treatment process at 1400 ℃ was chosen for the purpose of removing glue and preventing contamination of the furnace during pressureless sintering in an ultra-high temperature furnace. Pretreatment is not a sintering stage, and there is no densification process of sintering shrinkage. The volume density after pretreatment is consistent with the size of the formed volume density of the sample.

Figure 1 Relationship between apparent porosity and bulk density of the sample and sub micron level SiC fine powder content

From Figure 1, it can be seen that the volume density of the sample after pretreatment at 1400 ℃ decreases with the increase of sub micron level SiC fine powder content; After sintering at 2100 ℃, the volume density of the sample initially slightly increases with the increase of sub micron level SiC fine powder content, but a turning point occurs when the fine powder content reaches 50%, and the increase in volume density sharply increases after reaching 60%. After comparison, it can be found that when the sub micron level SiC fine powder content is 20%, the volume density of the sample after sintering at 2100 ℃ is lower than that after pre-treatment at 1400 ℃; After the sub micron level SiC fine powder content exceeds 30%, the volume density of the sample after sintering at 100 ℃ is greater than that after pre-treatment at 1400 ℃. In addition, the apparent porosity of the sample after sintering at 2100 ℃ showed a trend of first increasing and then decreasing. When the sub micron level SiC fine powder content was between 20% and 50%, the apparent porosity of the material after sintering was consistent with that after pretreatment. This indicates that when the sub micron level SiC fine powder content is less than 50%, the sintering densification effect has little effect on the refractory material; When the sub micron level SiC fine powder content exceeds 50%, the sintering densification becomes more pronounced and the apparent porosity decreases.

Figure 2 shows the linear shrinkage changes exhibited by the sintered sample with an increase in the amount of sub micron sized SiC fine powder. It can be seen that when the sub micron level SiC fine powder content is 20%, there is no significant linear shrinkage of the sample; When the sub micron level SiC fine powder content is 30%, the sample exhibits significant linear shrinkage. As the amount of submicron SiC fine powder increases, the linear shrinkage rate also keeps increasing, with all 0.45 added μ The full fine powder sample of sub micron sized SiC has the highest shrinkage rate.

In summary, when the sub micron level SiC fine powder content is within the range of 30% to 50%, the volumetric density of the sample after sintering at 2100 ℃ and pre-treatment at 1400 ℃ have a similar downward trend, that is, the volumetric density decreases with the increase of sub micron level SiC fine powder content; After the sub micron level SiC fine powder content exceeds 50%, the volume density of the pre treated sample continues to decrease, while the volume density of the sample increases sharply with the increase of fine powder content after sintering at 2100 ℃, and the trend of the two changes is opposite.

The relationship between the sintering linear variation of the sample and the sub micron level SiC fine powder content in Figure 2

2.2 Effect of submicron SiC fine powder content on mechanical properties

Figure 3 shows the effect of sub micron SiC fine powder content on the room temperature compressive strength of the sample after sintering. As shown in Figure 3, with the increase of sub micron SiC fine powder content, the compressive strength of the sample also continuously increases. Because the more sub micron level Si C fine powder there is, the more obvious the sintering effect is, and the stronger the bonding between the matrix and aggregate, its strength will also increase with the increase of sub micron level Si C fine powder.

Figure 3 Relationship between room temperature compressive strength and sub micron level SiC fine powder content of the sample

2.3 Effect of submicron SiC fine powder content on microstructure

Figure 4 shows the scanning electron microscopy images of samples S2, S3, S4, and S5 after sintering at 2100 ℃, where S31 and S32 are enlarged views of the two details of S3.

Figure 4 Morphology of sintered fracture surface of the sample

From Figure 4, it can be seen that the bonding between the sub micron sized SiC fine powder matrix and the sintered aggregate of the new SiC refractory material. The sub micron level SiC fine powder content of S2 and S3 is relatively small, and the bonding strength between them and the aggregate is also relatively low. In S2 and S3, aggregates are the main component, while submicron sized SiC fine powder as a matrix is only weakly dispersed in the narrow gaps between aggregates, making it easy for damage to occur between aggregates without matrix distribution or in areas with weak bonding between aggregates and matrix, resulting in lower strength. The distribution of the aggregate in S4 is relatively balanced with the sub micron level Si C fine powder matrix, and almost all of the aggregates are filled with the matrix, but they are not tightly encapsulated by it. In S5, the matrix can completely envelop the aggregates. After the sample is sintered without pressure, the sub micron sized SiC fine powder matrix shrinks, while driving shrinkage movement between the aggregates, making the material more dense. The matrix in S5 serves as the main component of the material, forming a dense "core-shell" structure. In the magnified images of S3 (S31 and S32), it can be seen from the morphology of the bonding part between the aggregate and matrix that submicron sized SiC fine powder is firmly coated on the surface of the aggregate as the matrix. It can also be seen from the figure that growth phenomenon occurs after sintering of submicron sized SiC fine powder. From this, it can be seen that the new SiC refractory material has characteristics between ceramics and traditional refractory materials, and is a special type of self composite directly bonded SiC refractory material.

The influence of particle size of 2.4 SiC fine powder on sintering performance

According to the experimental formula of S3 in Table 3, 0.45 with a matrix content of 30% is added μ Replace all sub micron Si C fine powders with 2.5 μ M micron SiC fine powder, all other conditions remain unchanged, and the obtained sample is recorded as μ B3. take μ The B3 sample was sintered at 2100 ℃.

When using 0.5-0 mm SiC and 30% fine powder in the aggregate section, μ B3 sample (with 2.5 μ M micron SiC fine powder as matrix and S3 sample (with 0.45 μ The sintering performance of sub micron sized SiC fine powder as matrix is shown in Figure 5. As shown in Figure 5, the linear shrinkage rate and apparent porosity of S3 sample are significantly higher than μ B3, but its volume density and compressive strength are lower than μ B3. This indicates that when the matrix content of the new SiC refractory material is 30% and there is little linear change in sintering, although the sintering activity of micrometer level fine powder (2.86 g/cm 3) is lower than that of submicron level fine powder (3.02 g/cm 3, see Table 2), the sintering volume density of micrometer level fine powder samples can still be higher.

Figure 5 Sample μ Comparison of performance between B3 and S3 after sintering

Effect of 2.5 micron and submicron SiC fine powder composite matrix on sintering performance

According to the theoretical research results of SiC particle size distribution optimization by Li Xiaochi et al. [3], it can be concluded that the optimal grading can be achieved when the aggregate fraction is 1.43-0.5 mm, 0.5-0 mm, and 240 mesh, with 35%, 20%, and 5%, respectively. Combined with the research of Zhu Hongxi et al. [4], D 50=3.5 μ m. D 50=2.5 μ m. D 50=0.45 μ The influence of HB3-1, HB3-2, and HB3-3 mixed fine powders as the matrix on the sintering performance of the sample was studied by mixing these three types of SiC fine powders. The experimental formula is listed in Table 4.

Table 4 Grading Experiment of Fine Powder Matrix

Table 4 Grading Experiment of Fine Powder Matrix

The linear changes and bulk density of H1, H2, H3, and C (control group) samples after sintering in Table 4 are shown in Figure 6.

Figure 6 Performance comparison of sintered samples

From Figure 6, it can be seen that the linear shrinkage rate of the sintered samples is almost all greater than 2%. The reference group C has the highest linear shrinkage rate, which means that the sintering activity of the new SiC refractory material sample with sub micron level SiC fine powder as the matrix is better than that of the H1, H2, and H3 samples using mixed fine powder as the matrix. When the fine powder content is 40%, the change in volumetric density of the sintered sample is consistent with the change in volumetric density during molding (the formed volumetric densities of H1, H2, H3, and C are 2.59 g/cm 3, 2.52 g/cm 3, 2.56 g/cm 3, and 2.52 g/cm 3, respectively, and the sintered volumetric densities are 2.72 g/cm 3, 2.64 g/cm 3, 2.69 g/cm 3, and 2.65 g/cm 3 as shown in Figure 6); There is also a significant change in the linear shrinkage rate between H1, H2, and H3, which is due to the different proportions of SiC fine powder with different particle sizes in the matrix. There is a certain gradation effect between micrometer and submicron SiC fine powder, resulting in different linear shrinkage rates of the sample during sintering.

The comparison of the room temperature flexural strength of the sintered samples is shown in Figure 7. From Figure 7, it can be seen that the reference group C has the highest room temperature flexural strength, which is 0.45 for all materials used μ After sintering with sub micron sized SiC fine powder B3 as the matrix, although its bulk density is not as high as H1, H2, and H3, its mechanical properties are the best.

Figure 7 Comparison of flexural strength of sintered specimens

3 Conclusion

(1) 0.45 μ When sub micron sized SiC fine powder is added as a matrix with 0.5-0 mm aggregate, the sintering performance of the sample will undergo significant changes. The sintering linear shrinkage rate increases with the increase of its content, and the turning point of sintering volume density and apparent porosity is at 50%. The mechanical properties of the sample also increase with the increase of sub micron sized SiC fine powder content.

(2) When the content of micrometer and submicron SiC fine powder is 30%, 2.5 μ Replacing 0.45 with m micron level fine powder μ Although the linear shrinkage rate of the sample decreases after m sub micron level fine powder, the volume density of the sample increases to a certain extent after sintering, and the mechanical properties also improve after sintering.

(3) When the micro and submicron Si C fine powder content is 40%, it will be simply 0.45 μ Replace sub micron level fine powder with 3.5 μ m. 2.5 μ m. 0.45 μ When these three different particle sizes and gradations of mixed fine powders are used, although the linear shrinkage rate of the samples is reduced to varying degrees, their formed volume density and sintered volume density are both improved to a certain extent, and all of them are 0.45 μ The mechanical properties of the sample with sub micron sized SiC fine powder matrix are optimal.