The Influence of Two Different Molding Methods on the Properties of Silicon Nitride-Bound Silicon Carbide Bricks

Silicon nitride-bonded silicon carbide refractories (Si₃N₄-SiC) possess excellent properties such as high strength, high temperature resistance, high thermal conductivity, and low coefficient of expansion, making them suitable for applications in marine pressurized boilers, particularly in areas subject to intense thermal shock, such as tuyeres, observation holes, and wall bricks. However, due to the irregular shape of furnace refractory bricks, uneven pressure transmission occurs during traditional machine pressing, leading to uneven density distribution in the finished product. When areas with lower density overlap with areas of concentrated thermal shock stress, these overlapping areas become weak points and crack initiation sites, reducing the refractory brick's thermal shock resistance. Therefore, achieving structural homogenization in irregularly shaped refractory brick products and improving their thermal shock resistance is crucial for enhancing their service life and safety.

silicon nitride-bonded silicon carbide brick

How to improve the thermal shock resistance of silicon nitride-bonded silicon carbide refractories?

Currently, there are two methods for evaluating the thermal shock resistance of refractories: one is the number of thermal cycles required to reach a certain failure condition (cracks, 20% mass loss, etc.), but this method has an excessively long evaluation period and large data dispersion. The second method compares the retention rate of a certain property (flexural strength, elastic modulus, etc.) before and after thermal shock. This method reduces the number of thermal shock cycles, evaluates the product as a whole, and has higher data reliability.

Si₃N₄-SiC samples were prepared by vacuum vibration casting and conventional machine pressing. Their bulk density, thermal conductivity, and room-temperature physical strength were tested, and the density distribution was compared. A water quenching method combined with a residual fracture toughness test scheme was designed to analyze the fracture toughness retention rate of the two types of samples before and after thermal shock, providing a new approach to evaluating the thermal shock resistance of refractory materials.

Raw Materials and Sample Preparation

The main raw materials used in the experiment were industrial-grade SiC particles (purity >98% (w)), industrial-grade Si powder (purity >99% (w)), and high-purity nitrogen (purity ≥99.999% (w)). Si₃N₄-SiC samples were prepared according to the ingredient list using vacuum vibration casting and conventional machine pressing.

Performance Testing

The bulk density, apparent porosity, room-temperature flexural strength, room-temperature compressive strength, high-temperature flexural strength (held at 1400 ℃ for 0.5 h), thermal conductivity, and coefficient of thermal expansion of the samples were tested. The microstructure of the sample cross-section was observed.

Results and Discussion

Routine Performance

The routine performance test results of the vacuum vibration casting specimens and the machine-pressed specimens are presented. During the vacuum vibration casting process, the blank distribution is more uniform compared to the machine-pressed blank, and gas is more fully expelled, which is more conducive to densification. Therefore, the resulting specimens have relatively lower apparent porosity and slightly higher bulk density. Simultaneously, the room temperature flexural strength, high temperature flexural strength, and room temperature compressive strength of the vacuum vibration casting specimens are increased by 26.0%, 24.7%, and 33.2%, respectively, compared to the machine-pressed specimens. The room temperature thermal conductivity is 7.6% higher than that of the machine-pressed specimens, while the coefficient of thermal expansion is 13.3% lower. Overall, the routine performance of the vacuum vibration casting specimens is superior to that of the machine-pressed specimens.

Density Distribution

The density distribution of the specimens prepared by the two molding methods is compared and analyzed.

The density of different parts of the vacuum vibration casting specimens ranges from 2.68 to 2.72 g/cm³, with little difference, indicating high structural uniformity and therefore fewer crack initiation areas. The bulk density of the machine-pressed samples ranged from 2.52 to 2.76 g/cm³ in different areas. The density in the central region of the bottom surface corresponding to regions 1#-5# was significantly lower than that in the two sides corresponding to regions 6#-20#, making these areas weak points. This led to stress concentration in the central region of the bottom surface during sudden temperature changes, becoming a crack initiation point. In actual service, the fracture mode of the machine-pressed samples consistently showed initial cracks originating in the central region, eventually propagating to cause brick fracture, consistent with the density distribution analysis results.

Thermal Shock Resistance

The fracture toughness and retention rate of the samples before and after thermal shock. The fracture toughness of both types of samples decreased with increasing thermal shock temperature difference, especially after water cooling at 1600℃, where the decrease in fracture toughness retention rate was significantly greater. At this point, the fracture toughness retention rate of the vacuum vibration casting sample was 63.7%, while that of the machine-pressed sample was only 30.8%. Furthermore, the data dispersion of the fracture toughness of the vacuum vibration casting sample was lower than that of the machine-pressed sample, which is more beneficial for quality control of products in engineering production. Therefore, the thermal shock resistance of vacuum vibration casting specimens is better than that of machine-pressed specimens.

Fracture Analysis

For Si₃N₄-SiC refractories, SiC is the reinforcing phase with a relatively high mass fraction (>70%), while the Si₃N₄ phase mainly acts as a binder. When the interfacial bonding strength between the two phases is sufficiently high, the SiC phase mainly undergoes transgranular fracture during crack propagation, consuming more fracture energy. After thermal shock testing, the sample interface is damaged by oxidation and thermal stress, leading to a decrease in the bonding strength between the two phases and an increase in the proportion of intergranular fracture in the SiC phase. The fracture energy consumed by crack propagation in the Si₃N₄ phase is less, resulting in a decrease in residual fracture toughness. Microstructure of the fracture surface of the vacuum vibration-cast sample before and after thermal shock. The glass phase content at the fracture surface reflects the degree of oxidation of the sample. It can be seen that when the thermal shock temperature is <1400 ℃, oxidation traces are not obvious. When the thermal shock temperature is 1600 ℃, SiO₂ glass phase precipitates on the fracture surface, and the degree of oxidation of the sample increases significantly. Microstructure of the fracture surface of the machine-pressed sample before and after thermal shock. Therefore, it can be seen that the oxidation degree of the samples deepens with increasing thermal shock temperature, and the oxidation corrosion of the machine-pressed samples is more severe after thermal shock at 1600 ℃. The SiO₂ glass phase has covered the fracture surface, indicating that the internal structure of the samples has been severely damaged.

Conclusions

(1) Si₃N₄-SiC refractory materials were prepared by vacuum vibration casting and conventional machine pressing. The conventional properties of the vacuum vibration casting samples are superior to those of the machine-pressed samples.

(2) The vacuum vibration casting samples have a uniform volume density distribution and high strength. In contrast, the machine-pressed samples have an uneven volume density distribution, with the density in the central area of the bottom surface being significantly lower than that on the sides, becoming a weak point and crack source, resulting in lower strength.

(3) The residual fracture toughness of both types of samples decreases with increasing thermal shock temperature. At the same thermal shock temperature, the residual fracture toughness and fracture toughness retention rate of the vacuum vibration casting samples are significantly higher than those of the machine-pressed samples. Therefore, its thermal shock resistance is stronger than that of the machine-pressed samples.