Factors Affecting the Quality of Silicon Nitride-Bonded Silicon Carbide Products and How to Effectively Control Them
With the continuous development of the silicon carbide industry, its preparation process has become increasingly complex, and its performance has become more comprehensive and superior. In the preparation process of silicon nitride-bonded silicon carbide, controlling the factors that affect product quality is crucial for manufacturing enterprises to ensure the performance of the final product and achieve quality control goals. This article discusses relevant issues, analyzing and elaborating on aspects such as the properties of raw materials, binders, different forming processes, drying processes, kiln loading methods, and nitriding processes, providing reference for professionals in the industry.
Silicon nitride-bonded silicon carbide materials are a type of advanced refractory material. They can be further subdivided into categories, such as silicon nitride-bonded silicon carbide radiant tubes and silicon nitride-bonded silicon carbide bricks. Due to their excellent properties, these materials stand out from many products with similar performance, particularly for their energy-saving and environmental protection characteristics. They have a promising market development prospect in the future. The production process flow chart for silicon nitride-bonded silicon carbide products is shown in Figure 1.
Influence of Raw Material Properties
The main raw materials involved in the production of silicon nitride-bonded silicon carbide products include silicon carbide, silicon powder, nitrogen, and other additives. Unlike ordinary silicon nitride products, the raw materials required for silicon nitride-bonded silicon carbide products must have higher purity. The purity of silicon carbide should be above 98.5%, the purity of silicon powder above 99%, and the purity of nitrogen above 99.9%.
In addition to strictly controlling the purity of raw materials, it is also essential to carefully manage the particle size and particle gradation of the raw materials during the production and processing stages. An excessively large particle size will directly affect the bulk density of the green body during forming, reduce its compactness, and thus impact the final product quality.
Regarding raw material particle gradation, particular attention should be paid to the particle size of the silicon powder. Controlling the particle size of silicon powder ensures the reaction efficiency between the silicon powder and nitrogen. However, blindly reducing the particle size of silicon powder can have negative effects: the reaction rate between silicon powder and nitrogen will become too fast, and the violent reaction may lead to heat accumulation and a rise in temperature in the reaction device. Once the temperature exceeds 1400°C, it may induce silicon flow on the surface of the silicon carbide, which would be detrimental to product quality.
Furthermore, adding ZrSiO₄ (zircon) to the raw materials of silicon nitride-bonded silicon carbide products can help improve the oxidation resistance of the products.
Influence of Binders
In the production process of silicon nitride-bonded silicon carbide products, temporary binders need to be added. The addition of binders serves two key functions. First, it helps to fuse the raw materials to form a homogeneous mixture, improves the dispersibility of raw material particles, and creates favorable conditions for green body formation. Second, silicon nitride-bonded silicon carbide products undergo a heating process during the drying and firing stages. Under high-temperature conditions, the temporary binders in the products decompose. During the volatilization of gaseous substances, a large number of network-like pore channels are formed in the silicon nitride-bonded silicon carbide products. This not only facilitates the injection of nitrogen and improves the reaction efficiency between silicon powder and nitrogen but also enhances the stability of the final product.
Common temporary binders include organic dextrin, calcium lignosulfonate, and dispersants from Sika (Germany). Currently, the mass percentage of temporary binders added in the industry is usually within 5%.
Influence of Forming Processes
Currently, the forming processes for silicon nitride-bonded silicon carbide products mainly fall into two categories: semi-dry forming and slip casting. Among these, semi-dry forming is more widely used due to its higher production efficiency. In China, slip casting is primarily adopted, which requires good slurry performance. There are many factors that determine the quality of the slurry, and among them, the surface treatment of silicon carbide micropowder plays a very important role.
The source of this raw material was largely dependent on imports, such as from Saint-Gobain in France. In 2018, Shenyang Changxin Silicon Carbide Micropowder Co., Ltd. successfully overcame technical difficulties through independent research and development and developed silicon carbide micropowder specifically for silicon nitride-bonded silicon carbide products. This not only eliminated the dependence on foreign products but also greatly improved product performance: the bulk density increased from 2.52 g/mL to 2.57 g/mL, and the average flexural strength at 20°C increased from 90 MPa to 100 MPa.
Regardless of the process, the bulk density of the green body directly affects the nitriding quality.
Influence of Drying Regimes
In the drying process, both temperature and time have a significant impact on product quality. If the temperature is too low or the time is too short, residual moisture will remain in the green body. This residual moisture can induce the oxidation of silicon powder during the subsequent nitriding reaction, thereby reducing the nitriding efficiency and affecting product quality.
The rate of temperature rise also affects product quality. A temperature rise that is too rapid is not conducive to controlling the high-temperature environment, and excessively high temperatures can cause cracks to form on the surface of the green body.
Influence of Kiln Loading Methods
In the kiln loading process before the nitriding step, one factor that may affect quality is the gap spacing between the green bodies. A certain amount of space should be left between the green bodies to create favorable conditions for the smooth penetration and filling of nitrogen and to avoid the phenomenon of silicon flow caused by excessive kiln loading.
Influence of Nitriding Regimes
After the raw materials for silicon nitride-bonded silicon carbide products are mixed, kneaded, and formed, they are fired in a nitriding furnace at a high temperature of approximately 1400°C. The quality and performance of the final product are closely related to the temperature of the nitriding reaction.
During the reaction between silicon powder and nitrogen, two main temperature stages are generally experienced. The first is the temperature rise stage, and the second is the nitriding reaction stage of the raw materials. In the temperature rise stage, the temperature inside the furnace increases from the initial temperature to approximately 1100°C, while the temperature during the raw material nitriding reaction stage ranges from 1100°C to 1350°C.
Conclusion
The market demand for silicon nitride-bonded silicon carbide products is steadily increasing, presenting a highly favorable market opportunity for manufacturing enterprises. To build a strong market reputation and enhance the competitiveness of their products, enterprises must analyze the factors affecting the quality of these products and implement effective control measures. This will help ensure the quality of silicon nitride-bonded silicon carbide products and accelerate the achievement of enterprises' operational and strategic goals.