Silicon nitride ceramics are gaining prominence due to their exceptional high-temperature stability, making them ideal for use in various thermal applications such as ceramic firing batts, roll bars, and high-temperature bearings, as well as in turbine blades. This article delves into the intricate production processes of silicon nitride ceramics, which utilize silicon powder as a primary raw material, and explores the advanced techniques that enhance the material's properties for industrial use.
Silicon nitride ceramics are renowned for their resilience at high temperatures, resistance to thermal shock, and impressive mechanical strength. These properties make them increasingly popular in industries that demand materials capable of withstanding extreme conditions. For instance, in the aerospace sector, silicon nitride is used for turbine components due to its ability to endure the high temperatures and stresses within jet engines.
The production of silicon nitride ceramics begins with the transformation of silicon powder into a compacted green body. This body is then subjected to an initial nitriding process in a nitrogen atmosphere at temperatures around 1200°C. Following preliminary nitriding, the semi-finished product undergoes further refinement and is then placed in a high-temperature environment ranging from 1350-1450°C for secondary nitriding. This process, known as reaction sintering, yields ceramics that do not shrink during production, ensuring high dimensional precision.
To achieve the desired quality, manufacturers often start with high-grade crystalline silicon blocks, which are ground using a wet ball mill with alcohol as the grinding medium, achieving particle sizes below 0.07mm. The purified raw materials are then formed into adobe using various molding methods such as casting, compression molding, hot pressing, injection molding, or isostatic pressing, aiming to reach a specific density.
The green body is then treated in a nitriding furnace, which can be a molybdenum wire furnace or a molybdenum disilicide rod furnace, to ensure a vacuum-sealed environment for safety. The reaction between silicon powder and nitrogen commences at temperatures between 970-1000°C, with the rate increasing as the temperature rises. However, it is crucial to avoid temperatures above the silicon melting point to prevent structural collapse.
An alternative method involves first converting silicon powder into silicon nitride powder through nitriding, followed by the addition of a small amount of magnesium oxide. The mixture is then placed in a mold and subjected to hot pressing at temperatures between 1700-1800°C. To enhance the purity of the silicon nitride powder, it may undergo a secondary nitriding process and further purification through fine grinding before hot pressing.
Approximately 5% magnesium oxide or other additives are mixed with the silicon nitride powder using alcohol as a medium in a ball mill. The mixture is then placed in a graphite mold, coated with boron nitride powder to prevent sticking, and hot pressed in a furnace at pressures between 25-50Mpa and temperatures of 1750-1850°C.
According to metal powder suppliers, silicon nitride's exceptional properties make it an excellent material for various high-temperature applications. It is used as a separation ring in horizontal continuous casting and can also serve as a crucible, liquid metal transport pipes, pumps, valves, thermocouple protection tubes, slide gates, and other high-temperature products. In zinc refining furnaces, silicon nitride ceramics demonstrate their potential as a versatile and highly-developable material for high-temperature structural and refractory products.
For more detailed information on the properties and applications of silicon nitride ceramics, visit the American Ceramic Society or explore resources provided by NIST.
Source: Metal Powder Supplier (Note: The original source link provided is no longer available, and thus, a direct citation cannot be made. The information has been fact-checked and updated with current knowledge and practices in the field.)
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