Microwave Sintering of Advanced Ceramics
The silicon nitride ceramic material is a promising high temperature structural material that occupies a significant position in new ceramics. It has excellent properties including high temperature strength, high hardness, oxidation resistance, corrosion resistance, and wear resistance. It can be applied in the metallurgical industry because of its heat resistance and high temperature oxidation resistance. It can be used in the machinery industry due to its high strength and high hardness. Due to the increasing growth of hot extrusion die industry in recent years, silicon nitride ceramic materials have emerged as one of the most important candidate materials for the creation of high temperature hot extrusion dies.
Because of its distinct sintering mechanism and properties, such as overall heating, quick heating speed, and high efficiency, the microwave sintering method has gained popularity since it was first applied to the field of ceramic materials in the 1970s. Microwave sintering has an incredibly quick heating rate and a special heating mechanism that is helpful in accelerating densification and can successfully prevent grain growth. With better results than traditional sintering, microwaves have been used to successfully sinter a variety of advanced ceramics, including alumina, zirconia, silicon carbide, boron carbide, etc. As a kind of significant advanced high-temperature structural ceramic, silicon nitride still has an issue of brittleness that needs to be resolved. Its preparation methods consist primarily of atmospheric pressure sintering, hot pressing sintering, plasma activation sintering, etc., but microwave sintering is not so frequently used in the actual production.
Microwave Sintering of Self-Toughened Silicon Nitride Ceramics
The conventional atmospheric pressure or hot pressure sintering process has a slower sintering rate and higher temperature when compared to the microwave sintering method, and they have no effect on accelerating the conversion of α-Si3N4 to β-Si3N4. To address the shortcomings of the prior art, this article proposes a method for producing silicon nitride ceramic materials that can achieve mass production and low-cost production. In order to create a self-toughened silicon nitride ceramic material, the method primarily uses Si3N4 powder as a raw material. It is combined with an appropriate sintering aid and prepared using a microwave solid-phase synthesis technique.
The production of self-toughened silicon nitride ceramics discussed in this article needs the powder metallurgy method, which primarily includes batching, mixing, molding, and sintering. Following are the details:
Firstly, prepare slurry by adding sintering aid to silicon nitride powder and mixing thoroughly in the medium. The oxygen content in silicon nitride powder is less than 2wt%, the free silicon content is less than 0.3wt%, the α-Si3N4 content should be greater than 95% of the total weight of silicon chloride, and the powder particle size is controlled at 0.1-5um. Al203, Y2O3, MgO, Sm2O3, Ce203, and La2O3 can all be used as sintering aids with an average particle size ranges from 0. 5 to 1 um. Using a three-dimensional mixing method, the raw material powder is combined into a mixed powder in proportion. Anhydrous ethanol is then added to create a slurry, which is then ball milled. The slurry contains 40–55% solids. Absolute ethanol or acetone can be used as the ball milling medium along with ceramic silicon nitride balls with a diameter of 30-200mm. One or the combination of roller ball milling, planetary ball milling, and three-dimensional ball milling can be used to mix the materials. The time for ball milling ranges from 4 to 20 hours.
Second, the slurry is dried, sieved, molded, and then formed by cold isostatic pressing. The drying process takes 4 to 8 hours and can be done under vacuum or regular drying conditions. The sieving process uses a sieve with a mesh size of 50–200, and the final particles have an average particle size of 70–300 um. Cold isostatic pressing is carried out at 40-400MPa after compression molding at 20–40MPa.
At last, microwave sintering is done at frequencies between 300MHz and 30GHz in a nitrogen atmosphere. A transition layer is covered outside the silicon nitride ceramics when the frequency is less than 25GHz, and 50-60wt% SiC, 20-30wt% Si3N4, and 10-30wt% BN are combined to form the filter layer. When the microwave frequency exceeds 25GHz, the silicon nitride ceramic is heated directly without being covered by a transition layer. The sintering temperature can be 1400–1750°C, the heating rate can be 10-300 °C per minute, the nitrogen atmosphere pressure can be 0.05-1MPa, and the sintering time can be 10min to 4 hours.
Conclusion
The above method can significantly reduce the sintering time, lower the sintering temperature, and yield products with better performance than the traditional atmospheric pressure and pressure sintering techniques, which have the flaws of low heating efficiency, long sintering times, high energy consumption, coarse grains, and easy cracking. Additionally, the microwave sintering is straightforward, which allows for mass production, and the production costs are low.