Characteristics and Applications of Silicon Nitride Ceramics
The excellent mechanical properties of silicon nitride ceramics make them a promising material for use as high-temperature structural ceramics. These properties include resistance to thermal shock and creep at high temperatures, as well as high strength and hardness, self-lubricating qualities, and good stability. Ceramics made of silicon nitride are utilized in a variety of applications, including those that need refractory materials, high-performance bearings, ball valves that are resistant to heat and corrosion, sealing rings, and specialized cutting tools.
Causes of Oxidation of Silicon Nitride Ceramics
Pure Si3N4 ceramics can fulfill the demands of high temperature structural materials and have strong oxidation resistance. Pure Si3N4 ceramics, however, can only be produced using costly methods like CVD or high temperature hot isostatic pressing since they are very challenging to sinter. Multiphase bodies of Si3N4 ceramics, which are frequently utilized, are created by hot pressing, air pressure, or reaction sintering when sintering aids are added. Typically, MgO, Al2O3, Y2O3, La2O3, CeO2, and other materials are used as sintering aids. These ions are added, which aids in dissolution and encourages sintering. However, the oxidation resistance of Si3N4 ceramics at high temperatures is also significantly decreased by the presence of these impurity ions. Research reveals that the oxidation rate of silicon nitride ceramics with sintering aids is two orders of magnitude higher than the oxidation rate of silicon nitride generated by chemical vapor deposition (CVD) in the presence of water vapor, impurities, and other reactive gas or liquid phases. The oxidation rate will be further accelerated when above-mentioned substance present.
One of the primary problems affecting the dependability of silicon nitride ceramics and limiting applications is oxidation, which causes the development of cavities, fractures, weak grain boundaries, and increased wear rate on the surface of products. Due to the expensive expense of creating ultra-pure silicon nitride ceramics through chemical vapor deposition and their extremely restricted uses, silicon nitride ceramics with commercial application value are essentially aided by sintering aids.
The oxidation resistance of silicon nitride ceramics is of major relevance for their high-quality and low-cost manufacturing, and it is of great significance to lengthen the service life of silicon nitride ceramic components, enhance their dependability and practical use in various fields. Therefore, this article provides a novel, efficient, broadly adaptable, and cost-effective surface modification method to improve the high temperature oxidation resistance of silicon nitride ceramics by utilizing the oxidation rate control mechanism of Si3N4 materials.
Experimental Principle of the Surface Modification Method
The majority of the cations from sintering aids and impurities are localized at the grain boundaries, and high temperature heat treatment can remove a significant number of cations. According to this theory, it is suggested in this paper that silicon nitride can be oxidized to create an oxide layer with the proper thickness on its surface before being maintained at the proper temperature and atmosphere until a significant amount of sintering aids and impurity cations have been completely transferred from the grain boundaries to the surface. The surface oxide layer is eliminated once the oxide layer has been diffused.
After this process, a "extraction purification layer" will be formed, increasing the crystallinity of the grain boundaries, and the concentration of sintering aids and impurity cations in the near-surface area of the silicon nitride will be greatly reduced. The grain boundary is further crystallized by using an appropriate temperature and atmosphere system, which can ensure that the sintering aids and impurity cations that are on the surface of the silicon nitride ceramic after this process are treated. This process is carried out regardless of whether the ions are fully diffused into the oxide layer or after the oxide layer has been removed. The concentration has dropped dramatically, and the degree of crystallization at the grain boundaries is rather considerable. The oxidation resistance of silicon nitride ceramics is enhanced by an increase in the diffusion resistance of sintering aids and impurity cations from grain boundaries to oxide layers.
Silicon Nitride Ceramics Surface Modification Method
The surface modification method for enhancing the high temperature oxidation resistance of silicon nitride ceramics involves the following steps and process conditions:
Pre-generate Oxide Layer on the Surface
Silicon nitride ceramics are oxidized at a temperature of 700 to 1500 ° C for 1 to 40 hours;
Crystallisation of Grain Boundaries
For 1-50 hours, maintain a temperature of 700-1500°C under a combination of inert atmosphere, reducing atmosphere, N2 atmosphere, and CO2 atmosphere to ensure that the sintering aid's cations penetrate completely from the grain boundary to the surface oxide layer.
Etch the Oxide Layer
Following cation extraction, the oxide layer is etched using an etching technique in HF solution with a concentration ranging from 1 to 50 mol%.
In order to further improve anti-oxidation performance, silicon nitride ceramics can be etched in one or more mixed atmospheres comprising inert atmosphere, reducing atmosphere, N2 atmosphere, and CO2 atmosphere at 900-degree Celsius after the oxide layer has been deposited. Treatment for grain boundary crystallization requires keeping the temperature at 1500 degrees Celsius for 1-50 hours. The ideal time and temperature are 1400°C and 6 hours, respectively.
Conclusion
The surface modification method of silicon nitride ceramics suggested in this article has the benefits of being straightforward, affordable, and very reliable. The surface condition criteria for this procedure are not as strict as those for other methods, and it may be applied to any shape of the article. Additionally, the method is not constrained by the cation species of the additional sintering aid, and its impact may be combined with increasing the silicon nitride formulation's high-temperature oxidation resistance.