Silicon nitrides (Si3N4) are a series of advanced engineering ceramics characterized by high strength, hardness and excellent chemical and thermal stability.
While Silicon Nitride was discovered in the mid-nineteenth century, it not lend itself to easy fabrication until later, due to its covalently bonded nature. This led to the development of two types of silicon nitride: Reaction–bonded silicon nitride (RBSN) and hot pressed silicon nitride (HPSN). Since the 1970s two further types have been developed: Sintered silicon nitride (SSN), which include the sialons, and sintered reaction–bonded silicon nitride (SRBSN).
Reaction bonded silicon nitride (RBSN) is made by heating a compact volume of silicon powder in a nitrogen gas atmosphere. The reaction starts at 1200 °C, and Nitrogen-hydrogen or nitrogen-hydrogen-helium gas mixtures are used most often as they give faster and more easily controlled reactions, and result in higher strength materials.
This process of reaction bonding enables the production of complex, and near net shapes. Although silicon expands approximately 20% on conversion to the nitride, the expansion is accommodated by void spaces within the silicon compact. There is little change in the overall volume of components, allowing accurate dimensioned shapes to be created without expensive or time consuming finishing operations.
Reaction bonded silicon nitride components have lower density and higher porosity than those made by the hot pressing process, primarily due to the difficulty in ensuring complete reaction throughout the volume of the silicon pack. As a consequence, RBSN components have inferior mechanical properties such as strength and Young’s modulus.However, the material is relatively affordable to produce, and has found application particularly as kiln furniture.
Key properties of RBSN include good thermal shock resistance, resistance to wetting by molten metals, high electrical resistivity, and mechanical fatigue and creep resistance. The material also shows excellent oxidation resistance and the ability to be formed into complex shapes relatively easily. Common trademark names of the material include NITRASIL®, and its applications lie in industrial, mechanical and chemical settings, especially as bearing parts.