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Silicon nitride (Si3N4) is one of the toughest and most durable ceramics. It has excellent mechanical properties, high hardness, chemical and thermal shock resistance and a low coefficient of thermal expansion. This makes it suitable for applications requiring high loads and speed.
Corrosion resistance is an important characteristic of silicon nitride, especially when it comes to medical devices and implantable materials. Its resistance to corrosion is attributed to the formation of an oxide layer on the surface of the material, which provides a barrier against acidic and basic solutions. It is also resistant to molten metals and hot gases.
The atomic bonding in silicon nitride gives it a high Young’s modulus and Poisson’s ratio at room temperature. Its Young’s modulus is estimated at 280-290 GPa and its Poisson’s ratio at 0.22.
Its high hardness and wear resistance make Si3N4 a desirable material for tribological applications. Nevertheless, its intrinsic brittleness limits its use in many applications, such as in ball bearings and seals. In order to improve the tribological properties, particulate additions of TiB2 have been successfully used in Si3N4 matrices. Jones et al. tested the unlubricated sliding wear of hot-pressed Si3N4-40 vol%/TiB2 discs against BGSN balls in a ball-on-disc apparatus.
CVD Si3N4 coatings on RBSN and Si3N4 sintered bodies were developed in order to improve the oxidation and corrosion resistance of these two grades of silicon nitride. The results showed that RBSN with a CVD Si3N4 coating had the best performance in the oxidation and hot corrosion tests, while the SiCw/Si3N4 composite improved the bending strength and fracture toughness of the RBSN.