Silicon nitride ceramics, a material suitable for high-vacuum environments, are not only used as ceramic bearings, but also exhibit strong corrosion resistance in a variety of fields, including chemical machinery, food handling, and marine engineering. In particular, the self-lubricating properties of silicon nitride ceramics provide an effective way to solve the problem of vacuum contamination caused by lubricating medium in the application of steel bearings. By taking advantage of this unique self-lubrication, silicon nitride ceramic bearings have become more promising in a variety of environments, especially in high temperature and corrosive environments. Overall, silicon nitride ceramic materials offer greater possibilities for bearing applications in various fields, especially in harsh working environments.
The atomic structure of silicon nitride reveals an interesting phenomenon of electronic arrangement. The nitrogen atom possesses 5 electrons in the outer shell, and when it bonds with the silicon atom, one of the electron orbitals self-couples. This means that only three silicon atoms need to contribute one electron each to bond with the orbital of the nitrogen atom to achieve a stable number of electrons in the outer shell of the nitrogen atom to reach 8. So, in the structure of silicon nitride, there are three silicon atoms within the nearest distance around each nitrogen atom. This particular arrangement of electrons creates a planar hybrid orbital, which is why silicon nitride has extremely high resistivity. This electronic arrangement not only enhances the stability of the material, but also provides it with unique physical properties for specific applications.
In order to reduce the thermal stress of the material during thermal shock, the method of reducing the coefficient of thermal expansion and improving the thermal conductivity can be adopted. According to previous studies, the coefficient of thermal expansion of zirconia ceramic materials is closely related to the type and amount of stabilizer added. In particular, the CAO-stabilized zirconia has a relatively large coefficient of thermal expansion, which can be explained from the perspective of vacancy concentration. At atmospheric pressure, Si3N4 does not have a definite melting point, and it decomposes directly at about 1870. Si3N4 can tolerate oxidation up to 1400 °C, but in practice, its mechanical strength starts to decline above 1200°. These properties make Si3N4 have specific application limitations in high-temperature environments.
The erosion kinetic model curve of silicon nitride ceramics is in a straight line shape in HF acid, that is, the erosion rate increases gradually with the increase of hydrochloric acid concentration. In HCl aqueous solution, the erosion kinetic model curves of the two are hyperbolic shapes, and the erosion rate is the highest when the concentration of hydrochloric acid reaches 2m.
At present, there have been many scientific studies on the high-temperature etching of silicon nitride ceramics, and mainly focus on their surface oxidation behavior in high-temperature natural environments. At high temperatures, silicon nitride ceramics form a SiO2 passivation treatment layer, which effectively inhibits further oxidation and corrosion. Therefore, the degree of destruction of the SiO2 passivation layer determines the degree of erosion of silicon nitride ceramics.