Non-oxide ceramics
Non-oxide ceramics (for example nitrides , carbides or borides ) are distinguished from oxide ceramics by mixed bonds with predominantly covalent and only minor proportions of ionic bonds. Due to the strong binding energies , this results in high chemical and thermal stability, hardness and strength, but also low ductility and very high brittleness . In addition, the higher proportion of covalent bonds makes manufacturing by sintering more difficult, which requires a small particle size, sintering additives and increased pressures.
Technically important non-oxide ceramics include silicon nitride , silicon carbide , aluminum nitride and boron carbide .
Chemical properties
Non-oxide ceramics are subject to oxidation processes at high temperatures in an oxygen-containing atmosphere. However, in SiC and SiN ceramics, layers of silicon oxide form, which act as a diffusion barrier and slow down further corrosion of the material.
Electrical Properties
Compared to all oxide ceramics, the non-oxide ceramics are only slightly inferior electrical insulators . Only silicon carbide exhibits semiconductor properties to a significant extent , i.e. the electrical conductivity increases with increasing temperature (in the case of metals it generally decreases with increasing temperature).
Thermal properties
Compared to oxidic ceramics, non-oxide ceramics have comparatively high thermal conductivities :
- Silicon nitride: 10 to 35 W / (m K)
- Boron carbide: 50 W / (m K)
- Boron nitride : 50 W / (m K)
- Silicon carbide: 100 to 110 W / (m K)
- Aluminum nitride: 180 W / (m K)
The values differ due to different sintering methods and degrees of purity .
Other non-oxide ceramics
Other non-oxidic ceramics are: