Analogous to the oxide ceramics, the non-oxide
ceramics are made exclusively from synthetic raw materials.
The term non-oxide ceramics generally refers to carbides,
nitrides or oxynitrides. The non-oxide ceramics are characterised
by their truly unusual properties, as will be seen in the
following material descriptions. The high proportion of covalent
bonds in the carbide and nitrides crystal structures is responsible
for the remarkable combination of properties. Oxide crystal
structures, in contrast, have a relatively high proportion
of electrovalent bonds. This also implies that the process
from raw material powder to manufactured part is more costly
than for oxide ceramics. The raw materials generally have
to be extremely fine-grained, and the sintering process requires
an absolutely oxygen-free atmosphere. This implies firing
in a vacuum or inert gas in a gas-tight furnace at temperatures
that can reach well over 2,000° C!