Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 145
Process zone
Crack
Untransformed
particle
Transformed
particle
Transforming
particle
4.7 Mechanism of transformation toughening in zirconia ceramic.
Zirconia has other properties which make it a very interesting engineering
ceramic. As seen in Table 4.3, it has a very low coefficient of thermal
conductivity (1.5 W m –1 K –1 compared with 25.6 W m –1 K –1 in the case
of alumina) together with a very high thermal expansion coefficient (8 ×
10 –6 K –1 ), which is two or three times that of most ceramics and almost the
same as cast iron or steel. This makes zirconia a candidate for insulating
engine components, since any coatings will not have the severe problems of
thermal expansion mismatch found with other non-metallic surface layers.
4.3.4 Nitride ceramics
Silicon nitride
Silicon nitride (Si 3 N 4 ) is a covalently bonded crystalline ceramic. Its crystal
structure is based on the packing of tetrahedra: each tetrahedron has a silicon
atom at the centre and a nitrogen atom at each corner, just like the SiO 4
tetrahedra in silica. In silicon nitride, a nitrogen atom is shared by three
tetrahedra (in contrast to the oxygen sharing two tetrahedra in silica) forming
crystals of hexagonal symmetry. The tetrahedra are thus held more rigidly in
silicon nitride than in silica and less variation in bond length and angle is
allowed, leading to its stronger and stiffer mechanical properties.
Silicon nitride is strong, hard, wear-resistant, stable up to 1800 °C and
oxidation-resistant. With its low thermal expansion coefficient, it has excellent