Materials for engineering, 3rd Edition - (Malestrom)

Glasses and ceramics 143
ions. One third of the Al 3+ sites remain empty, so that the overall ionic
charges are in balance.
Reducing the grain size increases both the fracture strength and the
toughness of alumina (Fig. 4.5). The manufacturing conditions for fine grain
size (low temperatures, short time) are in conflict with those to minimize
porosity (high temperatures, long time) so that a compromise has to be made
by using sub-micron powder particles as a starting material and by the
addition of 0.05–0.2% magnesia (MgO) which prevents grain growth by
‘pinning’ the alumina grain boundaries. Hot pressing at 1350–1800 K may
enhance sintering, so that a shorter time is required at the sintering temperature
of 1850–2000 K.
4.3.3 Zirconia
Zirconia (ZrO 2 ), an engineering ceramic of growing importance, is again
an ionically bonded material and it exhibits three distinct crystal phases.
Above 2300 °C it is cubic, between 1150 and 2300 °C it is tetragonal and
below 1150 °C it has a monoclinic structure. The cubic form consists of
zirconium ions on a face-centred cubic lattice (Fig. 4.6) with oxygen ions
occupying certain holes in the structure. ZrO 2 undergoes a 3.5% volume
expansion during cooling below 1000 °C due to its change in crystal structure
to monoclinic and this causes catastrophic failure of any part made of pure
polycrystalline zirconia. Addition of CaO, MgO or Y 2 O 3 to the zirconia
600
Bending strength (MN m –2 )
400
200
0
1 2 5 10 20 50 100 200
Grain size (µm)
4.5 Showing increase in strength with decreasing grain size in alumina.