Abstract:
Tetragonal zirconia exhibits excellent hardness and fracture toughness due to its
transformation toughening mechanism. However, zirconia is susceptible to tetragonal to
monoclinic phase transformation at low temperatures, which leads to microcracks and
property degradation, a phenomenon known as low temperature degradation or hydrothermal
ageing. Retention of tetragonal zirconia at room temperature is paramount for high toughness
and can be achieved by the addition of stabilizers such as CaO, MgO, Y2O3 and CeO2. In this
study, hydrothermal ageing resistance of 3 mol.% yttria stabilized tetragonal zirconia (3Y TZP) and 10 mol.% ceria doped tetragonal zirconia (10CE-TZP) were compared by using two
step sintering method. 3Y-TZP and 10CE-TZP powder were uniaxial pressed and followed
by cold isostatic pressing. Both samples were initially heated to 1500 C and held at that
temperature for one minute and then cool down to 1300 C for 5 hours. Hydrothermal ageing
was conducted for 36 hours in an autoclave containing superheated steam at 180 C and 10
bar pressure. Phase transformation was analyzed by using X-ray diffraction (XRD) method.
It was found that the 10CE-TZP samples did not show any phase transformation within 36
hours of ageing. However, 3Y-TZP sintered with same profile exhibited the monoclinic phase
formation within 1 hour of exposure. The figure below shows the XRD profile of both samples
for exposure times up to 36 hours. The peak angle (2θ) around 30 belong to tetragonal and
the peak around 28 belongs to monoclinic zirconia. It can be noted that as the ageing time
increases, this was accompanied by an increased in the tetragonal to monoclinic phase
transformation in the 3Y-TZP ceramics. This was not the case for the 10CE-TZP sample, thus
indicating that the sintered ceramic was resistant against the aggression of water radicals in
destabilizing the zirconia lattice and inducing the phase transformation.
