  The tetragonal to monoclinic phase transformation of zirconia has been the
subject of extensive studies over the last 20 years [1-4]. The main features of
the transformation have been identified and its martensitic nature is now
widely recognised [5-8]. More specifically, the relevance of a nucleation and
growth model to describe the transformation is widely accepted. Recent fracture
episodes [9] of zirconia hip joint heads were reported, failures related to the
t-m transformation degradation. Among the materials solutions considered for
decreasing the sensitivity to t-m phase transformation, the possibility of
adding silica as a dopant appears as an appealing one. Previous studies have
revealed the beneficial effect of silica addition by the formation of a glassy
phase at the grain boundaries and triple points. This glassy phase has been
proven to reduce the residual stresses level [10], slowing down the
transformation kinetics. Preliminary quantitative investigations by XRD have
shown these materials are less susceptible to transformation. However, the
mechanism by which the transformation propagated has still to be assessed.
