  We demonstrate in this paper the influence of surface finish on the ageing
kinetics of biomedical grade zirconia. The critical influence of polishing has
been systematically investigated by optical microscopy, atomic force microscopy
and X-ray diffraction. The stress state around polishing scratches gives rise
to preferential transformation in the zone of the induced scratches and
consequently to accelerated ageing. The influence of residual stresses is
analyzed semi-quantitatively by preparing samples with various surface finish,
thus with various stress states. Rough polishing produces a compressive surface
stress layer beneficial for the ageing resistance, while smooth polishing
produces preferential transformation nucleation around scratches. When a
thermal treatment of 2 h at 1200 {\deg}C is applied to relax the residual
stresses, all the surfaces states exhibit the same sensitivity to ageing. These
results demonstrate that roughness alone cannot be used for ensuring a
long-term stability. The variation of ageing sensitivity is indeed related
indirectly to the surface roughness via the induced surface stress state. The
current ISO standards are not able to take these effects into account. Indeed,
great variations in ageing kinetics were observed for samples with different
surface states, although all well below the ISO requirements.
