Influence of surface finish and residual stresses on the ageing sensitivity of biomedical grade zirconia

TL;DR

This study investigates how surface finish and residual stresses influence the ageing behavior of biomedical zirconia, revealing that surface stress states, rather than roughness alone, significantly affect long-term stability.

Contribution

The paper provides a systematic analysis of how polishing-induced residual stresses impact zirconia's ageing, highlighting the limitations of current ISO standards.

Findings

Rough polishing induces beneficial compressive surface stresses.

Smooth polishing accelerates ageing due to stress concentration around scratches.

Thermal treatment relaxes residual stresses, equalizing ageing sensitivity.

Abstract

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.