Ageing of zirconia (zirconium dioxide) based materials is associated with surface structural changes caused by humidity at elevated temperatures. Tetragonal phase is the stable structure of zirconia at the sintering temperature. Yttria, magnesia or other oxides are added to zirconia materials to maintain this favourable crystalline form also at room temperature. With variable temperatures in the mouth, catalyzed by humidity, the structure at the surface will slowly transform to a monocline phase. A tetragonal to monocline phase transformation is assumed to reduce the bond strength between zirconia cores and veneering ceramics. This transformation is also regarded as detrimental to monolithic zirconia prostheses or implants due to the volumetric expansion of the crystals at the surface.

A study of the strength was performed for three different materials designed for different sintering techniques after artificial ageing with an accelerating ageing method. Discs of these materials were immersed in hot, 80 °C, acetic acid solution with eccentrical movement for one week to simulate many years of clinical service. The results showed that the materials still retained a high strength. However, despite negligible solubility, the biaxial flexure strength was reduced by 100 to 200 MPa for all materials.


Figure 1: Equipment and a fractured specimen from the biaxial flexure testing.
Figure 2: Specimens immersed in diluted acetic acid in polypropylene tubes were placed in a thermo bath with grape seed oil at a temperature of 80 °C.

The specimens were made from blocks intended for computer aided design and manufacturing, CAD/CAM. Two of the materials were yttria-stabilized tetragonal zirconia polycrystalline, Y-TZP, one milled in presintered condition and thereafter dens sintered, the other HIPed (hot isostatic pressed). The third material was dens sintered, magnesia partial stabilized zirconia, Mg-PSZ. A correlation of dissolution and strength data may be of clinical importance for the long-term outcome of FPDs.

The HIPed Y-TZP material had the highest strength values, but the least reliability (Weibull modulus). The milling procedure of the dens-sintered material seemed to have created a secondary fracture mechanism, probably caused by microcracks at the surface, resulting in lower values for some of the specimens. The Mg-PSZ exhibited the lowest strength, but the highest reliability.

The full article can be found here
Kvam K, Karlsson S.
Solubility and strength of zirconia-based dental materials after artificial aging.
The Journal of Prosthetic Dentistry. October 2013 (volume 110 | issue 4 | pp. 281–287).

NIOM Newsletter October 2013