Ageing and strength of zirconia materials
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.