Importance of tetragonal phase in high-translucent partially stabilized zirconia for dental restorations

For the use of partially stabilized zirconia (PSZ) as high-translucent zirconia, the importance of cubic phase (c-ZrO₂) is commonly emphasized without much attention for the remaining tetragonal phase (t-ZrO₂). The aim is to understand whether the crystal structure and microstructure of t-ZrO₂ have paramount influences on the properties of PSZ.

Two grades of 5mol% yttria-stabilized PSZ ceramics were prepared by different processing routes. A 5mol % yttria co-precipitated zirconia (T5Y) was compared with a 3 and 8mol% yttria-stabilized powder mixture grade with a bimodal grain size distribution (B5Y). The phase composition and lattice parameters were studied with x-ray diffraction (XRD) and Rietveld analysis. Mechanical properties were compared in terms of hardness, toughness and biaxial bending strength with Weibull analysis. Their translucency and aging stability were also characterized, and the microstructure before and after aging were followed by scanning electron microscopy (SEM).

T5Y and B5Y having similar phase composition (about 40% t- and 60% c-ZrO₂) showed the same basic properties (density, hardness and toughness), but the lattice parameters of t-ZrO₂ and the concomitant microstructure were completely different. The t-ZrO₂ phase in T5Y had a higher yttria content and a lower tetragonality (c/a) of 1.0126±0.0002, whereas the t-ZrO₂ phase in B5Y was comparable to that in conventional 3Y-TZPs with a tetragonality of 1.0153±0.0002. Consequently, B5Y showed a lower strength with lower Weibull modulus, being less translucent and more aging susceptible as compared to T5Y. The translucency and aging susceptibility of B5Y were even comparable to those of conventional 3Y-TZP.

Not only the phase composition with high cubic phase content but also the lattice parameters and the yttria content in the remaining t-ZrO₂ played a determinant role for PSZs to be used as 'high-translucent' zirconia.