Phase Transformations in Yttria-Partly Stabilized Zirconia Induced by Dental Polishing Regimes

The study examined how three polishing methods, using equipment from NTI CeraGlaze (NTI), Komet Dental (Komet), and EVE Diacera (EVE) and employing either wet or dry grinding, affect the texture (roughness) and phase composition of Y-PSZ dental crowns. Dental crowns made from VITA’s 3Y-/4Y-/5Y-partly stabilized zirconia (Y-PSZ; YZ-HT/ST/XT), utilizing a standard CAD/CAM process, underwent both wet or dry grinding and polishing. The effects of distinct polishing treatments on Y-PSZ surface phase content were investigated using X-ray diffraction (XRD) and Rietveld refinement, the grain size was measured by field emission scanning electron microscopy (FE-SEM), and confocal scanning laser microscopy (CLSM) was used to determine the surface roughness as the arithmetical mean height (Sa). To analyse the different mode of action, the components of the polishers were analysed using XRD, along with micro X-ray computer tomography (µXCT), FE-SEM, and CLSM for microstructural examination. The Komet and NTI polishing regimes reduced roughness significantly better than the EVE regime for the 3Y and all wet specimens, but caused a rhombohedral phase fraction. A possible explanation for this result is the overall finer structure of the EVE coarse polisher (abrasive particle size and content, texture density), which probably results in a lower force on the Y-PSZ surface. Therefore, the rhombohedral phase boundary would not be reached. Due to rhombohedral phase having larger volume expansion and shear than the monoclinic phase, it may result in enhanced transformation toughening or detrimental low-temperature degradation effects.