Abstract
Objectives: All-ceramic dental prosthetic materials are in high demand for their aesthetic and biocompatible properties. There is, however, a number of critical clinical problems both in terms of the aesthetic properties and “chipping” fracture of veneering porcelains, especially for porcelain-zirconia restorations. The former area has received minimal attention while the latter has resulted in various structural designs and cooling procedure considerations reduce such behaviour.
This thesis initially addresses the importance of firing of veneering porcelain on its subsequent aesthetic and mechanical properties. The problem of “chipping” of porcelain zirconia frameworks is initially investigated by considering the metastability of the tetragonal phase of yttria tetragonal zirconia polycrystalline (Y-TZP) ceramics during the preparation of porcelain-zirconia restorations. One specific problematic area is the interface between the porcelain and the Y-TZP framework and whether the associated veneering procedures and reactions result in a phase transformation of the zirconia and lead to weakening of interface bonding. The role of temperature gradients during fast and slow cooling for conventional and anatomical zirconia framework designs are compared as well as their influence on residual stress within the porcelain.
Methods: The effect of firing on mechanical, thermal expansion and solubility of porcelains were examined on the ISO 6872:2008 standards.
Dry and moist veneering porcelain powders were built up on the zirconia framework. High-resolution SEM observations of the zirconia at the porcelain zirconia interface were made to identify the zirconia microstructure. XRD2 micro diffraction measurements were carried out on the interface area to generate locally resolved zirconia phase information.
The temperature gradients between the inner and outer surfaces of crowns were measured with thermocouples during two different cooling methods with uniform and anatomical frameworks. Direct observation of the residual stresses for the uniform framework crown following fast and slow cooling was made with an optical polarimeter.
Results: For sintering of veneering porcelains the key outcomes were the critical role of the maximum temperature, firing time, vacuum and pre-drying of the porcelain on the physical properties and also for differences of the shade of a dental restoration. The importance of sintering time and temperature were critical for the resultant properties, most noticeably translucency (value) and strength.
Under moist firing conditions the phase composition of zirconia grains at the interface revealed both monoclinic and tetragonal structure. These observations indicate that destabilisation of the tetragonal phase of zirconia can occur at the interface during the veneering.
Temperature gradients generated during cooling showed that slow cooling decreases the temperature differences between the inner and outer surfaces of the restorations. For fast cooling, temperature differences of more than 140°C at temperatures above the glass transition temperature were recorded. Optical polarimeter observations indicated much lower stresses within the porcelain layer upon slow cooling of the crown. Rapidly cooled crowns resulted in higher stresses within the cusp area, which is the area where “chipping” is most frequently observed.
Conclusions: A simple firing temperature-time matrix readily enables the optimisation of the aesthetic and physical properties of veneer porcelains.
The porcelain veneering process, especially with a wet veneer during firing, results in a localized tetragonal to monoclinic structural transformation at the surface of the zirconia framework of all-ceramic dental restorations
Slow cooling of the porcelain during the final veneering step on porcelain-zirconia restorations reduces the temperature gradients and residual stresses within the porcelain layer, which represent one of the possible causes for “chipping”.