| Because of its good chemical properties, dimensional stability, high mechanical strength and toughness, zirconia had been used to make dental crowns and FPDs for ten years. It was suggested for used in any part to full-arch restorations, but till now, no long term clinical report was found, however, according to the available 3-5 year clinical observations and some laboratory scientific tests, no zirconia framework fractures was found, while veneering porcelain chipping and delaminating was frequently observed. All these phenomena indicated that there is a relatively weak link between zirconia and veneering porcelain, also the veneering porcelain itself has no enough strength to compatible with zirconia core. In addition, because of the low-temperature degradation properties, which makes zirconia more sensitive to surface treatment manners or thermal procedures. Therefore, the mechanical properties of zirconia can be influenced by many dental laboratory and clinical procedures like grinding, polishing, veneering firing, and sandblasting, etc, which could determine the population, size, and distribution of microcracks in the materials.In order to get a more comprehensive understanding for the bonding mechanisms and properties between zirconia and veneering porcelains, we intended to observe the porcelain Y-TZP interface through SEM and TEM, and try to give a more clearly picture about the bonding structures, and we analyzed the influences of some common clinical and laboratory procedure on the bonding strength between zirconia and dental porcelain, some main results were summarized as follows.1. The effects of surface treatments on the microstructural and crystallographic changes of dental 3Y-TZP ceramic:The purpose of this study was to observe the influence of different surface treatments, namely surface grinding, polishing, alumina abrasion, simulating dental porcelain firing and annealing on the microstructural and crystallographic changes of dental 3Y-TZP ceramics.Results:1) The results demonstrated that grinding, polishing, alumina abrasion promoted t→m phase transformation on the sufaces which generated a transformation compressive stress, while simulating dental porcelain firing and annealing reversed m to t transformation, especially in annealing procedure.2) Clinically grinding shoud be avoided, if any, shoud be heat treated.2. SEM and TEM Observations of porcelain Y-TZP interface1) SEM show that there is good compatibility between zirconia and veneer porcelain, the interface was formed by zirconia and noncrystal glass matrix.2) TEM show that there is no any change in zirconia lattice at the interface, which indicated zirconia lattice was not be influenced by veneering firing due to the small thermal mismatch, the interface is a noncrystal structure.3. The influence of multiple firing on the bending strength of zirconia-pocelain bilayered dental ceramicsBilayered three-point bending strength specimen of KAVO zirconia-Vita VM9 veneering porcelain were made according to ISO6872 standard, the core to porcelain thickness ratio was 1:1. Specimen were subjected to multiple dentine porcelain firing cycles. Three-point bending strength were tested, SEM was used to observe the characteristics of the failure mode.Results:1) Results show that no influence on the bending strength by multiple firing, the three-point bending strength were 996MPa-1020MPa after multiple firing up to 8 times.2) No obvious defects was found at the interfaces, but in veneering porcelain side, it looks more porous with the increasing firing times while the size and morphological structure of crystal phase were changed significantly.4. Effects of three types of veneering porcelain on bending strength of KAVOTM Y-TZP/porcelain bilayered structure:The purpose of this study was to investigate the effect of three types of veneering porcelain on the bending strength of KAVOTM Y-TZP/porcelain layered structure.Results:1) For covering KAVOTM zirconia core material, conventionally applied veneering slurry-porcelain combined with liner or wash firing has significant higher bending strength than pressed-on porcelain. SEM showed that the main failure type at the interface was adhesive.2) Thin layer sintering as applying washbake program or liner on KAVO TM zirconia surface increased the surface wettability, this procedure may be indispensable when veneering on the surface of dental zirconia.5. Effect of pigmentation on strength of dental Y-TZP/porcelain bilayered structureThe purpose of this study was to investigate the effect of two methods of pigmentation on the flexural strength of dental Y-TZP/porcelain layered structure. KaVoTM zirconia substructures were pigmented by dipping presintered blocks in coloring solution VITA LL1 and LL5, and colored TZ-3YS zirconia substructures were fabricated by adding pigments before isostatically pressing, color No 1. and No.5 were used for test. Specimens were made in monolithic or bilayered forms. The flexural strength was tested. XRD and SEM with EDX were used to analyze the characteristics of the surface structure. Results:In KaVoTM group, There were no significant differences of flexural strength between white and LL1 and LL5 colored monoclinic materials, which also in bilayered structures. While in TZ-3YS group, There was significant differences of flexural strength between color No.5 with white and color No.1 monoclinic materials, no difference was found in the latter two subgroups. The flexural strength were significant lower when veneered with porcelain in both kinds of zirconia groups, similar trends was observed with that of monoclinic materials counterpart. The XRD spectra showed only tetragonal phase was detected in both kinds of zirconia groups. Pigmentation has no apparent effects on the bonding strength between veneering porcelain and zirconia. Both coloring methods are appropriate when the concentration of pigments is under deliberate controlled.
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