Effect of Thermocycle Aging on Color Stability of Monolithic Zirconia

Background: The color stability of dental restorative materials is important for long-term clinical success. Objectives: The objective of this study was to examine the effect of thermocycling on the color and translucency stability of monolithic zirconia. Materials and methods: A total of 80 disc-shaped specimens (1 cm diameter) were produced from monolithic zirconia material, Katana High Translucent (Kuraray Noritake Dental, Kurashiki, Japan). The specimens were prepared in four different thicknesses: 0.5 mm, 1 mm, 1.5 mm and 2 mm. Before thermocycling, color measurements of the specimens were made by a spectrophotometer (Spectro Shade TM MICRO;MHT Optic Research AG, Milan, Italy). After the thermal aging procedure, the color measurement was repeated. Data obtained from the study were analyzed with descriptive statistics, correlation analysis, one-way ANOVA and Tukey’s tests. Results: After thermocycling, the L*, a*, b* values decreased at all thicknesses. The maximum change in the L*, a* and b* values was observed in 0.5-mm-thick specimens, while the least change was observed in 2-mm-thick specimens. The amount of color change in the specimens after thermocycling was found to be the highest in 0.5-mm-thick specimens (ΔE = 0.91 ± 0.02), and the lowest in 2-mm-thick specimens (ΔE = 0.85 ± 0.01). While a statistically significant color change (ΔE) was observed in 0.5-mm-thick specimens (p < 0.05), a statistically insignificant color change (ΔE) was observed (p > 0.05) in 1-mm, 1.5-mm, and 2-mm-thick specimens. After thermocycling, the translucency parameter (TP) values decreased at all thicknesses. The highest change in the TP values was observed in 0.5-mm-thickspecimens (1.09 ± 0.03), while the lowest change was observed in 2-mm-thickspecimens (0.40 ± 0.04). While a statistically significant change in the TP values was observed in 0.5-mm-thick specimens (p < 0.05), there was a statistically insignificant change in the TP values of 1-mm, 1.5-mm and 2-mm-thick specimens (p > 0.05). Conclusion: Although the color and translucency values after thermocycling exhibited statistically significant changes in the 0.5 mm thickness group, a statistically significant difference was not observed in the other thickness groups.

INVESTIGATIONS ON THE FORMATION OF INITIAL CRACKS IN THERMAL BARRIER COATINGS PREPARED BY EB－PVD

This paper deals with the mechanism of the formation of initial cracks in the yttria partially stabilized zirconia thermal barrier coatings prepared by EB-PVD method.The microcracks were only recognized inside the ceramic top coat of the thermalcycled TBCs. SEM/EDS observations indicated that some special oxides exist in the area just below the cracks.It seems that the formation of the initial cracks can result from the oxidation stress as well as the thermal stress.

^(99m)Tc in the evaluation of microleakage of composite resin restorations with SonicFill^(TM).An in vitro experimental model

Introduction: The composite SonicFillTM (Kerr/Kavo) is indicated for posterior restorations, with a single increment up to 5 mm due to reduced polymerization shrinkage, thus reducing working time. Aim: Evaluation of marginal microleakage with SonicFillTM. Method and Materials: There were sectioned sixty noncarious human molars in the occluso-cervical direction. Class V cavities were prepared on each tooth with gingival margin walls in a standardized way. The specimens were divided into 4 groups: group 1—restored with SonicFillTM (Kerr/Kavo), group 2—restored with FiltekTM SupremeXTE (3M ESPE), group 3—the cavities were not restored;group 4—restored with SonicFillTM (Kerr/Kavo). In groups 1, 2 and 4 the enamel was conditioned with 37% orthophosphoric acid and applied the self-etch adhesive system Clear- fillTM SE BOND (Kuraray). The specimens were stored in distilled water at 37?C for 7 days. After, the specimens, were immersed in a solution of 99mTc-Pertechnetate and the radioactivity was assessed with a gamma camera. The nonparametric Kruskal-Wallis and Mann-Whitney test with Bonferroni correction at a significance level of 5% were used for the statistical analyses. Results: There are significant differences between the positive and negative control groups and between these and experimental groups (p TM and FiltekTM SupremeXTE. Conclusion: The new composite SonicFillTM and FiltekTM SupremeXTE showed no difference concerning dye penetration. The Sonic- FillTM restorative system showed no influence in concerning microleakage.

Effect of thermocycling on the microstructure of Ti-6Al-4V alloy in simulated low Earth orbit space environment

The study of material damage and its physical mechanisms, the deep understanding of material performance degradation processes, and the improvement of key materials and devices used in a space environment are undoubtedly of critical scientific importance. This work investigated the mechanical properties and microstructures of the Ti-6Al-4V alloy subjected to thermocycling in a simulated low Earth orbit(LEO) space environment by using microhardness tests, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results revealed an initial increase in hardness with the number of cycles, followed by a subsequent decrease. After 300 thermal cycles, many cavities were formed at the two-phase interface of the Ti-6Al-4V alloy. The dominating structures were transformed into dislocation cell substructures of larger size. After 500 cycles, the central structures in the sample were subgrains evolving by dislocation cells, as well as intragranular twins and jogs structures. The relationship between microstructural evolution and thermal fatigue in LEO space environment was discussed, which may help predict the fatigue damage of materials.

Influence of Aging and 2-hydroxyethyl-methacrylate Content on the Color Stability of Experimental 2-hydroxyethyl-methacrylate-added Dental Glass Ionomers

Color stability of dental resin modified glass ionomer （RMGI） has been a challenge to dentistry; therefore, systematic changes in 2-hydroxyethyl methacrylate （HEMA） content were performed experimentally to find an idea to enhance the color stability. Changes in color （△E＊ab） and color coordinates （△L＊, △a＊ and △b＊） of experimental 10-50 wt pct HEMA-added dental glass ionomers （HAGIs） and corresponding RMGIs were determined after 5000 cycles of thermocycling. Color changes of HAGIs were not influenced by the HEMA content while △L＊, △a＊ and △b＊ values were influenced by the HEMA content. Color stability of 30% or 40% HEMA-added HAGIs was not different from those of the commercial RMGIs. Since the influence of HEMA itself on the color stability of HAGIs was limited, compositional modification to increase the color stability of these materials should be developed.