Effect of heat treatment and in vitro aging on the microstructure and mechanical properties of cold isostatic-pressed zirconia ceramics for dental restorations

Effect of Multiple Firings on the Microshear Bond Strength Between a Translucent Zirconia and a Resin Cement

PURPOSE

This study investigated the microshear bond strength between a resin cement and a translucent zirconia subjected to multiple characterization firings.

METHODS AND MATERIALS

5Y-PSZ zirconia blocks (Katana UTML) were sliced, sintered (1550°C, 2 h), and randomly divided into six groups (n=10) according to the number of characterization firings (0, 1, 2, 3, 5, or 10) and aging (baseline or after thermocycling). Each characterization firing was performed at 750°C for 1 minute. The ceramic surfaces were all sandblasted with 50 μm Al2O3 and silanized. Then, cylinders of resin cement (0.96 mm diameter × 2 mm height) were bonded onto their surfaces. The baseline samples were immersed in distilled water for 24 hours before the microshear bond strength (μSBS) tests. The aged samples were tested after 5000 thermocycles in water (5°C-55°C). The failure modes were classified as adhesive, predominantly adhesive, or cohesive. Scanning electron microscope images of the failure modes and the ceramic surfaces after the firings were taken. The μSBS data were analyzed by two-way ANOVA and Tukey's test.

RESULTS

The number of characterization firings and aging affected the bond strength. The highest bond strength values were observed from the 2-firing group at baseline. The μSBS results after 1, 2, or 3 characterization firings were similar at baseline and after aging. On the other hand, 0, 5, and 10 firings revealed the lowest bond strengths. The most frequent failures were adhesive and predominantly adhesive. Zirconia grains were not affected by the multiple firings.

CONCLUSION

One to three characterization firings after sintering improve the bond strength of 5Y-PSZ to the resin cement when compared to none or several (five or ten) firing cycles.

Influence of Preaging Temperature on the Indentation Strength of 3Y-TZP Aged in Ambient Atmosphere

Yttria-stabilized zirconia (3Y-TZP) containing 0.25% Al₂O₃, which is resistant to low temperature degradation (LTD), was aged for 10 h at 130–220 °C in air. The aged specimens were subsequently indented at loads ranging from 9.8 to 490 N using a Vickers indenter. The influence of preaging temperature on the biaxial strength of the specimens was investigated to elucidate the relationship between the extent of LTD and the strength of zirconia restorations that underwent LTD. The indented strength of the specimens increased as the preaging temperature was increased higher than 160 °C, which was accompanied by extensive t-ZrO₂ (t) to m-ZrO₂ (m) and c-ZrO₂ (c) to r-ZrO₂ (r) phase transformations. The influence of preaging temperature on the indented strength was rationalized by the residual stresses raised by the t→m transformation and the reversal of tensile residual stress on the aged specimen surface due to the indentation. The results suggested that the longevity of restorations would not be deteriorated if the aged restorations retain compressive residual stress on the surface, which corresponds to the extent of t→m phase transformation less than 52% in ambient environment.

Impact of multiple firings and resin cement type on shear bond strength between zirconia and resin cements

PURPOSE

The aim of this study was to evaluate the effect of multiple firings on the bond strength between yttria-tetragonal zirconia polycrystals (Y-TZP) and 2 types of resin cements.

MATERIALS AND METHODS

Sixty 3Y-TZP specimens (LAVA Frame Multi) were divided into 3 groups depending on the following firing procedures: (1) 2-firing cycles, (2) 5-firing cycles, (3) 10-firing cycles. Two samples from each group were investigated by using SEM to determine the morphological changes. All specimens were treated with 125 µm airborne-particle abrasion and the surface roughness of each specimen was measured. The specimens from each firing group were then further divided into 2 subgroups (n = 9) to apply 2 types of resin cement (MDP-free resin cement: RelyX Unicem-RU, and MDP containing resin cement: Panavia F 2.0-PA). The shear bond strength (SBS) test was performed and failure types of all the debonded specimens were classified by using a stereomicroscope as adhesive, cohesive, and mixed. The statistical analysis of surface roughness and SBS data were performed by using 1-way ANOVA and 2-way ANOVA followed by Tukey-HSD tests (α=.05). Failure modes were calculated as a percentage for each group.

RESULTS

The bond strength of RU and PA to the specimens obtained with 2 firings were not statistically different from each other (P=.1). However, the SBS values of PA were found to be significantly higher than RU for the specimens obtained with 5 and 10 firing cycles (P=.001 and P=.02, respectively). Surface roughness analysis revealed no statistical difference between groups (P=.2). The SEM analysis of samples fired 5- and 10- times showed irregularities and boundary loss in zirconia grains, and empty spaces between zirconia grains.

CONCLUSION

The bond strength of PA cement was higher than that of RU to the zirconia subjected to repeated firings (5 and 10 firing cycles). When zirconia is subjected to multiple firings, using MDP-containing resin cement can be recommended.

The effects of repetitive firing processes on the optical, thermal, and phase formation changes of zirconia

PURPOSE

The aim of this study was to investigate the effect of different numbers of heat treatments applied to superstructure porcelain on optical, thermal, and phase formation properties of zirconia.

MATERIALS AND METHODS

Forty zirconia specimens were prepared in the form of rectangular prism. Specimens were divided into four groups (n = 10) according to the number of firing at heating values of porcelain. Color differences and translucency parameter were measured, and X-ray diffraction (XRD) analysis and differential scanning calorimetry (DSC) were performed. Data were analyzed with analysis of variance (ANOVA).

RESULTS

There were no statistically significant differences in ΔE, TP, L, a, and b value changes of the zirconia specimens as a result of repetitive firing processes (P>.05).

CONCLUSION

Although additional firing processes up to 4 increase peak density in thermal analysis, additional firing processes up to 4 times can be applied safely as they do not result in a change in color and phase character of zircon frameworks.

Effects of Silica Coating by Physical Vapor Deposition and Repeated Firing on the Low-Temperature Degradation and Flexural Strength of a Zirconia Ceramic

PURPOSE

To examine the application of physical vapor deposition (PVD) silica coating as an approach to retard low temperature degradation (LTD) for dental applications. Accelerated aging characteristics of heat- and surface-treated zirconia material were also investigated by exposing specimens to hydrothermal treatment.

MATERIALS AND METHODS

The specimens (90 disc-shaped specimens [15 mm ×1.2 mm]) were divided into 9 groups (n = 10) according to the test protocol: Ctrl, control (no surface treatment); Ag, autoclave aging; GrAg, grinding + aging; SiAg, silica coating + aging; GrSiAg, grinding + silica coating + aging; 3FAg, 3-time firing + aging; Gr3FAg, grinding + 3-time firing + aging; 5FAg, 5-time firing + aging; Gr5FAg, grinding + 5-time firing + aging. Accelerated aging was performed in a steam autoclave (134°C, 2 bars) for 12 hours. Following each treatment protocol, X-ray diffraction (XRD) analysis was used to estimate the relative amount of monoclinic phase and corresponding transformed zone depth (TZD). Additionally, a biaxial flexure test was used to calculate the flexural strength. Statistical analysis was conducted with one-way ANOVA and Fisher's LSD test (p < 0.05).

RESULTS

The tetragonal-to-monoclinic transformation was retarded by PVD silica coating only on ground surfaces. Ground and heat-treated specimens exhibited the lowest monoclinic content after aging. The biaxial flexural strength value of the GrAg group was significantly higher than the values in all of the other groups except the SiAg group. The flexural strength value of the GrSiAg group was significantly higher than that of the 3FAg group. There was no statistically significant difference between the other groups (p > 0.05).

CONCLUSIONS

Grinding decreased the susceptibility of zirconia to LTD and increased the flexural strength. PVD silica coating and repeated firing decreased the monoclinic content only in ground specimens during aging.

Effect of grain size on the monoclinic transformation, hardness, roughness, and modulus of aged partially stabilized zirconia

OBJECTIVE

Low-temperature-degradation (LTD) has been reported to cause property changes in yttria-tetragonal zirconia polycrystals (Y-TZP). The current study measured monoclinic phase transformation of Y-TZP with different grain sizes and corresponding property changes due to artificial aging.

NULL HYPOTHESIS

the grain size of aged Y-TZP will not influence its transformation, roughness, hardness or modulus of elasticity.

METHODS

Four groups of Y-TZP were examined with differing grain sizes (n=5). The line intercept technique was used to determine grain sizes on SEM images (100,000×). Artificial aging was accomplished by autoclaving at 2 bar pressure for 5 h. X-ray diffraction (30 mA, 40 kV) was used to measure tetragonal to monoclinic transformation (t→m). Surface roughness analysis was performed using a non-contact surface-profilometer. Nano-hardness and modulus of elasticity were measured using nano-indentation.

RESULTS

SEM analyses showed different grain sizes for each sample group (0.350 μm, 0.372 μm, 0.428 μm, and 0.574 μm). The fraction of t→m transformation increased as grain size increased; furthermore, aging of zirconia caused increased roughness. Modulus and hardness after aging displayed no significant correlation or interaction with grain size.

SIGNIFICANCE

Smaller grains caused less transformation, and aging caused increased roughness, but grain size did not influence the amount of increased surface roughness. Future studies are needed to determine the effects of grain size on the wear and fracture properties of dental zirconia.