In vitro evaluation of low-temperature aging effects and finishing procedures on the flexural strength and structural stability of Y-TZP dental ceramics

https://doi.org/10.1016/j.prosdent.2006.08.004Get rights and content

Statement of problem

Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic material has been reported to possess exceptionally high flexural strength; however, the strength of Y-TZP ceramics can be affected by surface treatments performed by dental laboratory technicians and by intraoral conditions.

Purpose

This study investigated the influence of low-temperature-degradation (LTD) treatment, airborne-particle abrasion, and polishing on the flexural strength and structural stability of a Y-TZP ceramic material.

Material and methods

By sectioning zirconia ceramic blocks (Vita In-Ceram YZ blocks) into 25 × 4 × 2-mm bars, 310 specimens were fabricated and divided into 9 groups: Group C, as-sintered (control); group B24h, boiled in water for 24 hours; group B7d, boiled in water for 7 days; group H6h, stored in humidified air at 250°C for 6 hours; group H24h, stored in humidified air at 250°C for 24 hours; group H7d, stored in humidified air at 250°C for 7 days; group P, polished; group A, airborne-particle abraded; and group AB, airborne-particle abraded and boiled for 7 days. The flexural strengths (MPa) of all specimens were determined by using a 3-point bending test. The variability was analyzed by using Weibull statistics. Pairwise differences among the 9 groups were evaluated by using confidence intervals (95%) for scale and shape parameters. From every block used, 8 specimens were obtained and every specimen was marked according to the location of its origin from the original block. Surfaces of the specimens were evaluated by using scanning electron microscopy (SEM) at various magnifications. X-ray diffraction analysis was performed to identify possible tetragonal-to-monoclinic phase transformations. Energy dispersive spectroscopy (EDS) was used to obtain information on the chemical composition.

Results

Aging or “finishing” treatments had no significant negative effects on flexural strengths. Mean flexural strengths ranged from 796.7 to 950.2 MPa. Group A exhibited the highest mean flexural strength (950.2 MPa). Weibull modulus values ranged between 5.6 and 9.3. Tetragonal-to-monoclinic phase transformation was detected for a specimen boiled for 24 hours. More tetragonal-to-monoclinic phase transformation was detected for specimens boiled for 7 days and stored in humidified air at 250°C for 7 days. The SEM examination of fractured surfaces revealed sintering defects, and EDS analysis showed less yttria concentration on Y-TZP specimens that were boiled for 7 days.

Conclusion

Within the limitations of this study, the results suggested that the LTD procedures and the polishing treatment used did not reduce the flexural strengths of zirconia bars. Airborne-particle abrasion increased the flexural strength of specimens.

Section snippets

Material and methods

Bars (L: 25 ± 0.2 × W: 4 ± 0.1 × H: 2 ± 0.1 mm) of presintered zirconium oxide stabilized with yttrium oxide (Vita In-Ceram YZ; Vita Zahnfabrik) were cut from zirconia ceramic blocks for the Cerec system (Vita In-Ceram YZ-40; Vita Zahnfabrik), then sintered to full density according to the manufacturer's instructions. The chemical composition and mechanical properties of the In-Ceram blocks are listed in Table I, Table II. Groups tested and experimental procedures are shown in Table III.

The

Strength degradation

Three-point bending flexural strength values are presented in Table IV. Weibull statistical analysis (maximum likelihood method) of the flexural strength data provided 2 parameter estimates for each group: the Weibull modulus m and the normalizing parameter σ0 strength, which corresponds to the 63.2 probability of failure, shown in Table IV. Based on the strength data (Table IV), the airborne-particle–abraded group (group A) was the strongest group (950.2 ± 126.7), which differed from all the

Discussion

The purpose of this in vitro study was to determine the influence of LTD and surface finishing procedures, such as airborne-particle abrasion and polishing, on the flexural strength and the microstructural stability of In-Ceram Y-TZP ceramic material. There were 310 specimens in all, with 61 specimens in the control group and 60 or more specimens in each experimental group.

Data indicated that the material tested performed with homogeneous behavior, because there was no significant difference in

Conclusions

Within the limitations of this in vitro study of the effects of aging and surface treatments on the properties of Y-TZP dental ceramics, the following conclusions were drawn.

  • 1.

    LTD, polishing, and airborne-particle abrasion did not produce strength degradation effects on the Y-TZP ceramic material examined. However, the airborne-particle-abrasion process is responsible for strength degradation if not neutralized by a region of compressive stresses.

  • 2.

    Airborne-particle abrasion significantly increased

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