Adjustments by grinding in 3Y-TZP frameworks should be performed with water cooling, and regeneration firing should be undertaken to obtain a more reliable material.
This study evaluated the effect of grinding protocols and resintering on flexural fatigue limit and surface characterization of LavaTM Y-TZP. Bar-shaped specimens (20×4.0×1.2 mm, n=40; 20×4.0×1.5 mm, n=80) were obtained. Half of the thinner specimens (1.2 mm) constituted the as-sintered group (AS), while the thicker ones (1.5 mm) were ground with diamond burs under irrigation (WG) or not (G). The other half of thinner and half of ground specimens were resintered (1000 ºC, 30 min), forming the groups ASR, WGR and GR. Fatigue limit (500,000 cycles, 10 Hz) was evaluated by staircase method in a 4-point flexural fixture. Data were analyzed by 2-way ANOVA and Tukey's test (α=0.05). Surface topography (n=3) and fracture area (n=3) were evaluated by SEM. X-ray diffraction data (n=1) was analyzed by Rietveld refinement. ANOVA revealed significant differences (p<0.001) for the grinding protocol, resintering and their interaction. Grinding increased the fatigue limit of non-resintered groups. There was no significant difference among the resintered groups. Resintering significantly increased the fatigue limit of the AS group only. Both protocols created evident grooves on zirconia surface. The failures initiated at the tensile side of all specimens. The percentages (wt%) of monoclinic phase were AS (8.6), ASR (1.2), G (1.8), GR (0.0), WG (8.2), WGR (0.0) before, and AS (7.4), ASR (6.5), G (3.2), GR (0.2), WG (4.6), WGR (1.1) after cyclic loading. Grinding increased the fatigue limit of non-resintered Y-TZP and formed evident grooves on its surface. Resintering provided significant increase in the fatigue limit of as-sintered specimens. In general, grinding and resintering decreased or zeroed the monoclinic phase.
This study evaluated the effect of grinding and regeneration firing on the flexural fatigue limit and surface characterization of Lava™ Y-TZP ceramic. Forty bar-shaped specimens with 20 × 4.0 × 1.2 mm constituted the as-sintered group (AS = control group), and 80 specimens with 20 × 4.0 × 1.5 mm were ground with cylindrical laboratory stone under water-cooling (WG) or in a dry condition (G) to reach 1.2 mm in thickness. Half of specimens were submitted to regeneration firing (1000 °C, 30 min), forming the groups AS/R, WG/R and G/R. Fatigue limit (500,000 cycles, 10 Hz) was determined by staircase method in a 4-point flexural fixture. Data were analyzed by 2-way ANOVA and Tukey HSD tests (α = 0.05). The surface topography (n = 3) and fracture area (n = 3) were evaluated by SEM. Samples were also analyzed by Rietveld refinement from X-ray diffraction data. ANOVA revealed significant differences (P < .001) for grinding protocol, regeneration firing and their interaction. In the groups not submitted to regeneration firing, the mean flexural fatigue limit of WG was higher (P < .05) than that of G and AS, with no statistical difference between each other (P > .05). After regeneration firing the inequality WG>AS>G (P < .05) was observed. The regeneration firing increased the fatigue limit of AS group and decreased those of G and WG groups (P < .05). Grinding protocols created evident grooves on zirconia surface. Failures initiated on tensile side of all specimens. The percentages (wt%) of monoclinic phase before cyclic loading were: AS (7.4), AS/R (6.5), G (2.8), G/R (0.0), WG (4.4), WG/R (0.0); and after cyclic loading: AS (8.6), AS/R (1.2), G (2.4), G/R (5.7), WG (6.3), WG/R (0.0). Wet grinding did not compromise the fatigue limit of zirconia, increasing its mechanical strength. Regeneration firing reduced the fatigue limit of ground samples, despite reducing the amount of monoclinic phase in all experimental conditions.
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