Objectives : To evaluate the strength degradation of five ceramics (yttria stabilized zirconia, YZ; alumina polycrystal, AL; glass-infiltrated alumina–zirconia, ICZ; and porcelains VM7 and VM9, all from Vita) after in vitro aging procedures. The null hypothesis is that these procedures cause strength degradation in all ceramics.
Materials and methods : For YZ, AL and ICZ, ceramic bar-shaped specimens were obtained by sectioning commercial blocks. Porcelain specimens were produced by sintering. Specimens were divided in three groups: (a) control ( n = 30); (b) cyclic fatigue ( n = 10, specimens fatigued in a chewing simulator, under artificial saliva at 37 °C); and (c) low temperature hydrothermal cycle ( n = 10, aging procedure in steam, using an autoclave at 134 °C, 2.2 kgf/cm 2 for 5 h). In the cyclic test the frequency was 2 Hz and in each cycle the load ranged from 0 to the maximum load in 0.25 s and then went back to 0 in the next 0.25 s. YZ, AL and ICZ were subjected to 10 5 cycles (maximum load of 80 N) and VM7 and VM9 were subjected to 10 4 cycles (maximum load of 45 N). All specimens were fractured in three point bending and statistical analysis was performed with ANOVA and Tukey test (alpha = 0.05).
Results : Results are shown in the table, in MPa (standard deviation). YZ showed mean strength statistically higher than those of the other materials. AL and ICZ showed similar flexural strength, and these values were statistically higher than those obtained for porcelains, which showed similar strength. Cyclic fatigue caused a significant decrease in strength only for AL, and it also caused an increase in its coefficient of variation (CV). Although the strength of ICZ, VM7 and VM9 also decreased after the mechanical cycles, this effect was not statistically significant. For YZ, a numerical increase in the mean flexural strength was observed after the cyclic fatigue, but it was not statistically significant. None of the materials had their strength significantly affected by the autoclave cycles, although an increase in the CV was observed for YZ.
Conclusions : The null hypothesis could only be accepted for the experimental group comprising AL associated with cyclic fatigue, as this material showed significant strength degradation after mechanical cycling. For all the other materials, no significant change in the flexural strength was observed after any of the aging treatments.
|Control||869 (92) a||463 (59) b||409 (33) b,c||69 (12) d||65 (9) d|
|Cyclic fatigue||920 (76) a||389 (103) c||372 (34) c||63 (9) d||63 (9) d|
|Autoclave cycle||875 (222) a||424 (53) b,c||392 (15) b,c||62 (7) d||62 (11) d|