Fatigue Behavior of Crystalline-Reinforced Glass-Ceramics

Purpose To evaluate the fatigue behavior of two crystalline-reinforced ceramics: leucite-reinforced (VL) and lithium disilicate-based (VD) glass-ceramics. Materials and Methods Bar-shaped specimens (16 x 4 x 1.2 mm) were produced for each ceramic using prefabricated CAD/CAM blocks. For each group, 30 specimens were subjected to a three-point flexural strength test in a universal testing machine. For VL and VD, 36 and 41 specimens were subjected to a cyclic fatigue test, respectively. The cyclic fatigue test was performed with a pneumatic mechanical cycling machine (1 Hz; 37 degrees C distilled water). Specimens were tested at two stress levels for each preset lifetime (10(3) and 10(4) cycles for VL; 10(4) and 10(5) cycles for VD) following the boundary technique. Fractography was performed with a scanning electron microscope. Data were analyzed with Weibull analysis. Results There were significant differences among groups for characteristic strength (sigma(0)) and Weibull modulus (m), as the confidence intervals did not overlap. The VD group presented the highest values of sigma(0), but the lowest Weibull modulus. Both groups showed a reduction of approximately 60% of the initial flexural strength (sigma(f)) after cycling for 10(4) cycles. For VD tested in fatigue, there was no degradation of sigma(f) when the number of cycles was increased from 10(4) to 10(5). The VL group showed an 18% decrease in sigma(f) when the number of cycles increased from 10(3) to 10(4). Flexural strength values estimated for a 5% probability of failure were 36 MPa for VL and 55 MPa for VD, after 10(4) cycles. Conclusion Both glass-ceramics showed similar strength degradation (60%) after a lifetime of 10(4) cycles, despite their distinct mechanical properties. Mechanical cycling in humid conditions proved to be an important factor for the degradation of the mechanical properties of crystalline-reinforced glass-ceramics.