Fracture Toughness of Dental Zirconia Before and After Autoclaving

Materials and Methods: Y-TZP powders (Tosoh, TZ-3YB-E) and Ce-TZP/Al₂O₃ nanocomposite powders (NANOZR, Panasonic Electric Works, MACZ100) were pressed, fired at 1350˚C for 2hr and 1450˚C for 2hr, respectively, and cut to specimen bars (3x4x40 mm). Notch, 1.5 mm in depth, was prepared on the one side of the bars. Half of the specimens were autoclaved at 134˚C for 5 h. Fracture toughness KIC were determined using Single Edge V-Notch Beam (SEVNB) method according to ISO-6872 and using the indentation fracture method (IF) after Marshall and Evans Equation. For SEVNB, three-point bending test span was 16 mm. For IF, the specimens were tested under 30 kgf for 15 sec, and the elastic moduli for Y-TZP and NANOZR were employed as 210 and 245 GPa, respectively. Ten specimens were tested for each group.

Results: KIC of Y-TZP determined by SEVNB slightly decreased from 6.31 to 5.79 MPa·m^1/2 after the autoclaving (p<0.01). KIC of NANOZR showed no change as 11.03 MPa·m^1/2 before and after the autoclaving (p>0.1) and was 1.75-1.91 higher than those of Y-TZP (p<0.01). On the other hand, KIC determined by IF of Y-TZP, 5.63 and 5.67 MPa·m^1/2 before and after the autoclaving, respectively, were slightly smaller than the values by SEVNB (p<0.01), whereas KIC of NANOZR, 20.71 and 20.00 MPa·m^1/2 before and after the autoclaving, respectively, were significantly larger than the values by SEVNB (p<0.01). There were no changes in monoclinic ZrO₂ content of NANOZR and Y-TZP after the autoclaving (p>0.1).

Conclusions: It is conceived that NANOZR is susceptible to stress-induced transformation from tetragonal to monoclinic, but quite stable against LTAD.