Effect of Sintering Temperature on Apparent Fracture Toughness of 3Y-TZP

Objectives: Previous work has shown that sintering temperature strongly affects the grain size of zirconia ceramics stabilized with 3 mol.% yttria. Our goal was to investigate the effect of sintering temperature on the apparent fracture toughness of 3Y-TZP.
Methods: Disc-shaped specimens (n=10) were sectioned from 3Y-TZP blanks and sintered at temperatures ranging between 1350 and 1600°C/2h. Specimens from two commercially available zirconia ceramics, Bruxzir (BZ) and ZPex (ZP) were also prepared and sintered according to manufacturer’s recommendations. Specimens were polished to a 1-micrometer finish and indented with a Vickers indenter under a 500N load. Micrographs were taken immediately. The apparent fracture toughness was calculated from the measured crack sizes using established equations for the indentation technique. The mean grain size was measured by SEM after thermal etching. A value of 220 GPa was selected for the modulus of elasticity. Results were analyzed by ANOVA and Tukey’s test.
Results: As expected, the grain size increased with sintering temperature from 0.23±0.019 to 0.61±0.075µm. Grains sizes for BZ and ZP were within expected values corresponding to their sintering temperatures, 1450°C and 1580°C, respectively. The apparent fracture toughness increased with sintering temperature from 7.73±0.75 at 1350°C to 9.38±0.58 MPa.m^0.5at 1550°C and decreased slightly at 1600°C, to 8.95±0.96 MPa.m^0.5. However, this decrease was not significant. Groups sintered at 1350 and 1450°C had a significantly lower fracture toughness than all other groups (p<0.05). The fracture toughness of groups BZ and ZP was within the expected range, in line with their respective sintering temperatures.
Conclusions: The apparent fracture toughness of 3Y-TZP ceramics was affected by grain size. The observed increase in toughness with sintering temperature is in line with flexural strength data and likely due to the reported increase in the ability of tetragonal grains to transform to the monoclinic phase as the grain size increases.