Residual compressive surface stress increases the bending strength of dental zirconia

To assess the influence of surface treatment and thermal annealing on the four-point bending strength of two ground dental zirconia grades.

Fully-sintered zirconia specimens (4.0×3.0×45.0mm³) of Y-TZP zirconia (LAVA Plus, 3M ESPE) and Y-TZP/Al₂O₃ zirconia (ZirTough, Kuraray Noritake) were subjected to four surface treatments: (1) 'GROUND': all surfaces were ground with a diamond-coated grinding wheel on a grinding machine; (2) 'GROUND+HEAT': (1) followed by annealing at 1100°C for 30min; (3) 'GROUND+Al₂O₃ SANDBLASTED': (1) followed by sandblasting using Al₂O₃; (4) 'GROUND+CoJet SANDBLASTED': (1) followed by tribochemical silica (CoJet) sandblasting. Micro-Raman spectroscopy was used to assess the zirconia-phase composition and potentially induced residual stress. The four-point bending strength was measured using a universal material-testing machine.

Weibull analysis revealed a substantially higher Weibull modulus and slightly higher characteristic strength for ZirTough (Kuraray Noritake) than for LAVA Plus (3M ESPE). For both zirconia grades, the 'GROUND' zirconia had the lowest Weibull modulus in combination with a high characteristic strength. Sandblasting hardly changed the bending strength but substantially increased the Weibull modulus of the ground zirconia, whereas a thermal treatment increased the Weibull modulus of both zirconia grades but resulted in a significantly lower bending strength. Micro-Raman analysis revealed a higher residual compressive surface stress that correlated with an increased bending strength.

Residual compressive surface stress increased the bending strength of dental zirconia. Thermal annealing substantially reduced the bending strength but increased the consistency (reliability) of 'GROUND' zirconia.