Flexural Strength of Two Multilayered and Monochromatic High Yttria Containing Zirconia Materials Following Different Sintering Parameters

Abstract

Objectives: Sintering parameters influence the properties of zirconia. This study examined the effect of altering sintering temperature and time of monochrome and multilayer 5 mol% yttria-partially stabilized zirconia (5Y-PSZ) on flexural strength.

Materials and methods: Three hundred specimens (width × length × thickness = 10 × 20 × 2 mm) were prepared from monolayer (Z_X) and multilayer (Z_M) 5Y-PSZ and randomly sintered at decreasing (T_D: 1,450°C), regular (T_R: 1,500°C), and increasing (T_I: 1,550°C) sintering temperature, with extremely short (H_E: 10 minutes), ultrashort (H_U: 15 minutes), short (H_S: 30 minutes), and regular (H_R: 135 minutes) sintering time (n = 15/group). The precrack was induced on the tension side before testing for flexural strength (σ).

Statistical analysis: Analysis of variance and Tukey's test were used for significant differences of σ at p < 0.05. The microstructure and crystalline (monoclinic; m, tetragonal; t, cubic; c) phase were evaluated by scanning electron microscope (SEM) and X-ray diffractometer (XRD).

Results: Z_XT_IH_S indicated the highest σ for Z_X (315.81 ± 18.91 MPa), whereas Z_MT_IH_S indicated the highest σ for Z_M (335.21 ± 36.18 MPa). There was no significant difference for σ between Z_X and Z_M (p > 0.05). Sintering zirconia at T_I or H_R indicated significantly higher σ than sintering at T_D or T_R or with H_S, H_E, or H_U for both Z_X and Z_M (p < 0.05). There was no significant difference for σ between T_RH_R and T_IH_S, T_IH_U, and T_IH_E (p > 0.05). SEM indicated intergranular and transgranular fractures. XRD revealed predominately c- and t-phases and minor amounts of m-phase.

Conclusion: Increasing sintered temperature with decreasing time offers acceptable strength to regular sintering. Raising sintering temperature with decreasing time is suggested to facilitate chairside restorative reconstruction.