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

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(X)T(I)H(S) indicated the highest σ for Z(X) (315.81 ± 18.91 MPa), whereas Z(M)T(I)H(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(R)H(R) and T(I)H(S), T(I)H(U), and T(I)H(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.