Effect Of Sintering Temperature on the Microstructural Characterization of Conventional and Translucent Monolithic Zirconia, with and Without Hydrothermal Degradation.

Abstract

Zirconia polycrystals are widely used as dental restorative materials due to their excellent mechanical performance. However, their intrinsic opacity has traditionally restricted their use to substructures in prosthetic restorations, limiting aesthetic applications. Recent advances in manufacturing technology have enabled faster sintering cycles through elevated temperatures, potentially enhancing clinical efficiency without compromising material integrity.

Purpose: This study aimed to assess the mechanical, physicochemical, and optical properties of two types of monolithic zirconia-conventional and translucent-subjected to varying sintering durations, surface treatments, and hydrothermal aging conditions.

Material and methods: A total of 320 specimens, produced in two thicknesses, were divided according to zirconia type and allocated across different experimental groups based on sintering time, polishing protocol (with or without glazing), and evaluation period (before and after aging).

Results: Translucent zirconia demonstrated slightly greater surface roughness (Ra = 0.62 ± 0.04 µm) compared to conventional zirconia (Ra = 0.52 ± 0.03 µm; p < 0.05). The highest microhardness values were observed in the ZCSC and ZCSR groups (≈1350 VHN), significantly surpassing those of translucent zirconia with conventional sintering (ZTSC) (p < 0.05). Surface free energy was also higher in ZCSC and ZCSR, particularly in specimens that were neither chemically polished nor aged. Glazing treatment led to a reduction in specimen dimensions and flexural strength in translucent zirconia. Fractographic analysis revealed compression curl features on the tensile side, indicating stress-induced crack propagation consistent with three-point bending.

Conclusions: The mechanical and optical behavior of zirconia was significantly affected by sintering parameters and hydrothermal aging. These factors should be carefully considered during material selection and fabrication to optimize clinical outcomes.