Effect of Modified Speed Sintering Temperature and Dwell Time on the Optical and Mechanical Properties of Low-Alumina 3Y-TZP Zirconia

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Objectives The aim of this study was to evaluate the effect of peak sintering temperature and dwell time within a speed-sintering protocol employing fixed heating and cooling rates on the optical, microstructural and mechanical properties of low-alumina 3Y-TZP zirconia. Methods Disc-shaped Zpex specimens were sintered at 1500 or 1550°C with dwell times of 0, 5, 10, or 20 minutes using heating and cooling rates of 70°C/min. Relative density, X-ray diffraction with Rietveld refinement, scanning electron microscopy-based grain size, total forward transmittance and indentation fracture resistance were evaluated. Based on optical and fracture resistance results, 1500°C for 10 minutes and 1550°C for 5 minutes were selected for biaxial flexural strength testing. Results All modified speed-sintering protocols produced relative densities greater than 99.5%. Increased thermal exposure was associated with progressive phase evolution, consistent with yttria redistribution from the transformable tetragonal phase to the yttria-enriched tetragonal phase and caused temperature-dependent grain growth, particularly at 1550°C. Total forward transmittance increased with thermal exposure and remained close to the manufacturer-reported benchmark. Indentation fracture resistance showed limited variation (approximately 5.3-5.6 MPa·m¹/²), while biaxial flexural strength remained approximately 1100 MPa. Weibull analysis indicated that mechanical reliability differed between the 2 selected sintering conditions, despite their comparable mean biaxial flexural strengths. Conclusions Sintering at 1500°C with a 10-minute dwell provided the most favourable balance between near-complete densification, total forward transmittance, indentation fracture resistance and biaxial flexural strength within the investigated processing window. Clinical Relevance A controlled modified speed-sintering protocol may reduce processing time while maintaining optical transmittance and mechanical performance, supporting efficient clinical processing of low-alumina 3Y-TZP.