Effect of Glazing Dwell Time and Cooling Duration on Flexural Strength and Surface Microhardness of 3 mol% Yttria‐Stabilized Tetragonal Zirconia: An In Vitro Study

OBJECTIVE: To evaluate the effect of glazing dwell time and cooling duration on the flexural strength and surface microhardness of high-translucency 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP). MATERIALS AND METHODS: Total of 108 rectangular zirconia specimens (20 × 4 × 2 mm) were fabricated from preshaded monolithic blocks and sintered at 1530°C. Specimens were divided into nine groups (n = 12). The control group (NG) remained unglazed. Experimental groups underwent conventional firing (CF;1-min dwell) or extended firing (EF; 3-min dwell) at 850°C, each combined with four cooling durations (4,7,9, or 12 min) between 850°C and 600°C. Flexural strength was evaluated using a four-point bending test (ISO 6872:2015) with Weibull analysis, while surface microhardness was measured using the Vickers method (50 g, 10 s). Data were analyzed using two-way ANOVA and Tukey's HSD test (α = 0.05). RESULTS: Significant differences were observed in flexural strength and microhardness (p < 0.001). The unglazed group showed the highest strength (698 ± 18 MPa; m = 46). Among glazed groups, EF3 (3-min dwell/9-min cooling) exhibited the highest characteristic strength (σ₀ = 657 MPa). EF1 (3-min dwell/4-min cooling) showed the lowest strength (466 ± 23 MPa), below ISO requirements. Microhardness decreased with longer cooling durations. CONCLUSIONS: Glazing parameters significantly affect 3Y-TZP properties. Extended firing with controlled slow cooling (9 min) optimizes strength and reliability, while rapid cooling compromises performance.