This study evaluated the effects of the sintering temperature on Vickers microhardness and three-point flexural strength values of two multi-layered zirconia materials. Multi-layered zirconia systems with four distinct layers were selected: DD cube ONE ML (4Y-TZP) and DD cubeX² ML (5Y-TZP). In total, 96 plate-shaped A2-shade specimens were obtained using individual layers of these two zirconia materials. The individual layers were then divided equally into batches with three different sintering temperatures (1300, 1450, and 1600 °C), and the Vickers microhardness was assessed. Another group of 72 bar-shaped specimens was prepared from the same materials. These were similarly divided into three different sintering temperatures, and the flexural strength was assessed. SEM was used to conduct fractographic analyses. The data were analyzed using SPSS 24.0 software with a p-value < 0.05. The microhardness and flexural strength of 4Y-TZP were higher than those of the 5Y-TZP at all the sintering temperatures. A significant difference was found in the microhardness and flexural strength values between groups sintered at different sintering temperatures (p < 0.05). The highest microhardness and flexural strength values were found at 1450 °C (p < 0.05). The microhardness values of different layers were not significantly different (p > 0.05). The sintering temperature and type of ceramic material significantly affected the microhardness and flexural strength. However, the layers did not significantly affect the microhardness.

Adequate polymerization is essential for the mechanical performance and longevity of resin-based composites; however, recently introduced chemical cure, bulk-fill composites remain underexplored. This in vitro study compared the degree of conversion (DC) of a chemical cure bulk-fill composite (Stela, SDI) with those of a nano-hybrid light-cured composite (Filtek™ Z250, 3M). Sixty standardized disc-shaped specimens (5 mm × 2 mm; n = 30 per material) were prepared according to the manufacturer’s instructions. We measured DC using Fourier-transform infrared spectroscopy (FTIR) immediately after polymerization and at 5, 10, 20 min, and 24 h. Both materials showed a progressive increase in DC over time. However, the light-cured composite consistently achieved higher conversion values, reaching 62.39 ± 4.23% after 24 h compared with 47.58 ± 3.04% for the chemical cure composite (p < 0.001). These findings indicate that DC alone should not be considered predictors of composite performance.

Adequate polymerization is essential for the mechanical performance and longevity of resin-based composites; however, recently introduced chemically cured bulk-fill composites remains insufficiently investigated. This in vitro study compared the degree of conversion (DC) and surface microhardness (SMH) of a chemically cured bulk-fill composite (Stela, SDI) with those of a nano-hybrid light-cured composite (Filtek™ Z250, 3M). Sixty standardized disc shaped specimens (5 mm x 2 mm; n= 30 per material) were prepared according to the manufacturers instructions. The DC was measured using Fourier-transform infrared spectroscopy (FTIR) immediately after polymerization and after 5, 10, 20 minutes and 24 h. SMH was evaluated using Vickers microhardness tester before and after artificial aging by thermocycling (5,000 cycles). The light-cured composite exhibited significantly greater SMH then the chemical cured composite (95.01 ± 0.59 HV vs. 51.21 ± 0.50 HV, p < 0.001). Both materials demonstrated progressive increase in DC over time. However, the light-cured consistently achieved higher conversion values, reaching 62.39 ± 4.23% after 24 h compared with 47.58 ± 3.04% for the chemically cured composite (p < 0.001). These findings indicate that the DC and SMH alone should not be considered a predictor of composite performance.