Effects of heat treatment on the microhardness of direct composites at different depths of restoration
AbstractPurpose: The mechanical properties of direct composites may be improved by heat treatment. This study aimed to verify whether heat treatment influences the Knoop microhardness (KHN) of direct composites and whether there are differences among photoactivated areas before and after the application of heat. Methods: A total of 84 rectangular cross-sectional specimens (10 x 2 x 2 mm) was prepared, 14 per composite. After photoactivation (600 mW/cm² - 40 s), seven specimens per composite were heat-treated (170 ºC/10 min), whereas the others remained unheated (control). KHN values were obtained for three sites on each specimen at specified distances from the irradiated surface (0.2, 1.0, and 2.0 mm). The data were analyzed by two-way ANOVA and Tukey’s test at with a significance level set to 5%. Results: The improvement in the KHN values depended on the composites studied. The KHN values of TE-Economic, Natural Look, and Prisma were significantly improved by heat treatment (P < 0.05), whereas FillMagic and Glacier remained unchanged (P > 0.05) after heating. The distance from the irradiated area significantly influenced the KHN of all of the composites, regardless of heating application (P < 0.05). Sites that were closer to the irradiated surface exhibited a higher KHN (P < 0.05). Conclusion: The choice of the composite is important when intending to improve the KHN by heating. Sites that are closer to the irradiation source exhibit greater hardness, even after thermal treatment.
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