Evaluation of the Effect of Different Light-Curing Protocols on the Microhardness of Contemporary Bulk-Fill Resin Composites

Background: The study investigates newly developed composite materials with advanced filler technology and modified resin matrices, designed to enhance esthetic quality, clinical efficiency, and mechanical properties. This study evaluated the effect of two light-curing protocols—a conventional low-voltage (LV) protocol (10 s at 1200 mW/cm2) and a high-voltage (HV) protocol (3 s at 3000 mW/cm2)—on the microhardness (MH), bottom/top MH ratio, and the correlation between filler content (wt%, vol%) and MH of bulk-fill resin-based composites (RBCs). Four RBCs were tested: Tetric PlusFill (TPF), Tetric Plus Flow(TPFW), Tetric PowerFill (PFL), and Tetric PowerFlow (PFW). Materials and Methods: Samples were fabricated in the laboratory using specially designed cylindrical molds (diameter = 8 mm, height = 4 mm). Initial MH was measured on the top and bottom surfaces of composite specimens 24 h after light curing using a digital microhardness tester (QNESS 60 M EVO, ATM Qness GmbH, Mammelzen, Germany). The correlation between the filler content (wt%, vol%) and the MH of the RBCs was tested. For the calculation of depth-dependent curing effectiveness, the bottom/top ratio for initial MH was used. Conclusions: The MH of bulk-fill RBCs was found to be influenced by both material composition and the applied light-curing protocol. An increase in filler content resulted in higher MH values under both protocols, with the filler volume fraction exhibiting a stronger correlation than the weight fraction. While both flowable and sculptable Tetric Plus composites exhibited higher MH values under the HV protocol, Tetric Power composites demonstrated greater initial hardness under LV protocol. The flowable composite PFW showed the most pronounced reduction in MH under HV curing. The bottom/top MH ratio exceeded 80% in all tested materials, confirming adequate polymerization throughout the composite layers.