Objective: evaluate the microhardness and depth of cure of sonic bulk fill resin composite material for cementation of CAD/CAM composite block with different thicknesses versus the traditionally used dual cured resin cement by assessing microhardness at the top and bottom and calculating bottom/top ratio to determine the depth of cure. Materials and methods: CAD/CAM resin composite blocks (Grandio) restorative materials were used in the present study. For cementation options, two resin

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... were selected: SonicFill 2 bulk fill resin composite (Kerr) and Dual-link universal Resin cement (Bisco). Four sections were prepared from each CAD/CAM blocks thickness of 1 mm, 1.5 mm, 2 mm and 3 mm respectively. Sectioned specimens of each CAD/CAM blocks were then divided according to their thickness into three groups (n=5).Portions of each composite cement were placed on a 1-mm thick transparent glass slide and squeezed to a 100µm film thickness Each specimen was covered with one of the CAD-CAM composite block slabs or left uncovered (control) and then light-cured for 40 sec that was positioned perpendicularly on top and in direct contact with the CAD-CAM slabs or the top glass (control).Micro-hardness was obtained for the top and bottom surface of specimens. The mean microhardness values and hardness ratio % of the specimens were calculated and tabulated. Microhardness measurements were performed twice; one immediately and after 24 hours. Statistical analysis was performed. Significance of the difference within the same group was evaluated using one way analysis of variance (ANOVA) test, followed by Tukey's post hoc test when ANOVA indicated a significant difference. T test was used to compare between both groups. The level of significance was set at P < 0.05.