Influence of Adhesive Layer Thickness on Wear Damage of the Bonded Interface Between Enamel and Composite Inlays

The objective of this study was to evaluate the influence of adhesive layer thickness on wear damage of the bonded interface between enamel and composite inlays using self-adhesive resin cement. Sixty bonded interface samples consisting of four different adhesive layer thicknesses (10 ± 10, 50 ± 10, 100 ± 10 and 150 ± 10 μm) were prepared from resin composite (Tetric N Ceram Bulk Fill; Ivoclar Vivadent AG), self-adhesive resin cement (RelyX U200; 3M ESPE) and tooth enamel. Ball-on-flat sliding wear tests (20 N, 1 Hz, 5 × 10⁴ cycles) were performed on four groups of samples (n = 15). The wear depth and wear track morphology of each sample after 5 × 10², 5 × 10³, and 5 × 10⁴ cycles were recorded by white light interferometry and scanning electron microscopy, respectively. Optical microscopy was also used to evaluate cracks and their propagation in the samples. Stress distribution of the interface with four different adhesive layer thickness was analyzed by Finite Element Analysis. All the four groups had similar wear evolution on the bonded interface and the adjacent enamel showed characteristic brittle fracture and microcracks. But the extent of wear damage varied among different groups. The 50 ± 10 μm group owned the slightest damage after 5 × 10², 5 × 10³, and 5 × 10⁴ cycles. Stress concentration always happened on the enamel near the interface and thinner adhesive layer owned more stress concentration. In conclusion, for self-adhesive resin cement, 50 ± 10 μm was the best thickness of adhesive layer for the enamel–composite inlay interface from the aspect of tribology.