## 2 Materials and Methods
### Specimens Preparation and Surface Treatments  
All resin composite discs were prepared using split Teflon mold with a central hole of 7 mm diameter and 2 mm thickness. The mold was placed on a piece of polyester strip (Stripmat, Polytentia, Mezzovico, Switzerland) over a glass slide. Resin composite material was packed inside the central hole of the mold until the material was slightly over filling the mold. Afterwards, the repair surface treatments were assigned as follows:  
1. **Oxygen-inhibited layer (OIL, n = 32 discs):** In this group, a piece of Teflon tape was placed over the resin composite disc and gently pressed with a glass slide to extrude excess material and create a smooth flat surface. The glass slide and the Teflon tape were removed, and the material was photo-polymerized in contact with air for 10 s and 20 s (\(X\)-tra fil and Admira Fusion X-tra, respectively), following the manufacturer's instructions, using LED photo-polymerization unit (Elipar, 3M ESPE, Seefeld, Germany; light output: 1200 mW/cm\({}^{2}\)). The discs were removed from the mold and no further surface treatments were performed. These discs were used to test the cohesive strength of each material (positive control). 2. **Matrix (M, n = 32 discs):** After the material was packed inside the mold, a piece of polyester strip was placed on the top of the resin composite and gently pressed with a glass slide as described for OIL. The glass slide was then removed and the material was photo-polymerized using the LED photo-polymerization unit for 10 s and 20 s (\(X\)-tra fil and Admira Fusion X-tra, respectively), following the manufacturer's instructions. The discs were removed from the mold and no further surface treatments were performed. These discs served as negative control group. The remaining 192 discs (n = 96/each resin composite material) were light-polymerized against the polyester strip and removed from the mold. They were then wet-ground over #600 SiC papers for 10 s, rinsed for 5 s and air-dried for 5 s, before the application of the following surface treatments:  
3. **Futurabond M+ adhesive (FBM+):** The adhesive was applied with a microbrush (Single Tim, VOCO) and rubbed over the surface for 20 s, air-dried for 5 s and photo-polymerization for 10 s according to the manufacturer's instructions. 4. **Silane/Futurabond M+ adhesive (S/FBM+):** Silane was applied with a microbrush and left to react for 2 min. The silane was not air-dried according to manufacturer's instructions. The adhesive was applied and rubbed for 20 s, air-dried for 5 s, and photo-polymerized for 10 s.
5. **Admira Bond (AB):** Each disc was etched using 34.5% phosphoric acid etchant gel for 15 s, rinsed for 20 s and air-dried for 10 s. Adhesive was applied to disc surface using a microbrush and left \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material (Description)** & **Composition (Batch \#)** & **Manufacturer** \\ \hline \multirow{6}{*}{Cimara (ceramic repair system)} & SiC bur: SiC grinding low-speed bur & \\  & Adhesive: dimethacrylates, carbon acid-modified & \\  & dimethacrylates, CQ, BHT, Amime, acetone (1414216) & \\  & Silane: Reactive silane, isopropanol, acetone, amine & \\  & (1415052) & \\ \hline Bis-GMA: Bis-phenol A glycidyl methacrylate; UDMA: urethane dimethacrylate; TEGDMA: Triethylene glycol & \\  & dimethacrylate; CQ: camphorquinone; BHT: butylated hydroxytoluene. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._undisturbed for 30 s. The adhesive was gently air-dried for 5 s and photo-polymerized for 10 s according to the manufacturer's instructions. 6. **Silane/Admira Bond (S/AB):** Silane was applied and left to react for 2 min and no air dryness was performed. Admira Bond was applied for 30 s, air-dried for 5 s and photo-polymerized for 10 s.
7. **Ceramic Repair System (CRS):** Each composite disc was conditioned using a silicon carbide (SiC) grinding bur supplied by the manufacturer (Cimara bur, VOCO), in one direction for 5 s [(20)].