2.3 Shear bonding strength test and Fracture analysis

Plastic boxes made by 3D printer with the size of 8 × 10 × 5mm were used as molds to mount the enamel cube-ceramic plate specimens. The epoxy resin was used as mounting material. The mounted specimens were stored in a humid chamber at 37°C for 24 h prior to testing. Each ceramic plate with its enamel cubes was fixed to a microshear device adapted to a miniature load testing machine (Instron 3367, Norwood, MA, USA). The shear force was applied at a crosshead speed of 1 mm/min until debonding.

After debonding, the fractured surfaces were evaluated with an optical microscope (AMscope, California, CA, USA) to classify the failure modes into one of the following categories: (A) adhesive failure at the interface between the ceramic and resin cement or between the resin cement and the enamel interface; (C) cohesive failure within the ceramic, within the resin cement or within the enamel only; and (M) mixed failure for adhesive and cohesive failure at the same site. The fractured surfaces and cross sections of selected debonded specimens were evaluated using a field-emission scanning electron microscope (SEM).