Effect of bonding on load bearing capacity of brittle material

Objective: To investigate the effect of bonding on load bearing capacity and failure mode of a brittle dental material under Hertzian indentation. Methods: 10 mm diameter discs of 0.4, 0.8, 1.2, 1.6, 2.0, 2.4, 2.8, 3.2, 3.6, 4.0, 4.4, 5.0, 6.0, 7.0 mm thickness were fabricated for a typical dental material: amalgam, 20 replicates for each thickness group. In each thickness group, specimens were randomly divided into two subgroups, 10 for each. In the first subgroup (B), the discs were bonded with the substrate (30% glass fibre-reinforced polyamide-nylon 6,6, E=10 GPa), while in the other subgroup (NB), the discs were tested resting freely on top of the substrate. Hertzian indentation test was done through a universal testing machine after 7 d's storage in the air at 23±oC, using a 20 mm diameter hard steel ball. Load at the first failure was detected acoustically; mode was determined visually. At least 1/3 of specimens in each case were examined under scanning electronic microscope for corroboration. Results: In both NB and B subgroup, the main failure mode of amalgam shifted from bottom-initiated radial cracking to near-contact cone cracking or subsurface plastic deformation accordingly with the increase of thickness. Failure loads increased initially with thickness, but levelled off (or tended to level off) when the thickness reached 4.0 mm. The failure loads of B subgroups were significantly higher than NB subgroups in small thickness groups (≤2.8 mm), but no significant difference was found between NB and B subgroups for thicknesses greater than 3.2 mm. Conclusion: Bonding greatly improves the load bearing capacity of thin brittle coating material, but has little effect on thick coating under Hertzian indentation test. Failure load of the brittle material is closely related with the corresponding failure mode.