Fracture Mechanisms of Resin Based Dental Restorative Composites

Objectives: The aim of the present study is to investigate the microscopic fracture and toughening mechanisms for two different particulate reinforced resin based dental restorative composites, a hybrid (Filtek^TM Z250) and nanofill (Filtek^TM Supreme).

Methods: To observe fracture mechanisms, double notched four point bending beams of size 2 x 3 x 33 mm³ were cured for 80s using visible light (Triad II) and were loaded until fracture occurred at one of the notches. Optical and SEM observations of the surviving notch tip were used to determine the interaction of cracks with the underlying microstructure. In addition, resistance curve (R-curve) experiments were also performed to determine the fracture resistance of razor blade created pre-cracked compact tension specimens. Specimens were tested dry or hydrated (soaked for ~60 days to achieve >99% hydration).

Results: Microscopic observations of cracked samples show clear evidence of an interparticle crack growth mechanism where the crack propagates along the particle-matrix (or cluster-matrix) interface in both of the composites independent of the environmental conditions. Crack deflection by the filler particles is one observed toughening mechanism, and the micron scale solid particles in the hybrid composite appear to be more effective in deflecting the crack than similar sized agglomerates of nanoparticles in the nanofill. Crack bridging, another toughening mechanism where locally intact regions of composite near the crack tip sustain a portion of the load that would otherwise contribute to crack advance, was also observed. Preliminary results suggest that the latter toughening mechanism could be responsible for the observed rising R-curve behavior (crack extension toughening) of the dental restorative composites.

Conclusion: A novel method for investigating the micromechanisms of fracture in resin based dental composites provides evidence for enhanced toughening with crack extension, possibly attributed to crack bridging and deflection from filler particles.