Mechanistic interpretations of fracture toughness and correlations to wear behavior of hydroxyapatite and silica/hydroxyapatite filled bis-GMA/TEGDMA micro/hybrid dental restorative composites

The fracture toughness and wear behavior of designed and fabricated dimethacrylate (bis-Glycidyl methacrylate in combination with Triethylene glycidyl dimethacrylate as resin-matrix system) based restorative visible-light cured composites filled with hydroxyapatite (micro-filled) and silica/hydroxy-apatite (micro-hybrid) combinations were evaluated. Two series of composites were prepared with filler loadings in the range of 20-50 wt%. Conditional fracture toughness (KQ) values were determined using pre-cracked single-edge-notch-beam (SENB) specimen in three-point-bending test (mode-I fracture). Micro-hybrid restorative composites showed fracture toughness values that were comparable to micro filled dental composites. Irrespective of the nature of the filler or their combinations, the composites with 20 wt% fillers showed maximum value of k(Q) while those with 50 wt% filler showed minimum value of KQ. The fracture toughness (K-Q) values determined experimentally were compared to theoretical predictions from micromechanical modeling, which revealed that KQ of these composites primarily attributed to crack-tip shielding mechanism. The sliding wear assessment of the composites on a pin-on disk set-up was carried out as a function of applied-load, sliding-duration and sliding-velocity. The specific wear rates remained comparable in their trends for both micro-filled and micro-hybrid composites though the absolute values of wear resistance and fracture toughness have been observed to be relatively higher for hybrid composites. Dental composites with 30-40 wt% of filler-loading showed highest wear resistance. Worn surface morphology revealed intensive traction marks in case of composites with 50 wt% of filler. A suggestive correlation between Brittleness parameter (B) and fracture toughness and wear behaviors indicated that highly-filled composites are more prone to exhibit poor dimensional stability. (C) 2017 Elsevier Ltd. All rights reserved.