Nanomechanical properties of dental resin-composites

Objective. To determine by nanoindentation the hardness and elastic modulus of resin-composites, including a series with systematically varied filler loading, plus other representative materials that fall into the categories of flowable, bulk-fill and conventional nano-hybrid types. Methods. Ten dental resin-composites: three flowable, three bulk-fill and four conventional were investigated using nanoindentation. Disc specimens (15 mm x 2 mm) were prepared from each material using a metallic mold. Specimens were irradiated in the mold at top and bottom surfaces in multiple overlapping points (40 s each) with light curing unit at 650 mW/cm(2). Specimens were then mounted in 3 cm diameter phenolic ring forms and embedded in a self-curing polystyrene resin. After grinding and polishing, specimens were stored in distilled water at 37 degrees C for 7 days. Specimens were investigated using an Agilent Technologies XP nanoindenter equipped with a Berkovich diamond tip (100 nm radius). Each specimen was loaded at one loading rate and three different unloading rates (at room temperature) with thirty indentations, per unloading rate. The maximum load applied by the nanoindenter to examine the specimens was 10 mN. Results. Dependent on the type of the resin-composite material, the mean values ranged from 0.73 GPa to 1.60 GPa for nanohardness and from 14.44 GPa to 24.07 GPa for elastic modulus. There was a significant positive non-linear correlation between elastic modulus and nanohardness (r(2) = 0.88). Nonlinear regression revealed a significant positive correlation (r(2) = 0.62) between elastic moduli and filler loading and a non-significant correlation (r(2) = 0.50) between nanohardness and filler loading of the studied materials. Varying the unloading rates showed no consistent effect on the elastic modulus and nanohardness of the studied materials. Significance. For a specific resin matrix, both elastic moduli and nanohardness correlated positively with filler loading. For the resin-composites investigated, the group-average elastic moduli and nanohardnesses for bulk-fill and flowable materials were lower than those for conventional nano-hybrid composites. (C) 2012 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.