Subsurface deformation associated with hardness measurements of composites.

This study was prompted after a series of composites had been hardness-tested with the Vickers (pyramidal diamond indenter) and Barcol tests (flat-ended steel cone indenter). The results were found to differ markedly from the supplied Barcol/Vickers conversion chart. A sectioning and silver staining technique was therefore developed to examine the modes of subsurface deformation of four composites under the two indenters. Under the Vickers indentations, the large particle hybrid, Occlusin, showed a clear hemispherical area of subsurface disruption while the small particle hybrid, Brilliant Dentin, had a less distinct zone of disruption. The microfilled materials, Heliomolar and Isosit, showed minimal silver uptake, suggesting a less disruptive mode of plastic deformation. Under the Barcol indentations, Occlusion and Brilliant Dentin produced clear hemispherical zones of deformation, while Heliomolar and Isosit were badly crazed with distinctive patterns of subsurface flaws. These consistent patterns of failure to some extent explain the comparison of hardness measurements and may be of use in explaining the wear mechanisms of composite restorations.