Statistical Nanoindentation To Explore Filler-Matrix Influence On Dental Composite Macro-Fracture

Objectives: The purpose of this study is twofold: (1) to characterize the organic and inorganic constituents, nano-mechanical properties, and structural features of various commercial composites by exploiting a novel statistical nanoindentation method, and (2) to understand the effect of organic-inorganic signatures on the macro-facture of dental composites.
Methods: Twenty composite brands were tested to determine their structural features and nano-mechanical properties. Elastic modulus (EM), hardness (H), and stiffness (S), were measured using a nanoindenter with a Berkovich diamond indenter. The constituent phases per composite brand were identified via grid nanoindenation, where EM, H, and S were mapped using a total of 3,600 indents per an area of 20 x 20 µm². The mass, volume, and density of the constituent phases per composite brand were also verified via Thermogravimetric Analysis (TGA) and Archimedes principle. Fracture toughness and flexural strength values were measured using a 3-point bend test. Composite microstructural features were analyzed by scanning electron microscopy and laser diffraction particle size analyzer. Statistically deconvolution technique was applied to derive the mechanical phases and their corresponding percentages. Data were analyzed with ANOVA/Tukey (α=0.05) and regression.
Results: The filler’s or matrix’s percent area measured via the grid nanoindentation is linearly related (r²=0.90) to the filler’s or matrix’s percent volume measured via the traditional TGA/Archimedes techniques. A positive linear correlation between fracture toughness and Weibull characteristic strength (r²=0.74) is identified. The inorganic constituent of modern dental composites is found to exhibit one of three distributions: unimodal, bimodal, and multimodal. Bimodal filler distribution responds with significantly higher EM, H, and S than unimodal and multimodal distributions.
Conclusions: In general, filler percent weight values directly correlated with EM, H, and S. Although filler percent weight and fracture toughness were independent properties, bivariate regression analysis demonstrated filler EM directly correlated with fracture toughness. Grid nanoindentation is a viable option to measure dental composite systems with heterogeneous phases.