In this study, a brittle epoxy resin has successfully been toughened by very small concentrations of SiC and diamond nanoparticles. The tensile stress-strain response (based on the ASTM tensile tests for polymers) and the fracture energy release rate, GIC, (based on the ASTM Single-Edge-Notched-Bending, SENB, tests for polymers) of neat and nano-reinforced epoxy were characterized over a range of nanoparticle concentrations. The maximum elevation of the fracture toughness, GIC, occurred at a very small particle concentration of about 0.2% by weight, for both diamond and SiC nanoparticles). This was also manifested as higher tensile failure stress and strain. The elevation of fracture toughness is most likely due to crack front trapping of the particles that promoted subsequent local plastic deformation. Scanning electron micrographs of the fracture surfaces for samples tested in tension and fracture showed the transition of epoxy behavior from brittle-to-ductile-to-brittle with increasing weight percentage of nanoparticles. At higher particle concentrations, flaky fracture surface was observed and the fracture toughness dropped, attaining values similar to the unreinforced polymer, which is attributed to agglomeration of the nanoparticles.

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