Strengthening and stiffening carbon fiber epoxy composites by halloysite nanotubes, carbon nanotubes and silicon carbide whiskers

•Halloysite nanotubes strengthens and stiffens carbon fiber polymer composites.•This is shown for cross-ply epoxy-matrix composites under flexure.•17% increase in strength and 11% increase in modulus were obtained.•Halloysite tubes were less effective than multiwalled carbon nanotubes.•Halloysite tubes were more effective than silicon carbide whiskers.

Low-cost natural halloysite (Hal) nanotubes (0.1 μm diameter) were effective for strengthening and stiffening continuous fiber epoxy composites, as shown for cross-ply carbon fiber (5 μm diameter, ~ 59 vol.%) epoxy nanocomposites under flexure, giving 17% increase in strength, 11% increase in modulus and 21% decrease in ductility. They were less effective than expensive multiwalled carbon nanotubes (0.02 μm diameter), which gave 25% increase in strength, 11% increase in modulus and 14% decrease in ductility. However, they were more effective than expensive silicon carbide whiskers (1 μm diameter), which gave 15% increase in strength, 9% increase in modulus and 20% decrease in ductility. Each filler, at ~ 2 vol.%, was incorporated in the composite at every interlaminar interface (interface between adjacent fiber laminae) by fiber prepreg surface modification. The flexural strength increase due to Hal nanotubes incorporation corroborated with the interlaminar shear strength increase. The measured values of the composite modulus agreed roughly with the calculated values based on the Rule of Mixtures. The interlaminar interface thickness was higher for the SiC whiskers case than the carbon nanotubes or Hal nanotubes case. The lamina thickness was not affected by the fillers. The composite density was 2% higher for the Hal nanotubes and SiC whiskers cases than the carbon nanotubes case.