Abstract
The concentration and small size of nanodiamonds (NDs) plays a crucial role in the mechanical performance of epoxy-based nanocomposites by modifying the interface strength. Herein, we systemically analyzed the relation between the high concentration and small size of ND and the fracture properties of its epoxy-based nanocomposites. It was observed that there is a two-fold increase in fracture toughness and a three-fold increase in fracture energy. Rationally, functionalized-NDs (F-NDs) showed a much better performance for the nanocomposite than pristine NDs (P-NDs) because of additional functional groups on its surface. The F-ND/epoxy nanocomposites exhibited rougher surface in contrast with the P-ND/epoxy, indicating the presence of a strong interface. We found that the interfaces in F-ND/epoxy nanocomposites at high concentrations of NDs overlap by making a web, which can efficiently hinder further crack propagation. In addition, the de-bonding in P-ND/epoxy nanocomposites occurred at the interface with the appearance of plastic voids or semi-naked particles, whereas the de-bonding for F-ND/epoxy nanocomposites happened within the epoxy molecular network instead of the interface. Because of the strong interface in F-ND/epoxy nanocomposites, at high concentrations the de-bonding within the epoxy molecular network may lead to subsequent cracks, parallel to the parent crack, via crack splitting which results in a fiber-like structure on the fracture surface. The plastic void growth, crack deflection and subsequent crack growth were correlated to higher values of fracture toughness and fracture energy in F-ND/epoxy nanocomposites.
Highlights
Epoxy resins are thermosetting polymers mostly used as adhesives [1]
The pristine NDs (P-NDs) and F-ND powders were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS)
F-ND/epoxy nanocomposites exhibit a rougher surface than that of P-ND/epoxy nanocomposites which indicates the presence of a strong interface
Summary
Epoxy resins are thermosetting polymers mostly used as adhesives [1]. The advantage of epoxy resins is that they are in liquid form at room temperature and can be transformed into rigid form by adding a suitable hardener and heating them for a while. The epoxy/hardener ratio and the curing temperature strongly affect the mechanical properties of epoxy resins [2]. Shukla et al [6] reported the effect of volume fraction and functionalization of alumina nano-platelets on the mechanical and fracture properties of epoxy while Zhao et al [1] observed significant improvements in the ductility and modulus of epoxy by using alumina nano-particles as a filler. Carbon nano-structures are potential candidates as filler and, in the past, several carbon structures have been used to enhance the mechanical properties of epoxy resins These carbon nano-structures include carbon nano-fiber, carbon nano-tubes (CNTs), graphene, and nanodiamonds, etc. The large surface-to-volume ratio gives more space to attach functional groups on the surface of ND These functional groups effectively changed the mechanical properties of the epoxy resin [12,13].
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