Figure 5. Scanning electron micrographs of the epoxy matrix composite materials reinforced with single-walled carbon nanotubes.
The scanning electron micrographs of the epoxy matrix composite materials are illustrated in Figure 6 which are reinforced with multi-walled carbon nanotubes. The use and development of carbon nanotubes has expanded [69, 70]. These materials are valuable in next generation industries including the fields of electronics and chemistry [71, 72]. The further development of carbon nanotube technology allows organized structures or intertwined randomly oriented bundles of carbon nanotubes to be formed [73, 74]. Techniques have been developed to controllably build organized architectures of carbon nanotubes having predetermined orientations, such as vertically aligned carbon nanotubes [75, 76]. Although such structures may be useful for a variety of purposes, the structures by themselves may be limited in terms of function and application [77, 78]. The structure is a combination of dispersed multi-walled carbon nanotubes and micron-scale agglomerates. On the fracture surface, multi-walled carbon nanotubes are pulled-out of the matrix, indicating that there is likely energy dissipation from interfacial debonding and pullout of the multi-walled carbon nanotubes. Outside the area where multi-walled carbon nanotubes are locally agglomerated the fracture surface is quite smooth, similar to that of the unreinforced epoxy. Where the multi-walled carbon nanotubes are present it is clear that there is an increase in surface roughness. In the areas where multi-walled carbon nanotubes are agglomerated, tail-like structures are formed. These features indicate that the multi-walled carbon nanotubes interact with the crack path and result in crack deflection and a more torturous fracture path. Some multi-walled carbon nanotube pullout as well as texturing of the fracture surface, but the formation of the larger-scale tail structures is not present. A possible reason for the overall higher fracture toughness for the structure that contains both dispersed and agglomerated features may be a result of larger agglomerates being able to more effectively interact with the crack front.