Tensile strength and toughness of carbon nanotube-graphene foam composite materials and the corresponding microscopic influence mechanism

Carbon nanotube (CNT)-graphene foam (GF) composites (CGFCs) have garnered considerable attention due to their unique properties. However, the factors and mechanisms influencing their tensile strength and toughness remain unclear. Here, the influence of graphene stiffness, CNT length, and CNT number on the tensile strength and toughness of pure GF and CGFC was investigated using coarse-grained molecular dynamics. It shows that CGFC exhibits improved strength (66.5 MPa) and toughness (34.4 MJ/m³) compared to pure GF (18.0 MPa and 11.2 MJ/m³) due to enhanced interconnectivity among graphene flakes through additional graphene-CNT-graphene contacts. Notably, the effect of graphene stiffness on the tensile properties reveals an interesting observation: lower stiffness of the graphene correlates with higher strength and toughness of the foam. This phenomenon arises from the increased deformability of softer graphene, promoting the formation of more graphene-graphene and graphene-CNT-graphene contacts. As a result, the interconnectivity is significantly enhanced, leading to improved strength and toughness. Furthermore, greater CNT length and number contribute to the increased strength and toughness. This effect can predominantly be attributed to the growing number of graphene-CNT-graphene contacts, which enhances the connectivity between graphene. These insights inform the design of high-strength and tough materials using CNTs and graphene.