In the material engineering process, due to the weak interface between the nanofiller and the matrix nanocomposite, the strengthening effect of the nanofiller is far lower than the theoretical prediction value.
In a paper published a few days ago in "Science Advances", Ningning Song and a team of scientists from the Department of Mechanical and Aerospace Engineering at the University of Virginia demonstrated graphene-coated boron carbide (B4C) nanowires (B4C-NWs@graphene). This structure makes the nanowires have excellent dispersibility in the matrix, and helps the nanowires to super-strongly bond with the matrix.
B4C-NWs@Graphene enhances the epoxy resin composite material, and at the same time enhances the strength, elastic modulus and ductility. By using graphene to customize the composite interface, Song et al. effectively used nanofillers to increase the load transfer efficiency by 2 times. They used molecular dynamics simulation technology to solve the shear hybrid self-assembly mechanism of graphene/nanowire structures. This low-cost technology opens up a new path for the development of tough nanocomposites to improve the interface and achieve efficient high load transfer.
Nanofillers including nanowires and nanoparticles have a larger specific surface area than microfillers. Therefore, in theory, they provide ideal reinforcement materials for special connections that enhance strength and toughness. However, in materials science and engineering, nanocomposites have still not achieved this goal due to the weak interface between the filler and the matrix.
Boron carbide (B4C) is the third hardest material known in nature, and is often praised for its key physical and mechanical properties. However, when boron carbide nanowires (B4C-NWs) are used as reinforcing materials in nanocomposites, due to their weak dispersibility in the matrix and weak interface bonds, the B4C nanowires alone cannot show Enhancement. Therefore, the engineering of the nanocomposite interface is very important to realize its full potential.
Among the many methods being researched and previously explored in the fields of materials science and nanomaterials, Song et al. reported a graphene interface engineering technology. In this mechanism, they bonded B4C-NWs and graphene together to unusually enhance the strength and toughness of the resulting material. They converted high-quality graphene sheets into graphite, and at the same time wrapped them on B4C-NWs through shear mixing to obtain the B4C-NWs@graphene structure.
Song et al. first uniformly grow B4C-NWS on the surface of carbon fiber cloth through a vapor-liquid-solid process, where cotton is used as a carbon source, and amorphous boron powder is used as a boron source, and a catalyst is used at the same time. The research team separated B4C-NWS from the substrate by ultrasonic vibration, and used X-ray photoelectron spectroscopy (XPS) to study the chemical bond states in the material, confirming that high-quality B4C-NWs were produced.
In order to directly synthesize and self-assemble B4C-NWs@graphene, Song et al. mixed graphite powder and B4C-NWs, and then used transmission electron microscopy (TEM) to show how graphite was successfully exfoliated into graphene. B4C-NWs are still intact in the mixture. During the synthesis process, the graphene sheets self-assembled onto the surface of B4C-NWs at the same time. Using high-resolution transmission electron microscopy (HRTEM) detection and the corresponding fast Fourier transform (FFT) mode, Song et al. confirmed that the self-assembly quality of graphene on B4C-NWs is very high, while maintaining single-layer and multilayer features.
The scientists dispersed B4C-NWs@graphene onto epoxy nanocomposites, and performed three-point bending tests on the composite and epoxy materials. Compared with the original epoxy resin sample, the B4C-NWs@graphene nanocomposite material undergoes greater plastic deformation before fracture. The results show that graphene acts as an interface agent to strengthen the bond between B4C-NWs and the epoxy matrix, and a series of mechanisms that are beneficial to bending together promote the toughness of B4C-NWs@graphene composites. In this way, graphene enables the nanofillers to have better dispersibility in the matrix, provides better load transfer, and combines strength and toughness. In order to better understand the dispersion quality of B4C-NWs@graphene components, Song et al. calculated the theoretical elastic modulus of composite materials. The results show that compared with other composite materials reported in the literature, the composite material retains excellent strength and toughness.
The team also performed molecular dynamics (MD) simulations to first understand how graphene sheets edit the B4C-NW surface, and how graphene disperses B4C-NWs and enhances load transfer in composite materials. Then, they conducted MD simulations to test the process of pulling the nanofillers from the epoxy matrix to understand the bonding strength between the nanofillers and the matrix. The MD simulation results are consistent with the experimental observations and reveal the details of the graphene customized B4C-NWs to enhance the interaction barrier to improve the dispersion performance. Song et al. conducted a simulation study on the process of pulling nanofillers from the epoxy matrix and calculated the interaction energy to understand the bonding strength between the nanofillers and the matrix. Due to the presence of graphene, the nanofiller has a higher surface area, the interaction energy between B4C-NWs@graphene and epoxy resin is higher, and the pull-out force peak is larger. In addition, the number of interacting atoms in the composite material is large, and the geometric shape is complex, which improves the interface strength and load transfer efficiency.
Through the above methods, Ningning Song and colleagues used graphene sheets to customize the interface between B4C-NWs and epoxy materials. The nanocomposite material (B4C-NWs@Graphene) was synthesized by shearing and mixing graphene powder and B4C-NWs in diluted water. The obtained suspension presents a uniform dispersion state in the water and the epoxy material, improves the load transfer efficiency, and at the same time improves the mechanical properties of the composite material. This low-cost and high-efficiency graphene coating technology will open up a new way for the development of nanocomposites with high strength and good toughness, which can be used in medicine, pharmacy and drug delivery.
The title of the paper is "Tailoring nanocomposite interfaces with graphene to achieve high strength and toughness".