Interfaces in Carbon Materials: Experiment and Atomistic Simulation

In this thesis work, we have explored the possibility and the limitations of using atomistic simulation in studying structure and properties of carbon/ carbon interfaces and its integration with appropriate experimental techniques. In doing so, we have tested several interatomic potential functions for carbon and concluded that the potential functions tested are not ideally suited for all applications. One has to pay careful attention in choosing an empirical potential function for a given application for carbon. The Tersoff function performed best for diamond in calculating both structural and mechanical properties while the Brenner function appears to be suitable for most types of carbon if confined to studying structure and energetics. Thus, the Tersoff function was used to produce a-C structures by melting and quenching of the diamond lattice. The a-C structures based on the Tersoff's function reasonably matched the experimental data although failure of the Tersoff's function to treat -n bonds leads to discrepancy in the high pressure aC. The a-C/graphite interface has been selected as a model system in this work. The methodology developed here can be extended to study the structure and properties of other types of carbon/carbon interfaces which can be found in monolithic carbon or carbon/carbon composites. To create an a-C/graphite interface, the low-pressure a-C structure is joined with the graphite crystal which is modelled by the Brenner function and a pair potential function to handle the interplanar interaction. The interface is created by compressing the amorphous carbon with perfect crystalline graphite terminated to expose (1120) planes. The planar structure and weak interplanar bonding allow the graphitic planes to deform in order to accommodate the bonds formed at the interface. The simulation also indicates that the generated interface mostly consists of nearly sp 2 hybridized bonding connecting the two sides. The bonds across the interface when formed are likely to maintain their equilibrium configurations. Due to the large interplanar spacing, both the graphite and aC sides have a high density of undercoordinated atoms (24%) leaving the interface energetically unfavorable with respect to the bulk. These undercoordinated atoms probably weaken the structural rigidity of the interface providing a fracture path under stress. HRTEM study of the a-C/pyrolytic carbon interface suggests that the a-C deposition process induces defects in pyrolytic carbon to enhance bonding between two materials. HTREM of the interface demonstrates that the basal planes will distort or bend at the interface, which qualitatively agrees with the simulation observation. To further validate the simulation result, a method to perform mechanical testing of a-C/pyrolytic carbon is devised. SEM and XPS study of the fracture surfaces show that fracture occurs predominantly through the interface, thus, confirming the simulation prediction that the interface is likely to be weak compared to the bulk phases in spite of presence of the reactive edge atoms from graphite at the interface. The mechanical testing also showed that the interface strength is sensitive to the surface roughness and chemistry and the fracture path can be altered depending on the interface conditions. Lastly, the (11211 twin interface in graphite is studied using the analytical model for graphite developed earlier because such twin interface represents an example of graphite/graphite interfaces found in carbon/carbon composites and the twins could also serve as an important source of plastic deformation of new carbon materials such as carbon foam. The simulation study indicates that the (1121) twin interface may consist of a special local atomic structure, namely, the 8-4-8 polygons.The boundary composed of such structure has the energy of 0.09 J/m and the activation energy of -3 eV for migration (mostly due to the formation of a kink along the boundary line). The result suggests that existence of 8-4-8 structure at the twin interface is not improbable compared to the dislocation model. Thesis Supervisor: Janez Megusar Title: Research Associate, Materials Processing Center Thesis Co-Supervisor: Sidney Yip Title: Professor, Department of Nuclear Engineering