ATOMISTIC MODELING OF SLIP ON INTERNAL AND EXTERNAL INTERFACES

Erman Guleryuz

Internal interfaces in polycrystals and external interfaces between contacting solids play an important role in mechanical phenomena such as plastic deformation and frictional sliding. The understanding of relaxation mechanisms at these interfaces is critical in functionality control, particularly at the nanometer-scale where interfaces dominate. This thesis is concerned with the atomistic modeling of relaxation mechanisms on two types of interfaces: (1) grain boundaries (GBs) in polycrystals under high nanoindentation stresses; (2) atomic-scale single-asperity contacts during stick-slip friction. Molecular dynamics simulations of nanoindentations into Cu bicrystals are performed to investigate the influence of GB structure, as represented by GB excess volume, in the plasticity of GBs. We have probed the local mechanical response in the vicinity of two asymmetric tilt boundaries: the dense GB (with small excess volume) and the loose GB (with large excess volume). The propensity for the activation of specific inelastic relaxation mechanisms at the GB depends on the GB excess volume. The dense GB supports larger elastic loads until a GB dislocation is nucleated. Then, GB sliding by dislocation glide occurs. For the loose GB, a structural transformation facilitated by the re-arrangement of GB atoms is the primary relaxation mechanism at the GB. Activated slip systems in the bulk of the crystal depend on the GB excess volume. These results indicate that GB excess volume is a critical parameter in the plasticity of GBs under characteristically high nanoindentation stresses. A multiple time scale method is developed to study the atomic-scale single-asperity stick-slip friction. The temporal evolution of stick-slip friction can be characterized by the presence of two distinct phases: (1) a slow phase when the system’s configuration is evolving at a relatively slow pace, and, (2) a fast phase, when substantial changes occur rapidly. An atomistic modeling technique is developed to take advantage of this multiple time scale nature of the system’s dynamics. The slow phase is effectively modeled using a quasi-static energy minimization procedure, while the fast phase is modeled dynamically. This method allows the modelling of more realistic loading rates while capturing the correct physics.

ATOMISTIC MODELING OF SLIP ON INTERNAL AND EXTERNAL INTERFACES

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