The contribution of plastic sink-in to the static friction of single asperity microscopic contacts

We report microscale friction experiments for diamond/metal and diamond/silica contacts under gigapascal contact pressures. Using a new nanoprobe technique that has a sufficient dynamic range of force and stiffness, we demonstrate the processes involved in the transition from purely interface sliding at the nanoscale to the situation where at least one of the sliding bodies undergoes some plastic deformation. For sliding of micrometre-sized tips on metallic substrates, additional local plastic yielding of the substrate resulting from tangential tractions causes the tip to sink into the surface, increasing the contact area in the direction of loading and resulting in a static friction coefficient higher than the kinetic during ploughing. This sink-in is largely absent in fused silica, and no friction drop is observed, along with lower friction in general. The transition from sink-in within the static friction regime to ploughing in the sliding friction regime is mediated by failure of the contact interface, indicated by a sharp increase in energy dissipation. At lower contact pressures, the elastic interfacial sliding behaviour characteristic of scanning probe or surface force apparatus experiments is recovered, bridging the gap between the exotic realm of nanotribology and plasticity-dominated macroscale friction.

An Analytical Solution for the Initiation and Early Progression of Fretting Wear in Spherical Contacts

This work derives analytically solutions to calculate the wear volume at the initiation of fretting motion, and its progression over the first few oscillation cycles. The Arcahrd-based model considers a deformable hemisphere that is contact with a deformable flat bock. The material pairs investigated are special alloys, the Inconel617/Incoloy800H and Inconel617/Inconel617. The analytical study begins with a unidirectional frictional sliding contact, where the local interfacial sliding distance and the nominal sliding distance at the initiation of gross slip are derived. The obtained analytical expressions for unidirectional sliding are then used to derive the corresponding wear volume for the initiation and progression of gross slip and the wear volume for a general fretting cycle under pure elastic conditions. These analytical derivations are all verified by finite element analyses (FEA). The FEA method and the analytical solutions render virtually identical results for both similar and dissimilar material pairs. The effects of plasticity on the wear volume under elastic-plastic conditions are also investigated. It is found that the fretting wear volumes obtained from the FEA simulations, which include plasticity, are close to those obtained from the analytical expressions for purely elastic regimes. All the results are presented in normalized forms, which can easily be generalized and applied to 3D fretting wear of other material pairs.

Nanomechanical Properties and Failure Mechanisms of Two-Dimensional Metal-Organic Framework Nanosheets

Nanoscale mechanical properties measurement of porous nanosheets presents many challenges. Herein we show atomic force microscope (AFM) nanoindentation to probe the nanoscale mechanical properties of a 2‑D metal‑organic framework (MOF) nanosheet material, termed CuBDC [copper 1,4‑benzenedicarboxylate]. The sample thickness was ranging from ~10 nm (tens of monolayers) up to ~400 nm (stack of multilayers). In terms of its elastic‑plastic properties, the Young’s modulus (<i>E</i> ~ 22.9 GPa) and yield strength (𝜎_Y ~ 448 MPa) have been determined in the through-thickness direction. Moreover, we have characterized the failure mechanisms of the CuBDC nanosheets, where three failure mechanisms have been identified: interfacial sliding, fracture of framework, and delamination of multilayered nanosheets. Threshold forces and corresponding indentation depths corresponding to the failure modes have been determined. To gain insights into the failure mechanisms, we employ finite-element models with cohesive elements to simulate the interfacial debonding of a stack of 2‑D nanosheets during the indentation process. The nanomechanical AFM methodology elucidated here will be pertinent to the study of other 2‑D hybrid nanosheets and van der Waals solids.

Nanomechanical Properties and Failure Mechanisms of Two-Dimensional Metal-Organic Framework Nanosheets

Nanoscale mechanical properties measurement of porous nanosheets presents many challenges. Herein we show atomic force microscope (AFM) nanoindentation to probe the nanoscale mechanical properties of a 2‑D metal‑organic framework (MOF) nanosheet material, termed CuBDC [copper 1,4‑benzenedicarboxylate]. The sample thickness was ranging from ~10 nm (tens of monolayers) up to ~400 nm (stack of multilayers). In terms of its elastic‑plastic properties, the Young’s modulus (<i>E</i> ~ 22.9 GPa) and yield strength (𝜎_Y ~ 448 MPa) have been determined in the through-thickness direction. Moreover, we have characterized the failure mechanisms of the CuBDC nanosheets, where three failure mechanisms have been identified: interfacial sliding, fracture of framework, and delamination of multilayered nanosheets. Threshold forces and corresponding indentation depths corresponding to the failure modes have been determined. To gain insights into the failure mechanisms, we employ finite-element models with cohesive elements to simulate the interfacial debonding of a stack of 2‑D nanosheets during the indentation process. The nanomechanical AFM methodology elucidated here will be pertinent to the study of other 2‑D hybrid nanosheets and van der Waals solids.