Microstructural evolution in dry and lubricated CuNi sliding contacts - an experimental approach

Abstract

Friction and wear of polycrystalline materials are linked to the microstructural response given to interfacial stresses. Even though most of the governing mechanisms of microstructural evolution in sliding contacts are generally understood, it is still uncertain which mechanisms matter under which conditions. For that reason, several studies were carried out over the years to investigate the microstructure and simulate the dislocation dynamics in the near surface zones during sliding. An attempt was made by Eder et al. to derive a deformation mechanism map for CuNi alloys, by analyzing deformation mechanisms such as twinning, time-dependent grain size development and shear in several fcc CuNi alloys subjected to sliding with molecular dynamics (MD) simulation. The motivation of the present study is to experimentally verify the predictions of this deformation mechanism map. The map has defined five distinct regions according to the dominant deformation mode of the structure: (1) elastic deformation; (2) twinning and grain refinement; (3) more twinning with grain boundary driven processes; (4) shear layer formation beside massive twinning; and (5) surface shearing. The microstructural evolution of CuNi alloys under distinct loading conditions was investigated. Experiments were conducted on pure Cu, CuNi10, CuNi30 and pure Ni probes in dry and lubricated reciprocating sliding contact. For the friction tests, the parameters were chosen similar as possible to those in the MD simulation. All test were conducted at room temperature (22C) and in an argon atmosphere to reduce oxidation. Exceptions are only the lubricated measurements and those conducted at low normal pressure. Friction curves were analysed and the friction surfaces were carefully examined using scanning electron microscopy (SEM) after friction. Cross-sectioning of the specimens after the friction tests were performed in the transverse direction (perpendicular to the direction of friction) in the middle of the track using a focused ion beam (FIB). The cross-sectional microstructure was analysed using a SEM/FIB dual-beam microscope. This deformation map may serve as a tool for finding optimum material compositions within a specified operating range.