Normal Load and Counter Body Size Influence the Initiation of Microstructural Discontinuities in Copper during Sliding

Abstract

Near the interface of two contacting metallic bodies in relative motion, the microstructure changes. This modified microstructure leads to changes in material properties and thereby influences the tribological behavior of the entire contact. Tribological properties such as the friction coefficient and wear rate are controlled by the microstructure, while the elementary mechanisms for microstructural changes are not sufficiently understood. In this paper, the influence of the normal load and the size of the counter body on the initiation of a tribologically induced microstructure in copper after a single sliding pass is revealed. A systematic variation in the normal load and sphere diameter resulted in maximum Hertzian contact pressures between 530 MPa and 1953 MPa. Scanning electron microscopy, focused ion beam, and transmission electron microscopy were used to probe the subsurface deformation. Irrespective of the normal load and the sphere diameter, a sharp line-like feature consisting of dislocations, the so-called dislocation trace line, was identified in the subsurface area at depths between 100 nm and 400 nm. For normal loads below 6.75 N, dislocation features are formed below this line. For higher normal loads, the microstructure evolution directly underneath the surface is mainly confined to the area between the sample surface and the dislocation trace line, which itself is located at increasing depth. Transmission Kikuchi diffraction and transmission electron microscopy demonstrate that the misorientation is predominantly concentrated at the dislocation trace line. The results disclose a material rotation around axes roughly parallel to the transverse direction. This study demonstrates the generality of the trace line phenomena over a wide range of loads and contact pressures and the complexity of subsurface processes under a sliding contact and provides the basis for modeling the early stages in the microstructure evolution.

A non-local methodology for geometrically necessary dislocations and application to crack tips

“…the dislocation free zone) due to the strong stress concentration, which drives dislocations to leave the materials from crack surfaces (Huang and Li, 2004) , at the latter position. Hence the peak GND density (manifesting itself as lattice rotation) is observed with an offset distance from the crack tip, which is similar to the discussion in the microstructure change under tribological constraint (Chen et al, 2018b;Ruebeling et al, 2021). Besides, an asymptotic analysis (Le and Tran, 2016) The stored energy density G proposed in (Wan et al, 2014) is given as:…”

Subsurface Microstructural Evolution during Scratch Testing on Bcc Iron

“…TEM foils were prepared by thinning with the Ga + -ion beam after covering the area of interest with electron-and ion-beam-assisted Pt deposition. See Greiner et al [25] and Ruebeling et al [26] for detailed method and parameter descriptions.…”

“…It was recently observed that this dislocation self-organization is also the location where a localized simple shear of the subsurface material occurs in the sliding direction [24]. Moreover, it was found to be present for a wide range of experimental parameters, such as the counterbody size and normal load [26], as well as for various sliding speeds [29], meaning its occurrence warrants a closer look. It is believed that the DTL is the demarcation line between the development of a nanocrystalline layer and the bulk material underneath as sliding progresses.…”

“…Compared to the testing that has been performed for multiple cycles, there is significantly less research on the onset of plastic deformation under a tribological load, although there have recently been attempts to investigate this regime in some detail. For these experiments, high-purity copper samples were tribologically loaded with sapphire spheres for a single trace [24][25][26]. The results demonstrated that after a mild singular tribological load, dislocations self-organize at depths of between roughly 100 and Metals 2021, 11, 1258 2 of 10 500 nm under sliding contact in a fashion that leads to a horizontal line-like contrast in TEM foils cut parallel to the sliding direction.…”

On the origin of plasticity-induced microstructure change under sliding contacts

“…Recent experiments on face-canter-cubic (FCC) samples have observed an abrupt and highly localized microstructure change (termed as "dislocation traceline" (DTL)) [28,29] at ~100 nm depth from the contact during one-stroke sliding. This microstructural change eventually leads to recrystallization of the single crystal in subsequent cyclic loadings when the material is more heavily loaded [30]. Approaches based on continuum assumptions [31] to identify microstructural changes, albeit providing good qualitative insight as to where such activity may take place, are hardly able to fully explain the mechanistic drivers for the formation of the dislocation traceline and the evolution of the discrete features associated with microstructural evolution.…”