Sequence of Stages in the Microstructure Evolution in Copper under Mild Reciprocating Tribological Loading

Abstract

Tailoring the surface properties of a material for low friction and little wear has long been a goal of tribological research. Since the microstructure of the material under the contact strongly influences tribological performance, the ability to control this microstructure is thereby of key importance. However, there is a significant lack of knowledge about the elementary mechanisms of microstructure evolution under tribological load. To cover different stages of this microstructure evolution, high-purity copper was investigated after increasing numbers of sliding cycles of a sapphire sphere in reciprocating motion. Scanning electron and focused ion beam (FIB) microscopy were applied to monitor the microstructure changes. A thin tribologically deformed layer which grew from tens of nanometers to several micrometers with increasing number of cycles was observed in cross-sections. By analyzing dislocation structures and local orientation changes in the cross-sectional areas, dislocation activity, the occurrence of a distinct dislocation trace line, and the emergence of new subgrain boundaries could be observed at different depths. These results strongly suggest that dislocation self-organization is a key elementary mechanism for the microstructure evolution under a tribological load. The distinct elementary processes at different stages of sliding identified here will be essential for the future modeling of the microstructure evolution in tribological contacts.

Solids Under Extreme Shear: Friction‐Mediated Subsurface Structural Transformations

“…[66] Focused ion beam cross-sections for all cycle numbers from the middle of the wear tracks are presented as cross-sectional SEM images in Figure 8. A fundamental understanding of the elementary mechanisms acting at different stages of the microstructural evolution is still lacking.…”

“…The microstructural investigations were based on focused ion beam (FIB) and scanning electron microscopy (SEM) in combination with cross-sectional EBSD. [66] Focused ion beam cross-sections for all cycle numbers from the middle of the wear tracks are presented as cross-sectional SEM images in Figure 8.…”

“…The color bar in the GND density mapping represents the number of GNDs per unit area (m 2 ). a-d) Reproduced with permission [66]. Copyright 2018, American Chemical Society.…”

Early deformation mechanisms in the shear affected region underneath a copper sliding contact

“…Experimentally, the elementary mechanisms governing the early stages of tribological loading have been studied less extensively, even though they seem elemental in deciding the future fate of a tribological contact. We have previously reported a distinct horizontal line feature at a uniform depth of about 150 nm below the surface of copper samples contacted in dry sliding conditions by a sapphire sphere 12 . Since this line was found to consist of dislocations, it was named dislocation trace line (DTL).…”

Linking microstructural evolution and macro-scale friction behavior in metals

“…More interestingly, pure metals exhibit an apparent bi-stability in their friction behavior, where in contact conditions associated with low friction the trends in friction appear to be converging, while the trends diverge for higher friction. Various authors [8,[10][11][12][13][14][15] have reported that this unusual and highly desirable macro-scale behavior for pure metals and alloys are associated with the formation and persistence of a thin layer of highly refined, ultra-nanocrystalline (10 to 20 nm grain size) metal at the sliding surface, though the conditions and mechanism through which this layer is formed and how these two characteristics are correlated remains unclear. We propose a hypothesis and supporting evidence for a correlation between surface grain structure evolution and friction behavior of metals that is both predictive and quantitative, relying exclusively on materials properties and contact mechanics models to predict the bounds of observed friction regimes.…”