Tribological loading induced microstructure changes have been widely reported to be of key importance to friction and wear. And a distinct microstructural discontinuity has been very often observed in the subsurface area of tribologically deformed materials. This discontinuity is the key feature in the microstructure which directly determines the mechanical properties and wear generation. Yet, the formation mechanisms and the origin of this discontinuity still remain elusive.
We investigate the microstructure changes in the subsurface area of pure copper in the very early stage of sliding contact with scanning electron transmission microscopy. A distinct dislocation structure is observed around 100 nm to the surface after the first sliding pass in a very mild sliding condition. This dislocation structure is the very beginning of the discontinuity in the subsurface area of materials under tribological loading and will evolve into the reported microstructure features in later stages of the sliding contact. A model is then proposed to describe and predict the behavior of this dislocation structure with a sign change in the stress field under sliding contact.
The origin of microstructural discontinuity is of significant importance to understand the reported discontinuity under tribological loading. And once a deep understanding of its formation mechanisms is acquired, it will allow for further tailoring materials surface for low friction and little wear.