Investigation into single dislocation wall formation in micro-wall sliding wear test
Plastic deformation dominates the tribological property during sliding metal contacts. However, due to the complex collective interaction of contacts, a physical understanding of the microstructure – tribological property relationship is still largely lacking. The rise of nano- and micro-mechanical instrumentation allows separating the interacting tribological processes. For instance, single microasperity sliding wear, i.e. nano/micro-scratch, testing has become a useful technique for understanding the mechanisms during the very initial run-in stage of tribology. Although the tribological contact has been dramatically simplified, the sliding-induced plasticity is still incompletely understood. In this work, we introduce a novel micro-wall wear test that aims at 1) further simplifying the stress state in the contact zone and 2) better identifying the active slip system in the entire deformation zone. Initially, we utilize the advanced Focused Ion Beam (FIB) to produce micro-walls with different directions in the different grains. We indent and slide on the microwalls using a wedge indenter. We observe that the dislocations prefer to be activated on the positively inclined slip-planes and then on the negatively inclined slip-planes during indentation. After sliding, the active slip-planes remain activated while the dislocation activity moves closer to the top surface. Interestingly, high-resolution electron backscatter diffraction (EBSD) results show that single dislocation-walls form after indentation in some of microwalls but after sliding in all investigated microwalls. Both sides of the dislocation-wall are misoriented by a large angle (~30° in some cases). The electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) confirm the high density of dislocations in the dislocation-walls. These results are compared to the work of Greiner at al. , who investigated dislocation trace formation under macroasperity contacts in a bulk material. The present study shows the grain orientation dependence and strong twin boundary effect of the dislocation-wall behavior. It is the first time to trace single dislocation wall formation in single grains during tribology. This micro-wall wear test has the potential to better study the origin of tribology-induced grain refinement in metals.
 C. Greiner, J. Gagel, P. Gumbsch, Solids Under Extreme Shear: Friction-Mediated Subsurface Structural Transformations, Adv Mater 31(26) (2019).
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