Dual-scale folding in sliding: a source for branched surface/sub-surface defect formation

Abstract Defect generation at the early stage of sliding is a serious issue in asperity contact and surface severe plastic deformation (SPD) processes such as burnishing, surface mechanical grinding, wedge sliding and friction stir processing, as it degrades the functional characteristics of the surfaces. Using in situ imaging and post-deformation characterization, the present work demonstrates that dual-scale surface folding, originating from deformation-induced two different length-scale surface perturbations/bumps generation, is a potential source for creating various branched crack-like surface/subsurface defects. The favourably oriented cluster of grains at the unconstrained surface promotes a big bump and subsequently a large fold, whereas individual grains within the cluster form small bumps and folds around the large fold. When the large fold surrounded by small folds crosses the wedge tip, it transforms into a branched crack-like defect. The activity of surface grains in developing small/big bumps is discussed, considering the grain orientation map and Taylor factor. Further, these types of defects can be characterized by the presence of alternate layers of refined and elongated grains around them. The implication of the work suggested that tailoring the size and orientation of grains should reduce the adverse effect of the dual-scale folding in sliding.