Nanoscratching of iron: A molecular dynamics study of the influence of surface orientation and scratching direction

Using molecular dynamics simulation we study scratching of Fe crystals by a hard repulsive tip. Three surface orientations - (100), (110), and (111) - are studied with two scratch directions in each case. We take care to analyze the projected normal and transverse contact areas properly to take the loss of contact of the tip to the substrate beneath it and its increased contact to the pile-up into account. We find that the form of the pile-up generated depends strongly on the surface orientation and scratch direction; this dependence is analyzed with the help of the dominant slip system in bcc Fe. The normal force to keep the tip at fixed depth and the transverse scratching force vary by only 15% between the different scratch systems investigated. The normal contact area decreases considerably during the initial stage of scratching as the trailing part of the tip loses contact with the substrate. The normal hardness shows similarly small variations between the different systems studied and is smaller than the hardness during the indentation process. The transverse contact area increases continuously during the scratch as the pile-up is generated. As a consequence the tangential hardness varies more strongly (by 26%) between the systems studied. For the (100)[011] scratch system we analyze the depth dependence of scratching. We find the friction coefficient to increase with scratch depth. The dependence of the normal and tangential hardness on depth are discussed with the help of the variation of the respective projected areas with depth. (C) 2015 Elsevier B.V. All rights reserved.