Deformation and abrasive wear mechanisms in nanomachining

Abstract

Machining is an important excess-metal removal process in manufacturing. In spite of the considerable effort on metal cutting research, the mechanics of this process at a nano scale are not fully understood. Hence metal cutting research is a challenging field and is currently attracting the attention of workers in the field of plasticity of metals as well as those concerned with the more immediate problems in practice. Nano scale grooves such as nano channels and nano slots have many applications in semiconductor and biomedical industries, which are growing to meet the increasing demands of nano technology. Whenever nanomachining is performed to produce a nano scale groove on a metal surface, abrasive wear occurs. The ultimate aim of the analysis of the mechanics of metal machining at a nano scale is to understand the basic phenomena, to predict how the materials deforms, which mechanism is dominant and what machining parameters (depth of cut, machining velocity) are required for a given material and for a given machining condition to produce a nano scale groove. In this research, nanomachining was utilized to investigate the abrasive wear mechanism that produces a nano scale groove on metal surfaces. Two different tools (Berkovich and Conical) with the same tip radius of 100nm but of different edge geometries were used to machine both polycrystalline materials and single crystal materials with a nano indenter equipped with a nano scratching attachment. During the machining operations, the generated normal and cutting forces were measured as a function of nano scale machining parameters. It was found that the generated forces (normal and cutting) increased with an increase in depth of cut; however, there was no significant effect on the generated forces due to the variation in machining velocities. In nano scale machining, the percentage of elastic recovery revealed that the deformation mechanisms were identified as elastoplastic in nature as opposed to the well-established completely plastic mode of traditional machining operations. The pile up volume due to elastoplastic deformation was utilized to distinguish between the ploughing and the cutting modes of abrasive wear mechanism. The percentage values of these two mechanisms were determined and utilized to obtain the dominant mode of an abrasive wear mechanism for producing a nano scale groove and to correlate this abrasive wear mechanism with the co-efficient of friction (µ) in different machining conditions. As a result of machining operations, the machined surface retains residual stress and strain. The type of residual stress, whether tensile or compressive, depends on the used machining parameters. It is however difficult to measure residual stress and strain at a nano scale machining level. Therefore, to understand the material deformation behaviour and their effect on the machined surface, numerical modelling of nano scale machining process was developed by using a novel mesh free modelling tool of Smoothed Particle Hydrodynamics (SPH).