Subsurface shear instability and nanostructuring of metals in sliding |
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| Authors: | S Tarasov V Rubtsov A Kolubaev |
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| Affiliation: | 1. Centre for Mechanics and Materials Technologies (CT2M), Universidade do Minho, Azurém, 4800-058 Guimarães, Portugal;2. Faculty of Mechanical Engineering, “Dunarea de Jos” University of Galati, Galati, Romania;3. Universidade do Minho, Dept. Eng. Mecânica, Azurém, 4800-058 Guimarães, Portugal;4. CEMUC – Department of Mechanical Engineering, University of Coimbra, 3030 Coimbra, Portugal;1. Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK;2. Institute for Metals Superplasticity Problems, RAS, 450001 Ufa, Russia;3. Belgorod State University, 85 Pobedy, Belgorod 308015, Russia;4. Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA;1. School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China;2. Metal Material Department, Suqian College, Suqian 223800, China;3. Engineering Research Center of Dredging Technology, Ministry of Education, Hohai University, Changzhou 213022, China;1. School of Mechanical Engineering, Nanjing Institute of Technology, Nanjing, Jiangsu 211167, China;2. Nanjing Boiler and Pressure Vessel Inspection Institute, Nanjing, Jiangsu 210002, China;3. Department of Mechanical Engineering, Nanjing Tech University, Nanjing, Jiangsu 210009, China;1. Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, UP 208016, India;2. Graduate School of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 5258577, Japan;3. Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 5258577, Japan |
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| Abstract: | Dry sliding wear conditions were used to obtain a 500 μm-thick layer of nanosize grains on copper samples. As shown, this layer reveals a flow behavior pattern similar to that of a viscous non-Newtonian fluid. Four structurally different zones were found in the longitudinal cross-sections of samples below the worn surface. Upper two of them are nanocrystalline and consist of many ～1 μm-thick sublayers, which show either laminar or turbulent flow behavior. These sublayers demonstrate different levels of elasticity as compared to each other and may be related to an interplay between work-hardening and thermal softening. Lower two zones undergo usual plastic deformation and severe fragmentation without viscous mass transfer. High level of Young's modulus in the fragmentation zone is evidence of insufficient thermal softening at that depth. We believe that viscous flow zones are the result of shear instability and subsequent shear deformation developed in subsurface layers due to thermal softening. Numerical study has been carried out to simulate friction-induced deformation and shear instability under conditions close to the experiment. As shown, such a situation is possible when deformation-generated heat is taken into account. Another interesting result relates to the sublayers’ strain rate distribution. It was found that 1 μm-thick sublayers may show either high strain rate gradient or zero strain rate as a function of depth below the worn surface. The latter case means that a pack of layers may exist and behave like an elastic body in ductile medium. |
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