Atomistic Insights on the Wear/Friction Behavior of Nanocrystalline Ferrite During Nanoscratching as Revealed by Molecular Dynamics

Tribology Letters, Jun 2017

Using embedded atom method potential, extensive large-scale molecular dynamics (MD) simulations of nanoindentation/nanoscratching of nanocrystalline (nc) iron have been carried out to explore grain size dependence of wear response. MD results show no clear dependence of the frictional and normal forces on the grain size, and the single-crystal (sc) iron has higher frictional and normal force compared to nc-samples. For all samples, the dislocation-mediated mechanism is the primary cause of plastic deformation in both nanoindentation/nanoscratch. However, secondary cooperative mechanisms are varied significantly according to grain size. Pileup formation was observed in the front of and sideways of the tool, and they exhibit strong dependence on grain orientation rather than grain size. Tip size has significant impact on nanoscratch characteristics; both frictional and normal forces monotonically increase as tip radii increase, while the friction coefficient value drops by about 38%. Additionally, the increase in scratch depth leads to an increase in frictional and normal forces as well as friction coefficient. To elucidate the relevance of indentation/scratch results with mechanical properties, uniaxial tensile test was performed for nc-samples, and the result indicates the existence of both the regular and inverse Hall–Petch relations at critical grain size of 110.9 Å. The present results suggest that indentation/scratch hardness has no apparent correlation with the mechanical properties of the substrate, whereas the plastic deformation has.

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Atomistic Insights on the Wear/Friction Behavior of Nanocrystalline Ferrite During Nanoscratching as Revealed by Molecular Dynamics

Tribol Lett Atomistic Insights on the Wear/Friction Behavior of Nanocrystalline Ferrite During Nanoscratching as Revealed by Molecular Dynamics A. T. AlMotasem 0 1 2 L. J. Holleboom 0 1 2 J. Bergstro¨m 0 1 2 A. Ga˚a˚rd 0 1 2 P. Krakhmalev 0 1 2 0 Department of Physics, Faculty of Science, Assiut University , Assiut 71516 , Egypt 1 Department of Mechanical and Materials Engineering, Karlstad University , Universitetsgatan 2, 65637 Karlstad , Sweden 2 & A. T. AlMotasem Using embedded atom method potential, extensive large-scale molecular dynamics (MD) simulations of nanoindentation/nanoscratching of nanocrystalline (nc) iron have been carried out to explore grain size dependence of wear response. MD results show no clear dependence of the frictional and normal forces on the grain size, and the single-crystal (sc) iron has higher frictional and normal force compared to nc-samples. For all samples, the dislocation-mediated mechanism is the primary cause of plastic deformation in both nanoindentation/nanoscratch. However, secondary cooperative mechanisms are varied significantly according to grain size. Pileup formation was observed in the front of and sideways of the tool, and they exhibit strong dependence on grain orientation rather than grain size. Tip size has significant impact on nanoscratch characteristics; both frictional and normal forces monotonically increase as tip radii increase, while the friction coefficient value drops by about 38%. Additionally, the increase in scratch depth leads to an increase in frictional and normal forces as well as friction coefficient. To elucidate the relevance of indentation/scratch results with mechanical properties, uniaxial tensile test was performed for nc-samples, and the result indicates the existence of both the regular and inverse Hall-Petch relations at critical grain size of 110.9 A˚ . The present results suggest that indentation/scratch hardness has no apparent correlation with the mechanical properties of the substrate, whereas the plastic deformation has. Atomistic; Polycrystalline iron; Scratch hardness; Wear; Dislocations; Twinning 1 Introduction Understanding wear, friction and mechanical properties of a material at nanoscale is crucial for further development in technological applications. Experimentally, nanoindentation and nanoscratching techniques are commonly used for nanoscale mechanical testing as they can provide accurate information of hardness, friction and wear. As a compliment to experimental technique, atomistic modeling becomes a powerful tool to deepen the understanding of wear and failure modes of materials at the atomic scale. In the literature, numerous studies of nanoscratching of metals are available both theoretically and experimentally and a comprehensive review can be found in [ 1, 2 ]. Molecular dynamics simulation has been used to investigate nanoscale machining and the factors governing the nanomachining process: tip geometry, machining speed, rake angle and surface roughness. However, most of these simulations usually adopt defect-free monocrystalline structures as the work material [ 3–9 ]. On the other hand, most engineering materials exist in polycrystalline forms and mechanical properties such as flow stress, yield stress and hardness of metals and alloys [ 10, 11 ] dramatically scale with grain size. Thus, grain size in polycrystalline structures is a controlling factor for material properties and material responses to deformation. For example, it has been shown that grain refinement to the nanometer scale leads to an increased yield stress Hall–Petch (H–P) relationship, while further refinement led to inverse H–P [ 12, 13 ]. The influence of grain boundaries (GBs) during nanomachining has been extensively studied [ 14–18 ]; nevertheless, most of the considered samples are either nc-fcc metals, nc-diamond or nczinc blend ceramics but rarely for nc-bcc materials. For instance, Shi et al. [ 15 ] performed MD simulations to investigate the effect of grain size and nanomachining parameters of polycrystalline copper. It was discovered that for all cutting conditions simulated, the polycrystalline structure requires smaller cutting forces compared with the monocrystalline structure. The authors attributed this behavior to the reduction in material strength with grain refinement. They also verified that the behavior of frictional and normal forces for different grains is coupled with the associated failure mode, i.e., (H–P) or inverse (H–P) as verified by tensile testing. Mishra et al. [ 18 ] simulated the wear of a nanocrystalline silicon carbide substrate by tools with a rounded end. They demonstrated that the primary mechanism for nanoscale wear of silicon carbide is GBs sliding and the compatible stress is accommodated by nucleation of partial dislocations, void formation and grains pullout. Furthermore, they compared the results with sc-silicon carbide wear response and demonstrated that nc-silicon ca (...truncated)


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A. T. AlMotasem, J. Bergström, A. Gåård, P. Krakhmalev, L. J. Holleboom. Atomistic Insights on the Wear/Friction Behavior of Nanocrystalline Ferrite During Nanoscratching as Revealed by Molecular Dynamics, Tribology Letters, 2017, pp. 101, Volume 65, Issue 3, DOI: 10.1007/s11249-017-0876-y