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Investigation of Wear Properties of Nanocrystalline through Multiscale Modeling of Nanoindentation and Nanoscratch Test

Chamani, Mohammad | 2017

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 49666 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhossein; Movahhedy, Mohammad Reza
  7. Abstract:
  8. Nanocrystalline materials have received increasing attention during the last decades. Polycrystalline structures with grain sizes less than 100 nm are referred as nanocrystalline (NC). Their mechanical properties differ significantly from polycrystalline structures. As an example, hardness and wear resistance of nanocrystalline structures are higher than those of polycrystalline structures. With the reduction of grain size, hardness increases based on the Hall–Petch relation. However, at the very small grain sizes the Hall–Petch relation breaks down and a fundamental shift takes place in hardening mechanism. Molecular dynamics (MD) simulation offers a powerful method for the investigation of materials behavior on the atomic scale because it can directly simulate grain growth, dislocations movement and effect of grain boundaries on deformation of nanocrystalline structures. It should be noted that despite the benefits of MD method, due to the high computational effort and simulation time, it is not possible to simulate a full-scale sample with MD simulation. Regarding the use of very small samples, boundary conditions have considerable effects on results of MD simulations. To reduce the effects of boundary conditions, concurrent multiscale method must be used. In concurrent multiscale methods, different scales are presented simultaneously and transferring from atomic to upper scales is done at the same time. Using the MD method and simulation of nanoindentation and nanoscratching tests, the parameters affecting wear behavior such as Young’ modulus, hardness and coefficient of friction (COF) are calculated. Also, by studying the nucleation and propagation of dislocations, deformation mechanism of a nanocrystalline material near the critical size grain is investigated. In addition to reducing the grain size, nanocrystalline multilayer of Al and Ni are researched as another hardening mechanism and it is found that layer thickness has more influence on the hardness of Al/Ni nanocrystalline multilayer rather than grain size. Reduction of the COF with the refinement of grain size in nanocrystalline nickel is observed by performing MD simulations of nanoscratching test, however, for a grain size of 5 nm this trend is observed to be reversed. To reduce the effects of boundary conditions, two types of concurrent multiscale methods are used. Using Finite Element and General Particle methods, the size of samples are increased up to a few hundred nanometers without increasing in computational time
  9. Keywords:
  10. Molecular Dynamics ; Nanoscratch ; Nanoindentation ; Nanocrystal ; Concurrent Multi-scale Model ; Wear Properties

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