Dependence of confined plastic flow of polycrystalline Cu thin films on microstructure
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Dependence of confined plastic flow of polycrystalline Cu thin films on microstructure Yang Mu, and Xiaoman Zhang, Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, USA J.W. Hutchinson, School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA W.J. Meng, Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, USA Address all correspondence to W.J. Meng at [email protected] (Received 18 May 2016; accepted 30 June 2016)
Abstract Axial compression was conducted on micro-pillars, in which polycrystalline Cu thin films were sandwiched between CrN and Si. Plastic flow of Cu was achieved, when the Cu films are inclined either at 90° or 45° with respect to the pillar axis. The texture of Cu films was altered by changing the template on which film growth occurred. The Cu microstructure was further altered by post-deposition annealing. The flow stress shows little dependence on the film texture in the as-deposited state. However, annealing influences the flow stress of confined Cu films significantly. The implications on strain gradient plasticity models are discussed.
Size effects in plasticity at small length scales have been demonstrated through different experiments, some by imposing a strain gradient,[1–3] others by changing the external sample size.[4,5] Simple shear of a thin-layer sandwiched between and well-bonded to two non-deforming substrates has been studied theoretically in the development of both strain gradient plasticity (SGP) and discrete dislocation plasticity.[6,7] Normal compression of a thin-layer bonded between rigid platens has offered another theoretical case study for SGP.[8] While such confined deformation geometries offer easy configurations for calculation within various plasticity models, experimental studies relevant to plastic deformation of confined thin layers have lagged behind theoretical developments. A previous experiment attempted to load thin Al layers diffusion bonded between Al2O3 adherents in shear.[9] With the Al layer thickness ranged between 10 and 50 µm, that experiment found no conclusive evidence of a dependence of strength on the Al layer thickness.[9] We recently demonstrated a new experimental protocol for evaluating the plastic flow response of metal thin films confined between two elastic–brittle solids.[10] Metal thin films are vapor deposited onto a ceramic substrate, followed by vapor deposition of another ceramic top layer. Such a ceramic/metal/ceramic sandwich structure is then fabricated via scripted focused ion beam (FIB) milling into microscale cylindrical pillars, between 2 and 5 µm in diameter. Axial compression loading on the ceramic/metal/ceramic micro-pillar then affects different kinds of loading on the metal film layer, depending on the orientation of the metal film with respect to the pillar axis. In the case of CrN/ Cu/Si micro-pillars, axial compression led to extensive plastic
deformation of Cu films in both
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