Thickness dependence of flow stress of Cu thin films in confined shear plastic flow
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Thickness dependence of flow stress of Cu thin films in confined shear plastic flow Yang Mu, Ke Chen and W.J. Meng, Department of Mechanical and Industrial Engineering, Louisiana State University, Baton Rouge, Louisiana 70803 Address all correspondence to W.J. Meng at [email protected] (Received 31 July 2014; accepted 09 September 2014)
Abstract Compression loading on CrN/Cu/Si(100) micropillars containing 45°-inclined interfaces yielded unequivocal evidence of shear plastic flow within Cu thin films confined between non-deforming Si and CrN. Confined shear plastic flow occurred over Cu thicknesses between ~100 and 1200 nm, with a monotonically increasing flow stress as the thickness decreases. The demonstration of a significant dependence of the shear flow stress on the confined Cu film thickness offers a new example of scale-dependent plasticity, and a new experimental test case for non-local plasticity theories.
Size effects in plasticity have been documented through many experiments over the past two decades.[1–3] Shearing of a wellbonded metal layer sandwiched between two substrates which do not deform plastically has been one of the canonical cases studied in the development of non-local plasticity theories.[4–8] In many respects, it captures in the simplest manner the essence of plastic strain gradients associated with geometrically necessary dislocations generated when plastic flow is blocked at the metal/substrate interfaces. Analytically, shear of confined layers is one of the simplest problems to solve.[4–8] Previous experiments performed in the configuration of shear loading of thin Al layers diffusion bonded between Al2O3 adherents were inconclusive.[9] Until now, experimental data for the confined shear problem have not been available to confront theoretical predictions. Shear plastic flow in confined thin layers, in addition to its scientific interest, is of significant technological interest. Over the past two decades, vapor phase deposition of thin ceramic coatings has become an important means for engineering surfaces of mechanical components[10] and manufacturing tools.[11] Satisfactory adhesion of coatings to substrates is critical to the lifetime of coated systems and often the deciding factor for their implementation. To improve adhesion, interfacial layers, often consisting of elemental metals, are deposited between the ceramic coating and the substrate. When coated components are subjected to external load, shear strength of the interface region can be the limiting factor on how mechanical load is transmitted from the substrate to the top coating.[12] Measurements of shear plastic flow in confined thin layers and correlation of flow stress to their structure and composition are therefore important for engineering improved coating/ adhesion-layer/substrate systems.
The dominant experimental means for determining yield strength of thin metal films at present is instrumented nanoindentation. However, mechanical property evaluation by indenting films on substrates encounters difficulties at small film thickne
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