Gaylord Guillonneau

Micromechanics of Amorphous Metal/Polymer Hybrid Structures with 3D Cellular Architectures: Size Effects, Buckling Behavior, and Energy Absorption Capability

By designing advantageous cellular geometries and combining the material size effects at the nanometer scale, lightweight hybrid microarchitectured materials with tailored structural properties are achieved. Prior studies reported the mechanical properties of high strength cellular ceramic composites, obtained by atomic layer deposition. However, few studies have examined the properties of similar structures with metal coatings. To determine the mechanical performance of polymer cellular structures reinforced with a metal coating, 3D laser lithography and electroless deposition of an amorphous layer of nickel-boron (NiB) is used for the first time to produce metal/polymer hybrid structures. In this work, the mechanical response of microarchitectured structures is investigated with an emphasis on the effects of the architecture and the amorphous NiB thickness on their deformation mechanisms and energy absorption capability. Microcompression experiments show an enhancement of the mechanical properties with the NiB thickness, suggesting that the deformation mechanism and the buckling behavior are controlled by the brittle-to-ductile transition in the NiB layer. In addition, the energy absorption properties demonstrate the possibility of tuning the energy absorption efficiency with adequate designs. These findings suggest that microarchitectured metal/polymer hybrid structures are effective in producing materials with unique property combinations.

Orientation-dependent mechanical behaviour of electrodeposited copper with nanoscale twins

The mechanical properties of electrodeposited copper with highly-oriented nanoscale twins were investigated by micropillar compression. Uniform nanotwinned copper films with preferred twin orientations, either vertical or horizontal, were obtained by controlling the plating conditions. In addition, an ultrafine grained copper film was synthesized to be used as a reference sample. The mechanical properties were assessed by in situ SEM microcompression of micropillars fabricated with a focused ion beam. Results show that the mechanical properties are highly sensitive to the twin orientation. When compared to the ultrafine grained sample, an increase of 44% and 130% in stress at 5% offset strain was observed in quasi-static tests for vertically and horizontally aligned twins, respectively. Inversely strain rate jump microcompression testing reveals higher strain sensitivity for vertical twins. These observations are attributed to a change in deformation mechanism from dislocation pile-ups at the twin boundary for horizontal twins to dislocations threading inside the twin lamella for vertical twins.

Is the second harmonic method applicable for thin films mechanical properties characterization by nanoindentation

The second harmonic method is a dynamic indentation technique independent of the direct indentation depth measurement. It can be used to determine near-surface mechanical properties of bulk materials more precisely than classical dynamic nanoindentation. In this paper, the second harmonic method is extended to the measurement of the mechanical properties of thin poly(methyl methacrylate) (PMMA) layers deposited onto silicon wafers. It is shown that this new technique gives precise results at small depths (less than 100 nm), even for films with a thickness lower than 500 nm, which was not possible to achieve with the classical continuous stiffness measurement method. However, experimental and numerical results obtained both with classical nanoindentation and second harmonic methods differ at high indentation depth. Using finite element (FE) simulations and AFM measurements, it is shown that the contact depth calculation with classical models can explain this difference.