Strain Ratio Effects In Mechanical Properties Of Supported Thin Films

Metallic thin films deposited onto polymeric substrates find important technological applications as stretchable microelectronic devices. Academic research on these systems has mainly focused on uniaxial tensile tests, though biaxial stress states are generally applied during service. We study in this article the strain ratio effect on nanocrystalline 50nm-thick gold thin films supported on a polyimide: equibiaxial applied strain vs uniaxial applied strain. The mechanical behaviors are compared with digital image correlation, x-ray diffraction, and relaxation tests, giving the true and lattice strains, insights into strain heterogeneities, and leading plasticity mechanisms. Moreover, two load-unload cycles were performed to deconvolute the different parts contributing to the signals. It is shown that though the samples were transversely isotropic (i.e., isotropic in the plane of the applied strains), the work-hardening properties are very different, the micro-plastic regime being greatly reduced in the equibiaxial test. This is illustrated with the elastic limits and with the fact that an equibiaxial test presents at equivalent von Mises strain higher stress relaxation and a decrease in strain heterogeneities.