Plastic deformation dominates chemical reactions in Ti/Si multilayered nanofilms

The purpose of the study was to clarify the exothermic chemical reaction mechanisms in Ti/Si multilayered nanofilms under mechanical loading. We conducted in situ compression experiments of truncated-cone specimens of polycrystalline-Ti/amorphous-Si multilayered nanofilms (bilayer thickness of ~ 34 nm) deposited by electron beam evaporation within a scanning electron microscope. The true stress increased almost linearly with increasing true strain and the tangent modulus began to decrease at ~ 3 GPa. Transmission electron microscopy of the deformed specimens confirmed that each layer was plastically compressed in the stacking direction and expanded in the in-plane direction, resulting in an increase in the Ti/Si interface area. Selected-area electron diffraction analysis revealed that a new crystal structure, proposed to be Ti5Si4 and/or TiSi, was generated on the Ti/Si interface and within the Ti layer. In addition, the volume of the specimens decreased with increasing strain, supporting the hypothesis of a chemical reaction occurring. The chemical reaction was induced at the new reactive Ti/Si interface by the partial fracture of preexisting compound layers due to tensile stresses in the in-plane direction, and/or induced by diffusion-induced mixing through the thinned compound layers. These findings present the possibility of controlling the chemical reaction by local mechanical loading. The observed exothermic reaction can be used for various applications, such as local heating in large-scale micro- and nanodevices.