On the mechanistic origin of nanoscale “cold welding”

When nano-objects are brought together into contacts without any applied load at or near room temperature, a solid-state bonding can be accomplished either instantaneously or within some time, which has been termed by many recent works as “cold welding” as opposed to the traditional fusion welding. Such phenomena are more often encountered at nanoscale and have been utilized to build nano-devices such as nanowire-based interconnects or electrodes. Clearly, cold welding could be merely driven by the elastic adhesive contact, the analysis of which is straightforward but does not involve a time dependence. Room temperature, nanoscale, and contact size growth might suggest the operation of the curvature-driven sintering mechanism, or result from our proposed interfacial diffusional process in this paper as driven by a stress gradient (i.e., Coble creep). Here, we fabricated silver nanowires (AgNWs) with radius of several tens of nanometers, placed them into interconnect-like contacts, and measured the contact size growth at selected temperatures up to 200 °C. Out of the two hypothetical mechanisms (sintering versus Coble), only the latter mechanism can compare favorably with our experiments and result into physically meaningful parameters such as the activation energy and stress gradient magnitude. Consequently, this work presents the first conclusive study for nanoscale cold welding, as being the Coble diffusional creep.