Thin metal films deposited on polymer substrates are widely used in flexible electronics applications as conductive lines and electrodes. Since the substrates are usually much thicker than the films and carry the major part of the applied load the failure of a functional film cannot be clearly deduced from a stress-strain curve. Thus, additional in-situ monitoring methods are required to gain the information about mechanically induced structural changes of polymer-supported thin films.
In this presentation an overview of mechanical behavior of polymer-supported films will be given using the monotonic and cyclic tensile tests with in-situ monitoring of electrical resistance. The resistance of brittle films (ITO, Cr, Mo) under monotonic tensile load clearly indicates the strain at which the first cracks are induced. In the case of ductile films the resistance follows the constant volume approximation and may grow insignificantly even at high strains depending on film thickness and grain size. Under cyclic loading conditions three distinct types of resistance response will be demonstrated and analyzed. During fatigue testing, the growth of resistance with the cycle number clearly indicates the propagation of through-thickness cracks. The decrease in resistance with the cycle number can be observed in nanocrystalline and UFG films if dynamic mechanically-induced grain coarsening occurs. Minor resistance increase is observed even at relatively high cyclic strains if the plastic deformation appears in the form of surface roughening, but without clear through-thickness cracks. Finally, the effect of crack re-bridging during the unloading segments of each cycle can be clearly detected by resistance measurements. To gain a quantitative relationship between electrical resistance and parameters of crack pattern simulations of electric current flow in quasi-3D material with cracks were performed. In-situ resistance measurements during mechanical testing of a thin film are shown to be a powerful tool which allows one to capture the topological changes of a thin film in a fast and reproducible way.