Influence of residual stresses of sputtered thin film electrodes for dielectric elastomer applications

This paper focuses on the electromechanical properties of novel sub-micron compliant metallic thin film electrodes for dielectric elastomer membranes. Electrodes with thicknesses between 10-20 nm and different residual stress states are explored. Either pure nickel films or sandwiches of nickel (Ni) and carbon (C) are deposited by DC magnetron sputtering onto pre-stretched silicone elastomer membranes. Both, 37.5 % biaxial pre-stretch and 57.5% pre-stretch under pure shear condition (PSC) are considered in the conducted investigation. After the coating process is completed, the elastomer is allowed to relax. In the contracted configuration, it exhibits a wrinkled surface. After this state is reached, the electromechanical characterization is performed. In the conducted experiments, all types of films reveal a low initial resistance (around 100 Ω/square). Depending on the kind of pre-stretch and the electrode material, a strain of 100 % without any major degradation is achieved. It is also shown how the residual stress of the layers can be influenced by suitable sputtering parameters. As a result, low residual film stress significantly improves the electromechanical properties of PSC pre-stretched elastomers, but have only minor influence on the biaxially pre-stretched ones, regarding the Ni and the Ni+C thin films. This phenomenon is directly connected to the failure mechanisms observed on the two types of pre-stretched membranes. For C+Ni electrodes, the residual stress state of Ni does not influence the electromechanical properties for both, the biaxially pre-stretched and the PSC pre-stretched coated membranes. Nevertheless, the results are of fundamental importance for understanding the role of residual stresses for the creation of electromechanically stable and highly conductive electrode films, to be used in DE applications.