Electrochemical analysis of strain-induced crack formation of bilayer barrier plasma polymer films on metal and polymer substrates

Barrier performance and crack formation of amorphous silicon a-Si(O, H)/diamond-like carbon (a-C:H) bilayer thin films were studied under uniaxial stretch forming. The plasma polymer films were prepared on bare and PET film coated stainless steel substrates to enable the investigation of the influence of different substrate mechanics on the crack formation. In-situ electrochemical impedance spectroscopy (EIS) results indicated that film thickness reduction and onset of crack formation depend on the substrate mechanics. The reduced residual compressive internal stresses on the PET coated metal substrate caused film failure as early as at 3% strain. On the contrary, for the bare metal substrate, first cracks were observed at 8% strain, typically at defect sites. Moreover, it was demonstrated that for such a bilayer film system, individual layers can fail independently, indicating the importance of local stress fields.