Analysis of strength and response of polymer nano thin film interfaces applying nanoindentation and nanoscratch techniques

Abstract

Polymer and hydrogel thin films on a compliant substrate are gaining significant attention these days in the fields of soft electronics, digital display, biomedical devices and sensors to name a few. The functional characteristics of these thin films depend on the performance of its interface with the substrate. The interface response is a non-linear function of surface energy and stiffness of a substrate beside other factors. The deviation of thin film properties under confinement from its bulk counterparts and its ultra-low physical dimension makes characterization of an interface extremely challenging. Probing and shearing of these interfaces with appropriate load value and up to the critical load rate can provide insight into the integrity of these interfaces under different service conditions. We have studied the response of PMMA thin film (150 nm) on a softer epoxy substrate and a stiffer glass substrate of increasing interfacial strength applying nanoindentation (NI) and nanoscratching. Pile up behaviour of the film material at confined state, the critical load at the interface proximity and the plastic work done during indentation as determined in this work can be successfully applied to characterize interface behaviours. These parameters also help in developing interface models and build qualitative correlations between thermodynamic work of adhesion and true mechanical work of adhesion at the interface. We hypothesize that the elasto-plastic region around load tip plays a significant role in determining pile up above film surface. We have discussed the possible molecular mechanisms in this region under stress leading to pile up.