Actuation in shape memory layer engineered film based architectures

Abstract

The design opportunities of shape memory properties as they are downsized from bulk to thin film architectures are analysed, based on the observation that thin films deposited on substrate offer a wider range for smart engineering design. Films attached to a substrate show a two-way memory effect in bimorph architectures, due to a combined contribution of the film and the substrate. Such an actuation is not only reversible, but more stable in terms of actuation, compared to the one in bulk alloys and free standing films (i.e. removed from the substrate). In fact, bimorph and trimorph architectures relate the film to the substrate and since the phase transformation in the shape memory alloys is governed by stress, temperature and/or magnetic field, it follows that by adjusting the initial stress (for magnetic transition) and the initial as well as the supplementary thermal stress (for thermally controlled transitions), one could shape the actuation behaviour. Thus, the actuation control is based not only on the composition of the films but also on the relative thickness of the film(s) and substrate, the surface architecture of the film or the structural and compositional distributions of the films on the substrate. Experimental results show how the actuation response of bimorph and trimorph architectures with shape memory films deposited on both parts of the substrate can be modelled to respond to specific requirements, based on the combination of the depositions of the sides of the substrate. A larger number of variants can thus be engineered for particular applications in micro-opto-electro-mechanical systems.