Small-scale mechanical characterization of viscoelastic adhesive systems

Adhesive materials are important in a wide range of engineering applications, including replacing mechanical joining techniques and adhering small micro-electrical-mechanical system (MEMS) devices. Adhesion is quantified as the amount of energy required to separate two surfaces per unit area, typically by performing large destructive mechanical tests.  However, as the scale of devices using adhesives decreases, new techniques for characterising adhesives must be developed. Indentation is one such technique, which is an easily repeatable, non-destructive, depth-sensing technique that can be performed across multiple length-scales.  For a time-dependent adhesive material, such as polymers, there are at least ten experimental parameters for a single indentation test that can affect the measure of adhesion.  This dissertation aims to examine the effect of these experimental parameters on time-dependent adhesive materials and relate the results back to typical large mechanical tests.  A systematic parametric study is performed on two cross-linking densities of polydimethylsiloxane in which the indentation system setup, length-scale and testing rates are independently considered.  A relationship is found between the length-scale of the test and the measured adhesion. The relationship between testing rates is shown to be complex as the load-, hold- and unload-times individually influence the measured adhesion.  An analytical model is developed to account for the effect of the time-dependent material properties on the measured value for adhesion, which is compared back to the fundamental surface energy.  A small-scale destructive testing technique is used to validate the indentation results. In this dissertation it is shown that lowering the cross-linking density of polydimethylsiloxane increases the measured work of adhesion, which is attributed to an observed change in deformation mechanism to allow for larger deformations to occur.