Indenter Tip Dependence in the Determination of Elastic Modulus in Polymers

The two most common outputs of nanoindentation experiment are hardness and elastic modulus. Length scale dependent deformation in polymers has however been observed in different experiments such as microbeam bending, torsional thin wires and indentation testing which may affect the mechanical testing. Unlike in metals where the size dependency is attributed to necessary geometry dislocations, the origin of length scale dependent deformation in polymers is not well understood. In this study, elastic modulus of polydimethylsiloxane (PDMS) is determined using both Berkovich and spherical tips. Observing different trends for elastic modulus upon the change of indentation depth using these two different tips brings up the question which tip should be used to get the real mechanical properties of PDMS which is discussed here. Surface roughness, surface effects and the imperfection of the Berkovich indenter tip are negligible at the studied length scale.

Length Scale Dependent Deformation in Polymers

Length scale dependent deformation of polymers has been observed in different experiments including micro-beam bending and indentation tests. Here the length scale dependent deformation of polydimethylsiloxane is examined in indentation testing at length scales from microns down to hundreds of nanometers. Strong indentation size effects have been observed in these experiments which are rationalized with rotation gradients that can be related to Frank elasticity type molecular energies known from liquid crystal polymers. To support this notion additional experiments have been conducted where Berkovich and spherical indenter tips results have been compared with each other.

Direct Observation of Crack Propagation in Copper–Niobium Multilayers

The failure of a cross-sectional 65 nm-thick copper and 150 nm-thick niobium multilayer thin film was investigated via an in situ transmission electron microscopy straining experiment. The fracture of the free-standing multilayer films was associated with confined dislocation slip within layers containing and preceding the crack tip. Four crack hindrance mechanisms were observed to operate during crack propagation: microvoid formation, crack deviation, layer necking, and crack blunting. Failure was observed to occur across and through the copper and niobium layers but never within the interfaces or grain boundaries. These results are discussed relative to the length-scale-dependent deformation mechanisms of nanoscale metallic multilayers.