Nanomechanics of thin films and surface

The nanoindentation and imaging capabilities of the interfacial force microscope (IFM) were used to explore how near surface and thin film mechanical properties correlate with microstructure and stress distributions (which are in turn determined by processing conditions). For single crystal Au samples, the critically resolved shear stress at yield was found to vary from 2 +/- 0.2 GPa for defect free regions, to 0.9 +/- 0.2 GPa in the presence of dislocations (intermediate values were recorded near grain boundaries and surface steps). The magnitude of the load-relaxation accompanying the onset of plasticity was found to scale with the strain energy stored in the material surrounding the contact immediately prior to yielding. Images and load-displacement curves demonstrate that yielding occurs in two stages: the first results in atomic scale permanent deformations while the second is characterized by large (greater than 10%) load relaxations and results in the pileup of material around the indenter. The images demonstrate that the onset of pileup is characterized by terraces raised in multiples of 0.25 nm above the initial surface and bounded by (111) planes. For 100 nm thick Au films, the measured elastic response was found to be reversibly dependent upon applied stress. Measurements of thin film elastic response appear to reflect a composite elasticity comprised of both bond and defect compliance. Measurements of the plastic deformation of individual grains in these films demonstrate that grain boundary sliding can accommodate plasticity at room temperature. Lower yield thresholds (2 GPa) were observed for intragranular deformation than for intergranular deformation (7 GPa). The application of tensile stress to the thin films was observed to favor grain boundary sliding. This work clearly demonstrates that once film thickness and deformation volumes shrink below 1 mum, a material's deposition and processing conditions (instead of the composition) become the principal determinate of the mechanical properties. These studies also highlight the utility of nanoindentation as a technique for measuring the mechanical properties on the nanometer scale.