Size effect in contact compression of nano- and microscale pyramid structures

An electrochemical etching approach was developed to fabricate self-similar nano- and microscale pyramid structures on single-crystal gold (1 0 0) surfaces. Using their unique self-similar characteristics, pyramids of (1 1 4) facets were compressed to study the length scale effects in the contact pressure and plastic deformation. At first, many pyramids were compressed simultaneously with a flat mica sheet to measure the ridge angle changes of the deformed pyramids with respect to the sizes of the flattened area. The ridge angle changes were scattered between approximately 2° and 13° for compression displacements of 50–350 nm, in contrast to the perfect plasticity prediction of −4.7°. Then, individual pyramids isolated with a focused ion beam were compressed with a flat tip nanoindenter for displacements of approximately 10–100 nm to obtain the relationship between the contact pressure and the compression depth. The plastic deformation-adjusted contact pressure evaluated by taking into account the initial 6–14 nm roundness offset of the pyramids is characterized by an initial increase up to approximately 2.5 GPa for a shallow compression depth within 10 nm followed by a gradual decay to approximately 450 MPa at a compression depth of 100 nm. This pressure seems to be still decaying towards an asymptotic value predicted by a continuum limit analysis. Given the size and self-similar nature of the pyramids, various mechanisms could possibly contribute to the observed scale dependence. The current study provides valuable experimental evidence for size-dependent material behavior at small length scales.