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Super Hardness of Nano-Gold

Super Hardness of Nano-Gold Pushing the limits of computational and experimental capabilities, IMNI researchers in the Solid Mechanics group within Brown’s Division of Engineering and their collaborators at the University of Washington and General Motors have shown experimentally and via simulation that pure gold, perceived to be one of the softest metals, can be extremely strong at the nanoscale. Their work is reported in a June 2009 issue of the Proceedings of the National Academy of Sciences in a publication entitled: “Size-Scaling of Plastic Deformation in Nanoscale Asperities” by DK Ward, WA Curtin, D Farkas, J Lian, J Wang, K-S Kim, Y Qi.

Many experiments in recent years have shown that plastic flow in metallic materials depends on the volume of material being tested, with smaller sizes being stronger, but the origins of this effect remain elusive, especially in the nanoscale regime below 1 micron (one millionth of a meter). This new collaborative work uses large-scale molecular dynamics methods to simulate crushing of gold nanopyramids up to 40nm x 40nm on the truncated top surface, while performing experiments on real gold nanopyramids as small as 60nm x 60nm. The simulations show that the strength of gold can reach 4-10 GPa, stronger than the strongest steels, and that the strength decreases with increasing contact area in a characteristic manner. The experiments reach strengths of 2-3 GPa, and match the strengths predicted by the parallel modeling effort.

Fig.1. Hardness of Gold nanopyramids as a function of the contact edge length.  Simulations show a characteristic scaling with contact size, reaching hardnesses of 4-10 GPa below 10nm.  Experiments show a similar size scaling, with hardness reaching 2-3 GPa at sizes below 100nm.

 

According to senior Brown authors Bill Curtin and K-S Kim, an important application of this work is in understanding and controlling the wear and fatigue degradation that occurs between surfaces of engineered materials in contact. All surfaces are rough at multiple scales, culminating at the smallest scale with asperities or “bumps” that can be on the order of nanometers in height and/or lateral dimensions. Thus, the macroscopic behavior of rough surfaces under normal loads and under sliding conditions is determined by the nanoscale asperity contacts and their deformation. This work demonstrates that the asperity strength is size-dependent, and identifies physical mechanisms controlling that strength.

Figure 2:  Views of the simulated atomic structure after various depths of compression; a) the first deformation is the injection of dislocations from the contact edges. b,c) injection continues, along with interactions among the dislocations that impedes their motion and makes subsequent injection difficult, giving rise to the size scaling of the hardness.   Colors are assigned based on the number of neighbors of an atom, so that red ~ surface atoms, yellow ~ stacking fault atoms, green ~ partial dislocation core atoms.