Nano scale creep and the role of defects

The modulating force method in nanoindentation gives a direct measure of contact stiffness, and being insensitive to drift, allows the accurate observation of creep in small indents to be carried out over long time periods. We present results for a range of metals at room temperature. Strain rate indices similar to those for macroscopic creep are found. Reverse creep occurs for step unloading greater than about half the starting load. For electropolished tungsten, we find quite different behaviour before and after the sudden pop-in. Afterwards, creep is as in other metals, but beforehand, it is essentially zero. The slight changes of stiffness observed at the very smallest loads are due to diffusion of adsorbed surface films into the contact zone. Our results show that the dislocations nucleated and multiplied at pop-in provide the mechanism of creep.