In this article we report the influence of surface oxides and relative humidity on the nanomechanical response of hydrophobic and hydrophilic Si surfaces. Depth-sensing nanoindentation combined with force modulation enabled measurement of surface forces, surface energy, and interaction stiffness prior to contact. Several regimes of contact were investigated: pre-contact, apparent contact, elastic contact, and elasto-plastic contact. Both humidity and surface preparation influenced the surface mechanical properties in the pre- and apparent-contact regimes. Meniscus formation was observed for both hydrophobic and hydrophilic surfaces at high humidity. Influence of humidity was much less pronounced on hydrophobic surfaces and was fully reversible. In the elastic and elasto-plastic regimes, the mechanical response was dependent on oxide layer thickness. Irreversibility at small loads (300 nN) was due to the deformation of the surface oxide. Above 1 μN, the deformation was elastic until the mean contact pressure reached 11 GPa, whereby Si underwent a pressure-induced phase transformation resulting in oxide layer pop-in and breakthrough. The critical load required for pop-in was dependent on oxide thickness and tip radius. For thicker oxide layers, substrate influence was reduced and plastic deformation occurred within the oxide film itself without pop-in. Elastic modulus and hardness of both the oxide layer and Si substrate were measured quantitatively for depths <5 nm.

For submicron-scale mechanical property measurements, depth sensing nanoindentation techniques are very successful and gaining much attention. However, for ultra-small volumes of materials below a length scale of 10 nm, measuring the quantitative mechanical properties of materials is still a problem. The atomic force microscope (AFM) has very good surface sensitivity and has been shown to measure nanomechanical properties. However cantilever instability, conventional force detection and displacement sensing make contact area measurement difficult, hence the measured mechanical properties are usually only qualitative. In this article, we show that combining force modulation with depth sensing nanoindentation allows measurement of the mechanical properties of materials on the nanometer scale. With this technique we have studied the role of oxide layers on the mechanical response of Si surfaces. We also present a novel quantitative stiffness imaging technique, which can be used to directly map the mechanical properties of materials with submicron lateral resolution.

The AC force modulation technique in nano indentation has been extended to study the dynamic viscoelastic properties of polymer materials. The frequency and temperature dependent loss and storage modulus, the time-temperature superposition, the mechanical relaxation and the activation energy for the relaxation process can all be directly measured from nano scale contact. The results obtained for PMMA and Polyisoprene are in good agreement with the reported literature results obtained from conventional dynamic testing. The technique is likely to be of particular value for polymer thin films.

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.