The frictional behavior of rocks in the laboratory is reasonably well described by rate- and state-variable friction laws, which reproduce a rich variety of natural phenomena when used in models of earthquakes. Despite the widespread adoption of the rate and state formalism in earthquake mechanics, the physical mechanisms that occur at microscopic contacting asperities on the sliding surface, which give rise to the observed rate and state effects, are still poorly understood. In an attempt to identify these underlying mechanisms, a series of nanoindentation experiments on quartz, an abundant mineral in the earth’s crust, was conducted. These experiments demonstrate the utility of using continuous stiffness measurements as a means of obtaining reliable indentation creep data on hard materials like quartz at room temperature. The projected area of indentation in quartz increases linearly with the logarithm of the time of indentation, in agreement with the increase in real area of contact with log time inferred from slide-hold-slide friction experiments on quartz rocks. However, the increase in fractional area with time in the indentation tests was larger than that inferred from friction experiments by a factor of 1.7. Differences between the rates of fractional area increase in the two tests may indicate that the increase in contact area during the hold portion of slide-hold-slide tests was modulated by slip that occurs during reloading after the hold, as was observed for other materials. The nanoindentation results suggest that the increase in frictional strength (i.e., the increase of state in the rate- and state-variable friction laws) during slide-hold-slide friction experiments was caused by creep of the highly stressed asperity contacts.

Recent experiments with the National Institute of Standards and Technology (NIST) Electrostatic Force Balance (EFB) have achieved agreement between an electrostatic force and a gravitational force of 10−5 N to within a few hundred pN/μN. This result suggests that a force derived from measurements of length, capacitance, and voltage provides a viable small force standard consistent with the Système International d’Unités. In this paper, we have measured the force sensitivity of a piezoresistive microcantilever by directly probing the NIST EFB. These measurements were linear and repeatable at a relative standard uncertainty of 0.8%. We then used the calibrated cantilever as a secondary force standard to transfer the unit of force to an optical lever–based sensor mounted in an atomic force microscope. This experiment was perhaps the first ever force calibration of an atomic force microscope to preserve an unbroken traceability chain to appropriate national standards. We estimate the relative standard uncertainty of the force sensitivity at 5%, but caution that a simple model of the contact mechanics suggests errors may arise due to friction.

Details of the elastic–plastic transitions in crystalline compound semiconductors have been examined using spherical indentation. Two cubic (InP and GaAs) and two hexagonally structured semiconductors (ZnO and GaN) have been studied. A series of indentations have been made in each material at a number of different loads. The resulting load–penetration curves exhibited one or more discontinuities on loading (so called pop-in events). The load at which the initial pop-in event occurred has been measured along with the corresponding indenter extension. The elastic and elastic–plastic response of each material to spherical indentation has been calculated and compared with the experiment. By taking the difference between the elastic and elastic–plastic penetration depths, it has been found that the pop-in extension at each load could be predicted for each material. The detailed deformation behavior of each of the materials during indentation has also been discussed.

An atomic force microscope was used to measure the loss tangent, tan δ, of a pressure-sensitive adhesive transfer tape as a function of frequency (0.01 to 10 Hz). For the measurement, the sample was oscillated normal to the surface and the response of the cantilever resting on the polymer surface (as measured via the photodiode) was monitored. Both oscillation amplitude and phase were recorded as a function of frequency. The atomic force microscopy measurement gave the same frequency dependence of tan δ as that measured by a dynamic shear rheometer on a film 20 times thicker. The results demonstrate that the atomic force microscope technique can quantitatively measure rheological properties of soft thin polymeric films.