We have studied the behavior of microwear and hardness of a superelastic nickel-titanium alloy using a triboindenter at various temperatures. Wear resistance was found to anomalously decrease with an increase in hardness. The observations are analyzed based on simple contact theory which suggests that the increase of hardness with the temperature is mainly due to an increase in phase transition stress, while the decrease of wear resistance with the temperature is due to an increase of the austenite elastic modulus and a decrease of the amount of phase transition that can be recovered.

Poly-12-hydroxystearic acid (PHSA) is widely used as a coating on colloidal spheres to provide a "hardsphere-type" interaction. These hard spheres have been widely used in fundamental studies of nucleation, crystallization, and glass formation. Most authors escribe the interaction as "nearly" hard sphere. In this paper we directly measure this interaction, using layers of PHSA adsorbed onto mica sheets in a surfaces force apparatus. We find that the layers, in appropriate solvents, have no long-range interaction. When the solvent is decahydronaphthalene (decalin), the repulsion rises from zero to the maximum measurable over a distance range of 15-20nm. The data is converted to equivalent forces between spheres of different diameters, and modeled using a hard core potential. Using zeroth-order perturbation theory and computer simulation, we demonstrate that the equation of state does not deviate from that of a perfect hard-sphere system under any relevant experimental conditions.

A device to study the friction of two molecularly smooth surfaces separated by an ultrathin liquid film is presented along with its design, calibration, and performance. The apparatus can move one of the surfaces and measure the friction force on the other one bidimensionally for both processes. A high mechanical impedance system (104N/m) measures continuous friction forces where only stick-slip was previously observed. The frequency and travel distance of the movement can be varied over a wide range (frequency from 10-4 to 7Hz and distance from1 to 800μm) to provide variations of the shear rate over seven orders of magnitude. The actual movement provided by piezoelectric bimorph drive can be affected by the friction forces and is measured by strain gauges. The friction forces are measured with an accuracy of ±2μN with a capacitance sensor. The mechanical design prevents the surfaces from rolling under force. The apparatus is tested with hexadecane. The potential applications of this apparatus and its limitations are discussed.

With a nano-indenter and a microhardness testing machine, nano-indentation hardness and microhardness are measured in a wide load range (0.1-19600mN) for five materials. Even fused silica and silicon almost have constant hardness during the load range, the nanoindentation hardness of copper, stainless steel and nickel titanium alloy shows obvious indentation size effect, namely that the hardness decreases with the increase of depth. For the measured materials, the nano-indentation hardness is about 10-30% in magnitude larger than the microhardness. The main reasons can be explained as the analysis of the nano-indentation hardness using the projected contact area at peak load Ac instead of the residual projected area Ar, as well as the purely elastic contact assumption describing the elastic/plastic indentation process. The analysis based on a simple model indicates that Ac is always smaller than Ar, and the more heavily the indent piles up (or sinksin), the larger the difference between the nano-indentation hardness and microhardness.

In this paper, we present our experimental results on the process of the in situ chemical modification of the silicon nitride atomic force microscopy/frictional force microscopy tip by n-octadecyltrimethoxysilane (OTE)/mica, OTE/SiO2, and SiO2. The modified tips have different friction and adhesion properties against mica reference samples compared to those before modification. The resultant tip modification depends not only on OTE self-assembled monolayer (SAM) but also on the substrates the OTE SAM is prepared on. In the case of OTE/mica, the friction of the modified tip against mica reference is greatly reduced; in the case of OTE/SiO2, the friction of the modified tip against mica reference is greatly increased. It is surprising that bare SiO2 can also chemically modify the Si3N4 tip to increase the friction against mica reference. In the case of OTE modification, it was found that the tips could be cleaned by repetitive friction scans on mica. However, a tip modified by SiO2 cannot be mechanically cleaned. Moreover, it was found that humidity and load could also affect the tip chemical modification. Ourresults are important for interpreting tribological data since the actual contact chemistry was often overlooked in the atomic force microscopy experiments in the past.