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Nanoparticles (NPs) can be highly beneficial as additives to lubricating fluids, and the tribotronic response of charged NPs tuned by external fields represents an area of great technological potential. Tribotronic response, however, is expected to be highly size dependent, which represents a significant design challenge. To explore this issue, quartz crystal microbalance and cyclic voltammetry were employed to characterize nanotribological and electrochemical behavior of platinum–nanofluid interfaces formed by aqueous suspensions of different-sized negatively charged titanium dioxide (TiO2) NPs. Suspensions of 5, 40, and 100 nm NPs were all observed to reduced interfacial frictional drag forces upon introduction into pure water in zero field conditions, with reductions for the 40 nm NPs about twice those of 5 nm particles at comparable concentrations. Suspensions of 100 nm NPs produced even greater reductions, but rapidly precipitated from the suspension when left unstirred. NPs were also driven to and from Pt electrode surfaces by applying external electric fields with varying amplitudes and modulation frequencies. For electric fields of sufficient amplitude and duration, the 40 nm TiO2 nanosuspension exhibited tribological properties consistent with a reversible electrophoretic deposition of the NPs, accompanied by changes in the electrochemical attributes and increasing interfacial drag. The 5 nm NP properties were consistent with progressive reductions in interfacial drag forces at the NP–suspension interface linked to field-induced increases in concentration. [ABSTRACT FROM AUTHOR]

We report a study of the frequency response of AT and BT cut Quartz Crystal Microbalances (QCMs) to laser illumination. AT and BT cut QCMs have a similar response to mass adsorption but the opposite frequency response to stress levels, allowing the individual contributions to stress and mass desorption induced by the laser light to be quantitatively investigated. A detailed analysis of the transient thermal behaviors in the quartz crystals has also been performed, which yields stress levels that are consistent with the observed results. Our results quantitatively confirm previous suggestions that radial compressive stress is a dominant contributor to the response of the QCM to laser illumination. They also establish a theoretical framework for the analysis of the transient phenomenon to infer temperature shifts and radial stress levels in the system irrespective of whether both AT and BT cut crystals are utilized. [ABSTRACT FROM AUTHOR]

This Resource Letter provides a guide to the literature on the fundamental origins of friction. Books, reviews, and journal articles are cited for the following topics: History of friction and tribology, the adhesion theory of macroscopic friction, first principles' treatments of frictional energy dissipation at the atomic level, experimental methods for studying friction at the atomic-scale, stick-slip phenomena and lattice commensurability effects. [DOI: 10.1119/1.1484153]

The contract mechanics occurring at the interface between a probe tip and quartz crystal microbalance (QCM) surface are investigated. It reflects the elasticity of the interface and the number and average size of contact regions.

Accurate measurements of the vibrational amplitude of a transverse shear mode quartz resonator were undertaken.

Electrical contact resistance measurements are reported for RF micro-electromechanical switches situated within an ultrahigh vacuum system equipped with in situ oxygen plasma cleaning capabilities. Measurements were performed on fused (permanently adhered) switches with Au/Au contacts and functioning switches with Au/RuO2 contacts in both the presence and absence of adsorbed monolayers of pentane and dodecane. For switches adhered in the closed position, adsorption occurs only in regions external to direct contact. For functioning switches, however, it can occur either within or exterior to the contact. The data are analyzed within the framework of two distinct geometries, to explore how the presence of adsorbed molecules in regions close to the contact may impact vacuum tunneling contributions to the experimentally measured resistance: (1) The resistance associated with direct contact in parallel with a vacuum tunneling path, which upon uptake of the monolayer is replaced by the molecular resistance and (2) a series connection of the direct contact resistance with the molecular layer after adsorption occurs, with the vacuum tunneling path assumed to be negligible. In all cases, the experimental results quantitatively favor scenario (1), whereby uptake of the molecular layer effectively shuts down the vacuum tunneling path, in this case approximately 30 Ω in the absence of an adsorbed film. The methods described herein thus constitute a new and original approach to documenting vacuum tunneling levels in regions of close proximity. [ABSTRACT FROM AUTHOR]

A series of experiments were performed in vacuum environments to investigate the impact of rf micromechanical system switch contact voltage versus resistance for gold-on-gold contacts at cryogenic temperatures. The purpose of this work was twofold as follows: (1) to examine whether asperity heating models already validated for high temperature contacts were also applicable at cryogenic temperatures and (2) to explore the implications and validity of prior suggestions that contact temperatures between 338 and 373 K are high enough to dissociate adsorbed film and/or push them aside but low enough to prevent asperities from becoming soft and adherent. Measurements on two distinct switch types, fabricated at independent laboratories, were performed in the temperature range 79–293 K and for contact voltages ranging from 0.01 to 0.13 V. Contact resistance values at all temperatures were observed to be lower for higher contact voltages, consistent with the aforementioned asperity heating models, whereby increased contact currents are associated with increased heating and softening effects. In situ removal of adsorbed species by oxygen plasma cleaning resulted in switch adhesive failure. Switches that had not been cleaned meanwhile exhibited distinct reductions in resistance at contact temperatures close to 338 K, consistent with suggestions that films begin to desorb, disassociate, and/or be pushed aside at that temperature. [ABSTRACT FROM AUTHOR]

A variable temperature, ultrahigh vacuum atomic force microscope (AFM) was used to characterize interfacial friction for a single-asperity diamond contact on a diamondlike carbon (DLC) substrate over a nominal substrate temperature range of 90 to 275 K. Calibrated friction force measurements were obtained by analyzing lateral force hysteresis loops as a function of normal force. For sufficiently large normal forces, the lateral force was proportional to the normal force, and a friction coefficient μ could be identified. μ varied approximately linearly with substrate temperature, with μ=0.28 at T=90 K and μ=0.38 at 275 K. These results are compared to other recent variable temperature AFM friction measurements and to theoretical calculations based on the Tomlinson model. This comparison is obscured by large, experimentally uncontrolled temperature differences between the tip and the substrate which inevitably exist in conventional, variable temperature AFMs. A thermal model which can be used to quantitatively estimate these temperature differences is presented. [ABSTRACT FROM AUTHOR]

Contact resistance measurements are reported for radio frequency microelectromechanical system switches operating in an ultrahigh vacuum system equipped with in situ oxygen plasma cleaning capabilities. Ru-based contacts were prepared by means of standard sputtering techniques, sputtering followed by postdeposition oxidation, (surface RuO2) or reactive sputtering in the presence of oxygen (bulk RuO2). In situ oxygen plasma cleaning lowered the resistance of Ru contacts by two or more orders of magnitude but not lower than Au contacts, irrespective of whether the Au contacts were cleaned. The time dependence of the resistance was fit to power law extrapolations to infer contact creep properties and resistance values at t=∞. Time-dependent creep properties of mixed Au-Ru contacts were observed to be similar to those of Au-Au contacts, while the absolute value of the resistance of such contacts was more comparable to Ru-Ru contacts. Prior to, and for short oxygen plasma exposure times, bulk RuO2 resistance values exhibited much larger variations than values measured for surface RuO2. For O2 plasma exposure times exceeding about 5 min, the bulk and surface RuO2 resistance values converged, at both t=0 and t=∞, with the t=∞ values falling within experimental error of theoretical values predicted for ideal surfaces. The data strongly support prior reports in the surface science literature of oxygen plasma induced thickening of oxide layers present on Ru surfaces. In addition, they demonstrate that vacuum alone is insufficient to remove contaminants from the contact surfaces and/or prevent such contaminants from reforming after oxygen plasma exposure. [ABSTRACT FROM AUTHOR]

It is shown that measured and calculated time-dependent electrical resistances of closed gold Ohmic switches in radio frequency microelectromechanical system (rf-MEMS) devices are well described by a power law that can be derived from a single asperity creep model. The analysis reveals that the exponent and prefactor in the power law arise, respectively, from the coefficient relating creep rate to applied stress and the initial surface roughness. The analysis also shows that resistance plateaus are not, in fact, limiting resistances but rather result from the small coefficient in the power law. The model predicts that it will take a longer time for the contact resistance to attain a power law relation with each successive closing of the switch due to asperity blunting. Analysis of the first few seconds of the measured resistance for three successive openings and closings of one of the MEMS devices supports this prediction. This work thus provides guidance toward the rational design of Ohmic contacts with enhanced reliabilities by better defining variables that can be controlled through material selection, interface processing, and switch operation. [ABSTRACT FROM AUTHOR]