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The making and breaking of nanometer-scale contacts is an essential operation in MEMS devices with moving parts. The behavior of contacts in this size range is not well understood, especially if viscoelastic materials are involved. This article describes shear modulation spectroscopy, a new scanning force microscope technique especially well suited for quantitative studies of nanometer-scale contacts to viscoelastic materials such as lubricants and some polymers. The technique is illustrated by measurements and analysis of contacts to poly(vinylethylene)., The original document contains color images. Sponsored in part by the Department of Energy.

Friction is usually treated as a two-body problem, in which the two counterfaces move against each other and a "magical" parameter the friction coefficient comes into being. Not so. At some scale, from atomically thin surface films to chunks of wear particles, third bodies play an important role in friction. These third bodies are often born in the sliding contact and sometimes growing up to be wear particles. They might come about because the tribologist intended to lubricate one or both counterfaces, or they might arise simply from atmospheric gases. Either way, they play a far more important role in friction and wear than you would gather from treatments of friction and wear found in the literature. For the past fifteen years, our studies at the Naval Research Laboratory have focused on sliding behavior of "low wear" coatings and surface treatments in concentrated contacts. We have been interested mainly in "how" films transfer to the stationary counterface and "what" compositions and phases of films and third body particles form. Friction and wear tests have been carried out at relatively low speeds, typically 0.1 - 100 mm/s, with sphere-vs.-flat geometries at high normal contact stresses, 0.5 - 1.5 GPa, in unidirectional or reciprocating sliding. Surface topography, chemistry and microstructure are characterized before and after wear tests and, more recently, by in-situ and in-vivo studies; in the latter, we have focused our sights on the contact itself, watching third bodies form and move in the contact and using Raman microscopy to identify compounds in the sliding interface. Although original films wear away, subsequent films can grow if the environment provides a replenishing material (e.g., oxygen) or the sliding contact replenishes transfer films by forming third body reservoirs.

Tribological coatings of yttria stabilized zirconia (YSZ), gold, diamond like carbon (DLC) and MoS2 were synthesized using magnetron assisted pulsed laser deposition (MSPLD). The coatings were synthesized in four-component and three-component combinations that included YSZ/Au/DLC/MoS2, YSZ/Au/MoS2, and YSZ/Au/DLC. A range of coating compositions was studied to explore coating optimization for low friction in varying environments (dry, humid and high temperature). For four-component YSZ/Au/DLC/MoS2 coatings, the optimal compositions for friction adaptation between dry nitrogen and humid air included relatively high concentrations of the soft phase, Au (>20 at. %), and low amounts of the hard phases, DLC and YSZ. Ex situ Raman spectroscopy analysis indicates that friction adaptation involves a combination of both lubricating species, MoS2 and carbon where transitions of DLC to graphitic-carbon and amorphous MoS2 to its hexagonal phase occurs after cycling between both room temperature humid air and dry nitrogen. In large carbon concentrations (>30 at. %), the DLC component was found to be detrimental for friction in dry nitrogen and humid air, but promoted a longer coating wear life at 500 deg C. The three-component coating of YSZ/Au/MoS2 performed well in both dry nitrogen and humid air, suggesting a synergism between Au and MoS2, where carbon was not necessary for lubrication in humid air.