Nanoindentation and quartz crystal microbalance based tribometer supports simple model for friction

Contact Area vs. Load

Traditional pin-on-disk tribometers allow evaluation of tribopairs at macro-scale, while Atomic Force Microscopy and similar techniques obtain the information at nanoscale. The microscopic range, however, is rarely described due to its complexity.

Recently, researchers from St. Olaf College in USA developed a new technique capable of performing tribological measurements at microscopic scale. The authors called the machine a ‘probe-QCM’ tribometer, and it consists of a nanoindenter and a quartz crystal microbalance. The former is used to apply the load, while the latter to perform measurements of resonance properties of the crystal. These measured quantities further transformed into real contact area and lateral force vsalues.

Probe-QCM tribometer was employed to explore the relationship between the real contact area, load and to analyze a simple friction model:

 F_{friction} = A_{real}*\tau

where  F_{friction} is the friction force,  A_{real} is contact area and  \tau is the shear stress. This equation is frequently associated with adhesion theory of friction, suggesting that the shear stress is developed by the adhesion forces. The equation itself, however, does not imply any specific nature of the shear stress forces.

By measuring the contact area at various contact loads, researchers obtained the value of the shear stress and found that this value is almost constant in a wide range of the applied pressures (120 to 860 MPa). This finding confirms a linear relation of friction force to the real contact area in nanoscale contacts.

As discussed by the authors of the research, the main advantages of the probe–QCM technique are the sensitivity to small contacts, high sliding speeds relevant for applications, and the simplicity to measure friction and contact area.

Further details can be found in the original article: Brian P. Borovsky, Christopher Bouxsein, Cullen O’Neill, Lucas R. Sletten, An Integrated Force Probe and Quartz Crystal Microbalance for High-Speed Microtribology, DOI https://doi.org/10.1007/s11249-017-0933-6.

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