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Bowen, James
(2021).
Abstract
This work reports the development of a new method for measuring tribological properties at the nanoscale. An atomic force microscope (AFM) has been used to continuously measure the friction between two orthogonally crossed cylinders over a 2 mm displacement. A human hair fibre was used as the lower cylinder, while the upper cylinder was a 0.5 mm section of 75 μm diameter tungsten wire.
Measurements were performed at a sliding velocity of 20 μm/s, the compressive normal load between the cylinders was 600 nN, and data were recorded every 2 nm. Topography and lateral load were recorded throughout the bidirectional measurement, with differential friction observed upon comparison of the ‘with cuticle’ and ‘against cuticle’ sliding directions.
To assess the influence of sliding velocity on the frictional behaviour a sequence of increasing velocities in the range 0.01-1.0 mm/s was programmed; the total displacement of this measurement covered 5 mm. The mean friction coefficient did not vary significantly with sliding velocity, although occasional high lateral loads were observed at velocities in excess of 0.1 mm/s.
These measurements were achieved through the use of novel cuboidal AFM cantilevers with spring constants in the range 100 to 10,000 N/m. The cantilevers offer mechanical stability when large probes, such as spheres or cylinders, are attached to their free end. This new series of cantilevers permits AFM adhesion, indentation, and tribological testing with normal loads approaching 1 mN. The combination of these cantilevers with the displacement sensitivity of the AFM represents a unique methodology for biotribology research. Applications include personal care systems, dental tribology, and systems involving highly compliant and/or highly adhesive contacts.
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