Friction and adhesion in boundary lubrication measured by microtribometers

The measuring and modelling of friction are critically important for the motion control in nanopositioning, particularly when bearings are employed to cover the wide working distances. Since the positioning system usually operates at very low speed to achieve fine positioning, the boundary lubrication is the dominant regime. A detailed characterization of the friction of boundary lubrication formed by Poly–α–Olefin (PAO) with and without surfactant and a suspension of MoS₂ in base oil has been performed in reciprocating sliding tests by steel/steel point contacts, and correlated with adhesion measurements by silicon/silicon point contacts. A microtribometer based on laser interferometers and glass springs, which can resolve 100 nN force in a speed range of 1–1000 μm/s was employed to detect the minute changes in forces. We find that a simple linear function instead of a logarithmic function is possible to describe the relationship between the friction force and operating speed for all the lubricants tested, though the gradients are quite different and under the influence of normal load. Comparing to PAO+surfactant and MoS₂ suspension, PAO shows a much higher load-dependent coefficient of friction. This result is further confirmed by the repulsion force measurements, which shows a higher increase of contact pressure with the increase of normal load for PAO.