Simultaneous in situ measurements of contact behavior and friction to understand the mechanism of lubrication with nanometer-thick liquid lubricant films

A high-performance microscopic observation system was developed and integrated with a purpose-built pin-on-disk type tribotester. This allows for simultaneous in situ measurements of the vertical displacement and friction force of a pin sliding on disks lubricated with nanometer-thick liquid films at velocities up to 0.1 m/s, with accuracies of approximately 0.6 nm and 10 μN. Upward pin displacement was observed, exhibiting an exponential increase followed by a slight increase with increasing sliding velocity. The pin displacement also increased with film thickness and lubricant viscosity. We conclude that even nanometer-thick liquid lubricant films can generate upward dynamic pressure. Further insight into the shearing process was gained by analyzing the measured friction forces using the Eyring thermal activation energy approach.