High-Speed Friction Measurements Using a Modified Surface Forces Apparatus

Methods of measuring friction forces in the surface forces apparatus (SFA) are presented for sliding velocities from 10 m/s. A feed-forward control (FFC) system for the piezoelectric bimorph slider attachment is introduced to allow experiments at velocities up to ~4 mm/s. For still higher speeds, a motor-driven rotating mini-disk setup using a pin-on-disk geometry is presented, with modifications to enable sliding velocities in the ranges 1 cm/s-5 m/s and 1-25 m/s. Example data sets demonstrate the applicability of the approach to modeling important tribological systems including hard-disk drives. We find that mechanical system parameters such as the resonant frequencies and mutual alignments of different moving parts become increasingly important in determining the tribological response at sliding velocities above ~1 cm/s (for SFA or bench top devices). Smooth or stick-slip sliding--common features of low-speed sliding--become replaced by large-amplitude oscillatory responses that depend on the load and especially the driving speed or rotational/reciprocating frequencies. Detailed recordings and modeling of these complex effects are necessary for fully understanding and controlling frictional behavior at high speeds.