Stick-slip is commonly ascribed to a difference between the static and dynamic coefficients of friction or at the least a strong negative slope of the friction/velocity curve at very small relative speeds. Other types of variable sliding have been ascribed to irregularities in local friction coefficients, to the excitation of resonance frequencies in oscillatory systems, and to local heating at contact spots. In the course of ongoing studies of metal-metal friction and contact resistance in the hoop apparatus, stick-slip, smooth sliding, somewhat disturbed harmonic oscillations, and a novel type of sliding dubbed “negative stick slip” have been examined. The coefficient of friction (μ) is again found to drop with decreasing ambient pressure, and the velocity dependence of μ determined directly from smooth sliding in vacuum shows an unexpected steep rise at low velocities. Both of these observations are in contradiction with the adhesive model of wear. Negative stick-slip suggests a still more complex μ(νrel) dependence which, however, appears to be compatible with the measurements. At any rate, negative stick-slip cannot be explained by any previously proposed models.

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