Significance of the normal degree of freedom and natural normal vibrations in contact friction

Abstract

An apparatus has been devised for studying contact friction and self-excited frictional vibrations at constant driving velocities ranging from~1−10 −9cm/sec. The essential features of the apparatus are a total absence of vibrations due to the actuating mechanism and the possibility of damping both natural and self-excited vibrations. It has been shown experimentally that both the negative friction-velocity slope and the frictional self-excited vibrations are closely associated with the freedom of normal displacements of the slider. Whenever the latter are absent the force of contact friction becomes practically independent of the relative velocity of sliding and the self-excited vibrations disappear. Direct interferometric observations have shown that the tangential “darts” of the slider in the process of self-excited vibrations are invariably accompanied by simultaneous upward jumps so that stick-slip sliding is a succession of bumps rather than mere forward darts. Self-excited vibrations in sliding vanish in two cases: (1) at velocities lower than a certain critical value below which the force-velocity curve acquires a positive slope and (2) when the normal vibrations are damped by external means. The value of the critical velocity may be roughly estimated in terms of the creep viscosity of the rubbing solids, their yield strength, the friction coefficient and both the height and spacing of the surface asperties. The sliding friction oscillograms obtained with and without normal vibration damping have revealed some facts which prove the existence of natural normal microvibrations whose frequency is determined by the contact stiffness and mass of the slider. It has been shown that these microvibrations strongly affect both the magnitude of the frictional force and the stability of sliding. Special interferometric measurements have been undertaken to show the high sensitivity of the frictional force to normal slider displacements. The same method has enabled the contact stiffness to be determined as a function of the normal slider displacement. The existence of the normal natural contact microvibrations of the slider and their influence on the frictional force suggests the possibility of sharply reducing friction by low-power forced vibrations at resonance frequency. This has been checked experimentally with the aid of Rochelle salt vibrators glued on the upper surface of the slider. The friction-frequency curves invariably showed a distinct resonance minimum, the percentage friction drop at resonance frequency ranging between 35 and 85%.