NATURE OF STATIC FRICTION TIME-DEPENDENCE

Magnetic storage systems, like disk-head and tape-head interfaces, machine tool guideways, and many other friction pairs, are vulnerable to so-called 'stiction'. Stiction is a contact phenomenon whereby static friction increases relative to kinetic friction. As a rule, this effect increases dramatically with time of stationary contact. Thus, the time dependence of static friction is an important subject of tribological studies. The motivation of this work is to understand the static friction mechanism at both extremes of the rest time or dwell range: short-term dwell is in the order of seconds or minutes, and long-term dwell is in the order of days or months. The stiction models reviewed cover a wide spectrum of static friction processes related to material deformation, meniscus formation and viscous lubricant flow. A complex stiction mechanism is proposed that includes all of the above as related to slider perpendicular micro-displacements. At rest the slider descends onto the counter surface; at that time, the liquid-solid bond enhancement and meniscus formation take place. At start-up, the slider has to ascend back, and the enhanced molecular force constitutes a perpendicular component of the friction force. The resultant excess of the static friction force over the kinetic one is due to both molecular (adhesion) and mechanical (deformation) processes. The driving force for the slider micro-descent could be either mostly external or mostly internal. The latter case takes place in a magnetic disk-slider interface, where long-term stiction is caused by the adhesion enhancement because of a lubricant inflow. A theoretical model of the processes is developed. Measurements of the friction and adhesion forces, as well as of the slider micro-displacements and the capacitance of a stationary contact, were done on various testers with various materials and lubricants. They confirmed the theoretical model convincingly.