Stick-slip in the scratching of styrene-acrylonitrile copolymer
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Stick-slip in the scratching of styrene-acrylonitrile copolymer Kangjie Li Department of Mechanical Engineering, Materials Science Program, University of Rochester, Rochester, New York 14627
Beta Yuhong Ni Eastman Kodak Company, Rochester, New York 14652
J. C. M. Li Department of Mechanical Engineering, Materials Science Program, University of Rochester, Rochester, New York 14627 (Received 20 January 1995; accepted 14 February 1996)
Stick-slip process occurred during the scratch test of styrene-acrylonitrile copolymer. For the first time, a bamboo-like morphology of the scratch track corresponding to the stick-slip phenomenon was observed. A “joint” was formed during the stick stage and during slip a uniform “stem” was made. The period and amplitude of the stick-slip both increase with the vertical load and decrease with the driving speed. A theoretical model is constructed based on the stiffness of the system and the plastic deformation of polymer both in the vertical and horizontal directions. The model assumes no distinction between the coefficients of static and kinetic friction and gives quantitatively consistent results with experiments.
I. INTRODUCTION 1
Scratch testing is fundamental to the understanding of grinding, polishing, and wear with respect to material transport and removal. Usually in a scratch test, a hard object such as an indenter is pressed onto the material under load and moved to generate a groove over the specimen surface. The question is how the groove is formed and what is the relation between the shape of the groove and the loading characteristics. Stick-slip is the intermittent relative motion of two sliding surfaces against each other. Bowden and Leben,2 while studying the nature of kinetic friction between dry metallic surfaces, observed that the friction force was not constant during sliding, but had a sawtooth wave form and the sliding motion proceeded by jerks instead of a smooth path. The stick-slip motion is a typical friction-induced vibration observed at small sliding speeds,3–5 and it is generally undesirable since it may cause unwanted self-excited oscillations, serious wear of the components, fatigue damage, and positioning errors. During scratching, if the sample or the indenter is driven through elastic members, stick-slip phenomenon may be observed also; i.e., the sample and the indenter may stick together until the elastic forces become sufficient to jerk them loose and then sliding occurs until they become stick again when the elastic forces decrease to below certain limits. Phenomenologically, the most widely accepted cause for stick-slip is that the sliding system has a horizontal degree of freedom and static friction 1574
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J. Mater. Res., Vol. 11, No. 6, Jun 1996
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exceeds kinetic friction3 or the friction force decreases as the sliding velocity increases.6,7 The state variable friction laws,8,9 which take into account memory effects (displacement or rate dependen
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