1 Copyright © 2012 by ASME SELF-ORGANIZATION AND FRICTION DURING SLIDING Pradeep L. Menezes 1, 3 , Kishore 1 , Satish V. Kailas 2 and Michael R. Lovell 3 1 Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India 2 Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India 3 Department of Industrial Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI 53201 ABSTRACT In self-organized sliding processes, the surfaces align to each other during sliding. This alignment leads to a more ordered contact state and significantly influences the frictional behavior. To understand the self-organization sliding processes, experiments were conducted on a pin-on-plate reciprocating sliding tester for various numbers of cycles. In the experiments, soft magnesium pins were slid against hard steel plates of various surface textures (undirectional, 8-ground, and random). Experimental results showed that the transfer layer formation on the steel plates increased with increasing number of cycles for all surfaces textures under both dry and lubricated conditions. The friction also increased with the number of cycles under dry conditions for all of the textures studied. During lubricated conditions, the friction decreased for unidirectional and 8-ground surfaces and increased for random surfaces with the number of cycles. Furthermore, the friction and transfer layer formation depend on the surface textures under both dry and lubricated conditions during the first few sliding cycles. Later on, it is less dependent of surface textures. The variation in the coefficient of friction under both dry and lubrication conditions were attributed to the self-organization process that occurred during repeated sliding. INTRODUCTION When two surfaces slide against each other, various phenomena take place at the interface. For example, surface roughness changes at the interface, transfer layer (i.e., secondary structures) builds up on the surfaces, and damage/shearing occurs on the surfaces. These processes depend on various tribological testing conditions. Friction and wear are irreversible processes which normally lead to material deterioration. Under certain situations, friction can lead to self- organization or alignment of surface profiles at the frictional interface. In the self-organized state, the surfaces adjust to each other, leading to a more ordered state and lower energy dissipation rates. In such a state, the surface structure evolves until it reaches a certain stationary state referred to as the equilibrium roughness distribution. This self-organization process leads to steady state friction and wear performance [1, 2]. Prior efforts have been made to study the self-organization process for surfaces [1] during sliding. The self-organization process was analyzed based on the change in roughness at the interface [1, 2]. The self-organization process during sliding as a function of surface texture is not well documented in the literature. The initial surface texture indeed plays an important role during the self-organization process. Moreover, the friction and transfer layer formation depends on initial surface texture [3, 4]. The formation of the transfer layer at the interface also contributes to the self-organization process. Hence, in this investigation efforts have been made to study the self- organization process as a function of surface texture during sliding. The self-organization process is very critical during running-in or repeating sliding contacts. One such example is in metal forming process where the sheet metal can alter the initial surface structure of the die during multiple sliding contacts. This can alter the friction values by more than 200% thus changing the initial surface structure of the die by the self- organization process. EXPERIMENTAL DETAILS To understand the self-organization process as a function of surface textures, three kinds of textures were produced on the steel plate surfaces. They are unidirectional, 8-ground and random. The unidirectional surfaces were prepared by grinding the plates against emery papers in a unidirectional fashion. The 8-ground surface was generated by moving the steel plate against emery papers along a path with the shape of an “8” for 500 cycles. For the unidirectional and 8-ground surfaces, the roughness was varied using different grits of empery papers. The random textures were generated on the steel plates by polishing the steel plate against the pad of a standard metallographic disc polishing machine. For the random surfaces, the roughness was varied using different abrasive powders. Experiments were conducted using a pin-on-plate reciprocating sliding tester [4]. In the experiments, the pins Proceedings of the ASME/STLE 2012 International Joint Tribology Conference IJTC2012 October 7-10, 2012, Denver, Colorado, USA IJTC2012-61219