Insights into sliding wear and friction behavior of copper in ethanol containing alkylphosphonic acid molecules

In this work, the friction and wear behavior of bare copper was investigated for the first time under lubricated sliding conditions in diluted ethanol solutions of butylhosphonic (C4P), octylphosphonic (C8P), dodecylphosphonic (C12P), and hexadecylphosphonic (C16P) acids. The technique aims towards a more environmentally friendly lubrication to be used in shaping of copper sheets. Bare copper samples were subjected to unidirectional sliding using a tribometer with ball-on-disk contact geometry. Copper substrates (20 mm(2) x 1 mm) were run against 100Cr6 empty set10 mm ball bearing counterbodies. All tests were conducted using the same sliding conditions with a normal load of 10 N, tangential velocity of 0.01 m/s, at room temperature of 20 degrees C. Worn surfaces were analyzed by Scanning Electron Microscopy, Optical Microscopy and White Light Interference Profilometry. When comparing to sliding tests in the pure ethanol solvent, significant decreases in terms of wear track dimensions, transferred material on the ball and friction coefficients are observed when active molecules are present in the solution. These form protective tribofilms exhibiting lubricating and anti-wear properties. Deeper studies on the tribological behavior of copper in C4P solution show that both low friction and low transfer of work material to the ball prevail in a specific range of low molecule concentration (5 x 10(-4) M; 25 x 10(-4) M). Even if the molecules are introduced during the test, after a few cycles, the tribological behavior improves, regardless of both friction level and copper surface degradation. Finally, specific friction tests were performed to further investigate the mechanisms. It was found that two mechanisms are involved; firstly molecules grafting onto the surface directly reduces friction, and secondly transformation of these grafted molecules into a tribofilm during the first mechanical contact cycles reduces it even further. (C) 2015 Elsevier Ltd. All rights reserved.