| AVS 61st International Symposium & Exhibition | |
| Tribology Focus Topic | Friday Sessions |
| Session TR-FrM |
| Session: | Applications of Novel Materials in Tribology |
| Presenter: | NityaNand Gosvami, University of Pennsylvania |
| Authors: | N.N. Gosvami, University of Pennsylvania J.A. Bares, BorgWarner Powertrain Technical Center F. Mangolini, University of Pennsylvania A.R. Konicek, ExxonMobil Research and Engineering A.M. Schilowitz, ExxonMobil Research and Engineering D.G. Yablon, ExxonMobil Research and Engineering R.W. Carpick, University of Pennsylvania |
| Correspondent: | Click to Email |
Zinc dialkyldithiophosphates (ZDDPs) are lubricant additives used nearly universally in engine oils. Despite the generation of volatile phosphorous- and sulphur-containing compounds in the downstream gases that can reduce the working life of the catalytic converter, the unrivaled wear protection of ZDDPs makes them essential to lubricant performance. ZDDPs work by decomposing under tribological sliding to form nanoscale anti-wear films whose growth mechanisms are still poorly understood due to the complexity of the macroscopic multi-asperity sliding interfaces and the multiple chemical species involved (1). Greater understanding of the formation of these films is essential to enable rational design of more environmentally-friendly and energy-efficient engine oil formulations (2, 3). Here we report the development and application of a novel experimental approach using atomic force microscopy (AFM) for visualizing and quantifying the formation of ZDDP anti-wear films in-situ in a single asperity contact with nanometer-scale spatial resolution. Experiments performed on iron-coated silicon surfaces at 80-140 °C in the presence of ZDDP containing polyalphaolefin oil show that thermal films grow on the substrate in the absence of tribological contact. These films are easily removed by sliding the tip at applied normal forces of only a few nanonewtons (contact pressure < 1.0 GPa). Continued sliding at higher normal loads (contact pressure ~ 2.0 -6.0 GPa) reveals the nucleation and growth of much more robust films with a pad-like lateral structure, similar to the morphology of anti-wear films formed by ZDDP in macroscopic contacts. The growth rate is nonlinear with time, and increases exponentially with temperature and contact pressure, in agreement with reaction rate theory. This is the first direct confirmation of asperity-level formation of such films, and the first quantification of the energetics of the tribofilm growth. Our findings provide new insights into the mechanisms of formation of ZDDP derived anti-wear films, enabling us to directly compare with atomistic predictions of pressure-induced cross-linking of zinc polyphosphates (4) and other possible proposed mechanisms (1).
(1) H. Spikes, Tribology Letters 17 (2004) 469.
(2) H. Spikes, Lubrication Science 20 (2008) 103.
(3) H. A. Spikes, Lubrication Science 20 (2008) 77.
(4) N. J. Mosey, M. H. Muser, and T. K. Woo, Science 307 (2005) 1612.