| AVS 60th International Symposium and Exhibition | |
| Tribology Focus Topic | Tuesday Sessions |
| Session TR+SE-TuM |
| Session: | Low Friction Materials |
| Presenter: | C. Minfray, Ecole Centrale de Lyon - LTDS, France |
| Authors: | C. Minfray, Ecole Centrale de Lyon - LTDS, France C. Matta, Ecole Centrale de Lyon - LTDS, France S. Loehle, Ecole Centrale de Lyon - LTDS, France T. Le Mogne, Ecole Centrale de Lyon - LTDS, France J.M. Martin, Ecole Centrale de Lyon - LTDS, France R. Iovine, Total, Solaize Research Center, France Y. Obara, Tohoku University, Japan R. Miura, Tohoku University, Japan A. Miyamoto, Tohoku University, Japan |
| Correspondent: | Click to Email |
Organic friction modifiers (OFMs) have been used for many years in metallic contacts to reduce friction under mild conditions. The interest for such friction modifiers is still very strong because environmental regulations ask for slightly hazardous lubricant additives. It is proposed here to revisit adsorption and friction behavior of fatty acids by coupling experimental and computational chemistry approaches.
The OFM studied is stearic acid, used as pure additive or blended at 1%w in PAO4 synthetic base oil. The surfaces of interest are iron-based materials (including pure iron and iron oxides) as they can be encountered in a steel/steel contact under mild or severe friction conditions. Adsorption and friction properties of such systems were studied experimentally (friction, XPS and PM-IRRAS surface analyses) and adsorption kinetics was also studied by computational chemistry (UA-QCMD).
Adsorption experiments of stearic acid (gas form) on the different surfaces (gold, pure iron, iron oxides) were carried out in an environmentally controlled chamber for different adsorption time (10 min to 2 hours). This was followed by in-situ XPS analyses of the surfaces. XPS analyses results, especially the position of the C1s peak contribution from the carboxylic group, show differences depending on the nature of surfaces.
Computational chemistry study was also performed to investigate the in vacuo interaction of stearic acid molecules with the different model surfaces (pure iron, Fe2O3, and FeOOH). An ultra-accelerated quantum chemical MD (UA-QCMD) simulator has been used in order to deal with chemical reaction dynamics for large complex systems. Different models have been built starting with one single molecule up to self-assembled monolayer (SAM) on iron based surfaces at 50 °C. We found differences in adsorption energy (physisorption or chemisorption) depending on the kind of surface and regarding the presence of one single molecule or a SAM.
Eventually, friction experiments were carried out on a cylinder-on-flat reciprocating tribometer using samples in AISI52100 steel. The lubricant was stearic acid blended at 1%w in PAO4 and temperature was set to 50°C, 100°C and 150°C with an applied load of 50 N (corresponding to a maximum contact pressure of 320 MPa). After the tribological tests, PM-IRRAS analyses were conducted on the tribofilms. Results show the presence of stearic acid in the wear track as well as carboxylate forms.
All experimental and simulation results of both adsorption and friction experiments are discussed for a better understanding of complex interactions between stearic acid and iron-based surfaces.