New Insights in adsorption processes of lubricant additives

Since the initial studies by Spikesin the 80s, adsorption mechanisms of lubricant oil additives gained significant attention, as can be seen by a total of approximately 20 publications in 2018 (see e.g. [1,2]).The presented theories are mostly based on ex-and post-mortem studies or simulations. Here, we present in-situ adsorption studies, using aquartz crystal microbalance with dissipation QCM-D(e.g. [3,4]).The QCM-D is a nanogram-sensitive technique to study adsorbed amounts (from frequency information) and viscoelastic properties (from dissipation) of theadsorbed molecules. Fig. 1 shows an exemplary frequency and dissipation shift during adsorption of a friction modifier from mineralbase oil. The frequency decreases after the media change from base oil to base oil + additive, which indicates a mass increase (adsorption). The dissipation increases after the medium change, so theadditive coating shows an increased viscoelasticity compared to thepure base oil. Interestingly, by changing the medium back to pure base oil the frequency decreases and the dissipation slightly increasesagain. ... mehrTo understand this phenomenonan additional method is needed. TheQCM-D gives no information exceeding quantitative adsorption and viscoelasticity. To describe the coatingmore precisely and also in a qualitative waywe used an imaging method to make the adsorption processes,as seen via QCM,visible. The combination of both methods gives interesting new insights in the behaviorof lubricant oil additiveson surfaces.

ACKNOWLEDGMENTS
The authors acknowledge the assistance of Daniel Bahl and Sven Janson.

REFERENCES
[1] Spikes, H. A. (1989): Additive-additive and additive-surface interactions in lubrication. In: Lubr. Sci. 2 (1).
[2] Bernat, S.; Armada, S.; Espallargas, N. (2018): Friction Mechanisms by Carboxylic Acids in Aqueous Lubricants. In: Tribology Letters 66 (3), S. 83. DOI: 10.1007/s11249-018-1035-9.
[3] J. Honselmann, M. Dienwiebel, T. Rühle, T. Umbach, W. Grabarse (2018): “Correlation of adsorption behavior and performance of corrosion inhibitors in synthetic base oils”. GETLUB 5. Internationaler Schmierstoffkongress. Hamburg, Forschungsvereinigung Antriebstechnik e.V.
[4] K.O. Evans (2010): “Quartz crystal microbalance investigation of the structure of adsorbed soybean oil and methyl oleate onto steel surface,” In: Thin solid films, v. 519, no. 2, pp. 900-905