23rd International Colloquium Tribology - January 2022 275 Influence of Humidity on Graphite Lubrication: the Road to Turbostratic Carbon Carina Morstein Karlsruhe Institute of Technology KIT, IAM-CMS Institute for Applied Materials - Computational Materials Science, MicroTribology Center µTC, Karlsruhe, Germany Fraunhofer-Institute for Mechanics of Materials IWM, MicroTribology Center µTC, Freiburg, Germany Corresponding author: carina.morstein@kit.edu Andreas Klemenz Fraunhofer-Institute for Mechanics of Materials IWM, MicroTribology Center µTC, Freiburg, Germany Martin Dienwiebel Karlsruhe Institute of Technology KIT, IAM-CMS Institute for Applied Materials - Computational Materials Science, MicroTribology Center µTC, Karlsruhe, Germany Fraunhofer-Institute for Mechanics of Materials IWM, MicroTribology Center µTC, Freiburg, Germany Michael Moseler Fraunhofer-Institute for Mechanics of Materials IWM, MicroTribology Center µTC, Freiburg, Germany University of Freiburg, Institute of Physics, Freiburg, Germany 1. Introduction Solid lubricants are used in applications where conventional liquid lubricants reach their limits. One of the well-known solid lubricants is graphite, showing distinct lubrication properties in normal atmosphere but severe wear and friction in vacuum. Currently, two main explanations for the good lubrication behavior of graphite are present: The first one is the deck-of-cards-model postulated by Bragg et al. [1], according to which the graphene layers are easily sheared against each other due to the weak Van-der-Waals-forces between layers. However, this model does not explain the humidity dependence of graphite’s lubrication properties and has already been disproven by experiments, e.g. X-ray diffraction [2]. The second model is the “adsorption model” postulated by Savage [3]. He suggested that water adsorbs as a layer on top of graphite and thus acts as a boundary lubrication film. One has to mention that both models have been mainly investigated under low loads and on surfaces with a nanoscale roughness. This work aims to investigate the graphite lubrication mechanisms and humidity dependence under high loads and on technical surfaces. 2. Experimental Procedure For friction and wear experiments, polished iron plates were coated with a graphite dispersion using an airbrush spray gun. In a microtribometer, the samples were tested against a small steel sphere in a linear-reversible fashion for 500 cycles. The normal force was 402 mN, resulting in a high Hertzian pressure of 1 GPa. To investigate humidity influence on friction and wear, the experimental chamber was continuously flooded with pressurized air, which was humidified to the desired values (between 0.24-44% rH). After the experiments, the wear volume and thus wear coefficient was quantified by confocal microscopy. For the investigation of structural changes and sp 2 content of the graphite, detailed scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), as well as electron energy loss spectra (EELS) analyses were conducted before and after the experiments. 3. Results and Discussion 3.1 Friction and Wear Generally, an influence of the humidity on the steadystate coefficient of friction (COF) is observed even at the high loads investigated in this work. With an increase in humidity from 0.24% to 24%, a slight decrease in friction from μ = 0.14 to 0.10 is observed, followed by a plateau at μ ≈ 0.10 for all higher humidities. Comparable literature experiments with graphite or graphene often do not specify the exact environmental conditions the experiment has been conducted in. However, when summarizing different publications on graphite and graphene lubrication, typical values for the COF vary between 0.08-0.25 [4-8]. Evidently, the friction yielded with the 276 23rd International Colloquium Tribology - January 2022 Influence of Humidity on Graphite Lubrication: the Road to Turbostratic Carbon presented graphite lubrication lies on the lower end of this range. When analyzing the wear coefficient of the plate, a parabolic trend is observed, with the highest value yielded with dry pressurized air (RH = 0.24%) and with highly humidified air (RH = 44%). At intermediate humidity, the wear coefficient shows only little variation and is an order of magnitude smaller. At high humidity, the rapid material transport could be due to the formation of capillary necks between the asperities of the tribopartners. This would result in an additional normal force component and consequently, an increase in adhesion of the graphite to the steel sphere. Hence, graphite transportation during sliding will be observed due to easier delamination of the coating from the substrate. The high wear coefficient at low humidity (dry pressurized air) can be explained with beginning “dusting” mechanisms, which are known to dominate the wear of graphite in vacuum [7]. 3.2 Structural changes For an analysis of the graphite structure, two TEM lamellae were prepared: one in the middle of the wear track after an experiment and one in an unworn area as a reference point. HR-TEM analysis of the unworn reference sample revealed that the graphite coating is a 3.5 µm thick, porous layer built out of randomly oriented graphene bundles. The content of sp 2 -hybridized carbon was analyzed with EELS measurements and determined to be at 98% for the whole layer. Hence, the airbrush coating process yields purely graphitic carbon. After sliding wear, the structure of the graphite coating is distinctly different: a gradient in regards of the sp 2 -content has developed. At the iron substrate, the content of sp 2 -hypridization is 77%, whereas at the sliding interface it is only 70%. This observation is also visible in the HR-TEM images themselves; at the iron substrate interface, the graphene layers are parallel to each other and to the substrate. At the sliding interface, the graphene bundles completely disappeared and a turbostratic structure is observed instead. In conclusion, shearing does not take place between the individual graphene layers at the top of the coating. If this was the case, a high sp 2 content and parallel graphene layers would be observed at the sliding interface. Instead, shearing at these high loads takes place in the bulk of the material, leading to the formation of turbostratic carbon. This contradicts both the deck-of cards-model and the adsorption model. As a result, a new model mechanism is needed for the lubrication mechanism under high loads. The experimental data hints to the fact that low friction with graphite under high load is achieved due to the shear-induced formation of turbostratic carbon. 4. Conclusion In summary, our results have shown that neither the popular deck-of-cards model nor the adsorption model is suitable for the explaination of the tribological properties of graphite under high loads. Sliding of individual graphene layers might be true for experiments under low loads with a basal plane as sliding interface. In technical graphite coatings as presented in this work, a gradual decrease of sp 2 -content is observed towards the sliding interface. This demonstrates that shear takes mainly place within the graphite layer, leading to a shear-induced formation of turbostratic carbon. References [1] Bragg, W. H.: An introduction to crystal analysis, (G. Bell and Sons, Ltd., London, 1928). [2] Arnell, R. & Teer, D. Lattice Parameters of Graphite in relation to Friction and Wear, Nature 218, 1155-1156 (1968). [3] Savage, R. H. Graphite Lubrication, Journal of Applied Physics 19, 1-10 (1948). [4] Berman, D., Erdemir, A., Sumant, A. V., Reduced wear and friction enabled by graphene layers on sliding steel surfaces in dry nitrogen, Carbon 59, 167-175 (2013). [5] Berman, D., Erdemir, A., Sumant, A. V., Few layer graphene to reduce wear and friction on sliding steel surfaces, Carbon 54, 454-459 (2013). [6] Bowden, F. P. and Young J. E., Friction of diamond, graphite, and carbon and the influence of surface films, Proc. R. Soc. Lond. A 208, 444-455 (1951). [7] Lancaster, K. K. and Pritchard, J. R., On the ‘dusting’ wear regime of graphite sliding against carbon, Journal of Physics D: Applied Physics, 13, 1551 (1980). [8] Buckley, D. H.; Johnson, R. L., Mechanism of Lubrication for Solid Carbon Materials in Vacuum to 10 −9 Millimeter of Mercury, ASLE Transcations 7, 91-100 (1964).