24th International Colloquium Tribology - January 2024 51 In-Operando Formation of Transition Metal Dichalcogenides - Instant Lubrication by Simple Sprinkling of Se Nano-Powder onto Sliding Contact Interfaces Philipp G. Grützmacher 1* , Maria Clelia Righi 2 , Ali Erdemir 3 , Carsten Gachot 1 1 Institute for Engineering Design and Product Development, Tribology Research Division, TU Wien, Vienna, Austria 2 Department of Physics and Astronomy, Alma Mater Studiorum − University of Bologna, Bologna, Italy 3 J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA * Corresponding author: philipp.gruetzmacher@tuwien.ac.at 1. Introduction There are many situations in which liquid lubricants come to their limits, e.g., for space applications [1]. In these cases, the attention turns to solid lubricants, such as graphite or MoS 2 . They have been used by industry for many years due to their excellent lubricating properties stemming from a layered structure. In the layers the atoms are bonded by strong bonds, while between the layers mostly act weak van der Waals forces. If these materials are subjected to shear forces the layers slide easily over each other, resulting in low friction. However, a major drawback of solid lubricants is their short wear life [2]. If solid lubricant layers are worn out their protective function ceases and friction and wear increases drastically. Additionally, the performance of many solid lubricants depends highly on the environmental conditions. Graphite works best in humid conditions, whereas MoS 2 shows lowest friction in dry environment [2]. In contrast, the in-operando formation of a solid lubricant has many advantages. If the lubricant is formed under operation conditions it is selectively formed at the locations where it is most needed, wear life in these cases is basically infinite, and the lubricant is formed only the environment where it is applied, thus reducing the environmental factor [3]. We describe an innovative and very unique approach that results in the formation of a solid lubricant in the form of a transition metal dichalcogenide (TMD), namely MoSe 2 or WSe 2 . The formation of the TMD is verified using Raman spectroscopy, X- ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (TEM). Furthermore, the mechanisms of the in-operando formation are unveiled by ab initio molecular dynamics simulations (AIMD). 2. Results and Discussion To observe the in-operando formation of TMDs, particularly MoSe 2 and WSe 2 , we performed ball-on-disk experiments in unidirectional, rotational sliding mode with an inert Al 2 O 3 counterbody at a load of 1 N and a sliding speed of 15 mm s -1 . The substrates were steel platelets that were coated by a 3-µm layer of either Mo or W. The chalcogen element was added to the sliding surfaces by simple sprinkling of a Se nano-powder (particle size distribution between 40 to 80 nm) before the start of the experiment. Without the application of the Se nano-powder, the coefficient of friction (COF) is relatively high with a steady-state COF after running in of 0.60 and 0.40 for the Mo and W substrates, respectively (Figure 1). This can be expected for non-lubricated sliding of a metal-ceramic friction couple. In contrast, the evolution of the COF changes drastically after adding the Se nano-powder to the sliding surfaces before the tests. While friction is comparable at the start of the tests, the COF continuously decreases over the course of the experiment. The final COF during the last 100 cycles of the experiment comes down to 0.09 and 0.13 for the Mo and W substrates with Se nano-powder addition, respectively. These COFs constitute low friction values, which are typically reached using fully formulated oils, which is remarkable considering the non-lubricated metal-ceramic sliding contact. Figure 1: Friction performance of the (a) Mo and (b) W substrates in unidirectional ball-on-disk experiments. Adapted from [3]. 52 24th International Colloquium Tribology - January 2024 In-Operando Formation of Transition Metal Dichalcogenides - Instant Lubrication by Simple Sprinkling of Se Nano-Powder onto Sliding Contact Interfaces It is worth mentioning that friction increases again if the experiments are continued for a longer time, which can be correlated with the degradation of the formed tribofilms under shear and load. However, sprinkling Se nano-powder again into the contact leads to the re-establishment of the lubricious layers and, hence, once more low friction. To verify that these substantial improvements in friction can be correlated with the in-operando formation of MoSe 2 and WSe 2 tribofilms TEM lamellae were prepared from the wear tracks of the substrates after tribological testing. The TEM analysis clearly shows continuous tribofilms on both surfaces with a thickness of 10 to 20 nm (Figure 2). In these tribofilms layered structures with inter-layer distances close to the ones corresponding to the (0 0 2) planes of hexagonal MoSe 2 and WSe 2 can be found. That these layered structures correspond to MoSe 2 and WSe 2 was further confirmed by Raman and XPS [3]. Figure 2: Transmission electron microscopy images of the tribofilms formed on the sliding interface of the (a,b) Mo and (c,d) W substrates. Adapted from [3]. Finally, our AIMD simulations unveiled the mechanisms responsible for the in-operando formation of the TMDs under sliding conditions. The results demonstrated that the formation of TMD layers from the Mo and W bulk constituents with Se nano-particles is highly favourable, associated with an energy gain of 2.0 eV and 1.1 eV per unit of MoSe 2 and WSe 2 , respectively. Furthermore, AIMD simulations under load and shear demonstrated the formation of TMD layers from metal and Se particles. First, bonds with trigonal prismatic coordination typical of TMDs are established between the metallic and Se particles. Then single metal atoms are detached from the metallic nano-particles and are surrounded and bond to Se atoms, leading to the complete disaggregation of the metallic nano-particle and finally, the formation of MoSe 2 layers. The simulations indicate that the formation of crystalline TMD layers during tribological loading of a Mo (W) substrate in presence of Se nano-powder. However, metallic nano-particles have to present for these reactions to happen. Extracting metal atoms from metal-oxide nano-particles or ideal Mo surfaces could not be observed. Therefore, we conclude that metallic particles are extracted from the surfaces thanks to wear-induced defects and debris, which then react with the Se nano-powder to form TMD layers. While, the direct reaction of metal-oxide nano-particles with the Se nano-powder seems not to be possible, the hard metal-oxide particles might accelerate wear of the surfaces, thus providing more metallic wear debris, which are then converted to the lubricious TMD layers. 3. Conclusion In this study, we have presented an innovative approach to form lubricious tribofilms based on MoSe 2 or WSe 2 TMD layers in-operando. The precursor for this formation, Se nano-powder, is simply sprinkled onto the sliding surfaces in ambient conditions. As a result, the COF is continuously decreasing down to 0.10 or even lower, levels typically reached only with fully formulated oils. The formation of the TMD layers is confirmed by TEM, XPS, and Raman spectroscopy. Ab initio molecular dynamics simulations give fundamental insight into the mechanisms of the TMD formation under sliding conditions. Our study clearly shows that the in-operando formation of highly lubricious TMD layers through tribochemical reactions between Mo (W) coatings and Se nano-powder is feasible. The approach by simple sprinkling of the Se nano-powder onto the sliding interface leads to highly reproducible results. This could be the solution for the low wear life of solid lubricant layers and is particularly interesting for maintenance-critical applications, where commonly used TMD coatings lead to component failure once they are fully worn. Additionally, our approach can inspire the in situ synthesis through tribochemical reactions of new compounds even different from TMD. References [1] Scharf, T. W., & Prasad, S. V. (2013). Solid lubricants: A review. Journal of Materials Science, 48(2), 511-531. [2] Zhang, S., Ma, T., Erdemir, A., & Li, Q. (2019). Tribology of two-dimensional materials: From mechanisms to modulating strategies. Materials Today, 26, 67-86. [3] Grützmacher, P. G., Cutini, M., Marquis, E., Rodríguez Ripoll, M., Riedl, H., Kutrowatz, P., Bug, S., Hsu, C.-J., Bernardi, J., Gachot, C., Erdemir, A., & Righi, M. C. (2023). Se Nanopowder Conversion into Lubricious 2D Selenide Layers by Tribochemical Reactions. Advanced Materials.