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15XV International Symposium on Self-Propagating High-Temperature Synthesis
GAS ENVIRONMENT INFLUENCED TRIBOLOGICAL BEHAVIOR OF TMD COATINGS
A. V. Bondarev*" and T. Polcar"
aCzech Technical University in Prague, Karlovo namesti 13, 121 35, Prague, Czech Republic *e-mail: andrei.bondarev@fel.cvut.cz
DOI: 10.24411/9999-0014A-2019-10021
Transition metal dichalcogenides (TMDs) are intrinsic solid lubricants with hexagonal layered structure and weak Van der Waals bonding between basal planes in crystallographic lattice, which provide low shear stress values under sliding conditions and associated with a low friction coefficient. Since friction and wear processes are mostly depends on surface properties, the TMDs can be considered as a surface modifier for tailoring of optimal tribological performance by deposition of thin coatings. The TMD coatings have been widely used in tribological applications where low friction coefficient is highly desirable, meanwhile presence of oxygen or/and water vapors in the ambient environment significantly reduce their tribological performance. In addition, low load-bearing capacity of pure TMDs coatings can cause their failure during operations. Subsequently, to produce coatings with enhanced mechanical and tribological properties TMDs have been combined with different additives to obtain composite structure, for instance with Ti, Zr, Cr, Au, and Pb, as well as non-metallic compounds, such as polymers C, N and F or even with multicomponent systems. While mechanical properties go up when nanocomposite structure is formed, the sensitivity to environment is also can be reduced but it is still the problem even for complex compositions of coatings. For tribotesting in controlled environment, air with different humidity, dry argon, dry nitrogen or even vacuum were utilized mostly. But published results indicate that still there are splits in over leading factor affects degradation of TMDs lubricity, it is ambient moisture-sulfur bonding or oxidation process. Behind this discussion about reasons of TMDs degradation exposed to different environment one more curious observation was misplayed - it was found that friction behavior of Mo-Se-C coatings in even oxygen and moisture free environments can be influenced by inert atmosphere. This phenomenon was also detected for carbon-based coatings and, moreover, it was reported that friction coefficient in vacuum was higher in comparison with inert gas atmosphere, which means physisorption and chemisorption processes, changes in surface energy or even intercalation of gas atoms in material structure have measurable impact on friction processes. Another point is that gas atoms can be located in cages in the amorphous TMD coatings and their release in molecular form during friction test also influence on tribological properties. But gas-involved tribological processes are still almost fully under-explored field of material science and goal of present work is to uncover phenomena occuder in different environments during friction of TMD-based coatings using advanced characterization and simulation methods. The TMD-based coatings were deposited by RF magnetron sputtering in Ar and in Ar + N2 atmosphere. Pin-on-disk tribological tests were performed in ambient air, vacuum, argon and nitrogen environment. For characterization of coatings, wear tracks and wear debris SEM, XRD, TEM, Raman and XPS were utilized, MD simulations were performed to highlight interaction of gases and solid lubricants during friction processes.
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A. V. Bondarev and T. Polcar
