ToF-SIMS investigation of MoS₂ after friction test in ultrahigh vacuum

G. Colas ^(1,2), A. Saulot ⁽²⁾, D. Philippon ⁽²⁾, Y. Berthier ⁽²⁾, D. Leonard ⁽³⁾

(1) Department of Mechanical and Industrial Engineering, University of Toronto, Ontario M5S 3G8, Canada

(2) Université de Lyon, CNRS, INSA – Lyon, LaMCoS UMR5259, F-69621, France

(3)Institut des Sciences Analytiques, Université de Lyon, CNRS, Université Claude Bernard-Lyon 1, 5 rue de la Doua, 69100 Villeurbanne, France

MoS₂ is a well-known lubricant for vacuum application and especially long life space applications. However, the role of contamination in achieving low friction and long wear life in industrial applications remains unclear. A former study [1] performed in ultrahigh vacuum (UHV) showed that in a sliding macro contact, the origin of low friction was primarily due to the formation of a 3^rd body layer, its trapping and its ability to flow plastically inside the contact. The study showed a homogenisation of the chemical composition and the internal coating contaminants throughout the contact and the 3^rd body. A second study [2] was performed in 4 environments (UHV, high vacuum, dry nitrogen, humid air) with the MoS₂ coating studied in the 1^st study but also with a MoS₂+Ti coating. It showed a similar homogenisation phenomenon (in UHV, dry nitrogen and humid air for MoS₂) but also some chemical selection, or even segregation, of chemical elements to create a bi-phasic like 3^rd body (in high vacuum for MoS₂, in dry nitrogen and humid air for MoS₂+Ti).

To further substantiate these conclusions, Time-of-Flight Secondary Ion Mass Spectrometry analyses were performed on PVD MoS₂ coatings after 3 tribological tests with exactly the same contact conditions but with different durations, i.e. after 3 cycles, 10 cycles and 150 cycles of friction, which were the main events and/or states of friction identified in previous work [1,2].

The analyses firstly showed that the MoS₂ coating is a complex Mo_xS_yO_z structure. Secondly, they showed that a chemical rearrangement of the material with the internal contaminants occurred in UHV and led to a specific MoS_xO_y phase. Finally, by comparing all real time measurements (friction forces, mass spectra, video) and post-test results (ToF-SIMS, but also SEM-EDS), the most probable tribologically induced chemical reactions were identified, confirming that internal contamination appears to be somewhat beneficial to friction in UHV on MoS₂.

[1] Colas G, Saulot A, Godeau C, Michel Y, Berthier Y. Wear 305 issues 1-2 (2013) 192-204

[2] Colas G, Saulot A, Bouscharain N, Godeau C, Michel Y, Berthier Y. Tribo. Int. 65 (2013) 177-189