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Tribochemical competition within a MoS2/Ti dry lubricated macroscale contact in ultrahigh vacuum: a Time of Flight Secondary Ion Mass Spectrometry investigation Guillaume Colas, Aurélien Saulot, David Philippon, Yves Berthier, and Didier Leonard ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02999 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018
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ACS Applied Materials & Interfaces
Tribochemical competition within a MoS2/Ti dry lubricated macroscale contact in ultrahigh vacuum: a Time of Flight Secondary Ion Mass Spectrometry investigation Guillaume Colas 1, Aurélien Saulot 2, David Philippon 2, Yves Berthier 2, Didier Léonard 3 * 1
Department of Mechanical & Industrial Engineering University of Toronto, 5 King’s College road, Toronto, ON M5S3G8, Canada 2 Univ Lyon, CNRS, LaMCoS UMR5259, INSA-Lyon, F-69621 Villeurbanne, France 3 Univ Lyon, CNRS, Université Claude Bernard Lyon 1, ENS de Lyon, Institut des Sciences Analytiques, UMR 5280, 5, rue de la Doua, F-69100 Villeurbanne, France *
Corresponding author:
[email protected] Tel: +33 437 42 35 54 Abstract Controlling and predicting the tribological behavior of dry lubricant is a necessity to ensure low friction, long life, and low particle generation. Understanding the tribochemistry of the materials as a function of the environment is of primary interest as synergistic effects exist between the mechanics, the physicochemistry, the thermodynamics within a contact. However, in most studies the role of the coating internal contaminants in the process is often discarded to the benefit of a more common approach in which the performances of the materials are compared as a function of different atmospheric pressure environments. The study focuses on the understanding of the tribochemical processes occurring between the materials and their internal contaminants inside a AISI440C contact lubricated by a MoS2/Ti coating. Time-of-Flight Secondary Ion Mass Spectrometry is used to study at the molecular level, the material before and after friction. Friction tests with different durations are performed in ultrahigh vacuum at the macroscale to stay relevant to the real application. The adsorption/desorption of gaseous species during friction is monitored by mass spectrometry to ensure reliable study of the tribochemical processes inside the contact. The study shows that a competition exists between the Ti and MoS2 based materials to create the appropriate lubricating materials via (i) recrystallization of MoS2 materials with creation of a MoSxOy material via reaction with internal contaminant (presumably H2O), (ii) reaction of Ti based materials with internal contaminants (mostly H2O, N2). The bi-phasic material created is highly similar to the one created in both humid air and dry N2 environments and providing low friction and low particle generation. However, the process is incomplete. The study thus brings insight in the possibility of controlling friction via a rational inclusion of reactant in a form of contaminants to control the tribochemical processes governing the low friction and long life. Keywords Tribochemistry, Vacuum, MoS2, MoS2/Ti, ToF-SIMS, Tribology, 3rd Body. 1. Introduction
Decades ago, the Jost report showed how friction and wear have significant impact on the British economy 1, Tribology was then created. Nowadays, even though significant research has been done in tribology with numbers of new lubricants (coatings, oils, greases, etc.), the impact remains significant worldwide and work is still needed, especially regarding the generation of fine particles, health and environment protection 2. In 2016, a bill was even proposed in front of the US House of Representatives to encourage research in tribology as it is of critical interest for the economy, the defense, the health care, etc. 3. However, the challenge with tribology, and more particularly the 1
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tribology of dry lubricated contacts which is of interest in this study, is that a comprehensive understanding of the processes governing the tribological behavior and wear lives to ultimately predict them, requires understanding the synergy existing between the mechanics, the physico-chemistry, and the thermodynamics within the contact 4. Space lubrication, the context of this study, is a really good example of that requirement. Lubricating space mechanism is incredibly challenging as lubrication must be sustained in numerous mechanical, physicochemical, thermal, and radiative environments on Earth and in space 5–14 . Moreover, the vacuum of space is not empty, it greatly differs from one orbit to another, but also from the vacuum space-like environment as modelled using UHV chambers 15. Indeed, 80% of the UHV atmosphere in vacuum chambers is comprised of water 16–18, while the vacuum of space is comprised of atomic ions, gamma rays, etc. 6 and the close environment inside and outside the satellite is comprised of water and chemicals due to material outgassing and motor emissions 18,19. The vacuum level used during the on-ground space qualification of the mechanisms is also of great importance. We demonstrated the non-equivalence of high and ultrahigh vacuum via the observation of tribochemical reactions between the contact and the residual gas contained within the chamber (mostly water) 20,21. A significant example is the quasi-inexistent particle creation with the AISI440C / AISI440C contact thanks to chemical reaction between Si (max 1% weight in AISI440C) and the residual water which created a protective Si+O compound 21. Similar segregation was observed for C within the crystalline MoS2 (c-MoS2) lubricated contact 20 and smaller worn area of high wear contacts were also observed 20–22. The effect of few ppm of water in a dry environment was also demonstrated in the past to have a significant effect 16,22. In previous studies 20,21,23,24, a comprehensive tribological investigation procedure to consider the tribological issue of dry lubricated contacts in a global view was proposed. The challenge to keep original materials and to make the procedure easily applicable by the use of common instrumentation tools (force sensors, video camera, mass spectrometry (MS), Secondary Electron Microscopy (SEM), and Energy Dispersive X-Ray Spectroscopy (EDS)) was met. Gathering all the mechanical and physico-chemical information was possible thanks to the formalization of the tribological investigation using the 3rd body concept, the tribological circuit (25–27 and Supplementary Information (SI)). Initially developed for solid 3rd body flows inside the contact, it was extended to the gaseous flows inside the contact 24. The gaseous tribological circuit (Figure 1) particularly considers the internal source flows (gQsi) that are mainly due to desorption of adsorbed species, decomposition of molecules into gaseous molecules under tribological and environmental stresses, and release of internal gaseous elements contained within the bulk of the 1st bodies (bodies initially in contact). Released gaseous species can then flow inside the contact (gQi) and be ejected (gQe) definitely (gQu) or temporarily to then re-enter the contact (gQr). Most importantly, it considers the differences between the reactive and non-reactive internal gaseous flows (gQiARC and gQiSRC). Reactive refers to flows of gaseous molecules chemically reacting (dissociation of molecules, exchange of electrons) with the 1st and 3rd bodies materials while non-reactive refers to gaseous species either physisorbing themselves or just going through the contact without staying in it, even temporarily. Note that the physisorbed species can have a significant impact on the tribological behaviors of the contact as it has been shown with the dry nitrogen (N2) experiments where the N2 molecules from the environment adsorb themselves (external source flow gQse) extending the lubricant wear life, and or reduce friction compared to the ultrahigh vacuum (UHV) and corrosive environments 16,20,28–31. In many cases, N2 is called neutral 31 but it clearly had an effect even though no chemical reaction occurred. Moreover, N2 is not a neutral gas as it can chemically 32,33 and tribochemically react 20. Hence we prefer referring to gaseous flows with and without chemical reactions (gQiARC and gQiSRC).
2
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ACS Applied Materials & Interfaces
Figure 1 - Gaseous tribological circuits
The downside of the conventional MS and EDS mentioned previously, is that they are limited to the detection of gaseous exchanges between the contact and the surrounding atmosphere (gQse, gQu) and changes in the elemental composition of the surfaces inside/outside the contact. Consequently, formally identifying chemical changes (gQiARC) of the materials at the molecular and structural level is lacking. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) has shown great capabilities for studying tribochemical reactions within wet 34–38 and dry lubricated contacts 31,34,35,37,39–43 , although having some limitation in terms of the quantitative nature of the measure on rough microscale samples 37. The use of ToF-SIMS appears sparse in tribology of dry contacts but few studies agreed and showed that superlow and ultralow friction can be achieved by a diversity of tribochemical reactions 31,34,37–43. Heavily used for carbon based materials in dry lubricated contacts under friction in various gaseous environments 31,34,35,39–42, only a few studies are looking at the tribochemical processes occurring inside the contact without an external supply of reactive gas (gQSE = 0) 43,44. Only ToF-SIMS analysis was able to provide us with the clear evidence of such interconnectivity between solid and gaseous materials during friction tests under UHV for a crystalline c-MoS2 lubricated contact 43. Coupled with the multiple measures and observations, it allowed determining a set of chemical reactions tribologically triggered inside the contact. The study also confirmed observations from other studies 28,45,46 suggesting that O in reasonable amount is not necessarily detrimental to low friction and long wear life of c-MoS2. In terms of flows it shows that minor gQiARC can occur without being detectable with a conventional mass spectrometer and that regular conclusion on MoS2 lubrication properties and mechanisms might not be as initially/commonly thought. Especially as substitution of S atom by O can only subtly change the lattice parameter of MoS2 47,48, and that under tribological stresses the c parameter of the crystal unit cell can vary 46. Consequently, ToF-SIMS can be a good complementary technique to XPS, Raman and XRD. Our previous study 20 on the tribological behavior of the MoS2/Ti coating showed that it exhibits catastrophic tribological behavior in UHV and excellent behavior in both dry N2 and humid air environments. It clearly appeared that contamination from the environment drove such behavior especially via specific chemical segregation and selection inside the contact to create a 3rd body with a bi-phasic nature (e.g. Ti+O+N material comprising plates surrounded by Mo+S+O material). That 3rd body provided low friction and long life by sharing the velocity accommodation modes: accommodation by plastic flow of the Mo+S+O 3rd body and by interface sliding on the Ti+O+N plates. Ti appears to concentrate the chemical reactivity of the coating materials with the environment and the contaminants, thus protecting the MoS2 based material from being oxidized. Such chemical segregation was particularly observable by EDS after tests conducted in dry N2 and 3
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air environments. However, due to only element based analysis, it was not possible to determine the involved processes. Similar phenomenon was observed with c-MoS2, and using ToF-SIMS on samples tested in UHV allowed better understanding the tribochemical processes involved between the MoS2 material and the coating internal contaminants 43. It showed that, contrary the commonly accepted tribochemistry of MoS2, the most favorable oxidation reactions are not the one taking place inside the contact as neither MoO3 nor MoO2 were created. Therefore, we decided to apply the technique to the MoS2/Ti coating to study tribochemistry of this coating with its internal contaminants, namely H2O and N2. The aim of the present paper is to report the detailed ToF-SIMS analysis of MoS2/Ti coatings after friction in UHV. The study demonstrates the existence of a tribochemical competition within the contact between the coating materials and its internal contaminants. By linking and discussing the results in comparison with the behaviors we observed in the 3 other environments in which the material has been tested 20, and with c-MoS2 43, it brings further understanding of the tribochemistry involved in N2 and air environments.. 2. Experiments
2.1. Friction tests The samples analyzed with ToF-SIMS underwent friction tests conducted on a fully equipped (video camera, quadrupole mass spectrometer and piezoelectric force sensors) reciprocating environmental pin-on-plate tribometer 23. Pin and plate samples are made of heat treated AISI440C with a hardness of 58 HRC. The roughness is Ra
