Reasoned use of contaminants to characterise the rheology of both solid 1st and 3rd bodies : Application to the tribology in space environments
From the very beginning of space exploration, “space tribology” became a specific field of competences in its own right because it implies lubricating a mechanism not only in the vacuum of space but also in several environments (simulated vacuum, dry and humid air, dry N2 environment, etc.) on Earth. Consequently, a lubricant, fluid or solid, must provide the desired tribological behaviour in all those successive environments, but it must also resist to the dynamic loading induced by the launch operations. Once in space, a space mechanism must be autonomous, with no possible maintenance, and its lubrication must be sustained during 15 to 30 years (i.e. up to 800 millions cycles for a ball bearing) in space while sustaining high reliability and great precision (to within a few µrad). Despite those stakes, the mastery of space tribology is limited to empiricism and conservatism of technical solutions, especially when solid lubricants, whose are studied in this study, are used. In facts, only directions for use exist without allowing predicting the tribological behaviour of the solutions. That prediction requires the reconstitution of “the contacts’ life”. The latter requires evaluating the 1st and 3rd bodies flows. Those flows depend on a multiscale and multiphysical coupling effect “forced” by the tribological stresses. To decrypt those flows, an experimental procedure which couples both real time and post-mortem mechano-chemical measurements (friction forces, gas analysis, photonic and electronic visualization,) has thus been developed. The simultaneity in the measure allows studying the relationship between mechanical (3rd body particles creation, plastic deformations of 1st bodies, etc.) and physicochemical (adsorption/desorption, chemical changes of 1st and 3rd bodies, etc.) phenomena. The procedure is applied to two tribological triplets respectively containing 1st bodies comprised of MoS2 and MoS2+Ti coatings whose tribological behaviours in vacuum and humid air are antagonistic. Such antagonism allows better understanding the responses of the 1st and 3rd bodies to the tribological stresses. The reconstruction of the contact life of those triplets allowed showing, among others: (1) the beneficial role of coating columnar structure in the obtaining of both a low friction coefficient and a long wear life. This is not because it is columnar but because such a structure localizes the 3rd body creation; (2) the beneficial role of contaminations, which is either internal or external to the 1st and 3rd bodies, by delocalizing the accommodation from the 3rd body volume to the surface complexes, which explains, for example, that dry N2 environment can not, even for financial reasons, mimic ultrahigh vacuum. In fine, the study allows specifying a procedure for designing “tribological triplets” which will be generalized at a latter stage thanks to the numerical tribology.