Factors Affecting Lubricant Lifetime in the Spiral Orbit Tribometer Under Vacuum

Abstract

Spacecraft mechanism engineers are severely limited in their lubricant selection, primarily due to the harsh environments in which they operate. With ever increasing mission complexity, combined with the reality that mission failure is commonly attributed to failure of essential mechanisms onboard the spacecraft, great desire exists to improve the mechanical reliability of spacecraft during flight. Just as in terrestrial applications, a comprehensive understanding of the relevant tribology is essential to achieve this goal, and particularly a thorough appreciation of the root causes of lubricant failure.

This thesis presents research into identifying and characterising the significant factors influencing the working lifetime of lubricants employed within European spacecraft mechanisms. Experimental testing was performed using a unique facility, The Spiral Orbit Tribometer (SOT), employing oil plating techniques to characterise the degradation behaviour of existing vacuum lubricants, to attempt to mitigate their known poor-performance issues, and to identify and characterise the next generation of lubricants for future spacecraft missions, under representative environmental conditions.

Experimental results provide validation of the shear-induced tribo-chemical degradation model for perfluorinated polyether (PFPE) based fluids, with the rate of this chemical degradation shown to be dependent upon temperature, contact pressure, environment, and substrate material (in addition to other variables). Experimental data was used to produce a semi-empirical model for fluid lubricant lifetime on the SOT. Furthermore, it was demonstrated that in-situ monitoring of identified gas species, including CF3 and COF, provides a precursor to imminent lubricant failure prior to significant friction increase.

A new solution of hybrid lubrication combining PFPE fluid and thin films of physical vapour deposition (PVD) MoS2 is proposed, and was shown to be synergistic under vacuum, displaying extended lifetimes beyond those predicted from the individual lubricant constituents, without compromising friction. Bis(trifluoromethylsulfonyl)imide anion-based ionic liquids were shown as capable of providing lubrication under spacecraft mechanism representative conditions, with greater lubricating lifetimes in comparison with existing PFPE-based vacuum fluids. Failure of these ionic liquids was shown to occur as a result of tribo-chemical degradation.

Implementation of research here presented will allow improved design of spacecraft mechanisms and lubricants, reducing the rate of failure, and ultimately allowing for longer and more successful missions. Selected results from this thesis have led to developments in lubrication practice which have seen application in European spacecraft programmes.