Rheological and tribological characterisation of six wet lubricants for space

Dry lubricants are often used in space because of their excellent behaviour in a vacuum and their thermal stability. However, mechanisms which are heavily loaded, with a high sliding rate, a long life span, or needing a stable friction coefficient cannot use this kind of product. They can be replaced by wet lubricants specially developed for vacuum applications. Such lubricants have a low vapour pressure in order to minimise outgassing losses and avoid contamination. Since European experience in the field of wet lubrication in space is rather limited, the French Space Agency CNES, with the help of the LMC (Laboratoire de Mecanique des Contacts - Contact Mechanics Laboratory), is currently characterising several wet lubricants for use in space applications. The selected lubricants were tested on a vacuum friction test bench which allows the main contact parameters to be controlled: speed, load, and kinematics. The bench is equipped with vacuum facilities and, since it runs under air, neutral gas, or vacuum, it can be used to study the effects of atmosphere. Variations of physical properties (viscosity) versus temperature (−60°C to 100°) and pressure (from atmospheric pressure to 500 MPa) were studied for all the lubricants tested, and these characteristics take into account in the analysis of the tribological test results. This paper briefly lists the characteristics of a space environment (vacuum, microgravity, etc.) and their consequences for lubrication. It describes the equipment used and the tribological and rheological test results obtained on six wet lubricants (three oils and three greases). It also contains conclusions in terms of recommendations about the use of such lubricants.