Space tribology

Liquid lubricants for space applications need to have low vapor pressure (volatility) in order to withstand the high vacuum on orbit. Suitable lubricants include perfluoropolyethers, cyclopentanes and polyalphaolefins, mostly in the form of base oils for lubricating grease.^[2]

Since the rate of evaporation increases with temperature, the use of liquid lubricants is often limited to temperatures below 100 °C. On the other side of the spectrum, the viscosity of liquid lubricants increases with decreasing temperature; i.e., the lower the temperature, the more viscous the lubricant (see also viscosity index). Thus, the use of liquid lubricants is limited to temperatures of around -40 °C.^[2]

Solid lubricants are used for applications with extreme temperature or where evaporation of lubricants would cause damage to sensitive instruments.

Solid lubricants are applied in the form of coatings, or through self-lubricating materials. In the former case, sputtered molybdenum disulfide (MoS₂) and ion-plated lead (Pb) are commonly used; in the latter case, polyimide composite materials based on polytetrafluoroethylene (PTFE) are often employed, as well as leaded bronze.^[2]

Space tribology ensures the reliable operation of mechanisms aboard spacecraft, which can be broadly grouped into one-shot devices (such as deployable solar panels, deployable antennas and solar sails), and continuously and intermittently operating devices (such as reaction wheels, electric motors and slip rings).^[3]