The majority of lubricants are used to minimize friction and wear between contacting, moving surfaces and can be inthe form of "dry" (solids) or "wet"
(liquids or semi-solid greases). Some perform a combined role of lubrication and thermal coupling.
All moving parts under vacuum, either "one shot" or constantly operating items, shall be lubricated. These include mechanisms and slip rings of deployable and orientable solar panels, bearings of rotating antennae, mechanisms of orientable experiments, deployment systems and active thermal control louvres. Moving parts appear also in pressurized systems where the situation is more or less
similar to that of conventional ground use. An additional use for greases and compounds is the thermal coupling of boxes and structural elements.
Basic oils are hydrocarbons, silicones, diesters, polyglycols and fluorinated compounds. Commercial products normally contain several additives to improve their lubricating properties. Greases are based on the same oils thickened with organic or inorganic gelling agents (metal soaps, silica, arylurea, indanthrene blue). "Compounds" are
high-molecular-weight organics which do not need any gelling agent to make them semi-solid. Besides these "wet" lubricants, many "dry" types find a use in spacecraft. These are:
-laminar inorganic substances, such as MoS2 and WSe2 , which are applied by burnishing, molecular sputtering or as an inorganic or resin-bound curing compound;
-self-lubricating polymers, such as polyamide, Teflon or polyimide, sometimes reinforced or modified by a filler (copper powder, MoS2 or carbon fibres).
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