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PRODUCT DATA of 13: Lubricants
Material13: Lubricants
General InformationThe 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.
Use in SpacecraftAll 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.
Main CategoriesBasic 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).
Processing and AssemblyApplication of oil or grease is straightforward; except on bearings where a porous retainer (phenolic, polyamide) is used; in this case the retainer is first solvent extracted, then vacuum impregnated (in 100 Pa vacuum range) by dipping in the oil used to make the grease. Dry lubricants are more difficult to apply and some processes are proprietary. In the case of metals, chemical and electrochemical plating can be used, as well as vacuum deposition.

Molecular compounds such as MoS2, which are rather sensitive to heat, can be sputtered (ion-sputtering, RF-sputtering). Simple burnishing is also used. When binders are used incombination with MoS2, the application process resembles that of curing a paint, and the items to be lubricated should have increased clearance to compensate for the lubricant thickness. In any case, new lubricants should be “run-in” before operational use is commenced. Particles given off during running-in shall be removed by a stream of clean dry air.

PrecautionsThe main problem is to ensure that the lubricant stays where it is useful and does not migrate to places where it is not wanted. Wet lubricants can disappear by evaporation or creep. Dry lubricants are destroyed by wear or by lack of adhesion to the substrate. Replenishment is possible with wet lubricants (e.g. from a sintered reservoir).Oil lubrication is basically hydrodynamic and does not operate in low-speed devices or under high loads. When the lubricant has disappeared, mating parts weld readily in vacuum. Moreover, the evolved residues create a danger of contamination in the vicinity.

Lubricants shall be applied only on clean surfaces, and lubricated items shall be protected from dust and dirt. Some lubricants intended for use under vacuum are degraded by running in normal atmosphere (lead for example) or by humidity (some silicate combinations).When lubricants are used in devices which should be electrically conducting, problems of electrical noise appear and wear can be increased at high current density. Thermally conductive compounds used at interfaces are prone to creep: they shall be kept in place by a suitable seal placed around the area concerned.

Hazardous and PrecludedOils and greases, except certain special grades, shall never be exposed directly to space conditions: labyrinth seals should be applied and the “exhaust pipe” should always be far away from sensitive satellite parts. Graphite is not a lubricant in vacuum, but an abrasive (it can be used in combination with other lubricating materials such as silver or MoS2). Ester-type oils can develop corrosivity under high radiation: this occurs rarely in space. Nylon absorbs considerable amounts of water, which are released subsequently in vacuum; because of this, its dimensional stability is not good. Sintered nylon, however, can be vacuum impregnated with oil to serve as a reservoir.
Effects of Space environment
  • Vacuum effects are mainly the evaporation of oils and “dry-off” of greases. Surface “cleaning” due to vacuum encourages oils to creep out of their location; this is particularly so with silicones. Wet lubricants are also liable to contaminate optical and electrical parts under vacuum. Some dry lubricants (particularly the resin-bound types) also evolve contaminating substances. Thin metal films shall be paired with the rubbing part in order to avoid as far as possible the tendency to cold weld: a good criterion is to avoid pairing materials which alloy readily. Cold welding is particularly intense for pairs of cubic-lattice metals.
  • Under radiation, oils have a tendency to evolve gases or corrosive products, to foam or to gel, but this needs rather high doses (over 10 Mrad in general) which are not normally encountered in space except for very special applications. Greases show the same damage, but at a lower rate since they are partly protected by their gelling agents. Dry lubricants are quite resistant to all types of radiation. In any case, lubricants are normally screened from high radiation doses by the mechanical parts to which they are applied.
  • The main temperature effect is to encourage evaporation of wet lubricants. Temperatures high enough to degrade lubricants should not be encountered in correctly designed parts and, in any case, lubricants are more stable in space, owing to the absence of oxidation. Normally, friction generates higher temperatures in space than on the ground for the same part: this is due to the difficulty of eliminating heat under vacuum (no convective cooling, no conductance through atmosphere).Wet lubricants allow better cooling than dry lubricants, but the gain is minimal in comparison with the other dangers already mentioned.
  • Atomic oxygen can degrade MoS2 and similar solid lubricants which are exposed to it.
Some Representative ProductsIn sealed instruments (or semi-sealed when contamination is not a problem), many oils can be considered:
  • Silicones (e.g. Dow Corning (USA), General Electric (USA), Wacker (D), Rhone-Poulenc (France) and ICI (UK));
  • Diesters (e.g. Lehigh (USA) and Kluber (D));
  • Fluorocarbon (DuPont (USA));
  • Greases based on the above-mentioned oils also exist.
For direct space exposure, very few non-contaminating silicone oils, greases or compounds exist; solid lubricants are also useful materials for direct exposure to space.
The following materials, for which some data sheets are included, can be considered:
  • Apiezon L,
  • Bray cote 601,
  • DC 340,
  • Everlube 620C,
  • Fomblin Z25,
  • Kinel 5518,
  • MAPLUB SH050,
  • MAPLUB PF100a,
  • MAPSIL 210,
  • PTFE,
  • Tio-lube 460