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What to consider when selecting a space lubricant Outgassing
Outgassing is one of the most important properties in aerospace lubrication as it can impact lubricant longevity and its potential to contaminate nearby systems. Outgassing is akin to evaporation; it is the release of individual molecules from a bulk liquid or solid material. A lubricant’s ability to provide low outgassing performance in the vacuum environment of outer space is key. Lubricants typically operate under atmospheric conditions where evaporation losses can compromise a lubricant’s integrity over time, and these evaporation losses are accelerated when operating in vacuum. Low outgassing lubricants can also prevent undesired contamination of nearby components such as sensitive optics or other areas onto which outgassed materials can condense. Optical components including sensors, lenses, and solar cells are examples of applications where outgassing condensation can compromise mission performance.
Standard outgassing testing is conducted per ASTM E595 and is designed to quantify a material’s propensity to outgas and release condensable volatiles. This test provides percent Total Mass Loss (TML) and percent Collected Volatile Condensable Materials (CVCM) data for materials. Residual Gas Analysis can also be conducted during this test to determine the specific molecular composition of the volatiles. Together these results indicate a lubricants stability in a static vacuum environment.
Temperature
Space mechanisms experience a wide range of temperatures (-100 °C to 120 °C) depending on their orbital path. The lubricants selected for these mechanisms must be able to withstand these extreme temperatures with minimal changes in viscosity. At the high-temperature limit, a lubricant must remain chemically stable, avoid degradation, and have sufficient film strength to adequately prevent wear. At the lowest expected temperature, the lubricant must remain sufficiently fluid to lubricate all intended areas within the mechanism.
Certain base oil chemistries have a wider operational temperature range than others. Specialty perfluoropolyether (PFPE) lubricants can withstand temperatures from -90 °C to +250 °C. If a lubricant possesses some, but not all optimal qualities,
10 LUBE MAGAZINE NO.172 DECEMBER 2022
mechanical solutions may be recommended as a design fix. For example, multiply-alkylated cyclopentane (MAC) lubricants are often recommended for space applications because of their low outgassing properties and unmatched wear performance. MACs, however, have an operating temperature range of -50 °C to 125 °C. In this case, if a space application requires the low outgassing and excellent wear performance of a MAC lubricant that they are unable to get from a PFPE lubricant and the operating temperature is below -50 °C, engineers may, and do, add a heater to their design that raises the temperature into the operating temperature range of MACs.
Component life Operational design life can vary greatly in space. Satellites may have an operational life of 5 to 15 years while exploratory research missions require that space mechanisms and their components function for several decades. Engineers must also take into consideration the time (and environmental conditions) a lubricated component will sit in wait prior to launch. As the ability to service or replace these components is limited or non-existent, lubricants must be able to protect components for the entirety of their intended life/journey.
Validating the life expectancy of lubricants in a space environment requires specialised equipment. Testing a lubricant under vacuum and load is the only way to accurately gauge the expected life of a lubricant in space. Test rigs capable of performing tribological tests under high vacuum conditions are difficult to design and build, making them inaccessible to many.
Originally developed by NASA to evaluate space mechanisms, the Spiral Orbit Tribometer (SOT) (ASTM F2661) is a test rig designed to bridge the gap between tribo-contact testing and longer-term bearing tests. The SOT measures the coefficient of friction of a rolling element under high and ultra-high vacuum conditions and provides relative lubricant lifetime approximations based on the number of orbits made below a friction threshold which is normalised to the amount of lubricant applied to the test specimen. As this test is done in high and ultra-high vacuum (<1E-7 torr) and the materials in contact can be customised, the SOT can provide insight into the performance of application critical lubricants under a variety of test conditions outside of the ASTM method.
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