Lubricants aren’t only essential for smooth running here on Earth. machinery that leaves terra firma must also be lubricated under much harsher conditions. But the lack of atmosphere and gravity are major obstacles to effective application. Caitlin Jacobs launches off for a look at greasing up in zero gravity.
Liquids do not behave the same way in space as they do on Earth. Freed from atmospheric oppression and gravitational tyranny, free-floating fluids form into spheres that drift around inside space stations or disperse as frozen mist into the cosmic vacuum. But even in space, the force of friction cant be escaped, and aerospace engineers and lubricant formulators must find ways of protecting hardware outside of our planets familiar tribology.
Perhaps the answer is to ditch those unruly liquid lubes in favor of solid ones for some applications, proposed Andreas Merstallinger of Aerospace & Advanced Composites GmbH.
Both liquid and solid lubricants have benefits and drawbacks outside of Earths atmosphere. For example, any liquid lubricant used in space could outgas, or evaporate. This not only causes loss of lubricity, but also outgassed lubricants can infiltrate other equipment in the vicinity, such as cameras, or affect the readings of devices like antennas.
We have to remember that space is a vacuum, so we dont have ambient pressure, Merstallinger told a group gathered for the 21st International Colloquium Tribology in January. This means that we have to be aware that any fluid that is put in space is cooking. He described a public display in which Austrian company AAC placed a lubricant in a vacuum and it immediately began to bubble, appearing to boil at room temperature. (Once they have boiled in zero pressure, liquids freeze in the minus 270 degrees Celsius of space.)
While equipment can reach extremes of ambient and operating temperatures, fluid lubricants are only effective within a limited temperature range, typically between -20 and 70 C. The higher the temperature, the shorter the life, because you are steadily evaporating or outgassing, Merstallinger explained. Lower temperatures could cause freezing.
Fluid lubricants can creep out of place in a vacuum, so barriers need to be used. Radiation can also negatively affect liquid lubes. Such problems can arise with greases as well as oils, since greases lubricate by bleeding oil during operation.
Typical fluid lubricants used in space include perfluoropolyether (PFPE) and multiply-alkylated cyclopentane oils and greases. A particular risk when using PFPE grease is a reaction between PFPE and iron in bearings and gears, known as the Lewis reaction. The result is decomposition of PFPE at elevated temperatures.
By contrast, solid lubricants such as molybdenum disulfide (MoS2), polytetrafluoroethylene (PTFE, better known as Teflon) and lead coatings are not susceptible to outgassing or creeping, and there is no practical temperature limit for their use. Although PTFE and polyimide – a substance with multiple applications that can be combined with solid lubricants to create mechanical parts – suffer at temperatures over 250 C, most space applications do not exceed 150 C.
Wear can create debris from solid lubes, which might contaminate other parts, but this is typically controlled through the use of labyrinths that contain the sloughed-off material.
However, Merstallinger admitted, we can only lubricate by solid lubricants if it is feasible. There are certain limits where we have to use oils or greases. For example, we have to be aware that, when using solid lubrication, the lifetime is usually much lower than using liquid lubrication.
While it doesn’t pose any significant problem for liquid products, humidity can be a threat to solid lubricants. If you are using MoS2, humidity uptake is a very critical thing because it can reduce the lifetime in a vacuum by orders of grade, and not only by a few percent, Merstallinger explained.
Exposure to cosmic radiation is also a problem for PTFE and other polymers. The polymeric chains disintegrate, causing a loss of lubricity. We got a piece of polyimide from the International Space Station, Merstallinger recalled, that originally would have been too strong to tear or break. After about 10 years of exposure, the piece was so fragile it seemed to crumble under his gaze.
Whatever lubricant is chosen, it must meet strict standards. On top of the ISO standards, the European Space Agency has set up the ECSS standards, or the European Cooperation for Space Standardization, he pointed out. These complex requirements define what is acceptable for materials, mechanisms, software, electronics and other items being sent into space.
Its important to understand the best applications of different solid lubricant products, Merstallinger emphasized. For example, graphite products need ambient hydrogen or water to lubricate, and diamond-like carbon products do not function well in space. For solid lubricants, the lubricant material is typically held within a polymer or metal matrix.
Within a vacuum at very high temperatures, MoS2 products, such as Austrian plastics company Ensinger Sintimids polyimide-matrix product Tecasint, are functional up to 300 C.
For low to moderate temperatures in a vacuum, products such as MoS2 and PTFE with a polymer matrix, including Victrex products and DuPonts Vespel, lead with a bronze matrix, or bronze and PTFE inside a PTFE matrix, are available and work well.
However, some products can only be obtained from the United States, and we have recently had trouble getting products because space and defense is a critical issue, and sometimes the Americans are not allowed to sell us some things, noted Merstallinger. So the European Space Agency is always impressed to get all-European products.
MoS2 with a PTFE matrix is one such lubricant that could only be obtained from the U.S., so AAC, which is a testing house for space materials and a subcontractor to that industry, is developing a European version in partnership with Ensinger.
A Lifetime in Space
Without any maintenance, lubricants must perform throughout a satellite or surface rovers entire mission. Lifetime lubrication in space means 15 years, which is the point at which technology has fallen behind the state of the art, said Merstallinger.
A limited lifespan is one of the biggest drawbacks of MoS2 solid lubricant coatings, which are applied by physical vapor deposition, a process of applying a solid surface in very thin layers in a vacuum. AAC tried several approaches to remedy this problem. One was combination lubrication, which involved using grease along with the solid lubricant. PFPE grease was applied with an MoS2 coating on precipitation-hardened steels. During testing, the friction coefficient was higher than with an MoS2 coating alone – the same as if PFPE grease had been used alone – and operation was much noisier. Merstallinger theorized that when the metal surfaces are already coated in grease, flakes of MoS2 do not attach to the metal and are rubbed away. The Lewis reaction could also come into play. In our opinion, it is not a good idea to combine solid lubrication with grease, he advised.
Next was doping, or adding another molecule to alter the greases properties. Doping was found to increase the lifetime of MoS2 at a similar load, or increase the load-bearing capacity at a similar lifetime. Merstallinger did not specify what material AAC used to dope the lubricant.
Then the company found success engineering a lubrication system for bearings. In a ball bearing, there is always a place for a cage where you can store additional solid lubricant, said Merstallinger. Researchers began with bearings that had physical vapor deposition coatings on the raceway, then added a cage filled with MoS2 that could be replenished. As the bearings spun, they transferred the solid lubricant from the cage to the raceway, extending the life of the lubricant from a range of 10 million to 20 million revolutions up to hundreds of millions of revolutions, which is comparable to the lifetime of some long-term mechanisms.
Finally, they deployed matrix materials. Along with Ensinger, AAC has developed and commercialized a composite lubricant with bronze as the matrix. The challenge was keeping the MoS2 inside the bronze, Merstallinger stated.
The company is currently working on a self-lubricating polymer matrix composite. PTFE was chosen as the polymer because of the performance of a PTFE-based product called Duroid 5813, which worked well in space but has been off the market for almost 10 years. The replacement product can only be obtained from the U.S., which carries a supply risk for Europeans in additional to other drawbacks. Our intent is to understand how the different kinds of fillers work and to come up with a European-based product, Merstallinger said.
The advantage is that the PTFE matrix itself is already lubricating, he explained, but the challenge is to stabilize the PTFE for long-term use. So far, developers have learned they must be careful in their choice of lubricant filler inside the metal or polymer matrix.
The filler must also be combined with some kind of fiber, and glass fibers, mineral fiber, and carbon nanofibers were tested. For the carbon nanofibers, manufacturing is still a problem. The best performance came from PTFE filled with mineral fibers and high amounts of MoS2.
The new PTFE composite material is close to qualification, according to Merstallinger. Main applications will be cages in ball bearings, as well as bushings and other sliding elements. The final product, which Ensinger will commercialize, will be functional from -30 to 80 C, extending to cryogenic applications.
The next step is confirming performance results, Merstallinger said. We have identified a European company for mineral fibers. So REACH conformance is now the way forward.