KEY CONCEPTS
• Tribology will continue to play an increasingly important role in space commerce.
• Performance demands for liquid and solid lubricants in space applications require research that has its own limitations.
• Despite the challenges, research is resulting in promising new lubricants that will hold up in operational space environments. 
Space research is entering into a new and exciting frontier. Commercial space flights by multiple enterprises and NASA’s planned space missions indicate that there is significant ongoing activity in this field. In addition, space habitation research has paved the way for programs such as long-term missions and in-space manufacturing initiatives. With the potential commercialization of space, tribology and lubrication are positioned to play essential roles in these ongoing and future developments.

Dr. Guillaume Colas, researcher at France’s Centre National de la Recherche Scientifique, observes, “With the increase of satellite constellations, reusable rockets, micro/nano satellites, upcoming human space exploration and the expectation of a lunar base, new challenges are arising.”

The example he uses is the development of less expensive and easier to use lubricants for downsizing mechanisms and new designs that have new contact configurations and new substrate materials that need to maintain mechanism performance with smaller parts—providing sustainable lubrication in very specific dusty environments such as the moon and Mars and all of this for a larger range of applications and systems.

Lubricants requirements for space applications
STLE member Dr. Salil Bapat, research staff, School of Mechanical Engineering, Purdue University, says, “Managing critical static/dynamic surfaces and interfaces requires protection against friction and wear with the highest performance reliability, repeatability and reproducibility. These critical functional interfaces, in open and closed loop systems, could be in different operational conditions such as pressurized or unpressurized (vacuum) and temperature-regulated or non-regulated vessels, facing the sun (extremely hot) or not (extremely cold), exposed to regolith (dust on the moon) and micro-meteorites and other variants.” 

These extreme and extremely variable conditions pose a challenge to developing a framework for even a basic set of requirements. Notwithstanding, STLE Life Member Dr. Christopher DellaCorte, University of Akron, observes that the primary requirement for designing lubricants for space applications is to first develop the right set of lubricant requirements for a specific application. Some areas of inquiry he suggests include:
• Is the lubricant exposed to the vacuum of space, or is it sealed inside another environment?
• Does the application include exposure to radiation?
• Is toxicity a concern?
• Does the lubricant only need to function briefly (e.g., a single use deployment mechanism) or does it have to function for years of continuous use (e.g., lubricant grease used in a reaction wheel bearing)?

“Space is no different in terms of requirements than a terrestrial application in that one needs to consider all the needs and then make the choice accordingly, followed by testing and more testing and maybe a backup lube system for redundancy,” DellaCorte says.

Colas advises that both fluid and solid lubricants for space applications come with demanding design parameters (see Challenges and Considerations for Solid Lubricants). While those requirements may differ from one mechanism to another, what they have in common is that they have to sustain all the qualification steps of the mechanisms (from the component level to the mechanism level) and the launch operations. 

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Challenges and considerations for solid lubricants^A
Per STLE member Dr. Jeffrey Lince, president, Space Tribology Consulting, Inc., most solid lubricants used in space are in the form of thin coatings on mechanism surfaces. Bonded coatings primarily consisting of micron-sized molybdenum disulfide (MoS₂) powder mixed with an epoxy or inorganic binder are used widely. 

Tungsten disulfide (WS₂) and MoS₂ coatings less than 0.1 µm thick can be fabricated using an air impingement process, with friction coefficient values as low as 0.02. They are inexpensive and easy to apply. However, the lack of a binder significantly limits their endurance. As such, they are generally used as anti-seize lubricants or for other low-cycle applications.

The newest solid lubricant coating technology involves sputter-deposition of MoS₂-containing coatings. These have been available for several decades but have been slow to catch on because of the space industry’s reliance on traditional bonded coatings. However, their use is attractive because they are much thinner than bonded coatings (1 µm versus 10 µm) and can achieve similar endurance with friction as low as 0.01. As such, they are increasingly being used for tolerance-sensitive applications like ball and roller bearings.

A key challenge for the future of space tribology is to develop hard, low friction materials—in contrast with typical solid lubricants that are soft and exhibit a non-negligible wear rate, which limits their cycle life. Diamond-like carbon (DLC) and other hard carbon-based coatings show promise for filling this niche, but like graphite, often rely on the presence of water vapor to perform adequately. In contrast, highly hydrogenated DLC coatings (HH-DLC; >40% H) developed at Argonne National Laboratory exhibit very low friction and wear in vacuum. Preliminary work on ball bearings in a vacuum environment showed promise, both dry and when used with spacecraft lubricants.

In the future, mixtures of various lubricating species will be fabricated to form adaptive or “chameleon” coatings to meet requirements for operation of a single material over a range of environments and temperatures, such as turbo-ramjet engines and articulating flight control surfaces in hypersonic vehicles. Many of these materials have been developed at the Air Force Research Laboratory. An example is a WC/DLC/MoS₂ nanocomposite coating, which provides low friction in both air and vacuum environments. Another example is yttria-stabilized zirconia (YSZ)-Ag-Mo composites that exhibit a moderate friction coefficient of 0.4 over a large temperature range (25-700 C). 

In addition to coatings, bulk composite materials are used for applications like ball bearing retainers, including solid lubricants like MoS₂ and/or polytetrafluoroethylene (PTFE) mixed with structural materials like polymers or bronze. These are best used in combination with low friction coatings on the bearing races.

There are some design problems that are beginning to be met with a mixture of liquid and solid lubricants, both oils with solid lubricant additives, and oils/greases used with parts that are coated in solid lubricants. These are especially useful for boundary applications. This approach has shown promise, but care must be taken to ensure that the liquid does not prevent the all-important MoS₂ transfer film from forming. 

A challenge for solid lubricants is the increasing number of planetary missions, including return to the lunar surface. The surface of planets introduces a new mix of environmental issues, such as the presence of vast amounts of abrasive impurities, including dust as well as ice (the latter, for example, on Titan, a moon of Saturn).

REFERENCE
A. All material in this section is attributed to Dr. Jeffrey Lince, president, Space Tribology Consulting, Inc.

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“An essential requirement is reliability,” Colas says. “Once in space, there is no maintenance, so we need to ensure that friction remains stable all along the mechanism’s lifetime. For mechanisms reaching years of service life, ball bearings may need to achieve hundreds of millions of rotations.”

DellaCorte agrees, adding, “One cannot generally service, relube or repair a space system. Once launched, it has to work for life. Thus, making sure you build in life margins or have redundancy plans is essential.”

Colas observes, “Qualification tests include testing in air (clean rooms), in dry environments, in vacuum chambers, at room temperature—high and low, even to cryo temperatures (if this is a cryo-mechanism)—and vibration testing that emulates launch. During launch, the mechanism undergoes violent accelerations (up to several 10s of g-force [gravitational acceleration]) and is subjected to noise (up to 150 dB) and vibrations (lower than 10 kHz). Even one-shot mechanisms (e.g., solar panels and antenna deployment mechanisms) are tested multiple times (up to several 10s of times) in the clean room’s ambient air and on the ground.”

He continues, “The choice of lubricants can then be driven based on the ratio of expected working life in space/on planets versus ground life (especially in air). The proximity of the lubricated part relative to critical parts such as optics also will be considered to avoid contamination from outgassing molecules or wear debris. There also is the question of torque noise to ensure that no tribologically induced vibrations are impacting the instruments. Sometimes one might prefer a lubricant offering slightly higher friction but demonstrating extremely good stability. Last, but not least, we also have to consider the toxicity of the lubricants.” 

Lince observes, “It is very difficult to meet all these requirements using mineral oils—even heavily refined ones—necessitating synthesizing entirely new molecules tailored to the application. This is where the creativity of the lubricant designer comes into play, because the oil molecule structure is just as important as molecule size in minimizing volatility, while optimizing viscosity, viscosity index (VI) and chemical stability. It is with these considerations in mind that lubricants like multiply alkylated cyclopentanes (MAC) and perfluorinated polyalkyl ethers (PFPE) have been developed for space use. In addition, the development of new lubricant additives that work with these new lubricants is paramount.”

Challenges of designing lubricants for space applications
Colas is most familiar with solid lubricants and dry contact configuration. He says that the two major issues he faces are related to: 
1. Maintaining the friction performance in different environments 
2. Launch operation challenges, which may lead to cold welding between mechanical parts. 

“Regarding the first point, we came across very interesting tribological behaviors of MoS₂-based lubricants and self-lubricating polymers used in space applications,” he says. “We found that, even if MoS₂ is sensitive to humidity, performing friction in moderate vacuum (10^-5 mbar) may result in better performance than in ultra-high vacuum (10^-8 mbar). The former is sometimes encountered as the vacuum level for qualification tests, while the latter is more representative of the vacuum level of space (low earth orbit and close satellite environment).” 

Not taking these variables into account can lead to overestimation of the friction and wear life of the lubricant and consequently of the mechanism itself. 

According to Ajay P. Malshe, R. Eugene and Susie E. Goodson distinguished professor, School of Mechanical Engineering, Purdue University, acute space conditions and the intricate interfaces demand extreme lubrication properties, which include:
• Thermally stable under extreme and variable temperature conditions while maintaining structural integrity 
• Exhibiting optimal vapor pressure to operate under variable or steady-state conditions in pressure and temperature
• Environmentally safe in case of leakages 
• Chemically compliant with materials of mating surfaces over an extended period of time 
• Able to withstand abrasive conditions created due to environmental contaminants like lunar dust.

Malshe adds, “Currently, the lubricants designed for space applications, such as gimble joints and others, are derived organically or inorganically or in combination. They typically fall under the category of ‘specialty lubricants.’ The soft lubricants are typically from special grades of silicone and other extremely low vapor pressure materials. Coatings of dry lubricants could be applied by various processes including plasma coating, mechanical means, lasers and others.”

Regarding the vacuum environment in vacuum chambers, Colas observes that the characteristics of the near environment of a satellite differ from the working orbit environment. The differences come from motor emissions, such as nitrogen, hydrazine, ammoniac, etc., and outgassing phenomena such as carbon monoxide/dioxide, water vapor, condensate from polymers/electronic boards, etc. This outgassing is linked to the materials constituting the mechanism and pressure differences in and outside the satellite. 

It is critical to understand the mechanisms involved and how the close environment of the contact may impact the tribological response of the material. Colas points out that in low earth orbit, there is atomic oxygen that is highly reactive with the materials and may quickly damage the materials if they are exposed to it. 

“By investigating qualitatively this particular issue, a new coating was recently developed, based on MoS₂, which managed to provide low wear and low friction in humid air/dry nitrogen and vacuum environments—while maintaining the good tribological performance when transitioning from air to vacuum,” Colas says. “This was demonstrated to be a good avenue to pursue in the development of new space lubricants.” 

Regarding Colas’ second point (launch operation challenges), he searched for a solution to lubricate parts that had been damaged on a satellite flight model during a final qualification test. Colas explains that the satellite remained stuck in its dispenser during the ejection tests, which followed a vibration test that emulated launch. The local contact conditions in this particular situation induced fretting wear and significant surface damage. The debris agglomerated into an interfacial material sticky enough to prevent the ejection. Ultimately a solution was found based on space-qualified grease that allowed successful ejection onto orbit (see Barriers to Successful Lubricant Development). 

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Barriers to successful lubricant development
Colas cautions that the space environment can significantly impact contact behavior. Vacuum plays a major role in dissipation of heat that essentially occurs through conduction and irradiation. It also leads to outgassing of materials (mainly fluid and grease lubricants), which leads to loss of lubrication and contamination. 

The space environment increases adhesion between materials, i.e., cold welding if both materials in contacts are miscible. Temperature cycling (passage from light to dark) may cause microcracking of materials, detachment of coatings and local loads induced by differential expansion issues. Temperature cycling also may induce modifications of electrical and mechanical properties of materials and lubricants, which could be an issue for brushes in motors, for example. 

For fluid lubricants, depending on the temperature range, cycling may result in viscosity modification, dissociation of components and an increase of outgassing phenomena. Microgravity induces an absence of structural loads with only preloads remaining. This leads to the modification of lubricant distribution and supply and the migration of the fluid lubricant and wear particles of solid lubricants. 

Anti-migration barriers and seals may have to be used in order to maintain lubricants in the contact vicinity. Magnetic radiations, plasmas, etc., also play an important role. Depending on the exposure, they can result in the modification of mechanical and chemical properties of materials, dissociation of oil components, structural modification of polymers, ionization of materials, charging of external surfaces and sputtering from material surfaces. 

Finally, the atomic oxygen in low earth orbit is known to induce oxidation, gasification and erosion of materials at times, which may finally turn lubricant particles/materials into abrasives, and eventually lead to wear, particularly of lubricant thin films. Radiation and atomic oxygen are less often considered because most tribological parts are inside the satellite or shielded, which reduces the impact. However, in some specific cases, they have to be considered.

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Validating lubricants for space applications: Tests and equipment
Ground-based tests of the full system, inside a thermal-vacuum chamber, are typically required. “Vibe testing to simulate launch loads and the launch environment also are normally used to weed out problems before showtime,” DellaCorte says. “Unfortunately, some aspects of space, like the absence of gravity and natural convection and ambient heat transfer via convection, cannot be simulated on earth.”

Colas explains that the developed solutions are tested at different scales. First, they are tested at the material scale on a tribometer for friction and wear behavior. Tests are performed in different contact conditions, at different temperatures, to estimate the range of working conditions of the lubricants. The lubricant also has to undergo outgassing experiments to make sure that the vapor pressure of the lubricant (fluid or solid) is low enough to avoid lubricant loss once in space. 

Aging tests also are performed to ensure oxidation stability, perhaps over a span of years. Then there are the tests at the component level such as on ball bearings or gears. Dedicated bench tests are developed to run the components in conditions as relevant as possible to the expected applications (rotation speeds, loads, thermal cycling, environments, etc.).

“Some tests can be run continuously for several months to reach an expected wear life from which extrapolation to multiple years in space can be made,” Colas notes. “Any tests are run in accelerated conditions (higher speeds), but the acceleration factors have to be carefully determined in order not to fall into a different lubrication regime and temperature elevation. That is true with both solid and liquid lubricants. Once the material has passed all tests, it can be tested on engineering models of mechanisms and follow qualification tests.” 

Colas adds that proof of work in space also may be required. For that specific test, there needs to be an open spot on a mission satellite for mounting a mechanism lubricated with the new material. Then the mechanism and lubricant can be monitored to evaluate performance. 

Regarding hardware design for space applications, DellaCorte observes that the most important aspect is carefully understanding the engineering loads, duty cycle and thermal and environmental situation in order to avoid omission.

“One should consider what has been a traditional approach—referred to as ‘heritage’ in the space community—and use that as a basis for design,” DellaCorte says. “If you are proposing a radical departure from what others have done previously, you must be able to justify your rationale. Testing your design followed by a physical teardown and inspection is essential. Most space hardware failures I have seen followed a failure path that was not foreseen but could have been deduced from rigorous ground tests followed by detailed hardware inspections.”

“For completely new materials/technologies/mechanisms, we have to do more general testing,” Lince says. “These general tests utilize standard test protocols like pin-on-disk testing (ASTM G99 or G133) or block-on-ring testing (ASTM D2714 or G77). These tests are rarely TLYF¹ but are useful in developing a relative ranking for several different products. The chosen solution must then be tested in model spacecraft hardware.”

Lince continues, “Qualification tests are performed on an engineering unit incorporating the flight design and include a design life verification test and functional test, among others. Life tests for high cycle mechanisms are generally conducted for at least 1.5 times the total number of cycles expected in ground testing and flight; low-cycle mechanisms should be tested for at least 50 cycles. Finally, the actual flight unit must pass various acceptance tests to uncover potential workmanship problems.”

This testing is bearing fruit. Colas reports that there are lubricants that perform well in vacuum (most lubricants exhibiting good lubrication properties in vacuum are poor lubricants under ambient earth conditions). 

“The reason for this good performance lies with the presence of water in the environment for graphite, and on its absence in vacuum for MoS₂,” Colas explains. 

Colas reiterates that tribology and lubrication in space are as essential and pervasive as they are on earth. “Even bolts and screws are lubricated to ensure that each and every one of them are properly tightened,” he says. “High friction and low wear also are desirable sometimes, for example in the case of pads separating folded solar panels. High friction is helpful for keeping them in place, but low adhesion is required to ensure they will unfold and not suffer from fretting and cold welding during launch.” So, some of the issues are different and some are the same, to varying degrees, as they are on earth.

Then there are the lubricant production realities. “A big problem with future success in space is commercial availability of the right solution,” Lince says. “Manufacturers understandably shy away from products that are ideal for space but have limited applicability on earth. There just isn’t the volume to make it worth their while.”

REFERENCE
1. TLYF is an acronym for “Test Like You Fly”—an environment as close to the actual application as possible.

Jeanna Van Rensselar heads her own communication/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at jeanna@smartprcommunications.com.