Smooth Moves: Tribology in Action
Paul Fleischauer, Steve Didziulis, and Michael Hilton
Space system designers must minimize friction and wear in moving parts without losing lubricants to the vacuum of space.
Tribology is the study of friction, wear, and lubrication of surfaces in sliding or rolling contact. Tribology is a source of particular concern for space system designers because the space environment affects lubricants and therefore constrains the design and, potentially, the functionality of spacecraft mechanisms.
The importance of tribology can best be illustrated by comparing the lubrication needs of an automobile with those of a spacecraft. For example, a typical automobile engine operating 10,000 miles per year undergoes approximately 25 million cycles of operation per year. In contrast, the antenna-pointing mechanism on a spacecraft might undergo 30 million cycles per year, an optical scanner 200 million cycles per year, and a control moment gyroscope more than a billion cycles per year. The repeated mechanical, surface-chemical, and thermal forces in a car engine eventually break down the lubricating oils, and the same is true for the lubricants aboard spacecraft. Unlike car engines, however, these mechanisms must operate for several years without the prospect of an oil change.
Jeffrey Lince prepares to make pin-on-disk friction and wear measurements. This testing is performed to measure the sliding coefficient of friction between two materials under ambient or purged atmospheres. |
Recognizing the importance of tribology, Aerospace has developed considerable expertise in the science of tribology and in the design of spacecraft mechanisms requiring lubrication. Aerospace researchers have also conducted landmark studies and tests of lubricants and materials over the years. This work has led to major contributions in the performance and successful operation of mechanisms on practically every Air Force launch vehicle and satellite program.
Lubrication Types
Mechanisms on spacecraft range from those that move once to those that rotate billions of cycles. They may be exposed to cryogenic temperatures or to temperatures well over normal ambients on Earth, and typically must operate with no vibration-induced noise. So, the lubricants employed must provide protection over a wide range of operating conditions and must do so with little or no chemical and physical degradation. Also, to prevent evaporation into the space vacuum, lubricants on space systems must have low vapor pressures or be otherwise physically contained by the design of a given mechanism.
In the early years of the space program, it was believed that lubrication of mechanisms for operation in the vacuum of space must be done with nonvolatile solid materials such as graphite. But it was soon discovered that graphite lost its lubricating properties in vacuum because of the loss of adsorbed gases. Alternative compounds with similar lamellar structures—that is, with crystallographic planes that move or slide over one another—soon replaced graphite, providing effective lubrication without the assistance of Earth's atmosphere.
One of those compounds was molybdenum disulfide (MoS2). This solid lubricant is composed of two-dimensional sheets that are held together by very weak forces perpendicular to the planes. Nanocrystals of MoS2 slide past one another with extremely low resistance, providing friction coefficients below 0.01 in vacuum. During those early years, Aerospace conducted numerous laboratory tests and studies of MoS2 in conjunction with NASA, the Air Force, Naval Research Laboratories, and contractors to determine its optimal coating conditions and understand its attributes and limitations.
Today, dry-film solid lubricants are used on most deployable mechanisms, many hinges, pivots, latching devices, and on some gimbals and low-cycle bearings and bushings. Materials like steel and titanium alloys are provided with surface coatings that include both hard antiwear coatings, such as carbides and nitrides, and soft lubricating films, such as slippery solids or oils. Some ceramic materials can also provide some degree of self-lubrication. Nonetheless, lubricating oils patterned after those used in the automotive industry are necessary for the operation of long-term, high-cycle-life precision bearings, such as those used in reaction and momentum wheels, despin mechanisms, and many scanning sensors.
The two basic types of oil lubricants are mineral (refined from crude oil) and synthetic (derived from natural gas components). In the early 1960s, a superrefining process was developed involving the molecular distillation of mineral oil into different viscosity grades for use in high-precision bearings like those found in mechanical gyroscopes and momentum wheels. The oils that developed from this process were superrefined gyroscopic fluids, known as the "SRG" series. These oils were assigned numerical designations (e.g., 40, 60, 80) that indicated their viscosity at 100ûF. For many years, these oils were considered the gold standard for precision space mechanisms, and oil companies began to produce similar products under different trade names.
Synthetic oils were first produced in the late 1970s but were not accepted in either automotive or space applications until the late 1980s and early 1990s. An interesting fact about these oils is that in their pure state they do not interact with the seals in motors and gear assemblies; so, to prevent leakage and loss of the oils, it was necessary to blend them with other molecules (polyesters) that would swell the seals and stop leaks.
Testing
One of the earliest lubrication tests Aerospace conducted on mechanisms and bearing systems involved a despin mechanical assembly (also known as a bearing and power transfer assembly) for the Defense Satellite Communication System program (DSCS II). In this case, the contractor had planned a seven-year life test, and Aerospace wanted to use this opportunity to develop a predictive tool for future system design and operation. The test was configured so that the movement of oil throughout the entire mechanism could be measured by tracking the movement of isotopically labeled additives placed at different locations of the ball-bearing system. The results of this test were somewhat unexpected: The actual migration of oil within the device could not be demonstrated. In fact, the additives pretty much remained in their original locations. But this early test laid a firm foundation for future efforts that have helped resolve many anomalies in a variety of mechanisms for many different spacecraft programs.
Research associate Sandra Jackson injects an oil sample into the Supercritical Fluid Chromatograph. This instrument has been widely used to evaluate oil purity and to assess the impact of exposure to vacuum and tribological function on the molecular weight distribution of liquid lubricants. |
Although the oils being used during this time were superrefined and of high quality, they still had vapor pressures higher than the pressure (vacuum) of surrounding space. As a result, the most common problem affecting precision instruments in space was a loss of sufficient lubrication and degradation of the remaining oil because of heat generated through friction within the bearings. When a lubricant breaks down, the molecules either break apart and evaporate, or they react to form polymers and high-molecular-weight molecules (typically known as sludge or varnish). These "gummy" materials can react with metal surfaces, causing wear, or they can deposit on the contacting surfaces, increasing friction. The increased friction raises the contact temperatures and causes more reaction of the lubricant, resulting in a "death spiral" toward catastrophic failure.
This problem was effectively mitigated through sophisticated engineering techniques that minimized oil loss by supplying bearing cartridges with vapor or liquid oil. Aerospace scientists developed models for predicting the loss rates of oils—and thus, life expectancy of mechanisms—depending on measured vapor pressures and the partitioning of fluid mixtures over the life of a mechanism. Also, NASA and the Air Force sponsored programs to develop less volatile and less reactive synthetic oils based on fluorocarbon technology, which provided longer residence times within mechanisms. Fluorocarbons are made by similar processes as synthetic hydrocarbons but with different starting materials. These new fluids had excellent properties for thermal management, and continue to be ideal for lubrication where oil films separate rolling or sliding surfaces with very little solid-to-solid contact. (A good example is the hard-disk drive, where the head literally flies over the disk surface with no direct contact.)
However, these oils could not be formulated with antiwear and antifriction additives (because the additives would not dissolve in the oils), making them unsuitable for most high-precision bearings with long life requirements, especially when the rolling contacts had some degree of asperity or microroughness. Aerospace once again began testing the properties of these and other fluids to find solutions to demanding mechanical applications.
One of these studies involved the Operational Line Scanner for the Defense Meteorological Satellite Program (DMSP). In this case, the original manufacturer discontinued the bearings that were used for the support gimbal in the scanner. When new bearings were purchased from a different supplier, the silicone lubricant that had been used for the old bearings proved inadequate for them. (Even though the new bearings were fabricated to the same specification as the old ones, they apparently did not have the same surface-chemical or mechanical tolerances and could not be made to the same high-quality standards.) A synthetic fluorocarbon lubricant, known as perfluoropolyalkylether (PFPE), was examined as a replacement. Aerospace conducted tests that demonstrated minimal improvement with this oil, and recommended a new synthetic hydrocarbon oil, polyalphaolefin (PAO). Rigorous tests were conducted at Aerospace that simulated the conditions of the gimbal and showed that the PAO oil would provide a substantial life margin for the scanner. The new bearing-lubricant combination increased the operating life of the Operational Line Scanner by a factor of two or more, resulting in other subsystems becoming life limiting.
James Helt manipulates samples for analyses within an ultrahigh vacuum, variable-temperature scanning probe microscope. This instrument can probe friction and wear phenomena on the atomic and molecular scale, providing fundamental insight into these properties. The inset shows the atomic-scale structure that develops when a solid lubricant containing MoS2 undergoes sliding contact with the microscope probe tip. |
The successful demonstration of PAO oils for DMSP opened the door for their use in many other Air Force mechanical systems. For example, during prelaunch testing for another government program, the satellite reaction wheels exhibited erratic torque signatures. The satellites had been stored on the ground for a number of years before this testing. When the wheels were disassembled, engineers found that the spin bearings were practically dry because the original SRG 40 oil had migrated and evaporated from critical interface areas. Because of previous testing experiences, Aerospace was able to recommend an appropriate PAO oil and quickly conduct tests that showed its superiority over the existing oil and other potential candidates. The wheels were refurbished with the new oil, and the reaction wheels ended up exceeding their expected mission life. In turn, many of these satellites far exceeded their design lives.
Other studies have shown that the test environment of a device—whether in air, vacuum, or inert gas—can substantially alter that device's performance and life. For example, in the case of MoS2, humid air can attack the lubricant, causing adverse affects. On the other hand, in the case of thin films, some amount of oxygen can improve the overall behavior by strengthening the bonds between lubricants and surfaces. Aerospace has recommended low relative humidity levels for storage, and eliminating moisture during testing, of most devices that are lubricated with MoS2. Aerospace has also participated in work to develop doped or multicomponent coatings based on MoS2 that have less sensitivity to humidity and other environmental variables, such as contact loading and operating speed.
Today, many hybrid bearings are made from a combination of high-strength steel rings and silicon-nitride ceramic balls. This combination is particularly robust, because the use of dissimilar materials at tribological interfaces helps prevent unwanted surface-chemical reactions—for example, the induced adhesion (cold welding) that can exacerbate surface wear in some metal-on-metal designs. On the other hand, the use of dissimilar materials presents greater challenges for lubrication. Aerospace has conducted metallurgical and tribological tests of different steel-ceramic combinations and used ball-bearing models to help determine the best lubrication system for this configuration. A type of synthetic oil, multiply-alkalated cyclopentane, has been used in combination with hybrid bearings to produce high-speed control moment gyros. This oil, Pennzane, is a hydrocarbon—as are PAOs and mineral oils—with remarkably low vapor pressure; it's also suitable to carry space-proven antiwear additives and can be formulated into greases with predictable properties, making it the present standard for effective space lubrication. Unit tests conducted at a momentum-wheel supplier's facility demonstrated that hybrid bearings lubricated with Pennzane have achieved high reliability and long life.
Conclusion
The success of these and other tests for the design and lubrication of moving mechanical assemblies are the results of a synthesis of testing philosophies with increasing knowledge of surface-chemical and physical phenomena. Aerospace maintains many analytical instruments for the study of surface science, and has applied these techniques to many problems in tribology, such as the effective use of antiwear additives in oil formulations and the performance of solid lubricants under a variety of operating conditions.
Acknowledgments
Much of the work discussed in this article was performed in conjunction with Aerospace scientists David Carre, Ann Bertrand, Jeffrey Lince, and Peter Frantz, whose contributions are gratefully acknowledged.
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