Guest post by SHI EN KIM
MSU SciComm blog contest winner
Climax mine in 2007 Photo credit: Jerry and Roy Klotz MD Wikimedia commons
In the late 1800s, settlers in the west chanced upon a deposit of a flaky, silvery mineral tucked away on the slopes of Bartlett Mountain, Colorado. This was no ordinary mineral—unlike other rocks, it felt slippery to touch when the settlers rubbed it between their fingers, and its flakes also stuck to the skin, just as would modern day glitter. The settlers had run out of animal fat to lubricate their wagon wheels, so the mystery mineral was a perfect substitute.
Modern day miners in the west swear by this tale—how molybdenum disulfide (MoS2) was accidentally discovered, and that it is an excellent lubricant. The mineral deposit in question became Colorado’s Climax Mine, the world largest molybdenum mine during its heyday, and is currently in operation despite seesawing between closure and revival over the past few decades.
At first, MoS2 had no demand in the market. In 1927, MoS2 first entered the patent records as a solid lubricant when mixed with other materials. It was only during the space race in the late 1940s, when MoS2 lubrication research and use skyrocketed, after scientists found that it retains excellent lubrication properties in hostile environments.
When a door hinge squeaks or a bicycle chain catches, we apply oil or grease between the two solid surfaces rubbing against each other. However, outer space opens up a new set of challenges that disqualifies the use of conventional liquid lubricants. Liquid lubricants boil off in low pressure environments and may contaminate other surfaces. Furthermore, lubricants need to be stable at extreme temperatures. Outside the protection of Earth’s atmosphere, such as on the surface of the International Space Station, temperatures can swing from -250-250 F. Common household lubricants are typically not stable at these temperatures—they freeze up or evaporate.
Enter solid lubricants—a counterintuitive type of lubricant that can ease up sliding when sandwiched between two solid surfaces. The key to solid lubricants is that it can be sheared easily. That is, solid lubricants can tolerate—or even promote—within itself the sliding motion similar to your rubbing your palms together. The friction between your palms warms your hands up; solid lubricants instead reduce this friction.
MoS2 makes a good solid lubricant because of its unusual layered structure. It is made up of atomically thin layers that are weakly held together, so these sheets can slip past one another along any direction. MoS2 is chemically stable for a wide range of temperatures, and unlike other layered solids, performs best as a lubricant under vacuum. Researchers have measured extremely low friction levels for between individual sheets of MoS2—under vacuum, they can be slicker than a banana peel!
MoS2 and other solid lubricants can be applied by spraying, or transferred atom-by-atom from a chunky source in a process known as sputtering. Sputtering works by kicking up MoS2 from the source using a beam of ions, then the gasified atoms migrate to a hard surface to form a uniform layer roughly a hundred times thinner than human hair. Unlike liquid lubricants, solid lubricants cannot be replenished. This limits the applications of solid lubricants to one-time or low duty operations, such as the unfolding of an antenna. Even then, care needs to be taken so as not to wear out the solid lubricant by the slightest vibrations while a spacecraft is still earthbound. The space probe Galileo had a rough start when its high-gain antenna did not fully unfold soon after it was deployed. Investigations revealed that the MoS2 lubricant had worn thin in the joints when Galileo was shuttled back and forth across the country on the trailer of a humming 18-wheel semi. During transportation, the jostling of the probe used up most of the MoS2, leaving insufficient amounts for the actual unfolding of the Galileo antenna in space.
For all the characteristics that make MoS2 suited for space, MoS2 suffers from several drawbacks. MoS2’s lubricity deteriorates in the presence of moisture and air. In certain applications involving high speeds, alternative lubricants to MoS2 are preferred. The solid lubricant Teflon, the same waxy coating on non-stick pans, has been added into the moving parts themselves for the turbopumps on rocket engines such as SpaceX’s Falcon 9 which launched last May. Researchers have also devised a slew of greases that can withstand vacuum environments. These greases are more suited for continuous movements over a long lifetime such as the long-term exploration of a rover, but they typically work in a narrow range of temperatures.
“Space is not a single-issue environment,” says Chris Dellacorte, a senior technologist who specializes in the study of friction at NASA’s Glenn Research Center in Cleveland, Ohio. “[The study of friction and lubrication] runs across a wide spectrum of applications, and each application has different requirements.”
Every space mission poses a unique set of criteria; there is no one-size-fits-all lubricant. The development of the Mars rover Curiosity is one example in which researchers had to make tough calls over the lubricant choice. MoS2 was initially selected to lubricate the gears, given that the temperatures on Mars dipped as low as -80˚F, ruling out most liquid options. But during the testing stage, researchers discovered that MoS2 wore out too quickly. The final design: grease and heaters for the gearboxes. The heaters on Curiosity today consume 30% of the power just to warm the grease and add an extra 100 kilograms to the rover. “We try to use solid lubricants,” says DellaCorte, explaining that solids are more stable at extreme temperatures. “But sometimes we [have to] pay the engineering penalty of using a more conventional lubricant.”
DellaCorte’s research focuses on developing lubrication technologies for new hostile environments. For example, Venus has an atmosphere of CO2 at a pressure above 1000 psi and also hosts clouds of sulfuric acid. Surface temperatures soar to 900˚F. What are the possible lubricant candidates for a Venus probe?
Not oils and greases—for which CO2 is an excellent solvent. In fact, pressurized CO2 is used in dry cleaning. CO2 might spare plastic lubricants such as Teflon, but instead they will be corroded by Venus’s strong acids. DellaCorte suspects that MoS2 or metal films such as gold might work as lubricants as they are the most chemically resistant and temperature stable contenders.
The average Earthling doesn’t think much about lubrication in his or her daily life, but in space, lubrication is a non-trivial engineering challenge that can make or break a mission. As humankind seeks out new worlds further away from Earth, lubrication technologies need to match the pace of our otherworldly ambitions. Engineers like DellaCorte at NASA’s Glenn Research Center and countless others around the world are striving to do just that.
“At the end of the day, we're trying to get the job done so the scientists can collect the scientific information they want.”
SHI EN KIM is a graduate student in molecular engineering at the University of Chicago and a freelance science writer. To see more of her writings, visit her Twitter profile @goes_by_kim or her website shienkim.wordpress.com.
ADDITIONAL IMAGE CREDITS:
A raw molybdenite ore embedded in quartz. Photo credit: John Chapman; Wikimedia commons
Galileo’s high gain antenna (the orange ribbed structure on the left) never fully opened. Illustration credit: Ken Hodges, NASA
NASA’s Curiosity rover on Mars, well-oiled and warmed. Credit: NASA
An artist’s impression of Venus’s inhospitable atmosphere, gouged by lightning strikes and streaks of sulfuric acid rain. Credit: European Space Agency