Around the turn of the decade, NASA's Kepler Space Observatory lived like a rock star.
Launched in 2009 into an Earth-trailing heliocentric orbit, it turned out an amazing amount of work over a short period of time, changing its fans' perceptions of their very place in the universe. But within four short earth years, Kepler's moment had passed: it seemed the $600 million wonder, like many a pop culture hero before it, had found it was indeed better to burn out, than to fade away.
By the middle of 2013, two of the space observatory's four reaction wheels had failed. Kepler had already exceed goals set by NASA, and the wheel failures came as little surprise to scientists familiar with the elevated friction levels experienced in space. Kepler could no longer lock onto faraway stars precisely enough to learn anything substantive, and was reprogrammed to take on a less strenuous set of tasks.
A satellite in orbit is a giant machine--as large as tens of meters across, according to Brandon Krick, assistant professor of mechanical engineering at Lehigh University--and this enormous, expensive, somewhat unwieldy device needs to be condensed down in size--anywhere between a watermelon and a Volkswagen--in order to withstand the forces set upon it during its journey into outer space.
But once it reaches its orbit, it needs to delicately unpack itself without damage. And once it sets up shop, the extreme low-pressure of the machine's surroundings presents yet another major challenge--one that requires a tribologist like Krick, who studies friction and its impact on matter and materials, to solve. If most traditional lubricants vaporize in space, how can the hundreds or thousands of tiny moving parts on a complex machine be spared from the "live fast, die young" ethos of Kepler's wheels?
Krick was recently granted Faculty Early Career Development (CAREER) Award from the National Science Foundation (NSF) to explore this and related questions. He and his team will be focusing on a material called Molybdenum Disulphide (MoS2) and its suitability as a lubricant in space and other extreme environments.
The CAREER Awards are NSF's most prestigious awards in support of early-career faculty. According to the NSF, the Awards support emerging academic leaders who have demonstrated potential to serve as role models in research and education. Formally kicking off on August 1, 2018, the award provides $500,000 over five years to support Krick's tribology-based research, educational, and outreach activities.
According to Krick, understanding the fundamental science of friction and wear doesn't just impact satellites--developed countries lose an estimated 2 to 7 percent of their gross domestic product to costs associated with friction and wear, including energy loss and replacement of worn-out machinery.
"The toll is approaching a trillion dollars in the U.S. alone," Krick says. "You need friction or you wouldn't have any grip or be able to react to force. But we need to be able to predict, control and in many cases reduce friction and wear, and that will lead to new materials and devices."
MoS2 is a solid lubricant in the form of a coating or powder, which means it won't vaporize in space like more familiar greases and oils will. It's also ideal, Krick says, because it's a lamellar material, which means its atoms are tightly bound in one plane, existing as microscopic layers. Two planes interact with each other, but do so weakly; as they slide against each other, the risk for excessive friction--and resulting machine failure--is low.
But MoS2 presents challenges as well. Krick says that while space is generally thought to be a vacuum, it's not a perfect one. His team's samples will ultimately be tested outside the International Space Station, where a significant amount of atomic oxygen exists, he says. This is a particularly corrosive agent, and when it reacts with MoS2, it can turn into Molybdenum Trioxide (MoO3), which can increase the shear strength between the two planes of the lamellar material, creating more friction and causing the system to fail.
To learn more before these samples go up in space, Krick will be working with the High Sensitivity-Low Energy Ion Scattering Spectrometer (HS-LEIS) at Lehigh to age samples with atomic oxygen and study how the surface chemistry changes. This NSF-funded machine offers a 3,000-fold improvement in sensitivity over conventional spectrometers, and is one of just two machines of its kind with that degree of sensitivity in the United States.
HS-LEIS identifies atoms on the outermost layer of a surface by firing noble gas ions at it. An ion then either bounces straight back or is deflected at an angle, and a fraction of its energy remains with the surface atom. The amount of energy lost is directly related to the atomic weight of the surface atom. The spectrometer then measures the energy of the rebounding noble gas ions to determine the identity of the atom from which it was scattered.
Krick also uses an ultra high vacuum (UHV) system in his experimental laboratory that allows him to simulate a space environment by pumping gas molecules out of the chamber. And with the new NSF support, he plans to build a small tribometer to use in conjunction with other systems, including electron microscopes and spectrometers, for in situ surface chemistry analysis of various materials when they interact with MoS2.
In addition to this unique instrumentation, Lehigh's vaunted alumni network is helping to fan the flames of Krick's research.
"Lehigh alumnus John Curry is one of the first students to graduate from my lab," Krick says. "Since successfully completing his Ph.D. studies earlier this year, John was quickly snapped up by Sandia National Laboratories, and I'm proud to say that we continue our partnership through this project. He performed a great deal of what turns out to be critical preliminary work for this project while he was on campus, including a new testing method for this type of material system. With this new support from the NSF, we're diving in even deeper."
"Sandia has some important analytical tools that we will be able to use," he continues, "as well as some state-of-the-art computational capabilities to perform molecular dynamics simulations. Together, we stand to gain some significant insight into the molecular mechanisms of friction in MoS2. Though this project, we hope to enable progress in the aerospace industry and space exploration, and perhaps find ways that this research can help fight friction right here on Earth as well."