The Contamination and Coatings Engineering Branch at NASA’s Goddard Space Flight Center in Greenbelt, MD, provides system-level support in contamination engineering and thermal coatings engineering from concept to mission end of life. This includes development, implementation, and management of instrument and spacecraft contamination control programs, technical consultation on contamination and coatings issues, thermal coatings applications, material property characterization, and coatings flight qualification.
Led by Randy Hedgeland, the branch maintains specialized laboratories for thermal coatings characterization and environmental testing, molecular kinetics testing, and surface effects measurements. Additionally, the branch designs and builds flight experiments, molecular adsorbers, custom thermal control materials, and protective coatings for astronaut visors. In support of current NASA initiatives, the branch also performs research and technology development in the areas of coatings development, extraterrestrial dust mitigation, planetary, laser damage, and advanced cleaning and verification techniques.
Recently, NASA Tech Briefs was given a behind-the-scenes tour of the branch’s Thermal Coatings Laboratory to find out how these coatings are developed, implemented, applied, and flight-qualified for NASA missions.
Two technologies for thermal control are featured in the lab: thin film coating technologies and spray coating technologies. The lab provides the coatings that provide thermal protection for the outside of the spacecraft.
“When you look at a spacecraft, you may see five surfaces: blankets, solar arrays, antennas, instrumentation, and then the exterior coatings that are on the spacecraft. That’s what we do inside this laboratory,” explained Mark Hasegawa, Thermal Coatings Application and Development Group Lead.
If you take a regular automotive or house coating and send it into orbit, it would darken very quickly because of particle radiation that you don’t see on Earth. The ultraviolet radiation is far more intense, and the ozone in the atmosphere stops that. Most of the stable spray coatings used at the lab are silicate-based. “It’s a very brittle coating,” said Hasegawa. “A lot of preparation is required. You can’t spray it on the way you spray an automotive coating.”
The coatings developed and used in the lab must be vacuum-stable. If not, they become a contamination hazard. “You don’t want optics fogging up because of contamination from coatings or materials,” added Hasegawa.
In addition, the spray coatings do not outgas or transport materials out. The silicate-based materials only transport out water, which usually is not a hazard. And since space is a very cold environment, the coatings will be subjected to very cold temperatures, and then be exposed to warm bodies such as the Sun. They need to be able to survive these extreme thermal cycles. Materials on the International Space Station, for example, go through 90-minute orbits of repeated hot and cold, for thousands of thermal cycles, so the coatings must bond well and adhere well.
Thin Film Coatings
The lab is responsible for environmental testing as well as thin coating deposition via vapor deposition chambers that are used to evaporate materials onto spaceflight hardware, blanket material, or whatever surface requires passive thermal control on the exterior of the satellite.
“We coat hardware as it’s rotating under vacuum with a coating developed in this lab in the 1970s, and we’re still the only ones who do it,” said George Harris, manager of thin film coatings technology. “It’s a silver sapphire quartz coating that reflects the vast majority of the Sun’s energy and also emits in the infrared interior energy created by the electronics going up in the satellite.”
The coating was used in a Hubble Space Telescope repair mission after astronauts reported that the existing thermal blanket material on the exterior of the telescope was falling apart. The silver Teflon material was degrading under the combination of ultraviolet and atomic oxygen. “We coated 30 sheets of stainless steel foil with this silver composite coating, and they made blankets out of it and put them on the Hubble,” explained Harris.
The silver sapphire quartz coating is being used in a robotic servicing mission to refuel on-orbit satellites in low Earth orbit for the Satellite Servicing Capabilities Office (SSCO) at Goddard. The mission satellites will experience heat from the Sun and some atomic oxygen and radiation, so the danger will be the fuel line overheating. Harris’s group will be coating the exterior conduit around the fuel line. “The shuttle tiles were made out of the same material, but they were much thicker,” he said. “Our coating is only about a couple of microns thick.”
Harris explained that the lab previously coated all of the astronaut visors prior to the end of the space shuttle program. But before the program shut down, they had the coating “stockpiled” so that they could continue to make visors for the astronauts for the remainder of the flights.
While additive manufacturing or 3D printing may not be a technology that comes to mind initially when you think of making coatings, the lab is using the technology in a number of ways. For example, additive manufacturing is being used to reduce masking and structural loading. There are multiple composite units that are circular, so only the outside can be coated, leaving the inside without coating. In order to coat the entire unit, 3D printed parts were made and the composites were stacked on top of each other to coat multiple parts in one run, as opposed to doing one at a time. This cuts down the masking requirements, along with touch time on the parts, by a factor of ten. So instead of having to mask each part, a number of them can be stacked together with minimal masking, and then they can be sprayed together.
According to Hasegawa, “For a program with 64 parts that are all the same size, it would require many hours of labor for masking and unmasking. We’re trying to use additive manufacturing for automated masking to reduce the touch time.” Additive manufacturing also can be used to manufacture replacement parts for equipment used in the lab. Older spray guns, for example, require replacement parts that may be hard to obtain. By manufacturing functional parts in-house using 3D printers, the lab is able to keep the spray guns working.
Research and development has resulted in a number of innovative coatings being created at the lab that have both current and potential future uses on a number of NASA missions. The Molecular Adsorber Coating (MAC) was developed to mitigate outgassing concerns. When you buy a new car, that “new car smell” is actually from chemicals outgassing from materials in the car. In spacecraft, those outgassed molecules can deposit on sensitive surfaces like telescopes and mirrors. MAC captures those molecules and prevents them from depositing on those sensitive surfaces.
MAC is also being used on the Ionospheric Connection Explorer (ICON) mission in the the Far Ultra- Violet (FUV) instrument to mitigate molecular outgassing. The FUV captures images of the upper atmosphere in the far ultraviolet light range. MAC can also be used inside vacuum chambers for testing, and is planned for use on the inside of the Mars 2020 rover.
Lotus Coating is a hydrophobic and dust-mitigation coating for planetary missions that prevents dirt and bacteria from sticking to and contaminating the surfaces of spaceflight gear. The need for the technology arose when astronauts came back covered with lunar dust that damaged their suits.
The name for the coating comes from the method it uses to prevent contamination from sticking. It mimics that of a lotus plant shedding water. Although the lotus leaf appears smooth, under a microscope, its surface contains innumerable tiny spikes that greatly reduce the area on which water and dirt can attach. This special quality is what the NASA team is attempting to replicate to prevent dirt from accumulating on the surfaces of spacesuits, scientific instruments, robotic rovers, solar array panels, and other hardware. The coating’s potential uses on Earth include car windshields, camera lenses, and eyeglasses — almost anywhere a need exists to repel dirt.
Metallurgy and Material Selection
For metallurgy and material selection for projects at Goddard, Tim Stephenson, Senior Metallurgist in the Materials Engineering Branch, focuses on three major technology aspects. The first, and most recent, is the athermalization of optical structures, part of which has been inspired by the James Webb Space Telescope (JWST). The low-expansion alloy used to maintain alignment of the optical structures — the four instruments that sit inside the JWST Integrated Science Instrument Module (ISIM) — was Invar, which was discovered 100 years ago. Invar is a dense, heavy iron material, and there is almost half a ton of it on JWST. “Since mass drives launch cost, the idea was to try to lightweight this material in any way possible and still have it behave the way we want it to behave — in other words, not change dimensionally with temperature,” said Stephenson.
A second aspect of his work is tailoring thermal expansion. Stresses developed in structures as temperatures change are minimized if they move in lockstep with each other. “We have an active effort in developing silicon optics using single crystal silicon. I can match thermal expansion to zinc solenoid, beryllium, and Schott glasses that were going to be used for x-ray optics, and that’s shifted back to silicon. That’s all part of the athermalization to minimize the distortion that occurs when we change temperature,” Stephenson explained.
The third area is tailoring the geometry of structures to select a natural frequency response; materials that will null out low-frequency vibrations and tolerate high-frequency vibrations. “This has a lot to do with sensitive instruments on the top of a rocket. We can put more sensitive instruments into space if we can tailor the materials around them to null out the damaging frequencies,” he said.
The promise of additive manufacturing technology also is evident in metallurgy. One of the challenging issues with additive manufacturing as far as making structural components, according to Stephenson, is trying to understand their damage tolerance — what is the critical flaw size? “In raw materials, that’s pretty easy to do. It’s more of a challenge with metal matrix composites and ceramic matrix composites,” he explained. “But with additive manufacturing, you’re essentially building things up using the material almost as if it’s a continuous weld. That’s a challenge. There’s a whole non-destructive evaluation staff that’s making rocket components using additive manufacturing.
“Once you look at the cost from idea to actual implementation, about 10% of the cost is in the development of the concept, and about 90% is in the scale-up to actually make it useful. There are a lot of custom things we can do in additive manufacturing that aren’t done in the commercial sector that are driven specifically to cut cost — mixing and matching materials, developing functional structures for natural frequency tailoring,” Stephenson said.
“A lot of the things I’ve worked on have been in response to the science people saying, ‘I wish we had a material that would …,’ and I say, ‘I can help you out with that.’”