Atomic Layer Deposition of Ultra-thin Boron Nitride
Space has a surprising amount of dangerous things in it. There are micrometeorites, solar particles, and of course all the space junk, everything from spent rocket stages to fragments of paint, zipping past satellites at up to 12.4 miles a second. They pose hazards to sensitive spacecraft optics, detectors, and solar panels in satellites and spacecraft.
Even though engineers have expanded different techniques to defend spacecraft from these fast-moving surprises, nothing gives 100-percent protection.
However, at NASA’s Goddard Space Flight Center in Greenbelt, Md., a technician is experimenting with an emerging technology that may provide another, possibly more effective technique for defending sensitive spacecraft components from these high-velocity bombardments.
Vivek Dwivedi and his colleague, Raymond Adomaitis, chemical engineering professor from the University of Maryland, are using atomic layer deposition (ALD), a rapidly evolving technology for coating plastics, semiconductors, glass, Teflon, and a surfeit of other materials, to create a new super-strong, ultra-thin coating made of tiny tubes of boron nitride, similar in appearance to the bristles on a toothbrush.
”Crystalline boron nitride is one of the hardest materials in the world,” Dwivedi says. This makes it perfect as a coating for sensitive spacecraft components. Crystalline boron nitride could make them less susceptible to damage when struck by space dust, tiny rocks, and high-energy solar particles.
Atomic Layer Deposition
The ALD technique, which the semiconductor industry has implemented in manufacturing of computer chips, involves placing a substrate material inside a reactor chamber and sequentially pulsing diverse types of precursor gases to create an ultrathin film. The layers are literally no thicker than a single atom.
ALD differs from other thin film application techniques since the process is split into two half reactions, run in sequence, and repeated for each layer. Consequently, technicians can precisely control the thickness and composition of the deposited films, even deep inside pores and cavities.
This gives atomic layer deposition a distinctive ability to coat in and around 3-D objects. That advantage, combined with the fact that technologists can create films at much lower temperatures than with the other techniques, has led many in the electronics, optics, textile, energy, and biomedical-device fields to replace older deposition techniques with ALD.
According to Dwivedi, if technicians use atomic layer deposition to coat glass with aluminum oxide, for example, they can fortify glass so it is over 80 percent stronger. The resulting thin films act like ”nano putty,” filling the nanometer-scale defects found in glass; the very same minuscule cracks that cause glass to break when struck by an object.
”This ALD application has profound possibilities for the next-generation crew modules,” Dwivedi said. ”We could decrease the thickness of the glass windows without sacrificing strength.”
”It’s really exciting,” said Ted Swanson, assistant chief for technology for mechanical systems at Goddard. ”This is an emerging technology that offers a wholly new way to protect spacecraft components, perhaps more effectively than what is possible with current techniques. Just as important, with ALD, we can lay down material less expensively.”
Hard Material to Work With
But that doesn’t mean the task is easy, Dwivedi said. Manufacturing an ALD-based coating made of boron and other precursor gases is remarkably difficult to do.
Currently, technologists manufacture boron films through reacting boron powder with nitrogen and a small quantity of ammonia in a chamber that requires heating to a blistering 2,552 degrees Fahrenheit. It is an expensive process. With atomic layer deposition, ultrathin boron-nitride film could be laid in a chamber no hotter than 752 degrees Fahrenheit.
”Our team has studied the difficulties and think we understand why they’re happening,” Dwivedi said. As a result, he judges the team will succeed at depositing boron nitride on a silicon substrate by next year. He believes instrument designers could one day use the technology to coat mirrors, spacecraft buses, and other components, if tests at Goddard and NASA’s Langley Research Center in Hampton, Va., validate the material’s effectiveness as a protective coating, Such testing could happen as early as summer of 2013.
Dwivedi and his team are not only creating a protective coating, they are using funding from NASA’s Center Innovation Fund and Goddard’s internal R&D program to test the technique as a possible way to coat X-ray telescope mirrors, which must be curved to collect high-energy X-ray photons that would otherwise pierce flat mirrors, and radiators needed to direct heat away from sensitive instruments.
”This technology can coat anything. It is perfect point-to-point. There are so many applications for this technology,” Dwivedi said. ”The only thing limiting its use is your imagination.”