Radiation-resistant coating developed by VCU engineer will protect future Artemis astronauts

By David Pulgar
VCU College of Engineering

Outside the protective field of Earth's outermost atmospheric layer, the magnetosphere, lies an invisible cascade of danger. Ionizing radiation from both the remnants of distant supernovas and from the sun can damage our cells, causing cancer and other harmful effects.

Shielding astronauts from those and other sources of ionizing radiation is an important step for manned spaceflight to Mars and beyond. Arvind Agarwal, Ph.D., professor and chair of the Department of Mechanical and Nuclear Engineering in Virginia Commonwealth University's College of Engineering, develops spacesuit coatings to protect astronauts from radiation and safeguard their suits against rogue rocks and dust on the moon and Mars.

Now, his work is on board the International Space Station, carried there by a Japanese cargo shuttle earlier this year.

"We started this work five years ago to develop shielding for future space missions, like Artemis, to protect spacecraft from radiation and physical impacts caused by lunar dust," said Agarwal, referencing NASA's mission to return humans to the Moon and ultimately reach Mars. "Astronauts living on the moon and Mars will go outside their habitats often, and taking that same kind of protection with them wherever they are is essential to their safety."

Setting healthy boundaries

Arvind Agarwal demonstrates the application of the polydimethylsiloxane boron nitride slurry to a fabric layer. (College of Engineering)

Neutron radiation is formed during nuclear fission or fusion, when unbound neutrons are released from atomic nuclei. Those neutrons interact with the hydrogen in our bodies and break down our DNA.

Lightweight elements, like boron, excel at creating a boundary against neutron radiation. Agarwal builds specialized coatings from those elements, which filter out radiation like how a water filter removes large impurities from drinking water.

Agarwal's coatings were developed in collaboration with former graduate student Sara Rengifo, now at NASA's Marshall Space Flight Center, who also served as a primary investigator on the project while in Agarwal's lab at Florida International University.

The pair have worked on two different applications for their material: Applying the coatings to metals and other hard surfaces, in order to cover a ship or habitat, and applying the coatings to spacesuits.

"Boron-nitride is a light material that absorbs the particles of neutron radiation capable of harming us," Agarwal said. "Its low coefficient of friction also makes it an excellent solid lubricant, meaning anything impacting the material slides off easily. Because of this, boron-nitride based coatings can be extremely abrasion-resistant and protect against impacts from small rocks and debris."

Researchers typically produce boron-nitride coatings in flat layers. Stacking these layers one atop the other, like a Post-it note, makes the material more resilient against repeated physical impact. The coatings can also be made as nanotubes - structures made from multiple cylinders inserted inside each other, and roughly 100,000 times thinner than a human hair.

That geometry makes the boron-nitride material even more durable. The researchers have also combined their boron-nitride substance with titanium, which further strengthens it.

"While it's an amazing material, boron-nitride is soft. Even as a nanotube, repeated impacts will eventually cause it to lose its abrasion-resistant properties," Agarwal said. "That's what we're experimenting with - trying to find the right recipe, the right combination of these materials and the right manufacturing technique to make the most effective product for a spaceship or habitat."

Protection from the elements

Arvind Agarwal's PDMS boron nitride samples are placed in an exterior testing frame, highlighted by the yellow box, and exposed to the vacuum of space. (NASA)

Agarwal's research at VCU tests the application of boron-nitride for polymer materials, like spacesuit fabric. Instead of mixing in titanium, the boron-nitride formulation is made into a slurry by combining it with a silicon-based polymer. The viscous material is then applied to fabric in a very thin layer.

As with the researchers' titanium surface coating for ships and habitats, finding the right ratio of polymer, boron and nitrogen is crucial to making a protective and durable material that remains flexible.

"The layer we're testing is about 10 microns thick. It's very, very thin. Think about dipping a white shirt into a colored dye, only the polymer slurry is more viscous than water," Agarwal said. "We still want abrasion resistance, but for a spacesuit the important thing is radiation protection and for the material to be flexible."

Before its rocket ride to the space station, Agarwal conducted a lunar storm erosion test of the polymer-boron-nitride material in a test rig in his lab at VCU. He exposed the coated fabric to high-velocity impacts, in order to simulate erosion with sharp lunar particles. After surviving the erosion test, he moved the fabric to a second phase of experimentation: Finding out how much radiation it would absorb while mounted outside of the space station, exposed to the vacuum of space.

Those tests paid off: a small sample of Agarwal's work is currently circling the Earth on the outside of the space station. The team is monitoring it from Earth, using a camera mounted outside the space station to observe the material.

"Radiation exposure makes polymer material hard and brittle, causing it to crack," Agarwal said. Astronauts on the space station are also physically checking on the coating. "They're weighing the samples at intervals to measure atomic oxygen. If enough oxygen has entered the sample that the weight has changed then that means the coating has degraded."

Studying degradation over time helps the researchers predict the coating's lifetime. A fixed expiration date could signal when a spacesuit needs to be replaced or have its coating reapplied.

Applications on Earth

PDMS boron nitride samples are placed into the frame that will be exposed to space. (College of Engineering)

The sample material will splash down with a space station return mission in August. Once the samples return to Earth, the researchers will use electron microscopy and other high-end tools to study how it held up in space.

But the coating's applications don't stop once it re-enters the atmosphere. Boron-nitride is a good thermal conductor and electric insulator, and the researchers think it has applications for everyday electronics. Think of your phone: if you hold it up to your ear for a long conversation, it gets noticeably warmer. A boron-nitride nanotube could channel heat away more effectively, making the device more comfortable for extended use.

In its layered sheet form, boron-nitride is a cheap and common solid lubricant, but its nanotube form is significantly more expensive. However, Agarwal said, "The work we're doing on the space station could eventually lead to several new applications of boron-nitride nanotubes, resulting in nanotubes being cheaper."

That could lead to technological improvements for everyone - not just astronauts.

A version of this story was originally published on the College of Engineering website.