Self-healing metal oxides could protect against corrosion

Researchers find an ultrathin layer of aluminum oxide, though solid, can flow like a liquid instead of cracking.

Researchers have found that a solid oxide protective coating for metals can, when applied in sufficiently thin layers, deform as if it were a liquid, filling any cracks and gaps as they form. Image: Christine Daniloff/MIT
Researchers have found that a solid oxide protective coating for metals can, when applied in sufficiently thin layers, deform as if it were a liquid, filling any cracks and gaps as they form. Image: Christine Daniloff/MIT

MIT scientists have recently discovered that a strong oxide defensive covering for metals can, when connected inadequately thin layers, disfigure as though it were a fluid, filling any breaks and holes as they frame.

The thin covering layer ought to be particularly helpful to forestall spillage of little atoms that can enter through most materials, for example, hydrogen gas that could be utilized to control energy unit autos, or the radioactive tritium (a substantial type of hydrogen) that structures inside the centers of atomic power plants.

Scientists used exceedingly particular instruments to see in detail the surface of metals covered with the special oxides including aluminum oxide, chromium oxide, and silicon dioxide to perceive what happens when they are presented to an oxygen domain and set under pressure.

While most transmission electron magnifying instruments (TEMs) require that examples be considered in a high vacuum, the group utilized a changed variant called a natural TEM (E-TEM) that enables the example to be contemplated within the sight of gases or fluids of intrigue. The gadget was utilized to consider the procedure that can prompt a kind of disappointment known as pressure consumption breaking.

The team, led by MIT graduate student Yang Yang explained, “Metals under stress from pressure inside a reactor vessel and exposed to an environment of superheated steam can corrode quickly if not protected. Even with a solid protective layer, cracks can form that allows the oxygen to penetrate to the bare metal surface, where it can then penetrate into interfaces between the metal grains that make up a bulk metal material, causing further corrosion that can penetrate deeper and lead to structural failure. “We want an oxide that is liquid-like and crack-resistant.”

That suggests aluminum oxide at room temperature can have just that liquid-like flowing behavior. If it is made into a thin enough layer, about 2 to 3 nanometers (billionths of a meter) thick.

Yang said, “Traditionally, people think that the metal oxide would be brittle” and subject to cracking. No one had demonstrated otherwise because it is so difficult to observe the material’s behavior under realistic conditions. That’s where the specialized E-TEM setup at Brookhaven National Laboratory, one of only about 10 such devices in the world, came into play. No one had ever observed how it deforms at room temperature.”

Ju Li, a professor of nuclear engineering and science at MIT said, “For the first time, we’ve observed this at nearly atomic resolution. This approach demonstrated that an aluminum oxide layer, normally so brittle it would shatter under stress when made exceedingly thin is almost as deformable as a comparably thin layer of aluminum metal — a layer much thinner than aluminum foil. When the aluminum oxide is coated onto a surface of a bulk piece of aluminum, the liquid-like flow “keeps the aluminum covered” with its protective layer.”

“The researchers demonstrated inside the E-TEM that the aluminum with its oxide coating could be stretched to more than double its length without causing any cracks to open up. The oxide “forms a very uniform conformal layer that protects the surface, with no grain boundaries or cracks,” even under the strain of that stretching, he says. Technically, the material is a kind of glass, but it moves like a liquid and fully coats the surface as long as it is thin enough.”

Yang said, “People can’t imagine that a metal oxide can be ductile. For example, a sapphire is a form of exactly the same material, aluminum oxide, but its bulk crystalline form makes it a very strong but brittle material.”

The team also included research affiliate Akihiro Kushima at MIT, Weizhong Han at Xi’an Jiaotong University in China, and Huolin Xin at Brookhaven National Laboratory. The work was supported by the National Science Foundation.

The paper appears in the journal Nano Letters.