A research team recently discovered that aluminum oxide, a chemical compound commonly used as a protective coating for metals, can deform into a thin, liquid-like substance that can fill cracks and prevent leakage of corrosive or radioactive materials. The team consisted of researchers from the Massachusetts Institute of Technology (MIT) (Cambridge, Massachusetts, USA), Xi’an Jiaotong University (XJTU) (Shaanxi, China), and Brookhaven National Laboratory (Upton, New York, USA), and their work was supported by the National Science Foundation (Alexandria, Virginia, USA).
Most metals tend to oxidize when exposed to air and water, resulting in cracks and structural failure over the long term. However, aluminum is one of three elements (along with silicone and chromium) that produce an oxide that can protect metals against abrasion and corrosion. With that in mind, the researchers wanted to determine why aluminum oxide and silicon dioxide offer excellent corrosion resistance.
They used an environmental transmission electron microscope (E-TEM) to examine how samples of these oxides would react on metal surfaces that are exposed to an oxygen environment and placed under stress. The E-TEM enabled researchers to study the effects of certain gases or liquids on the samples to see if they would exhibit signs of stress corrosion cracking (SCC). Whereas other metals corrode quickly under SCC, the researchers found that aluminum oxide was not only able to protect the metal surface but could also penetrate between the metal grains, thereby preventing deep-set corrosion.
Because it had been impossible to observe how metal oxides behave at room temperature, it was assumed they were too brittle to assume a flowing, crack-resistant form. But thanks to the E-TEM donated by the Brookhaven National Laboratory—one of only about 10 such devices in the world—the team learned that aluminum oxide can, even at room temperature, be made into a liquid-like layer about 2 to 3 nm thick. As a result, a flowing layer of aluminum oxide coated onto an aluminum surface protects it from structural failure.
According to MIT professor Ju Li, an oxide-coated portion of aluminum “forms a very uniform conformal layer that protects the surface” and can be stretched to more than twice its length without signs of cracking. He adds that the liquid-like properties of this aluminum oxide coating and its ability to prevents the intrusion of cracks or grain boundaries could make it a viable material in a variety of potential applications.
Li was a senior author of a paper that details the team’s findings. Entitled “Liquid-Like, Self-healing Aluminum Oxide during Deformation at Room Temperature,” the paper was published in the journal Nano Letters.
Source: MIT News, www.news.mit.edu.