Does rust never sleep?

The findings, published in the Proceedings of the National Academy of Sciences, could be relevant to understanding and perhaps controlling oxidation in a range of materials, from catalysts to the superalloys used in jet engine turbines and the oxides in microelectronics.

The experiments were performed by a team led by Guangwen Zhou, Associate Professor of Mechanical Engineering at Binghamton University, in collaboration with Peter Sutter of the Center for Functional Nanomaterials (CFN) at the U.S. Department of Energy’s Brookhaven National Laboratory.

The team used a Low-Energy Electron Microscope (LEEM) to capture changes in the surface structure of a nickel-aluminium alloy as ‘stripes’ of metal oxide formed and grew under a range of elevated temperatures.

The metal Zhou wanted to study, nickel-aluminium, has a characteristic common to all crystal surfaces: a stepped structure composed of a series of flat terraces at different heights. The steps between terraces are only one atom high, but they can have a significant effect on material properties. Being able to see the steps and how they change is essential to understanding how the surface will behave in different environments, in this case in response to oxygen.

“The acquisition of this kind of knowledge is essential for gaining control over the response of a metal surface to the environment.”

Scientists have known for a while that the atoms at the edges of atomic steps are especially reactive. “They are not as completely surrounded as the atoms that are part of the flat terraces, so they are more free to interact with the environment,” commented Sutter. “That plays a role in the material’s surface chemistry,” said Zhou.

The study, supported by the Department of Energy Office of Science, showed that the aluminium atoms involved in forming aluminium oxide stripes came exclusively from the steps, not the terraces. But the LEEM images revealed even more: The growing oxide stripes could not ‘climb’ up or down the steps, but were confined to the flat terraces. To continue to grow, they had to push the steps away as oxygen continued to grab aluminium atoms from the edges. This forced the steps to bunch closer and closer together, eventually slowing the rate of oxide stripe growth, and then completely stopping it.

“For the first time we show that atomic steps can slow surface oxidation at the earliest stages,” said Zhou.

However, as one stripe stops growing, another begins to form. “As the oxide stripes grow along the two possible directions on the crystal, which are at right angles to one another, one ends up with these patterns of blocks and lines that are reminiscent of the grid-based paintings by Mondrian,” commented Sutter.

Still the details and differences of the two types of surfaces could offer new ways scientists might attempt to control oxidation depending on their purpose.

“Oxides are not all bad,” said Sutter. “They form as a protective layer against corrosion attack. They play important roles in chemistry, for example in catalysis. Silicon oxide is the insulating material on microelectronic circuits, where it plays a central role in directing the flow of current.”

Knowing which kind of surface a material has and its effects on oxidation, or how to engineer surfaces with desired properties, might improve the design of these and other materials.