PNNL

The Science

Developing oxidation-resistant materials requires understanding the initial oxidation of surfaces, including the formation of thin passivating layers that protect the bulk of the alloy. Researchers studied the oxidation of different crystallographic facets of a nickel-chromium (Ni-5Cr) model system. They found that the (001) surface has a higher initial oxidation resistance compared to the (111) surface despite the slower rate of steady-state oxidation for the (111) surface. Simulations show that the difference in initial oxidation rate is due to variations in ion diffusion dynamics at the nickel metal/oxide interface that result from an atomic sieving effect.

The Impact

Metals and alloys with oxidation resistance are highly sought after for applications ranging from kitchen utensils to industrial machinery. However, designing these materials requires understanding the atomic nature of oxidation processes at exposed surfaces and buried interfaces. By identifying the underlying reason behind the different oxidation rates at different crystal facets, researchers can develop tailored materials that are more oxidation resistant. These materials can be grown to only expose the facets that passivate the fastest and most fully, leading to improved corrosive resistance.

Summary

Oxidation is central to how many materials degrade and fail. Oxidation-resistant materials have potential applications across industries and consumer goods, making their development an active avenue of research. A research team used a model Ni-5Cr alloy to explore the differences in initial oxidation rates of two crystalline surface facets. They used a specially designed in situ environmental transmission electron microscopy approach to study multiple crystal faces in real time, simultaneously. They observed that the Ni(111) surface initially oxidizes faster than the Ni(001) surface. This rate difference occurs because of diverging diffusion dynamics for nickel and oxygen at the underlying metal/oxide interfaces for each crystal orientation. Oxygen diffuses faster across the (111) interface than the (001) interface with the underlying structure, leading to an atomic sieving effect. This effect selectively promotes the diffusion of certain atomic species. Despite its initial higher oxidation rate, the (111) surface is more oxidation resistant in the steady state. This work identifies an underlying mechanism of surface oxidation and clearly shows the difference in short- and longer-term oxidation at different crystal facets. This knowledge can help researchers tailor their synthetic approaches for creating oxidation-resistant materials.

PNNL Contact

Daniel Schreiber, Pacific Northwest National Laboratory, daniel.schreiber@pnnl.gov

Chongmin Wang, Pacific Northwest National Laboratory, Chongmin.Wang@pnnl.gov

Funding

This work was primarily supported by the U.S. Department of Energy (DOE) Office of Science, Basic Energy Sciences, Materials Science and Engineering Division, Mechanical Behavior and Radiation Effects program (PNNL FWP 56909). Environmental transmission electron microscopy measurements were conducted on a project award (DOI: 10.46936/staf.proj.2016.49687/60006123) from the Environmental Molecular Sciences Laboratory, a DOE Office of Science user facility sponsored by the Biological and Environmental Research program at Pacific Northwest National Laboratory.