Elegant Approach Treats Oxidation as a Design Element

Think back to when you were just a kid with your coloring book, and how excited you were to graduate from the Crayola® 8-pack to the 64-crayon box. That’s how materials scientists feel about design breakthroughs for materials used in aircraft and rocket engines, energy conversion technologies, heat shields and other aerospace and defense applications.

“We have very successfully expanded the materials palette for ultra-high temperature materials,” said Elizabeth J. Opila, professor of materials science and engineering and director of the Rolls-Royce University Technology Center on Advanced Material Systems at the University of Virginia School of Engineering.

Opila’s Advanced High Temperature Materials  research group investigates materials capable of operating at temperatures that can reach 3,000 degrees Celsius and the ways those materials degrade. They are looking for materials that don't melt at these high temperatures.

Opila addresses the shortage of these materials as a member of two multi-university research initiative teams funded by the U.S. Office of Naval Research.

Don Brenner, Kobe Steel Distinguished Professor of Materials Science and Engineering at North Carolina State, leads one effort to understand the science of entropy stabilized ultra-high-temperature materials. Because entropy stabilized materials are made with roughly equal amounts of four or more elements, they have a unique ability to harness disorder and stabilize new compositions with tailorable properties.

An $8.4 million grant awarded in 2015 supports this initiative, which is a collaboration among UVA Engineering, NC State, Duke University and the University of California San Diego. Patrick Hopkins, professor of mechanical and aerospace engineering with courtesy appointments in materials science and engineering and physics at UVA, is also a member of this team.

“Through this collaboration, we discovered that mixing different metal carbides and borides together can really affect the properties of the materials. We created a huge number of compositions that can be tried out to deliver the desired properties for a specific application,” Opila said.

Team members collectively predicted which materials could be made harder, with tailored thermal conductivity. Their work introduces new theoretical, computational and experimental tools to the ultra-high temperature materials community.

The more complex the material, the more elusive its behavior. The UVA team developed a simple method to identify what will happen when ceramics are exposed to oxygen at really high temperatures, working with five-component combinations of nine elements.

“We showed that thermodynamics are really important,” Opila said. “Things are happening so fast that the question ‘what’s happening’ is almost more important than how fast it happens.”

The eureka moment came when Opila and Lavina Backman, a 2020 Ph.D. of materials science and engineering advised by Opila, recognized that some groups of elements in these materials—group IV transition metals such as hafnium and zirconium—reacted with oxygen more readily than others. They also realized that each element, or ingredient, in the base material reacts with oxygen in a preferential order, according to thermodynamic principles.

“A good analogy is someone picking out specific colors of M&Ms from the bag. That person is oxygen, our bag of M&Ms is the material, and each color represents an element. Imagine all the yellow ones getting picked out, then blue, and so on,” Backman said.

The team then developed tools to predict just how this preferential oxidation process would progress. “Our tool can predict which color gets picked out first,” Backman said. “We can use this tool to determine which elements we want and in what amounts to achieve the desired properties of the base material and oxide.”

This research provided the centerpiece of Backman’s dissertation, with select methods and findings published in Acta Materialia and the Journal of the European Ceramic Society.

“This topic was very new in the field of high-temperature materials research. This essentially meant we had a blank canvas, and we were able to propose research directions where oxidation resistance was considered from the very beginning,” Backman said.

Backman, now a Karles Fellow at the U.S. Naval Research Laboratory, considers herself fortunate to have been a part of the MURI team at its formation. “The MURI broadened my horizons as a researcher; I was constantly in conversation with people from different universities and research groups, who introduced me to new ways of thinking, doing science and approaching research tasks,” Backman said.

Backman joined Opila’s group as a Ph.D. student in 2015. “The motivation for the MURI research aligned with my personal motivations for starting graduate school. I have always been inspired by and interested in pushing technology boundaries for space exploration,” Backman said. Opila’s research on high-temperature ceramics, a candidate material for hypersonics, was a perfect fit for Backman, who continues to pursue advances in high-temperature materials that could allow humankind to explore new worlds.

Within the past year, Opila joined a new multi-university research initiative, also funded by the Office of Naval Research, to understand how a specific class of metal alloys behave in high-temperature and/or corrosive environments. Mitra Taheri, professor of materials science and engineering and director of the Materials Characterization and Processing facility at Johns Hopkins University, leads the collaborative effort, which also involves researchers at UVA Engineering, Arizona State University and Northwestern University. A five-year $7.5 million grant supports this initiative.

Taheri’s multi-institutional team explores the relationship between the fundamental structure of alloys and their corrosion and oxidation products. The team will work with multi-principal element alloys, a material class known to exhibit exceptional combinations of strength, ductility and damage tolerance.

The full team will design assemblages of mixed metallic compositions and the oxide that is going to result. Their approach requires a new way of thinking about materials design. Oxidation is not an after-effect that must be controlled, but rather a material property that can be designed from the substrate up and the oxide layer down.

John R. Scully, Charles Henderson Chaired Professor of Materials Science and Engineering at UVA, brings his expertise in corrosion in wet environments to the team’s effort, focusing on alloys that can form a passive film or “skin” with distinctive anti-corrosion protections.

Opila’s dream is a new “super MPEA” alloy with good resistance to corrosion from wet environments and oxidation. To get there, Opila and Scully will investigate the formation of protective oxide films on metal surfaces that can reduce the rate of corrosion and high-temperature oxidation, a process called passivation. They aim to identify similarities and differences in “protectiveness” during high-temperature oxidation and aqueous corrosion.

“A way to think about it is the tortoise and the hare. The tortoise always wins at high temperature, while the hare is first to the finish line in the 50-yard dash. They are completely different creatures,” Scully said.

All metals want to be oxides; that’s their stable form when they are pulled out of the earth. When a metal is refined and put into a high-temperature oxidizing environment, it quickly reverts to its preferred form. If the oxide layer grows too fast, it consumes the metal.

“Ideally, we want a continuous oxide that stays on the surface and minimizes degradation underneath,” Opila said.

Opila is excited about the team’s collaboration and early progress. The principal investigators hold frequent “chalk talks,” involving their post-docs and students. Team members are sharing data sets, and Opila is optimistic that they will soon be able to show early data, adding, “It’s going to be a fun four-and-a-half years.”