Disorder, chemical variety key in search for super-hard materials

UB chemistry researcher Eva Zurek is a key partner on a new $7.5 million effort to discover inexpensive materials hard enough to join two pieces of steel together through a process called friction stir welding.

The team will also develop a suite of artificial intelligence (AI)-based tools for the on-demand design of similar materials with properties tailored to a range of applications.

The collaboration is led by Duke University’s Center for Autonomous Materials Design and is funded by a five-year grant through the Department of Defense’s Multidisciplinary University Research Initiative (MURI).

Zurek, professor of chemistry in the College of Arts and Sciences, will receive nearly $1.2 million.

The project also includes researchers from Pennsylvania State University, North Carolina State University and Missouri University of Science and Technology.

The focus of the research is on “high-entropy” materials that combine several elements to create a complex structure that derives enhanced stability from a chaotic mixture of atoms. After demonstrating this approach with carbides in 2018, the researchers will now look to add borides into the irregular self-organized structures to produce some of the hardest materials ever made. The goal is to mix at least two high-entropy materials together so that they interlock, similar to a grid formed from a variety of Tetris pieces. This configuration contributes to hardness.

“We’ve already developed the computational machinery needed to tell us when this phenomenon will produce these stable, super-hard materials,” said Stefano Curtarolo, professor of mechanical engineering and materials science at Duke and leader of the new MURI award. “Our goal now is to develop the necessary ‘cooking’ procedures, as well as AI-materials tools that can automate the discovery of new recipes to fit different needs.”

One material the team will explore is a combination of carbon, boron, nitrogen and five other inexpensive metallic elements, all stabilized by entropy.

That complexity is part of what makes the research so exciting, says Zurek, a theoretical and computational chemist. Her team at UB will use machine learning tools to develop interatomic potentials that can be employed to perform large-scale simulations of the interlocking boundaries between the two high-entropy materials, and facilitate the discovery of new, hard, synthesizable materials using AI.

“I’m really excited about this,” Zurek says. “I think this is a way to go in order for theoreticians to simulate the dynamic behavior of chemical systems of thousands of atoms for hundreds of picoseconds, a relatively long period of time in this field, so that we can present more interesting targets for experimentalists. In our newly funded project, we want to find new super-hard materials that are composed of up to 10 different chemical systems or elements.”

While high-entropy materials could be useful for many applications, one priority for the Department of Defense is called friction stir welding, which uses drill-like bits to join two pieces of metal without melting them.

As the bits rotate, they heat and soften the metal, allowing the surrounding material to swirl and mix as the machine moves along a line. The technique produces an extremely strong and durable joint with few defects.

For a friction stir welding tip to make a successful joint between two pieces of steel, it must be incredibly hard to avoid wearing too fast, thermally stable to withstand the high temperatures, chemically inert so that it does not pollute the weld, and inexpensive enough to mass-produce. Diamond is hard enough for the job, but sheds carbon atoms during the process, making the weld brittle. Polycrystalline cubic boron nitride — the current material of choice — wears down too quickly for how expensive it is to make.

“If the right material could make friction stir welding a viable choice for large projects involving steel, it could revolutionize the construction of ships and other defense equipment,” Curtarolo says.

The key to solving the friction stir welding problem, Curtarolo believes, is creating an interlocking, Tetris-like maze of high-entropy carbides and borides.

“Carbides don’t usually mix with borides, but if we can get them to form interlocking grains, we can make something that is harder than both,” he says. “That’s the trick.”

In addition to Curtarolo and Zurek, team members on the MURI project include William Fahrenholtz, Curators’ Professor of Ceramic Engineering at the Missouri University of Science and Technology; Donald Brenner, department head and Kobe Steel Distinguished Professor of Materials Science and Engineering at North Carolina State University; and Penn State professors of materials science and engineering Jon-Paul Maria and Douglas Wolfe, the latter of whom is also head of the Department of Metals, Ceramics and Coatings Processing at Penn State’s Applied Research Laboratory.