Researchers from Drexel University, Linkoping University, in Sweden and Imperial College London have now produced an aluminum-layered boride whose unique behavior at high temperatures keeps it one step ahead of nature’s slow march toward high-temperature chemical degradation.

To make their boride material, called molybdenum aluminum boride (MoAlB), Michel Barsoum, professor in Drexel’s College of Engineering, and his team combined a molybdenum-boron lattice with a double layer of aluminum. The key to its ability to resist oxidation at extremely high temperatures is the presence of aluminum between the molybdenum and boron layers.

Borides are hard, heat-resistant materials often used for coating structures that have to withstand high temperatures and pressures. Image credit: Drexel University.“When heated to high temperatures in air, the aluminum atoms selectively diffuse to the surface and react with oxygen, forming a surface aluminum oxide, or alumina, protective layer that slows down further oxidation considerably," Barsoum says. "So the material forms its own protective coating.”

Upon testing, the group also found that it retains its high conductivity at elevated temperatures.

The material's melting point has yet to be determined, but preliminary results have shown it to be higher than 1,400 degrees Celsius. Barsoum speculates that, because of these promising results, his team’s work has laid the foundation for the development of ultrahigh-melting-point borides that are also oxidation resistant.

“Most people were trying to make the binary borides—materials with two elements—oxidation resistant by adding other phases and coatings," says Sankalp Kota, a doctoral student in Barsoum’s research group. "One reason we have been this successful at making materials with interesting properties has to do with the number of elements one starts with. With only two elements, it is difficult; with three or higher, the chance of producing a material with a new combination of properties is greater.”