The new corrosion-resistant boride material comprises alternating layers of molybdenum boride and aluminum. Image: Drexel University.
The new corrosion-resistant boride material comprises alternating layers of molybdenum boride and aluminum. Image: Drexel University.

Borides are among the hardest and most heat-resistant substances on the planet, but their Achilles' Heel, like that of so many materials, is that they oxidize at high temperatures. Oxidation is the chemical reaction commonly known as corrosion or rusting, and can signal the end for a material's structural integrity. But researchers from Drexel University, Linkoping University in Sweden and Imperial College London in the UK have now produced an aluminum-layered boride whose unique behavior at high temperatures keeps it one step ahead of oxidation.

Because of their impressive durability, borides are already used as coatings for surfaces that must survive the harshest environments – from the inside of combustion engines to cutting tools for hard metals. But, according to Michel Barsoum, professor in Drexel's College of Engineering and lead author of a paper in Scientific Reports describing this work, borides can be improved.

"This discovery is quite significant because it is the first example in the history of mankind of a transition metal boride that is quite oxidation resistant," said Barsoum, who heads Drexel's MAX/MXene Research Group in the Department of Materials Science and Engineering.

To make their boride material, called molybdenum aluminum boride (MoAlB), Barsoum and his team combined a molybdenum-boron lattice with a double layer of aluminum, producing a material that is durable enough to resist oxidation at extremely high temperatures. The key to this remarkable characteristic is the material's nanolaminated structure, comprising alternating layers of molybdenum boride and aluminum.

"This resistance to oxidation is possible because of the presence of aluminum in layers between molybdenum and boron layers," Barsoum said. "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. So the material forms its own protective coating."

Upon testing, the group also found that the material retains its high conductivity at elevated temperatures. Although its melting point has yet to be determined, preliminary results have shown it to be greater than 1400°C. Barsoum speculates that because of these promising results, his team's work has now laid the foundation for the development of ultrahigh melting point borides that are also oxidation resistant.

"Now we know we're looking in the right place to make materials with this impressive set of properties," said Sankalp Kota, a doctoral student in Barsoum's research group and the paper's first author. "Most people were trying to make the binary borides – materials with two elements – oxidation resistant by adding other phases and coatings. 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."

This story is adapted from material from Drexel University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.