A computer model of the atomic structure of one of the new carbides. The jumbled mess of carbon and five metal elements gives stability to the overall structure.
Images courtesy of Duke University

Materials scientists have discovered a new class of carbides expected to be among the hardest materials with the highest melting points in existence.

Traditionally compounds of carbon and another element – carbides paired with metals such as titanium or tungsten – result in materials that are extremely hard and difficult to melt, making them ideal for coating the surface of cutting tools or space vehicle parts.

A small number of complex carbides containing three or more elements also exist but are not common. The new carbide class joins carbon with five different metallic elements, achieving stability from the chaotic mixture of their atoms rather than orderly atomic structure. Duke University researchers computationally predicted these materials to exist, and they were synthesized at University of California San Diego.

“These materials are harder and lighter in weight than current carbides,” says Stefano Curtarolo, professor of mechanical engineering and materials science at Duke. “They also have very high melting points and are made of relatively cheap material mixtures.”

Crystalline materials gain their stability and strength through regular, ordered atomic bonds, but imperfections in the structure can strengthen a material. Five-metal carbides rely on disorder for stability similar to how a pile of baseballs won’t stand on its own, but a pile of baseballs, shoes, bats, hats, and gloves might.

A microscopic look at the structures formed in two of the potential new carbides. The material below, left, forms large blocks of similar structures making it unstable, while the material on the right forms a broad range of structures, giving it stability.

The difficulty lies in predicting which combination of elements will stand firm. The team narrowed the field to eight metals known to create carbide compounds with high hardness and melting temperatures. They then calculated how much energy it would take for a potential five-metal carbide to form a large set of random configurations.

Kenneth Vecchio, professor of NanoEngineering at UC San Diego, attempted to combine the elements in fine powders, pressing them at temperatures up to 4,000°F with 2,000A of current.

Three recipes proved successful, but one stood out – a combination of molybdenum, niobium, tantalum, vanadium, and tungsten.

“Getting this set of elements to combine is basically like trying to squeeze together a bunch of squares and hexagons,” says Cormac Toher, an assistant research professor in Curtarolo’s laboratory.

“I wouldn’t be surprised if it turned out to be the hardest material with the highest melting point ever made,” Curtarolo adds.

This work was supported by the Department of Defense Office of Naval Research.

Pratt School of Engineering, Duke University
https://pratt.duke.edu

UC San Diego
https://ucsd.edu