Ultra-hardmaterials are used for everything from drills that bore for oil and build newroads to scratch-resistant coatings for precision instruments and the face ofyour watch.
UCLAscientists are now reporting a promising new approachto designing super-hard materials, which are very difficult to scratch orcrack. Their findings appear in the April 20 issue of the journal Science.
Diamond is the hardest material known, because its carbon atomsform very short covalent bonds, according to co-author Richard B.Kaner, UCLA professor of inorganic chemistry and materials science andengineering. Most of the diamond used in the world isactually synthetic and very expensive. Diamond powder is used for oil drillsand machines that build roads and cut holes in mountains. Diamond cannot be used, however, to cut steel withoutruining the diamond blade.
Cubic boron nitride is a diamond substitute used to cut steel; itis made synthetically under very high-temperature, high-pressure conditions,and is even more expensive than diamond, Kaner said.
There are two ways to make super-hard materials that are"ultra-incompressible," meaning they are resistant to shape deformation, whichis a necessary condition for hardness: One is to imitate diamond by usingcarbon and combining it with boron or nitrogen to maintain short bonds; theother is to look for metals that are already incompressible and try to makethem hard, said Kaner. He and his colleagues are developing the secondapproach.
"Our idea is to combine an incompressible metal, which happens tobe soft, with short covalent bonds to make it hard," said
In 2005, Kaner, Chemistry and Biochemistry Professor SarahTolbert (a co-author on the Science paper), and Materials Science andEngineering Adjunct Professor John J.Gilman combined the relatively soft element osmium,the most incompressible metal known, with small covalent-bond forming atoms to make a materialthat is almost as incompressible as diamond, yet isso hard that it scratches sapphire, which is ranked 9 on a hardness scale of 1to 10).
"Wefound that if we combine boron with osmium, we push the osmium atoms apart byonly 10 percent from where they were in osmium metal, which is very good; youwant to push them apart as little as possible," Kaner said. "Then we searched through the transition metals to see if wecould do better than osmium, to get an expansion of less than 10 percent. Theonly metal we could find that had the potential for doing this is rhenium;hence, we made rhenium diboride.
Rhenium is a fairly dense, soft metal, which is next to osmium onthe periodic table of chemical elements.
"We formed short covalent bonds, pushing the rheniums apart byjust 5 percent from where they were in rhenium metal, making it bothincompressible and very hard. The rhenium-rhenium distance expanded by only 5percent from the metal — that's the key to this Science paper. Rhenium diborideis as incompressible as diamond in one direction, and in the other direction,just slightly more compressible."
At low applied forces, the hardness of rhenium diboride isequivalent to cubic boron nitride, the second-hardest material known, Kanersaid. At higher applied forces, rhenium diboride is a little bit below that.
"Our material is hard enough to scratch diamond, and much harderthan osmiumdiboride," he said.
While other super-hard materials, including diamond and cubicboron nitride, are made under expensive, high-pressure conditions, "ourmaterial is made in a simple process without applying pressure," Kaner said.
Kaner's research is federally funded by the National ScienceFoundation. UCLA has a pending patent application on the research.
The co-authors on the Science paper are all from UCLA: Tolbert; Abby Kavner, assistant professor of Earth and planetary materials in the department of Earth and space sciences; Jenn-Ming Yang,professor of materials science and engineering at the UCLA HenrySamueli School of Engineering and Applied Science; Hsiu-Ying Chung, agraduate student in materials science and engineering studying with both Yangand Kaner; Michelle B. Weinberger,a graduate student in chemistry in Tolbert's laboratory; and Jonathan B.Levine, a graduate student of chemistry in Kaner's laboratory.
Speaking of thecollaboration, Kaner said, "The reason I came to UCLA, and a reason I love thisplace, is because whatever you do — in my own case, whenever you make a newmaterial — you often need equipment and expertise that you don't have. At UCLA,there will be an expert in that area who has the equipment, and every time I'veasked, everybody is happy to help you do experiments and excited to collaboratewith you."
Despite thepotential of new super-hard materials, they are not likely to replace diamondany time soon, Kaner said.