SCIENCE/TECHNOLOGY
New material may be harder than diamond Researchers at Northwestern Universi ty's materials science and engineering laboratories have developed a new ma terial that may be harder than dia mond, the hardest known substance. Preliminary evidence indicates that a carbon nitride, P-C3N4, is one of a pos sible series of compounds in which the bulk modulus is greater than that of di amond—a necessary requirement for a harder-than-diamond material. The work is being led by Yip-Wah Chung, a professor of materials science and engineering and director of the tribology center at Northwestern's McCormick School of Engineering & Ap plied Science. Chung described the work last month at the International Conference on Metallurgical Coatings & Thin Films in San Diego. He says that, although preliminary results don't conclusively prove that his materials
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THE CHEMISTRY PROSPECTS, THE NATURE EXISTS
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MAY 25,1992 C&EN
are harder than dia mond, they give a pretty good indication that they are. Chung is seeking ma terials with better tribological properties than those associated with di amond. His attention was drawn to CNX films by previous work by Amy Y. Liu and Marvin L. Cohen at the Univer sity of California, Berke ley, which suggested that, on theoretical grounds, such films should be harder than diamond. The prediction was based on an empirical scaling Bright-field transmission electron micrograph of carbon nitride relationship, in which the thin film. Lattice fringes identify crystalline phase bulk modulus is related to the structural characteristics of the The data indicate two trends: the first material, according to the formula, Β = is that density of the film increases with (19.71-2.21) A*35, where Β is the bulk sputtering power; the second is that modulus, d is the interatomic bond films grown with the appropriate bias length in angstroms, and J is a measure have higher densities. In all cases, the of the ionic character of the solid. The densities cluster around the value of 2.0 scaling rule was originally developed g per cc. All films are insulating films for solids with a cubic unit cell but has with a resistivity exceeding 1 million been found to work very well for other ohm-cm. types of solids as well. All indications are that the carbon The theoretical prediction was that, if and nitrogen are chemically bound to carbon were substituted for silicon in the gether. Examination with scanningmaterial P-Si3N4, the resulting bond electron microscopy discloses smooth length and other features would yield a films. Atomic-force microscopy exami bulk modulus of 4.83 megabar, greater nation reveals granular features on the than the experimentally determined val order of 0.2 μιη. The films deposited on silicon were ue for diamond, 4.43 megabar. The new material would then become the hardest used for hardness tests. The researchers hoped that these standard, so-called material known. Chung and his coworkers prepared nanoindentor tests would determine such films with a direct current magne the hardness of the carbon nitride coat tron sputtering system using nitrogen ing. The indentor uses a small diamond as the sputtering gas. A series of films point to poke holes in a surface. But the was thus grown under different condi results on the new films were "ambig tions. Several substrates were tried in uous"—the diamond made no impres cluding polycrystalline zirconium, so sion on the films under load. 'We're not sure what they mean," dium chloride, and special glasses. The resulting films were characterized by says William Sproul, one of the re scanning-electron microscopy, and com searchers in the group. "Normally we positional information was obtained by would see a dent. In these tests we saw Auger spectroscopy and infrared ab absolutely nothing. It's either highly elastic, or it's harder than diamond." sorption. Sliding friction measurements in triThe rate of film deposition increases with the concentration of nitrogen in the bological tests indicate that the coeffi sputtering gas. This happens because the cient of sliding friction against steel is atomic mass of nitrogen is closer to that comparable to that of diamond. New tests to attempt to determine of carbon than it is to that of argon, the other diluent gas. The maximum film the hardness and bulk modules are be formation rate was about 2.5 μιη per ing planned. Joseph Haggin hour in 100% nitrogen.
