Superhard Coating Materials
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Superhard Coating Materials
Yip-Wah Chung and William D. Sproul, Guest Editors Abstract “Superhard” coating materials are defined by hardness values that exceed 40 GPa. In this issue of MRS Bulletin, we focus on noncarbon-based superhard coatings, with the exception of a review of carbon nitride (CN) materials. Nanometer-scale multilayered nitride coatings were the first to show the superhard property, and these coatings have quickly made their way into industry as protective coatings for cutting-tool operations. Nanocomposite thin films also exhibit superhardness, and some of these materials have hardnesses approaching that of diamond. Cubic boron nitride (c-BN), which is naturally superhard, has proven very difficult to deposit at thicknesses exceeding 0.1 m, but it is now reported that chemical vapor deposition techniques based on fluorine chemistries can produce c-BN films up to 20 m thick. The search to produce cubic -CN has led to the development of noncubic, fullerene-like forms of CN that are both hard and elastic, a very interesting combination of properties that has already been put to use in the harddisk industry. Overall, the development of hard and superhard coatings during the past 20 years has been remarkable. We have progressed from trying (and often failing) to deposit hard coatings to now designing new nanometer-scale multilayered and nanocomposite coatings that exhibit excellent hardness properties and other highperformance characteristics. Keywords: carbon nitride, cubic boron nitride, nanocomposites, nanometer-scale multilayered coatings, superhard coating materials, superhard oxide materials, thin films.
Interest in “superhard” coating materials, defined by hardness values in excess of 40 GPa, has increased significantly during the past 10–15 years. In nature, there are only two materials that qualify as superhard: diamond and cubic boron nitride (c-BN). Diamond is the hardest known material, with a hardness of 70–100 GPa depending on crystallographic orientation and purity; c-BN has a hardness of 50 GPa. Many researchers have tried to produce coatings that match or exceed the hardness of these materials. Much of the work has been directed at the deposition of diamond or diamondlike coatings, and there has been much success in this area. In this issue of MRS Bulletin, however, we look at other superhard coating systems that are not carbon-based, with the exception of a review of carbon nitride materials. In the mid-1980s, researchers at Linköping University1 in Sweden showed that it was possible to produce nanometer-scale multilayered coatings of titanium nitride and
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vanadium nitride with hardness values in excess of 50 GPa. Their discovery led to much research activity in this area, and many other nanometer-scale multilayered coatings have been investigated since this initial development. In this issue, we look at two contributions that are directly related to work in this field. In the first article, Barnett et al. report on efforts to produce high-temperature stability in nanome
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