www.mrs.org/publications/bulletin work represents a rapid transfer of tech- nology from the research stage to the rou- tine use of these coatings in industry. Related to nanometer-scale multilayered Superhard Coating coatings are nanocomposite thin films. Some of these films have hardnesses ap- proaching that of diamond. In his article, Patscheider discusses the effect of nano- Materials structure on coating properties. These films usually have nanocrystalline grains of Yip-Wah Chung and William D. Sproul, transition-metal nitrides or carbides sur- rounded by amorphous hard nitrides. The Guest Editors immiscibility of the amorphous and transition-metal nitrides is key in develop- ing this structure. The amount of the amor- Abstract phous material and the size and shape of the nanocrystalline grains have a direct “Superhard” coating materials are defined by hardness values that exceed 40 GPa. influence on the hardness of the material. MRS Bulletin, In this issue of we focus on noncarbon-based superhard coatings, with The hardness enhancement is due to re- the exception of a review of carbon nitride (CN) materials. Nanometer-scale multilayered stricted dislocation movement, as it is with nitride coatings were the first to show the superhard property, and these coatings have nanometer-scale multilayered coatings. quickly made their way into industry as protective coatings for cutting-tool operations. It became apparent very early on that Nanocomposite thin films also exhibit superhardness, and some of these materials have depositing c-BN coatings was difficult. In hardnesses approaching that of diamond. Cubic boron nitride (c-BN), which is naturally order to create the cubic phase instead of superhard, has proven very difficult to deposit at thicknesses exceeding 0.1 ␮m, but it is the hexagonal phase, stress had to be now reported that chemical vapor deposition techniques based on fluorine chemistries applied to the film. However, this stress can produce c-BN films up to 20 ␮m thick. The search to produce cubic ␤-CN has led to caused delamination of the coating once the development of noncubic, fullerene-like forms of CN that are both hard and elastic, a the thickness of the film exceeded a few very interesting combination of properties that has already been put to use in the hard- tenths of a micron. Thick c-BN films have disk industry. Overall, the development of hard and superhard coatings during the past eluded researchers for a long time. In this 20 years has been remarkable. We have progressed from trying (and often failing) to issue, Zhang et al. report on a method for deposit hard coatings to now designing new nanometer-scale multilayered and depositing c-BN films at a thickness in ex- ␮ nanocomposite coatings that exhibit excellent hardness properties and other high- cess of 20 m. Their technique uses chemi- performance characteristics. cal vapor deposition based on fluorine chemistry. Thick c-BN films will find many Keywords: carbon nitride, cubic boron nitride, nanocomposites, nanometer-scale useful applications, particularly in metal- multilayered coatings, superhard coating materials, superhard oxide materials, thin films. cutting operations where the hardness and insolubility of c-BN in the work piece will lead to enhanced tool life. In high-temperature applications, all of Interest in “superhard” coating materials, vanadium nitride with hardness values in the nitride- and carbide-based superhard defined by hardness values in excess of excess of 50 GPa. Their discovery led to materials fail due to oxidation. One of the 40 GPa, has increased significantly during much research activity in this area, and goals in researching new superhard mate- the past 10–15 years. In nature, there are many other nanometer-scale multilayered rials has been to develop superhard oxide only two materials that qualify as super- coatings have been investigated since this materials. In his article, Lowther shows hard: diamond and cubic boron nitride initial development. In this issue, we look how predictive computer modeling is being (c-BN). Diamond is the hardest known at two contributions that are directly re- used to guide researchers in the quest to material, with a hardness of 70–100 GPa lated to work in this field. produce new superhard oxides. depending on crystallographic orientation In the first article, Barnett et al. report on The final article, by Hultman et al., is and purity; c-BN has a hardness of 50 GPa. efforts to produce high-temperature stabil- on resilient fullerene-like carbon nitride Many researchers have tried to produce ity in nanometer-scale multilayered coat- coatings. Carbon nitride first came to the coatings that match or exceed the hard- ings. The superhardness of these materials notice of most researchers through the ness of these materials. Much of the work depends on maintaining a distinct layer published work of Liu and Cohen.2 They ␤ has been directed at the deposition of dia- structure, which can be destroyed by dif- predicted that the cubic -C3N4 form of mond or diamondlike coatings, and there fusion at high temperatures. To overcome carbon nitride might be as hard as or has been much success in this area. In this this problem, Barnett et al. describe the harder than diamond. This led to a flurry issue of MRS Bulletin, however, we look at use of immiscible layers to produce super- of attempts to produce this elusive mate- other superhard coating systems that are hard coatings that maintain their hardness rial that continues even today. The guest not carbon-based, with the exception of a to temperatures in excess of 1000ЊC. editors of this issue were among those ␤ review of carbon nitride materials. In the second article, Münz et al. look at who have tried to deposit cubic -C3N4, In the mid-1980s, researchers at Linköping industrial applications for nanometer-scale and like most researchers, they found it 1 University in Sweden showed that it was multilayered coatings. Much success has difficult to do. However, CNx, where x is possible to produce nanometer-scale multi- been achieved in adapting this technology around 0.2–0.3, was found to be a very in- layered coatings of titanium nitride and for use in metal-forming operations. This teresting hard material. It may even be a 164 MRS BULLETIN/MARCH 2003 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.226, on 26 Sep 2021 at 08:13:21, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs2003.56 Superhard Coating Materials superhard material, but it is not as hard as elasticity, and this finding is important for as will hard oxide coatings once they are diamond. Normally, most hard materials the design of tough materials. available, and CNx has already found in- are very brittle, but CNx is very elastic, Overall, the development of hard and dustrial acceptance as a protective overcoat which seems a contradiction. Hultman superhard coatings during the past on virtually all hard-disk drives manufac- et al. show that CNx has a fullerene-like 20 years has been remarkable. We have tured today. structure, which gives this material its progressed from trying (and often failing) elastic property. They review the nature of to deposit hard coatings to now designing References the bonding between the carbon and ni- new nanometer-scale multilayered and 1. U. Helmersson, S. Todorova, S.A. Barnett, trogen atoms and the issues involved in nanocomposite coatings that exhibit excel- J.-E. Sundgren, L.C. Markert, and J.E. Greene, trying to determine the hardness of the lent hardness properties and other high- J. Appl. Phys. 62 (1987) p. 481. material. The bonding between carbon performance characteristics. Thick c-BN 2. A.Y. Liu and M.L. Cohen, Phys. Rev. B 41 and nitrogen atoms is key to the degree of coatings will quickly find uses in industry, (1990) p. 10727. ■ Yip-Wah Chung, Guest Editor of this issue of MRS Bulletin, is a profes- sor of materials science and engineering at Northwestern Univer- sity in Evanston, Ill. He served as director of the NSF Industry–University Cooperative Research Center for Engineering Tribology at Northwest- ern from 1987 to 1992 William D. Sproul Scott A. Barnett Igor Bello and then as department Yip-Wah Chung chair from 1992 to 1998. His research is focused Chung can be reached Coatings and Thin Films transparent conductors, at the University of on the physics and by e-mail at ywchung@ three times. He is a co- hard coatings, and solid- Salford, England. He chemistry of surfaces northwestern.edu. editor of Surface and oxide fuel cells. was an adjunct professor, and wear-protective Coatings Technology, and Barnett can be reached industrial consultant, coatings. Chung’s work William D. Sproul, he serves on the edito- by e-mail at s-barnett@ and research scientist in on nitrogenated carbon Guest Editor of this rial board of Vacuum. northwestern.edu. materials engineering led to its application as issue of MRS Bulletin, is Sproul can be reached and surface science a protective overcoating a senior scientist in the by e-mail at bill.sproul@ Igor Bello is an associate at the University of for computer disk drives Power Products busi- aei.com. professor of materials Western Ontario, in use today. He was ness unit at Advanced science at City Univer- Canada, from 1989 to honored with the Energy Industries (AE) Scott A. Barnett is a sity of Hong Kong 1996. Since 1990, he has Technical Achievement in Fort Collins, Col. professor and associate and a core member focused on the synthesis Award from the Na- Prior to AE, he worked chair of the Materials of the Center of Super- of diamond, diamond- tional Storage Industry at Reactive Sputtering Science and Engineering Diamond and Advanced like carbon, and cubic Consortium (now the Inc., Sputtered Films Department at North- Films (COSDAF).