Published Online: 15 MAR 2012
Copyright © 2011 John Wiley & Sons, Ltd. All rights reserved.
Encyclopedia of Inorganic and Bioinorganic Chemistry
How to Cite
Mohammadi, R. and Kaner, R. B. 2012. Superhard Materials. Encyclopedia of Inorganic and Bioinorganic Chemistry.
- Published Online: 15 MAR 2012
Materials possessing superior hardness can be divided into two main categories: ultra-hard and superhard. Diamond and cubic boron nitride (c-BN) are considered ultra-hard with hardness values ranging from 70 to 110 and 45 to 60 GPa, respectively. All other materials with a hardness of greater than 40 GPa are called superhard. Ultra-hard materials are traditionally used in industrial applications including cutting tools and wear-protecting surfaces. However, diamond is not a good choice for high-speed cutting of ferrous-based alloys because of its graphitization on the material's surface and formation of brittle carbides, which leads to poor cutting performance. In addition, the synthesis of both diamond and c-BN requires high pressure-high temperature conditions. Therefore, a great deal of effort has been made to address these shortcomings and to find a reliable substitute for these ultra-hard materials. Studies have been directed toward the development of either main group compounds (such as SiC and Si3N4) or transition-metal compounds (such as WC and HfN). Unfortunately, most of these compounds are not superhard (e.g., WC, SiC) or their synthesis needs high pressures and high temperatures (e.g., Si3N4) that entails expensive equipment and processing costs. Among these compounds, dense transition-metal diborides have exhibited very promising mechanical properties including great hardness, while remaining relatively easy to synthesize at ambient pressure, and may form the next generation of cutting tools and abrasion-resistant coatings. In this article, we will review the recent advancements in synthesis and characterization of single-phase ultra-hard and superhard materials, with a special focus on transition-metal diborides. We hope that this article may shed light on the inconsistencies of some of the data reported in the literature and make comparisons among materials possessing great hardness easier. Except for some rare cases, the hardness of thin films and composites will not be covered here and left for a future study.
- bulk modulus;
- transition metals;