Superhard Materials

Abstract

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 Si 3 N 4 ) 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., Si 3 N 4 ) 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.

Effects of Variable Boron Concentration on the Properties of Superhard Tungsten Tetraboride

“… a Hardness values were taken from Mohammadi et al b Hardness value was take from Gu et al c Hardness value is from current work. …”

Nonrandomly Distributed Tungsten Vacancies and Interstitial Boron Trimers in Tungsten Tetraboride

“…Transition-metal (TM) borides, as candidate compounds for new superhard materials, have attracted a great deal of attention in the past decades. − These compounds exhibit not only excellent mechanical properties but also many other outstanding behaviors (e.g., strong chemical inertness, rich stoichiometries, inexpensive constituents, and facile synthetic conditions). − Moreover, their hardness can be further enhanced by the creation of solid solutions with other transition metals. , Because of the structural and bonding complexities in superhard metals, maximized TM-boron bonding and structures that lack slip planes are recently considered essential to achieving optimal high strength and hardness . However, a full resolution of the crystal structures and bonding states in TM x B y compounds has been of great difficulty due to the dominating X-ray scattering of the heavy TMs and the versatile ability of boron atoms to form different hybridized and even multicenter bonds. − This situation becomes particularly more serious in the synthesized highest boride of tungsten, that is, tungsten tetraboride (WB 4 ) , and its solid solutions, ,− with the highest reported Vickers hardness of 43–58 GPa under an applied load of 0.49 N. In spite of the extensive experimental and theoretical attempts with reached consensus that the WB 4 crystal lattice consists of alternating hexagonal layers of W and B atoms (i.e., WB 3 with space group of P 6 3 / mmc ), − many incomplete and conflicting structural assignments have been generated yet concerning the interstitial arrangements of B atoms between the hexagonal B layers (Figure , Table S1, Supporting Information).…”

Superhard Rhenium/Tungsten Diboride Solid Solutions

“…For the past few years, transition metal diborides have attracted a great deal of attention among materials researchers due to their combination of outstanding physical properties, such as metallic electrical conductivity, high incompressibility, high shear strength, and exceptionally high hardness . All of these attributes are desirable in materials for structural and engineering compounds and indicate that diborides may be suitable replacements for current metal carbides in next-generation cutting tools . Generally, these properties are also correlated: a high bulk modulus (incompressibility) appears to be a necessary, if not sufficient, predictor of high hardness .…”