Harder than diamond: dreams and reality

Abstract

Analysis of correlations between various physical properties of solids provides a formulation of simple criteria applicable to the search for new superhard materials. The prospects for the synthesis of new substances with the key elastic moduli, whose values approach or even exceed those of diamond are also discussed. We introduce the concept of ideal hardness and strength, which relates the elastic properties of materials to the corresponding mechanical characteristics, the concepts that are less unambiguously determined and that depend on the conditions of measurements. The control of material nanostructure makes it possible to approach the ideal hardness. The formulation of trends interrelating physical properties is expected to allow an essential guidance in the synthesis of new classes of superhard materials.volume V m , bulk modulus K and cohesive energy E c (see the data from Young (1991)). In particular, the relationshipwas proposed by Aleksandrov et al. (1987). For elemental solids, the ratio KV m =E c c varies approximatel y from 0.5 to 16 (®gure 2), although for the overwhelming majority of elements it lies within the much narrower interval of 1±4. If the normalized energy and volume for a certain group of substances are related by the universal scaling function, EˆE 0 f …V=V 0 †, where E 0 and V 0 are the Harder than diamond: dreams and reality 233

Advancements in the Search for Superhard Ultra‐Incompressible Metal Borides

“…[2,4] It has a VED of 0.485 e À Å À1 which is comparable to the VED of ReB 2 (0.477 e À Å À1 ), suggesting that this compound might be ultraincompressible. Gu et al recently synthesized WB 4 by arc-melting and measured the mechanical properties.…”

“…[4] A survey of bulk moduli and cohesive energies as a function of atomic number is presented in Figure 1. [3,4] The natural logarithms of the bulk moduli are plotted in order to display a greater range in the variation of elastic stiffness on a single graph.…”

“…[4] A survey of bulk moduli and cohesive energies as a function of atomic number is presented in Figure 1. [3,4] The natural logarithms of the bulk moduli are plotted in order to display a greater range in the variation of elastic stiffness on a single graph. Note the similarities between the peaks and valleys for both the bulk modulus and cohesive energy that roughly depend on the periodicity of the elements with half-filled shells exhibiting maxima for both terms.…”

“…[3,4,18,24] Covalent materials generally display high bond-bending force constants, contain highly directional bonds, and generally have high shear moduli. In an ionic material, however, the electrostatic interactions are omnidirectional, resulting in low bond-bending force constants and consequently a low shear modulus.…”

“…Several recent reviews in the literature cover various aspects of this research field, including HPHT synthesis of superhard materials, superhard thin films and nanocomposites, and the ultimate goal of creating new bulk materials harder than diamond. [4][5][6][7] Progress is steadily being made in HPHT synthesis,whererecentdevelopmentsinclude the synthesis of diamond-like carbon, aggregate diamond nanorods, ultrahard fullerites, and boron nitride nanocomposites using pressures up to 20 GPa and temperatures approaching 2 000 8C. All of these materials possess hardness and bulk moduli values comparable to single-crystal diamond.…”