Ultrahard nanomaterials: myths and reality

The last 25 years has witnessed a wealth of publications on the creation of carbon materials whose compression bulk modulus and hardness are much higher than those of diamond. This review presents a critical analysis of these studies. Three groups of myths that have emerged lately are discussed. The first is related to the possibility of creating materials whose bulk moduli are significantly higher than those of diamond. The second group is devoted to ‘experimentally measured’ values of hardness, much higher than that of diamond. The third includes alleged ‘theoretical’ grounds for a several-fold (!) increase in the hardness of covalent substances due to the effects of quantum confinement. It is shown that materials whose elastic moduli significantly exceed those of diamond cannot in principle be produced under normal conditions. Issues surrounding the quantitative measurement of hardness are discussed; it is noted that the creation of obstacles to the movement of dislocations in nanomaterials may allow a 20–40 % increase in the effective measured hardness of ultrahard materials. It is emphasized that alternative hypothetical approaches to increase hardness, for example, due to quantum confinement, actually have no physical grounds whatsoever. The highest mechanical characteristics of diamond are associated with reliably established physical laws, and any assertions regarding possible obtainment of materials whose elastic characteristics or hardness are several times greater than those of diamond may not be regarded as reliable to any extent or even science-based.