Superhard Materials: Advances in the Search and Synthesis of New Materials

Abstract

Materials with superior hardness can be categorized as ultrahard (Vickers hardness, H v ≥ 80 GPa) and superhard ( H v ≥ 40 GPa). These materials are commonly used as cutting tools and abrasives in the machining and manufacturing industries. With its extreme hardness, diamond is the best known and most used ultrahard material for industrial applications. However, it is ineffective at cutting and drilling ferrous alloys due to diamond's high reactivity with iron and poor thermal stability in air. Additionally, the synthesis of diamond requires both high pressure (HP) and high temperature, making it an expensive process. These limitations have driven the search for alternative superhard materials that are capable of cutting steels and other materials at lower costs. This article reviews the concept of hardness and summarizes advancements in the synthesis and mechanical properties of hard materials. It begins with a review of methods to measure hardness, adding HP diffraction methods to more conventional hardness measurements. It then considers new ultrahard materials that exist within the B–C–N ternary system, with hardness approaching diamond but improved chemical stability. Finally, it surveys superhard nitrides, oxides, and borides as potential alternative materials, focusing on transition metal boride systems where the synthesis can be readily achieved at ambient pressure and scaled for industrial applications. We hope that this article serves as an overview of hard materials and guide for the comparison of data reported in the literature.

Hardening Effects in Superhard Transition-Metal Borides

“…Since this is a model based solely on composition and does not incorporate crystal structures, certain intrinsic values still need to be manually vetted, such as negative moduli, incompatible crystal types, and hydrogen-incorporated phases. 13 Attempts have been made to search and identify the structures of metal borides through a first-principles global structural optimization method. 57 In addition, new approaches based on the CALYPSO algorithm were used to predict the hardness versus energy map in order to narrow down the energetically favorable superhard materials for syntheses.…”

“…Although the search for new superhard materials can benefit greatly from automated discovery, it is still a tool, rather than a replacement, for the entire composition selection process. Since this is a model based solely on composition and does not incorporate crystal structures, certain intrinsic values still need to be manually vetted, such as negative moduli, incompatible crystal types, and hydrogen-incorporated phases . Attempts have been made to search and identify the structures of metal borides through a first-principles global structural optimization method .…”

“…To provide a basis for comparison, we list hardness values with the corresponding applied load whenever possible. The concept and measurement techniques of hardness and high-pressure behavior are discussed in a separate book chapter . The scope of this Account is to highlight progress on metal boride systems, outline the factors that contribute to their high hardness, and indicate future avenues in the field of superhard materials research.…”