Strengthening Superhard Materials by Nanostructure Engineering

Traditional superhard materials mainly including diamond and cubic boron nitride are indispensable to scientific research in many fields and numerous industrial applications, especially as cutting and machining tools. Relentless search for new superhard systems with mechanical properties surpassing those of traditional ones has inspired tremendous experimental efforts over the past few decades. On the other hand, enhancing mechanical properties of traditional superhard materials via nanostructuring proved to be an effective approach. A number of recent experiments towards fabricating nanostructured diamond and cubic boron nitride under high-pressure and high-temperature conditions have led to record hardness of similar to 200 and similar to 110 GPa, respectively, nearly twice those of their single-crystal counterparts. Armed with largely strengthened toughness and thermal stability as well, these nanocrystalline forms of traditional superhard materials have great potential for the next-generation superhard material applications. However, to date, knowledge on such nanostructured superhard materials is still scarce, and there are many mysteries surrounding the formation mechanism, effects of pressure and temperature on the growth of nanograins and defects (e.g., nano-twins), and underlying strengthening mechanism of nano effect on mechanical properties, and so on. A review of this hot topic is necessary and would provide important guidance for future studies of superhard materials.