Since it was made in the late 1950s, crystalline cubic boron nitride has been used in a variety of important technical applications similar to those of diamond. This is because it is thermally stable up to 1650 K, compared with just 950 K for diamond. Moreover, it is widely used as a superabrasive for machining hard ferrous steels. However, boron nitride has not been able to replace diamond completely because its hardness is half that of diamond (50 GPa compared with 100 GPa).

Now, Dubrovinskaia and colleagues have made the first thermodynamically stable boron nitride with a hardness that approaches that of diamond's. The researchers achieved their result by reducing the size of the grains in the material from micron-sized down to the nanoscale.

The Germany–France team used the 5000 ton scientific press of the Bavarian Geoinstitute at the University of Bayreuth in Germany to synthesize boron nitride at a range of high pressures and temperatures. The researchers made a series of boron nitride materials with varying grain sizes and then used high-resolution transmission electron microscopy to characterize the nanostructure of the material. They characterized the mechanical and structural properties using various analytical methods, including the Vickers hardness test. Independent measurements in two laboratories in Bayreuth and Paris were made to double-check the results.

Dubrovinskaia and colleagues found that the grain size of the boron nitride crystals had decreased to 14 nm and that two dense phases with hexagonal and cubic structures had formed at the same time. These changes result in a maximum Vickers hardness of 85 GPa, which is twice that of single-crystal boron nitride. Larger grain sizes in the material resulted in a reduced hardness (see figure).

The researchers also measured an unusually high fracture toughness of 15 MPam1/2 and a wear resistance of 11. These figures are better than those for polycrystalline diamond, which has a fracture toughness of 5.3–7.0 MPam1/2 and a wear resistance of 3–4.

According to the team, these properties, and the fact that it is stable above 1600 K in air, mean that the material is an exceptional superabrasive. It could also be used in drilling or mining applications and in the machining of hard alloys and ceramics. Moreover, it might help in the design of other ideal materials that are hard, tough and thermally stable – something that remains a goal for materials science.

The researchers say that the increased hardness is a result of two factors: the nanosize effect, which restricts the propagation of dislocations throughout the material that would otherwise weaken it; and quantum confinement effects, which increase the hardness of individual crystallites.

The work was reported in Appl. Phys. Lett..