Superhard nanocomposite of dense polymorphs of boron nitride: Noncarbon material has reached diamond hardness

Abstract

The authors report a synthesis of unique superhard aggregated boron nitride nanocomposites (ABNNCs) showing the enhancement of hardness up to 100% in comparison with single crystal c-BN. Such a great hardness increase is due to the combination of the Hall-Petch and the quantum confinement effects. The decrease of the grain size down to 14nm and the simultaneous formation of the two dense BN phases with hexagonal and cubic structures within the grains at nano- and subnanolevel result in enormous mechanical property enhancement with maximum hardness of 85(5)GPa. Thus, ABNNC is the first non-carbon-based bulk material with the value of hard-ness approaching that of single crystal and polycrystalline diamond and aggregated diamond nanorods. ABNNC also has an unusually high fracture toughness for superhard materials (K1C=15MPam0.5) and wear resistance (WH=11; compare, for industrial polycrystalline diamond, WH=3–4), in combination with high thermal stability (above 1600K in air), making it an exceptional superabrasive.

Intrinsic hardness of crystalline solids

“…(9)), which have a 1/d dependence, to the Hall-Petch equation improves the agreement with the observed trend significantly over a large hardness range on the crystallite size (14-2000 nm) (Fig. 8) [29]. …”

“…This finding contradicts the reverse Hall-Petch effect [25]. However, a fit of the measured hardness (H V ) to the crystallite size (d), according to the Hall-Petch equation [29,30] (H V = H 0 + K/ d ), which has an inverse square root relationship with d was found to be unsatisfactory. However, adding the quantum effects term (Eq.…”

“…Recently, a unique superhard aggregated boron nitride nanocomposites (ABNNCs) showing the enhancement of hardness up to 100% in comparison with a cBN single crystal has been synthesized and characterized [29]. The decrease of the grain size down to nano-and subnanolevel results in enormous mechanical property enhancement with maximum hardness of 85 GPa.…”

Large indentation strain-stiffening in nanotwinned cubic boron nitride

“…Interestingly, all the molecular dynamics (MD) studied nanostructured Cu samples show a maximum strength (2.5B3.7 GPa) that is lower than the ideal tensile strength (4.2 GPa) for single crystal Cu 16 , and the measured maximum strength (1.1 GPa) of nt-Cu 10,11 is lower than that predicted by MD simulations [12][13][14][15] . Similar results have been observed in covalent materials, such as strength enhancement by reducing grain size d down to 10 nm in cBN in the Hall-Petch regime 17,18 and strength reduction in nanostructured diamond with do4.5 nm (refs 4,5) whose peak tensile and shear strengths only reach less than half the values for single crystal diamond 19,20 , which is attributed to the weak interactions associated with the irregular bonds at the boundaries between nanocrystallites, a typical GB-dominated reverse Hall-Petch effect.…”

“…The situation is drastically different for nt-cBN, which at an ultrafine twining size l ¼ 3.8 nm has a greatly enhanced Vickers indentation hardness (95B108 GPa) 6 that is almost twice the value for single crystal cBN 17,18 . This sharp contrast of a giant strength hardening in the classic reverse Hall-Petch regime, where a strength softening is normally expected, indicates a new mechanism governing the mechanical property of nt-cBN.…”