New Developments in the Boron Nitride Family

Authors:

Wang Guangzu, Zhengzhou Research Institute for Abrasives & Grinding

1. A Material Harder than Diamond Discovered — Wurtzite Boron Nitride

Among natural materials, mankind had almost abandoned the search for substances harder than diamond. However, the discovery of wurtzite boron nitride (wBN) has challenged diamond’s long-held position. This extremely rare natural material, formed during volcanic eruptions, is even rarer than diamond and exhibits a hardness approximately 18% higher than that of diamond.

Although wBN is exceedingly scarce, it is only one member of the boron nitride family. Boron nitride exists in four polymorphs: hexagonal boron nitride (hBN), rhombohedral boron nitride (rBN), cubic boron nitride (cBN), and wurtzite boron nitride (wBN). All of them possess Vickers hardness values comparable to diamond. Boron nitride is a material with broad application potential, suitable for the electronics and chemical industries, and—when used as a superhard material—capable of replacing diamond in exploration, drilling, and cutting tools (Baidu News).

For further reference, see Chapter 10, “Shock Synthesis of Wurtzite Boron Nitride”, in Nanodiamond, edited by Wang Guangzu and published by Zhengzhou University Press in 2009. This chapter covers the isothermal equation of state of wBN, phase transitions under shock treatment, transformation mechanisms from wBN to hBN under high pressure, and the transformation from wBN to cBN.

2. Phase Transformations of wBN

2.1 Isothermal Equation of State of wBN

Jilin University employed two different high-pressure in situ X-ray diffraction techniques to investigate the isothermal equation of state of dynamically shock-synthesized wBN under room-temperature static compression. By comparing the results with those of other studies, the isothermal bulk modulus was determined to be 334 GPa, slightly lower than that of cBN (369 GPa). No additional phase transitions occur in wBN under static pressures below 50 GPa.

2.2 Phase Transformation of wBN under Shock Treatment

The raw materials were mixed with copper powder at a weight ratio of 5:59 and pressed into stainless steel molds to form compacts with dimensions of 12 mm in diameter and 5 mm in height, achieving a density of 85% of the theoretical value. The wBN used in the experiments was obtained by shock compression, with an average particle size of 1–10 μm. Shock treatment was carried out using a ratchet-type planar wave generator at impact velocities of 2.5–5.3 km/s, generating pressures of 60–200 GPa. The shock-treated materials were then used for subsequent experiments.

2.3 Transformation Mechanisms of wBN to hBN under High Pressure

Metastable wBN transforms into stable hBN and cBN upon heating. Below the hBN–cBN equilibrium line, hBN is formed, whereas at higher pressures, cBN is produced. When heated in vacuum, the transformation of wBN to hBN begins at 600–700°C and completes at 1300°C, following a martensitic transformation mechanism. During hot pressing, the wBN–hBN and wBN–cBN transformations typically occur simultaneously.

At 1800°C and 7.7 GPa, changes in phase composition and density during hot pressing of wBN polycrystals were observed. The variation in hBN content depends on heating time: the initial increase is governed by the wBN–hBN transformation, while the subsequent decrease corresponds to the hBN–cBN transformation.

Experiments showed that during the initial isothermal holding stage, the maximum hBN content reaches 12 vol%, while cBN remains below 5 vol%. Prolonged sintering causes nearly all hBN to be consumed in forming cBN, with at least 10 vol% of hBN transforming into cBN. When evaluating the role of the hBN–cBN transformation, kinetic processes during sintering must be considered, as volume reduction during transformation promotes further hBN formation, followed by reconversion to cBN. The formation of cubic variants during wBN sintering has a pronounced effect on the hBN–cBN transformation.

The kinetics of hBN formation and consumption are critical factors in densification. Further densification during sintering may suppress hBN formation. Similar behavior was observed in samples sintered at 7.7 GPa and 2000°C, where the transformation proceeded faster than at 1800°C. At 2000°C, hBN content initially reached 16 vol%, but nearly all hBN transformed into cBN within 60 seconds.

It has been established that the wBN–hBN transformation is martensitic and can occur via two mechanisms: prismatic-plane and basal-plane mechanisms. Under atmospheric pressure and vacuum at high temperatures, the prismatic-plane mechanism dominates, whereas under high pressure, both mechanisms operate, with the basal-plane mechanism prevailing.

3. “Chair” and “Boat” Models of Boron Nitride

Experimental studies indicate three possible pathways for the transformation of hBN to wBN:

Transformation via the “chair model” directly to wBN;

Transformation via the “boat model” to an ADA-type structure, followed by transformation to wBN;

Transformation via the “boat model” by directly overcoming the energy barrier through thermal activation.

Since the energy barrier of the chair model is lower than that of the boat model, the chair model is considered the dominant pathway.

Regarding the transformation from hBN to cBN, only the chair model corresponds structurally to cBN. Therefore, regardless of the transformation pathway, hBN must pass through the chair configuration to become cBN. Three possible mechanisms are proposed:

According to T. Akashi, compressed hBN first transforms into wBN and then into cBN;

Direct thermal activation across the energy barrier between hBN and cBN due to intense atomic vibrations during interlayer compression;

Transformation of hBN into cBN under strong shear stress, followed by conversion from rBN to cBN.

The first pathway requires extremely high pressures (>100 GPa, even 150–200 GPa), far exceeding typical experimental conditions. The critical issue remains the transformation mechanism of wBN to cBN. Static pressure theory suggests that wBN must first undergo graphitization—transforming into low-density BN—before converting into cBN. This graphitization can occur via basal-plane or prismatic-plane mechanisms.

Under atmospheric pressure and vacuum at high temperatures, wBN graphitization proceeds via the prismatic-plane mechanism, whereas under high pressure, the dominant pathway is wBN → hBN → cBN. The hBN–cBN transformation is thus the key step. Direct thermal activation under high pressure favors hBN → wBN rather than hBN → cBN, due to the much lower energy barrier. Therefore, hBN can only transform into cBN through shear-induced mechanisms, involving dislocation propagation and stacking fault formation, which act as nuclei for cBN growth.

4. Progress in cBN Biomedical Coatings

Researchers at Kyushu University, Japan, prepared high-quality cBN films with excellent in vitro biocompatibility using plasma-enhanced chemical vapor deposition (PECVD). Chemical treatment of the films in hydrogen and nitrogen plasmas removed terminal fluorine atoms from the cBN surface, significantly increasing surface free energy polarity and rendering the films superhydrophilic.

Successful proliferation, differentiation, and biomineralization of osteoblasts were confirmed on these superhydrophilic cBN films, with performance comparable to nanodiamond films. The results demonstrate the strong potential of cBN as a non-cytotoxic superhard coating for biomedical applications.

5. Successful Synthesis of Ultra-Transparent Polycrystalline cBN

Sichuan University’s High-Pressure Science Laboratory and Geophysics Research Group synthesized nano-polycrystalline cubic boron nitride (PcBN) with a hardness of 69 GPa, making it the second-hardest transparent superhard material after diamond.

The transparent PcBN bulk material, produced at 14 GPa and 1700–1800°C, exhibits approximately 70% transmittance at wavelengths of 1400–1500 nm, along with excellent mechanical properties. The transparency is attributed to ultra-thin grain boundaries (~2 nm), while grain refinement and plastic deformation under high pressure enhance hardness.

This work provides critical evidence for the synthesis of transparent superhard ceramics and offers a more economical transparent window material for extreme environments.

6. Ultra-High Thermal Conductivity: From Diamond to cBN Crystals

Thermal conductivity is a key parameter for heat dissipation. Diamond has long been the benchmark, with room-temperature thermal conductivity of ~2000 W·m⁻¹·K⁻¹. However, its cost limits widespread application.

In 2020, Song Bai and co-authors reported in Science that isotopically enriched cubic boron nitride crystals achieved thermal conductivities exceeding 1600 W·m⁻¹·K⁻¹, surpassing boron arsenide and becoming the best non-carbon isotropic thermal conductor. Isotopic enrichment increased thermal conductivity by nearly 90%, the largest isotope effect observed to date.

7. Ultra-High Thermal Conductivity cBN-Based Thermal Pads

In November 2013, Shanghai Aled Industrial Co., Ltd. filed a patent for an ultra-high thermal conductivity cBN-based thermal pad, achieving thermal conductivity values exceeding 18 W/(m·K) through optimized composite formulations.

8. New Mechanism for hBN Synthesis Revealed

Researchers proposed a vacancy-assisted growth mechanism for high-quality multilayer hBN on Fe₂B alloy surfaces, enabling controlled synthesis at relatively low temperatures (~700 K). This mechanism addresses long-standing limitations in nitrogen solubility and diffusion, expanding the potential of hBN in 2D nanoelectronics and emerging microelectronic devices.

9. Cubic Boron Nitride and the Diversity of Boron Nitride

Beyond cBN, boron nitride exists in multiple crystal forms, each with distinct properties. Hexagonal boron nitride (“white graphite”) shares structural similarities with graphite but remains electrically insulating while exhibiting excellent thermal conductivity, chemical stability, and wear resistance.

10. Coherent Control of Solid-State Spin Color Centers in 2D Materials at Room Temperature

Researchers at the University of Science and Technology of China demonstrated coherent control of single-spin color centers in hBN at room temperature, marking significant progress toward quantum information technologies based on 2D wide-bandgap materials.