Supplemental Material
Synthesis Technique
High-pressure experiments were performed using a two-stage 6-8 type large-volume multianvil system with a Zwick/Voggenreiter 5000-ton press. The sample assembly consisted of a MgO +5 wt % Cr
2
O
3
octahedron with 18 mm edge length containing a LaCrO
3
heater.
The octahedron was compressed using eighth 54-mm tungsten carbide anvils with a truncation edge length of 11 mm and pyrophyllite gaskets. The sample was either placed into hBN capsule of a cylindrical shape with a sample chamber of 2 mm in diameter and 3 mm in height. Sample temperature was monitored by a W3%Re-W25%Re thermocouple located axially with respect to the heater and with a junction close to the BN capsule without corrections for the pressure effect on the thermocouple emf. Sample pressure at high temperatures as a function of hydraulic oil pressure was calibrated using the P-T diagrams of
MgSiO
3
and Mg
2
SiO
4
. The pressure and temperature uncertainties were estimated to be 1 GPa and 50 K, respectively. Samples were gradually compressed to the desired pressure at ambient temperature, and then the temperature was stepwise increased with a heating rate of about 100
K/min up to the desired value. Duration of heating was maximum 2 min in different runs. The samples were quenched by switching off the power and then slowly decompressed. They came out as translucent cylinders of a black shining material in a cBN capsule (at HPHT hBN transformed to cubic phase).
Samples characterisation
After synthesis all the samples were subsequently studied using in-house and synchrotron Xray facilities. A high-brilliance FRD RIGAKU diffractometer installed in the Bayerisches
Geoinstitute (Mo K

radiation, 60 mA–55 kV, APEX charge coupled device (CCD) area detector) was used. Angle-dispersive X-ray diffraction study was performed using the highresolution powder diffractometer of the Swiss-Norwegian BM01 beamline at the ESRF.
Monochromatic beam with 0.7995 Å wavelength was used. The diffracted intensities were registered with a MAR345 image plate. Diffraction data were processed using GSAS program.
Raman spectra were collected using Raman spectrometers XY Dilor, operating with 514 nm laser, and LabRam, operating with 632 nm incident laser.

The chemical composition and texture of the samples was analyzed using a PHILIPS CM20
FEG analytical transmission electron microscope (ATEM), including high resolution
HRTEM, operating at 200 kV.
Vickers hardness (Hv) measurements were performed with a microhardness tester (M-400-
G2, LECO Corporation) under loads up to 20 N. The size of the imprints was measured using scanning electron microscopy (LEO-1530).
120
14(1) nm, 1870(50) K
70(5) nm, 2370(50) K
>100 nm, 2570(50) K
100
80
60
40
20
0
22.0
22.2
22.4
22.6
22.8

23.0
23.2
23.4
Fig. S1. Shape of (002) reflections of cBN synthesized at 18 GPa and various temperatures.
The lines in the diffraction patterns of the samples synthesized at moderate temperatures
(1750-2200 K) show considerable broadening that reflects changes in size of blocks of coherent scattering as a function of the synthesis condition. For 18 GPa synthesis pressure smallest crystallites of 14 nm are observed in samples obtained at 1870(50) K. The narrowest reflection and biggest blocks of coherent scattering (>100 nm) correspond to the sample synthesized at the highest temperature.

Fig. S2. Raman spectra of the materials synthesized simultaneously at P=20(1) GPa and T=
2270 K from different precursors. A cross-section of the sample chamber after the experiment is shown in the inset (millimetre paper in the background serves as a scale). The outer layer of the sample was prepared from hexagonal BN, which transformed to cBN (grey) at HPHT conditions. A central part of the sample, made of pyrolytic p-BN and isolated by a platinum foil, transformed to ABNNC (shining black). Always, when graphite-like hBN (Goodfellow
Inc. or Aldrich Inc.) was used as a starting material, only sphalerite structured cBN with various crystallite sizes (ranged from 25 nm, nanocrystalline, to >100 nm, polycrystalline) was produced.

5000
4000
3000
ABNNC (18 nm, 18 GPa, 2170 K) cBN (18 GPa, 2570 K)
Calculated, cBN
Calculated, ABNNC
2000
1000
0
1030 1040 1050 1060 1070
-1
Fig. S3. Transverse optical Raman peaks of polycrystalline cBN (blue crosses; material synthesised at 18 GPa and 2570 K; maximum at 1054.8 cm
-1
and FWHM 7 cm
-1
) and nanocrystalline ABNNC (red pluses; crystallite size 18 nm; material synthesised at 18 GPa and 2170 K; maximum at 1053.0 cm
-1
and FWHM 10 cm
-1
). Continues lines are profiles calculated according to the phonon confinement model (M. Rajalakshmi, A. K. Arora, B. S.
Bendre, S. Mahamuni, Optical phonon confinement in zinc oxide nanoparticles, J Appl. Phys.,
87, 2445-2448 (2000); D. Bersani, P. P. Lottici, Phonon confinement effects in the Raman scattering by TiO2 nanocrystals, Appl. Phys. Lett. 72, 73-75 (1998)):
I (

, d )

A

(
  
0
  exp(

 sin
2 q
2 d
16
( qa

/
2
2
)
4 ))
2 
(

0
/ 2 )
2 dq , where A is the intensity prefactor, Γ
0
=7cm
-1
is the Raman linewidth,

0
=1054.8 cm
-1
is zonecenter phonon,
 
= 150 cm
-1
is the difference between the zone-center and the zone-boundary frequencies of the phonon dispersion curve of interest, d is crystallite size, a is the lattice parameter, q is phonon of wave vector.

Fig. S4. An example of the bright-field TEM images of the sample synthesized at 18(1) GPa and 1870(50) K. The high resolution HRTEM image (inset in the right upper corner) shows that nanometer-size grains have even finer structure. Namely, the material, which we call aggregated BN nanocomposite (ABNNC), consists of blocks with 2-layered wurtzite and 3layered sphalerite structures altering at the level of a few to a dozen of nanometers. The inset in the left lower corner shows an electron diffraction pattern, where the lines attributed to cBN and wBN phases (denoted with symbols “ c” and “w” before crystallographic indexes of reflections) are clearly visible.
The results of transmission electron microscopy (TEM) are in good agreement with the X-ray data. Figure S4 shows an example of the bright-field TEM images of the sample synthesized at 18(1) GPa and 1870(50) K. The material consists of particles with the size range of 20 to 70 nm. Moreover, high resolution HRTEM (inset in the left upper corner, Figure S4) shows that these already nanometer-size grains have an even finer structure. Namely, the material consists of blocks with two-layered wurtzite and three-layered sphalerite structures altering at the level of a few to tens of nanometers in correspondence with electron diffraction data (inset in the left lower corner, Figure S4), and we further call it aggregated BN nanocomposite
(ABNNC).
We measured the Vickers hardness of the samples with different phase compositions (e.g. single-phase cBN and ABNNC) and the various crystallite size (estimated from X-ray diffraction data; within the precision of our measurements we do not see difference in crystallite size of cBN and wBN and we use average value). At least five indentations were made for each data point to provide good statistics. As recommended for hard and brittle materials
24
, hardness is reported in the asymptotic-hardness region (Figure S5).

Fig. S5. The load dependence of the Vickers hardness for the sample synthesized at 20(1) GPa and 1870(50) K. The inset shows a scanning electron microscopy image of a typical indentation produced by twenty-second loading.
Wear coefficient
Wear coefficient, W
H
characterizes the wear resistance of a material. A common expression for wear resistance is the volume of the materials removed as a function of the applied load, hardness, and fracture toughness of the material. The harder and tougher the material, the better is its wear resistance and the higher its wear coefficient, for which there are a number of empirical formulas. To compare ABNNC with natural diamonds, ADNRs, and commercially available polycrystalline diamonds we used the same formula that was used by one of the world largest PCD manufacturers (Element Six (Pty) Ltd.):
W
H
= K
1C
0.5
*E
-0.8
* H
K
1.43
, where E is Young’s Modulus, H
K is the Knoop hardness, and K
1C is the fracture toughness.
All of the properties (E, H
K
, K
1C
) were determined experimentally for ABNNC in the present work. In particular, E = 777 GPa was found from gigahertz spectrometry data and nanoindentation measurements, and K
1C
= 15 MPа m
0.5
was calculated based on its relationship with the length of radial cracks from corners of imprints. (The details of these experiments will be published separately elsewhere.)
The value of W
H
= 11 is extremely high in comparison with that of single natural diamond
(~2-5), PCD (~3-4), and even ADNRs (~10), which was recently clamed as superior one.

Wear (abrasion) resistance and toughness are the two most important properties in assessing the suitability of superabrasive materials. Using the same wear coefficient (W
H
) calculation method, which was applied for polycrystalline diamonds (W
H
=3-4) and ADNRs (W
H
=10), we found W
H
= 11 for ABNNC with a crystallite size of 14 nm. In combination with high thermal stability (no changes were observed in samples treated at temperature up to 1600 K in air for 4 h), superior wear resistance and extremely high hardness of ABNNC open for this material unique opportunities for industrial applications ranging from deep-drilling in oil-exploration and mining to precision machining of hard alloys and ceramics.