The influence of the h-BN morphology and structure on the c-BN growth
Introduction
Cubic boron nitride (c-BN) the second hardest material after diamond is extensively used in industry for tooling especially steel products, because, unlike diamond, they scarcely react with iron. Cubic BN is produced by heating hexagonal BN (h-BN) under high temperature and pressure [1]. This is a direct transformation from the hexagonal to the cubic phase. To lower significantly the synthesis temperature different catalysts M3B2N4 (M=Li, Mg, Ca, Sr, Ba) were used [2].
Hexagonal boron nitride (a=0.250 nm, c=0.666 nm) has a layered structure. B and N atoms form hexagons, which extend in two-dimensional layers, forming the (0001) planes of the structure [3]. Within the hexagonal layers the B and N atoms are strongly linked by covalent sp2 bonds. The (0001) layers are stacked along the c-axis and weakly bonded by van der Waals forces. In a well-crystallized specimen, the B–N sheets are stacked parallel to each other.
After synthesis, h-BN powders can have different degrees of graphitization, i.e. different degrees of three-dimensional order. To measure the degree of order, a structural figure of merit was introduced [4] called graphitization index (GI). GIs are derived from X-ray diffraction as the ratio between the area under the [(100)+(101)] peaks over the area under the (102) peak. As defined in [4], GI describes the degree of order in the stacking of the B–N layers along the c-axis. It can vary between 1.6 for well-graphitized BN and 40–50 for the so-called turbostratic BN, were the B–N layers show random rotations and translations about the layer normal. For the c-BN synthesis, usually well-crystallized h-BN powders are chosen, which have GI<7.
In this paper we present a structural study obtained by transmission electron microscopy (TEM) on h-BN powders with graphitization indices between 1.56 and 4.97. Some of these precursors with similar GIs exhibited very different behavior at the synthesis of c-BN with Mg-based catalysts. We will show that the modification in the microstructure of the h-BN powders, as revealed by high resolution electron microscopy (HREM), can significantly influence the nucleation rate of c-BN and consequently the quality and size of the obtained c-BN crystals.
Section snippets
Experimental
h-BN powders of different provenence were used in this study. Samples for TEM studies were prepared by dispersing the powders ultrasonically in ethanol and retaining a drop of the colloid on carbon holey grids. The observations in diffraction contrast were carried out with a Philips CM 20 electron microscope, while high resolution was performed with a JEOL 4000 EX instrument. High pressure/high temperature tests were performed to produce c-BN by subjecting h-BN and Mg-based catalyst powders at
Results and discussion
The TEM images of the h-BN powders showed that the crystallites agglomerate in aggregates, which were not broken in the ultrasound bath. Fig. 1 gives a characteristic image of such an aggregate. All the crystallites are oriented with the c-axis perpendicular to the plane of the figure. Many of the crystallites in the aggregate are rotated with respect to each other, giving characteristic moiré fringes in the TEM images. On images like in Fig. 1 it is possible to measure the diameter (D) of the
Conclusions
- 1.
By studying the influence of the structural characteristics of h-BN precursors on the HP-HT c-BN synthesis yields, it has been noticed that besides the well-known GI factor, a better assessment can be performed by examining the microscopic features of the h-BN particles.
- 2.
The c-BN synthesis in the Mg–B–N system showed that the nucleation rate is better controlled for h-BN powders with a good ordering along the c-axis, although they have relatively smaller average particle diameters and larger GI
Acknowledgements
This work has been performed under the INCO-COPERNICUS contract no. IC15CT970710 of the European Communities.
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