KINETICS AND MECHANISM OF CBN SYNTHESIS - IN SITU STUDIES
Vladimir L. Solozhenko
Institute for Superhard Materials of the
National Academy of Sciences of Ukraine, Kiev, Ukraine
(e-mail: vls@ismanu.kiev.ua)
Keywords: cubic boron nitride, high-pressure synthesis,
in situ studies, kinetics, mechanism
Cubic boron nitride (cBN), second to diamond hardest material, is widely used for machining and polishing ferrous metals and heat resistant alloys. Among wide band gap semiconductors, cBN has several distinct advantages that makes it a promising material for applications in high temperature - high power electronics and short wavelength optoelectronics.
cBN is usually produced from hexagonal graphite-like boron nitride (hBN) at high pressures and temperatures by the so-called "catalytic" synthesis in the presence of compounds that form relatively low-melting eutectic liquids with hBN. The notions of kinetics and mechanism of cBN synthesis are still inadequate and inconsistent as they are based only on experimental data obtained by quenching from high pressures and temperatures.
Recently we have in situ studied the cBN formation by both crystallization from BN solutions in melts of various systems and solid-state phase transformations up to 7 GPa and 2000 K using powder diffraction of synchrotron radiation. The use of high flux of HASYLAB white beam and fixed-angle, energy-dispersive diffraction permitted experiments in real time scale and observation of intermediate phases existing only in narrow p,T-ranges. Our findings made us to revise the generally accepted notions of phase diagrams of some systems used for commercial synthesis of cBN and allowed some conclusions about mechanism of cBN nucleation and crystal growth.
For instance, the
incongruent nature of melting of Li3BN2(O),
a previously unknown high-pressure phase [1], and existence of L
+ BN = Li3BN2(O) peritectic equilibrium at
1620 K (5.36.3 GPa) have been established [2]. Below this
temperature cBN liquidus in the Li3NBN quasi-binary
section of the Li-B-N ternary system does not exist and, hence,
cBN crystallization is impossible. These results argue against
the previous ideas of congruent melting of Li3BN2
at high pressures [3] and provide the explanation of
experimentally observed low-temperature threshold for cBN
crystallization in this system. In situ studies of
kinetics of the cBN crystallization from the Li3NBN
melt being in equilibrium with hBN indicate that the process is
controlled by BN diffusion in the melt [4]. Kinetics data might
be best fitted by a model that assumes an instantaneous
nucleation in the initial stage of crystallization and nucleation
at constant rate when hBN-to-cBN conversion degree is higher than
0.2. This fact indicates that the nucleation mechanism changes in
the course of cBN crystallization.