| Summary: | In recent years there have been great efforts towards the invention of materials aimed at
addressing the weaknesses of diamond as a superabrasive material. The low thermal
oxidation stability and inability to resist chemical attack by iron group metals (Fe, Ni, Co
and its alloys) makes it uneconomical to use diamond for high speed machining of steel
alloys. Although cubic boron nitride (c-BN) has in some instances sufficed as the
superabrasive of choice for high speed machining of steel alloys, its hardness value is about
half that of diamond.
In light of this, it is important to take into consideration what makes the traditional
superabrasives special inorder to design superabrasives that can successfully complement
diamond and c-BN. The synthesis of materials consisting of light elements such as boron,
carbon, nitrogen and oxygen possessing extremely strong and short covalent bonds forming
tight 3-D networks with extreme resistance to external forces will lead to major
breakthroughs in this endeavour. This justifies the efforts directed at exploring the B-C-N
system. Still the search has been hindered by several factors such as the inability to obtain
suitable starting materials and the prohibitively high synthesis pressures. It is however
envisaged that with persistent research a breakthrough will eventually be made. Thus this
will keep this field of study energised for some time.
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The present work investigates the possibility of obtaining bulk sintered cubic BN-C
materials over a range of P,T,t conditions, from a polymer derived t-BN-C starting material.
The choice of the starting material was arrived at after a consideration of several factors
such as level of atomic intermixing, % yield of the method, high temperature stability of the
starting material and the cost of production. The BN-C starting materials used in the present
study were synthesized by solid phase pyrolysis of piperazine borane, C4H10N2·BH3 at the
Darmstadt Institute of Materials Science, Darmstadt,Germany. A milled mixture of graphite
and h-BN in the molar ratio of 2:1 was prepared for comparison purposes. Piperazine
borane was obtained by the reaction of piperazine (99 %) with borane dimethyl sulphide
complex in a molar ratio of 1:1. The borane was first polymerized at 400 °C for 10 h in Ar
forming a yellow coloured polymer. In a second step the resulting infusible polymer was
thermally decomposed at 1050 °C and 2h holding time under Ar flow and with a heating
rate of 100 °C/min to give the ternary BN-C material in about 44 wt% ceramic yield. The
ceramic was subsequently heat treated under N2 atmosphere to give a nominal composition
of BC1.97N with 0.438(7) wt% oxygen. The starting materials were characterised by X-ray
diffraction, Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR),
Thermal gravimetry Analysis and X-Ray Spectroscopy (XPS). The starting BN-C materials
were found to posses a turbostratic type structure composed of mostly C-C and B-N type
bonds. The exact arrangement however could not be ascertained from the analyses above.
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Static high pressure and high temperature (HP/HT) studies show a thermal stability
threshold of 2000°C at 2GPa. An attempt to synthesise cubic phases under shock
compression using a gas gun and column type methods resulted in no transformation at
temperatures below 2000°C and pressures of up to 46GPa. However decomposition was
observed at higher temperatures with the formation of boron carbide materials besides the
BN-C starting materials.
A further static HP/HT synthesis was successfully conducted in a 1200 t Sumitomo and
large volume 5000t Zwick-Voggenreiter ‘6-8’ type multi-anvil presses at the Bayerisches
Geoinstitut, Bayreuth,Germany. Bulk cubic BN-C materials were synthesized under static
high thermobaric conditions (20 GPa/2000 °C/30 s and 60 s holding time and fast quench)
in the form of sample Z608 from the 5000t press and S4306 in the 1200t press respectively.
A partial transformation was observed at 1920°C at the same pressure and no
transformation was observed by compressing the precursor at room temperature and 20GPa
pressure. Furthermore, no phase transformation was observed at 15GPa and 2000°C
heating. The bulk samples were characterised by synchrotron XRD, Raman spectroscopy
and Transmission Electron Microscopy (TEM). Polycrystalline c-BC2N materials with an
F-43m space group were formed under the applied reaction conditions. The ternary boron
carbonitride bulk materials possess unique Raman spectra resembling those known for
boron-doped diamond samples. This has prompted a further investigation on the low
temperature resistivity properties of high pressure-high temperature synthesised samples.
Temperature dependence resistivity studies of sample Z608 have shown a characteristic
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semiconducting property typical of amorphous type materials. TEM studies revealed partial
decomposition of sample Z608 with some nanodiamond being observed in High
Resolution-TEM.
In conclusion, this work shows that the successful synthesis conditions of a cubic BN-C
phase from polymer derived BN-C ceramic are 20GPa,2000°C and 30s isothermal holding
time. The HP/HT synthesis of cubic BN-C from this particular polymer derived ceramic
has not been reported earlier, thus this work presents a novel method. Furthermore, the
project presents similiar findings reported earlier in referenced literature confirming the
very high thermobaric conditions required for obtaining the cubic BN-C materials. This
makes the commercialisation of c-BN-C synthesis a non-starter thus the most plausible
route would be to study the possibility of reducing the thermobaric conditions in the
presence of solvent catalysts.
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