A study on the synthesis of ultrahard cubic BC2N heterodiamond

摘要

In recent years there have been great efforts towards the invention of materials aimed atudaddressing the weaknesses of diamond as a superabrasive material. The low thermaludoxidation stability and inability to resist chemical attack by iron group metals (Fe, Ni, Coudand its alloys) makes it uneconomical to use diamond for high speed machining of steeludalloys. Although cubic boron nitride (c-BN) has in some instances sufficed as theudsuperabrasive of choice for high speed machining of steel alloys, its hardness value is aboutudhalf that of diamond.udIn light of this, it is important to take into consideration what makes the traditionaludsuperabrasives special inorder to design superabrasives that can successfully complementuddiamond and c-BN. The synthesis of materials consisting of light elements such as boron,udcarbon, nitrogen and oxygen possessing extremely strong and short covalent bonds formingudtight 3-D networks with extreme resistance to external forces will lead to majorudbreakthroughs in this endeavour. This justifies the efforts directed at exploring the B-C-Nudsystem. Still the search has been hindered by several factors such as the inability to obtainudsuitable starting materials and the prohibitively high synthesis pressures. It is howeverudenvisaged that with persistent research a breakthrough will eventually be made. Thus thisudwill keep this field of study energised for some time.udivudThe present work investigates the possibility of obtaining bulk sintered cubic BN-Cudmaterials over a range of P,T,t conditions, from a polymer derived t-BN-C starting material.udThe choice of the starting material was arrived at after a consideration of several factorsudsuch as level of atomic intermixing, % yield of the method, high temperature stability of theudstarting material and the cost of production. The BN-C starting materials used in the presentudstudy were synthesized by solid phase pyrolysis of piperazine borane, C4H10N2·BH3 at theudDarmstadt Institute of Materials Science, Darmstadt,Germany. A milled mixture of graphiteudand h-BN in the molar ratio of 2:1 was prepared for comparison purposes. Piperazineudborane was obtained by the reaction of piperazine (99 %) with borane dimethyl sulphideudcomplex in a molar ratio of 1:1. The borane was first polymerized at 400 °C for 10 h in Arudforming a yellow coloured polymer. In a second step the resulting infusible polymer wasudthermally decomposed at 1050 °C and 2h holding time under Ar flow and with a heatingudrate of 100 °C/min to give the ternary BN-C material in about 44 wt% ceramic yield. Theudceramic was subsequently heat treated under N2 atmosphere to give a nominal compositionudof BC1.97N with 0.438(7) wt% oxygen. The starting materials were characterised by X-rayuddiffraction, Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR),udThermal gravimetry Analysis and X-Ray Spectroscopy (XPS). The starting BN-C materialsudwere found to posses a turbostratic type structure composed of mostly C-C and B-N typeudbonds. The exact arrangement however could not be ascertained from the analyses above.udvudStatic high pressure and high temperature (HP/HT) studies show a thermal stabilityudthreshold of 2000°C at 2GPa. An attempt to synthesise cubic phases under shockudcompression using a gas gun and column type methods resulted in no transformation atudtemperatures below 2000°C and pressures of up to 46GPa. However decomposition wasudobserved at higher temperatures with the formation of boron carbide materials besides theudBN-C starting materials.udA further static HP/HT synthesis was successfully conducted in a 1200 t Sumitomo andudlarge volume 5000t Zwick-Voggenreiter ‘6-8’ type multi-anvil presses at the BayerischesudGeoinstitut, Bayreuth,Germany. Bulk cubic BN-C materials were synthesized under staticudhigh thermobaric conditions (20 GPa/2000 °C/30 s and 60 s holding time and fast quench)udin the form of sample Z608 from the 5000t press and S4306 in the 1200t press respectively.udA partial transformation was observed at 1920°C at the same pressure and noudtransformation was observed by compressing the precursor at room temperature and 20GPaudpressure. Furthermore, no phase transformation was observed at 15GPa and 2000°Cudheating. The bulk samples were characterised by synchrotron XRD, Raman spectroscopyudand Transmission Electron Microscopy (TEM). Polycrystalline c-BC2N materials with anudF-43m space group were formed under the applied reaction conditions. The ternary boronudcarbonitride bulk materials possess unique Raman spectra resembling those known forudboron-doped diamond samples. This has prompted a further investigation on the lowudtemperature resistivity properties of high pressure-high temperature synthesised samples.udTemperature dependence resistivity studies of sample Z608 have shown a characteristicudviudsemiconducting property typical of amorphous type materials. TEM studies revealed partialuddecomposition of sample Z608 with some nanodiamond being observed in HighudResolution-TEM.udIn conclusion, this work shows that the successful synthesis conditions of a cubic BN-Cudphase from polymer derived BN-C ceramic are 20GPa,2000°C and 30s isothermal holdingudtime. The HP/HT synthesis of cubic BN-C from this particular polymer derived ceramicudhas not been reported earlier, thus this work presents a novel method. Furthermore, theudproject presents similiar findings reported earlier in referenced literature confirming theudvery high thermobaric conditions required for obtaining the cubic BN-C materials. Thisudmakes the commercialisation of c-BN-C synthesis a non-starter thus the most plausibleudroute would be to study the possibility of reducing the thermobaric conditions in theudpresence of solvent catalysts.