3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item A study on the synthesis of ultrahard cubic BC2N heterodiamond(2010-07-20T12:43:32Z) Matizamhuka, Wallace R.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. iv 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. v 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 vi 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.Item Development of a potentially hard Ta1-xZr1+O1+xN1-x material.(2008-06-09T10:37:58Z) Matizamhuka, Wallace R.Abstract Theoretical investigations on the ZrxTa1-xO1+xN1-x system predict that some of its phases are likely to possess relatively high hardness values.(1) Such materials may be suitable for industrial application as cutting tools. The motive of the project was to investigate the best synthesis route and a method for obtaining well sintered, dense oxynitride phases with a nominal composition TaON (x=0), Ta0.8Zr0.2O1.2N0.8 (x=0.2) and Ta0.3Zr0.7O1.7N0.3 (x=0.7). This was achieved through three main steps, i.e. synthesis of the oxynitride powders, high pressure sintering and evaluation of mechanical properties. A sol gel method was used to obtain the precursor oxide powders. TaCl5 and 70wt% zirconium propoxide were used as the starting materials. Oxide gels were formed by dissolving precursor materials in absolute ethanol for 15minutes with continuous stirring, followed by subsequent hydrolysis to form gels which were aged for 24hrs at 800C. The gels were dried in air at 1000C for 12hrs in a drying oven followed by calcinations in a muffle furnace at 6000C for 6hrs to remove the alkyls and chloride ions. High surface area amorphous powders were obtained (~6.60 ± 0.02 m2/g in the case of Ta2O5) after milling with 4mm steel balls for 4hrs in a planetary mill. The respective oxynitrides were obtained by thermal nitridation using an ammonia (99.99%) flow method. A temperature of 9000C maintained for 4hrs in the presence of water vapour at an ammonia flow rate of 50cm3/min were found to be the optimum nitridation conditions. The water vapour pressure was realised by bubbling the ammonia through a water bath at room temperature prior to supply to the furnace. The water vapour pressure of such a set up was approximated to be ~3.1*103Pa. This nitridation process was carried out in a tube furnace using a silica tube of length 1200mm and external diameter of 40mm and an alumina boat as the holding vessel. Approximately 2g of oxide powder were used for each run. The dependency of nitridation on temperature and ammonia flow rate iii was investigated for the formation of TaON. Pure TaON formation was found to be more favoured by temperatures of 9000C with a heating rate of 200C/min and by an ammonia flow rate range of 40-50cm3/min. These conditions were also used for the mixed Ta-Zr oxynitrides. Ta0.3Zr0.7O1.7N0.3 formation was found to be dependent on the heating rate with ZrO2 forming beside the oxynitride solid solution above a heating rate of 100C/min. In the present work the phenomenon has been found to be dependent on the kinetics of the crystallisation reactions. At higher heating rates crystallisation of the separate phases is favoured leading to the formation of separate phases. On the other hand with an optimum heating rate the solid solution is maintained to the final nitridation temperature. The powders were found to be thermally stable in air above 6000C with TaON being the most stable with a weight change occurring at a temperature of ~6900C. The powders were stable in pure nitrogen well above 10000C. Sintering in a hot press in the temperature range of 900-14000C at a heating rate of 500C/min and a pressure range of 50-85MPa using previously heat treated h-BN crucibles in argon resulted in porous, partially densified materials. A maximum % theoretical density of 81.6% was obtained for TaON at 10000C and 85MPa pressure applied for 1hr. TaON oxidised to Ta2O5 above 10000C with an oxide phase transition being observed above 13000C. High pressure sintering was carried out in the temperature and pressure regime of 920- 12000C and 3-5.5GPa respectively in the case of TaON. The mixed Ta-Zr oxynitrides were sintered at 3GPa at a temperature of 11000C. No phase transitions were observed in all cases. An average hardness value of ~16.8GPa and fracture toughness of ~3.4MPam1/2 were obtained for the TaON phase. Ta0.3Zr0.7O1.7N0.3 and Ta0.8Zr0.2O1.2N0.8 were found to possess hardness values of 13.4GPa and 13.02GPa respectively under the same sintering conditions. It was observed that the hardness values obtained for TaON are higher than those for ZrO2 or HfO2 ceramics, due to the stronger covalent bonding in nitrogen present in TaON. On the other hand the fracture toughness values are as low as those of fully stabilised ZrO2 materials due to lack of phase transformation toughening.