3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
6 results
Search Results
Item Low pressure liquid phase sintered diamond composites(2018) Mkhize, Mandisa QueenethPolycrystalline diamond (PCD) materials form part of tool components used in automobile, aerospace and mining applications. These components are commonly prepared using high pressure high temperature (HPHT) techniques. The importance of PCD is due to properties such as very high hardness, toughness and wear resistance at extreme conditions in a reproducible manner. However, few studies have examined the feasibility of using liquid phase sintering aids, such as the Y2O3-Al2O3-SiO2 oxide binder system to sinter PCD at low pressures using the Spark Plasma Sintering (SPS) method. In this study we aimed to produce a dense, strong, liquid phase-sintered diamond composite without undergoing the diamond phase’s solution re-precipitation stage, under a low pressure of 30 – 70 MPa. Diamond composites using monomodal and bimodal diamond feedstock powders were fabricated using yttrium alumino-silicate additives, with compositions of 40wt%Y2O3-25wt%Al2O3-35wt%SiO2 (yttria-rich) and 30.78wt%Y2O3-13.65wt%Al2O3-55.58wt%SiO2 (silica-rich) labelled as LPI and LPII, respectively. Diamond powder and the yttria alumina silica powders were mixed using the planetary ball milling technique and the ad-mixed components were heated and pressed using the SPS furnace. This showed that the silica-rich liquid phase sintering aid produced low density composites due to amorphous grain boundary and the move of the softening point to high temperatures. However, the yttria–rich additive produced bimodal diamond composites of high relative density of ~97% and hardness of ~13GPa due to faster densification rates. All the samples were measured for density using the Archimedes' method. Characterization was performed using powder X-Ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersion Spectroscopy (EDS) and Vickers hardness measurement. Examination of fracture surfaces resulted in linking microstructural features such as intergranular cracks, crack branching and intergranular phases to the behavior of these additives under the sintering conditions used in this work. This study revealed that high densities were attainable using the yttria-rich binder under low pressures using an SPS furnace. The effect of the heating/cooling rates via the SPS were also observed to affect the microstructural behavior of the composites and consequently their properties.Item Development of PGMs-modified TiAl-based alloys and their properties(2017) Mwamba, Ilunga AlainTitanium aluminides Ti3Al (α2), γ-TiAl and TiAl3 have received much attention for potential applications where light weight for energy saving, room temperature corrosion resistance in aqueous solutions, high-temperature oxidation resistance, or where combinations of the above are needed. Gamma-TiAl of composition Ti-47.5 at.% Al with additions of platinum group metals (PGMs: Pt, Pd, Ru and Ir) was investigated for microstructure, hardness, room temperature aqueous corrosion, high-temperature oxidation resistance, mechanical alloying and consolidation by spark plasma sintering, and coating on titanium Grade 2 and Ti-6Al-4V substrates. Gamma-TiAl of Ti-47.5 at.% Al produced by melting and casting gave a microstructure consisting of γ grains and lamellar grains with alternating of α2 and γ phase lamellae. Additions of 0.2, 1.0, 1.5, and 2.0 at.% PGMs introduced new phases of high PGM contents. The γ and lamellar phases were still present. The additions of PGMs significantly improved the aqueous corrosion properties at room temperature, by improving the pitting corrosion resistance of the γ-TiAl alloy by modifying its hydrogen evolution of the cathodic reaction. The presence of PGMs also influenced the oxidation behaviour of γ-TiAl at 950°by forming the Z-phase which stabilized a continuous protective Al2O3 phase. However, Ti-47.5 at.% Al, being a two-phase alloy (α2+γ), PGMs could not sustain a stable Z-phase, as it transformed into an oxygen supersaturated Ti3Al, which subsequently led to the formation of TiO2+Al2O3, a non-protective oxide mixture. The optimal PGM addition to γ-TiAl was 0.5 at.%, with iridium giving the best room temperature corrosion and high-temperature oxidation resistance. Mechanical alloying of Ti and Al pure powders with PGM additions gave powders where α2 and γ were only identified after heat treatment. Consolidation of the mechanically alloyed powders by spark plasma sintering gave different microstructures from the cast alloys, with continuous α2 and γ phases and evenly distributed nanometer-sized alumina, and much higher hardnesses. Cold spraying the mechanically alloyed powders on to titanium Grade 2 and Ti-6Al-4V substrates gave coatings of irregular thickness, dense near the substrates with porosity at the top, giving poor oxidation protection.Item Hard, wear resistant Fe-B-C composites produced using spark plasma sintering(2017) Rokebrand, Patrick PierceFe-B-C composites were produced, from boron carbide and iron powders, using spark plasma sintering. This provided information on the effects of rapid sintering on densification, composition and the microstructure of the materials produced. The composition range included a selection high Fe contents (69.3, 78 and 80.9 vol. % Fe-B4C) and high B4C concentrations (1, 3, 5 vol. % Fe-B4C). The properties of the materials were investigated to determine the potential for using relatively cheap Fe and B4C powders to produce hard, wear resistant materials. High Fe-B4C composites were sintered at 900, 1000 and 1100°C at 60 MPa. Densification increased with increasing temperature and at 1100° each composition achieved ≥ 97 % densification. The materials reacted during sintering with the main phases observed being Fe2B and Fe3(B,C) whilst additional phases formed were FeB, C and Fe23(B,C)6.Comparing the phases that were produced to Fe-B-C phase diagrams showed deviations from expected compositions, indicating the non-equilibrium nature of producing the composites using SPS. Although the composites were not at equilibrium, all the B4C reacted and could not be maintained, even with fast heating and cooling rates. The properties of the materials were dependent on both densification and the phases that were present after sintering. Materials containing higher amounts of the Fe2B phase showed higher hardness and fracture toughness results, up to 13.7 GPa and 3.5 MPa.m0.5 respectively for the 69.3 vol. % Fe-B4C. The materials were sensitive to grain and pore growth which negatively affected properties at 1100°C. The transverse rupture strength of 388.3 MPa for 80.9 vol. % Fe-B4C composite was the greatest, and showed evidence of both intergranular and transgranular fracture. The strength was affected by a fine dispersion of porosity at the grain boundaries, throughout the material, and free carbon in the structure was detrimental to the strength of the 69.3 % Fe-B4C. The wear rates were lower using Si3N4 wear balls compared to stainless steel balls, where 69.3 vol. % Fe-B4C showed the best wear rates, 8.9×10-6 mm3/Nm (stainless steel ball) and 1.77×10-6 mm3/Nm (Si3N4 ball), due to the higher Fe2B composition and free carbon acting as a lubricant during sliding. 1, 3 and 5 vol. % Fe-B4C composites were sintered to densities above 97 % of theoretical at 2000°C and 30 MPa. The formation of a transient FeB liquid phase assisted densification. 1 % Fe-B4C attained hardness and fracture toughness up to 33.1 GPa and 5.3 MPa.m0.5 with a strength of 370.5 MPa. Thermal mismatch between the FeB phase and B4C caused high residual stresses at the interface which led to cracking and pull-out of the FeB phase. Residual carbon at the grain boundary interface exacerbated the pull-out effect. Increasing Fe and the subsequent FeB phase had an embrittling effect. The materials suffered severe wear of up to 36.92×10-6 mm3/Nm as a result of the pull-out with the remaining porosity acting as a stress raiser. 20 vol. % of the Fe in each system was substituted with Ti to reduce the presence of residual carbon. Although in some case the properties of the respective compositions improved, residual carbon was still present in the composites.Item Investigation into the microstructure and tensile properties of unalloyed titanium and Ti-6Al-4V alloy produced by powder metallurgy, casting and layered manufacturing(2016) Masikane, Muziwenhlanhla ArnoldABSTRACT Solid titanium (Ti) and Ti-6Al-4V (wt.%) materials were fabricated from powders using spark plasma sintering (SPS), cold isostatic press (CIP) and sinter, layered (rapid) manufacturing, centrifugal and vacuum casing. ASTM Grade 4 Ti, Al and V, 60Al-40V (wt.%) and the pre-alloyed Ti-6Al-4V powders were used as starting materials. The solid Ti and Ti-6Al-4V materials produced by the SPS were compared to the CIP and sinter method on the basis of density, microstructure and chemistry. The materials produced by the CIP and sinter method were also compared to those produced by vacuum casting method on the basis of microstructure, oxygen pick-up, chemistry and room temperature tensile properties. Centrifugal casting was compared to the vacuum casting technique on the basis of microstructural homogeneity. Rapid manufacturing was compared to SPS and CIP and sinter on the basis of microstructural homogeneity, density and tensile properties. The tensile properties of all materials were also compared to their commercial counterparts to investigate the effect of interstitial oxygen. The technology resulting in materials with superior properties was finally identified as most promising for commercial production of Ti-based materials. On the basis of densification, the SPS method appears superior compared to the CIP and sinter and rapid manufacturing method due to the benefit of pressure aided sintering, while the rapid manufacturing method is superior to the CIP and sinter method due to the use of a high power laser resulting in high densification rates. In cases where microstructural homogeneity is the key requirement, the CIP and sinter and rapid manufacturing methods appear superior compared to the SPS method due to longer isothermal holding time and higher sintering temperature and the use of pre-alloyed Ti-6Al-4V powder, respectively. On the basis of oxygen pick-up and additional contamination, the vacuum casting route is inferior due to the tendency of melt-crucible interaction, resulting in the dissociation of ZrO2 and subsequent pick-up of O and Zr. Based on the homogeneity of the microstructure, centrifugal casting is better than vacuum casting. The ductility of vacuum cast Ti was better than that of CIP and sintered Ti, possibly due to limited diffusion of oxygen from the crucible compared to oxygen absorbed from the controlled atmosphere during CIP and sinter. The vacuum casting of the Ti-6Al-4V alloy resulted in dissolution of oxygen and Zr due to melt-crucible interaction. Hence the ductility was worse compared to the alloy produced by CIP and sinter. The rapidly manufactured Ti-6Al-4V specimens exhibited superior ductility and strength compared to all alloys produced by other methods due to the use of high purity starting powder. The tensile properties of these specimens were also comparable to standard requirements. The similarity of the tensile properties of wrought Ti-6Al-4V alloy reported in the literature was an indication of limited oxygen pick-up during rapid manufacturing. Therefore based on low oxygen pick-up, microstructural homogeneity, high density and superior tensile properties, the rapid manufacturing route appears to be the most promising approach for commercial processing of titanium based materials.Item Improvement of alumina mechanical and electrical properties using multi-walled carbon nanotubes and titanium carbide as a secondary phase(2013-10-04) Nyembe, Sanele GoodenoughThe objective of this research was to improve alumina (Al2O3) mechanical and electrical properties by reinforcement using multi-walled carbon nanotubes (MWCNTs) and titanium carbide (TiC). The objective of the study was achieved with interesting and challenging difficulties along the way. The MWCNTs were initially coated with boron nitride (hBN) in order to improve the Alumina-CNTs interface which was previously discovered to be weak and also to protect them from reacting with Al2O3 during sintering. The coating of CNTs with hBN was done using nitridation method. This method was unsuccessful since it was not possible to coat each CNT individually. Dispersing hBN coated CNTs proved to be impossible without pealing the off the hBN coating. The “flaking off “of the hBN coating from the CNTs revealed that the CNT-hBN interface was weak; therefore uncoated CNTs were used for this study. The starting powders (Al2O3, TiC and CNTs) were individually dispersed before they were mixed together. TiC and Al2O3 were dispersed using an ultrasonic probe which was done successfully. The CNTs were dispersed by an ultrasonic probe and then attritor milled with the use of polyvinylpyrolidone (PVP) as a dispersant. The dispersed Al2O3 and TiC (30 wt%) powders were mixed in a planetary ball mill. The composite powder was sieved and sintered using SPS with temperature and pressure programmed to be 1700˚C, 35MPa respectively. In making the Al2O3+CNT composite powder, the already dispersed Al2O3 and CNTs (1 wt%) were mixed in a planetary ball mill, after sieving the powder it was sintered using SPS at 1600˚C, 35MPa (programmed conditions). Lastly in making the Al2O3+CNT+TiC composite, the already dispersed TiC, CNTs and Al2O3 were all mixed in a planetary ball mill, after sieving it was sintered using SPS at 1650˚C, 35MPa (programmed conditions). For comparison of properties, dispersed monolithic Al2O3 was also sintered using SPS at 1600˚C, 35 MPa. The density results showed that the monolithic Al2O3 was 99.8% dense, , Al2O3+CNTs was 99.4%, Al2O3+TiC+CNTs was 99.2% and Al2O3+TiC sample was 99.0%. The mechanical properties of the samples were measured using the indentation method. The hardness and fracture toughness of the samples were; Al2O3= 3.3MPa√m (17 GPa), Al2O3+CNTs = 4.2MPa√m (18 GPa), Al2O3+TiC = 4.8 MPa√m (23 GPa) and Al2O3+TiC+CNT= 5.0 MPa√m (23 GPa). The electrical properties showed that incorporating CNTs and TiC into Al2O3 improved Al2O3 electrical conductivity. The measured electrical conductivity of the ceramic samples were; Al2O3 iii ≈ 0 Sm-1, Al2O3+CNTs= 30 S.m-1, Al2O3 +TiC + CNTs = 6855 S.m-1 and Al2O3+TiC = 9664 S.m-1. The CNTs improved Al2O3 mechanical properties slightly inhibiting grain growth by pinning the grain boundary movement and also by crack bridging. The Al2O3 electrical conductivity was increased by the CNTs network that was located along the alumina grain boundaries. The TiC improved Al2O3 mechanical properties slightly inhibiting grain growth and through crack deflection mechanism. The addition of TiC into Al2O3 increased the electrical conductivity by serving as a conducting continuous secondary phase. The results show that the CNT-hBN interface is weak. The addition of CNTs and TiC into monolithic Al2O3 slightly improved its mechanical and electrical properties but it density was slightly compromised. CNTs and TiC slightly improved monolithic alumina hardness by in inhibiting Al2O3 grain growth and the fracture toughness through crack deflection and crack bridging mechanisms. The CNTs network located at the Al2O3 grain boundaries not only aided in improving Al2O3 hardness but also served as transport medium for electrons hence increasing the Al2O3 electrical conductivity. Addition of TiC into Al2O3 increased its electrical conductivity by conducting electrons from one TiC grain to the adjacent grain. The large increase in electrical conductivity upon addition of TiC is due to the presence of a continuous TiC phase within Al203.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.