Made available in DSpace on 2014-10-09T12:51:30Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:10Z (GMT). No. of bitstreams: 1 11328.pdf: 13067508 bytes, checksum: 886d399df69a8146f0353092e99aa16a (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:51:30Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:10Z (GMT). No. of bitstreams: 1 11328.pdf: 13067508 bytes, checksum: 886d399df69a8146f0353092e99aa16a (MD5) / O objetivo deste trabalho foi desenvolver potenciais materiais para aplicações automotivas, em particular como insertos para assento de válvula em motores de combustão interna à gasolina. O desenvolvimento compreendeu as etapas de processamento dos materiais via metalurgia do pó, tratamento térmico, caracterização microestrutural e mecânica, usinagem e desgaste dos mesmos. O desenvolvimento objetivou principalmente a redução de custo e a aplicação de elementos menos poluentes, excluindo-se, por exemplo, a aplicação do Co devido ao seu alto custo e do Pb devido aos seus efeitos toxicológicos e prejudiciais ao meio ambiente. A realização de uma pesquisa minuciosa na busca de patentes relacionadas indicou que os materiais estudados apresentam composições particulares, e, portanto inéditas. Os resultados encontrados nos ensaios de dureza e de resistência à compressão radial nas amostras tratadas termicamente apresentaram resultados superiores aos da liga comercial atualmente em uso. Testes de usinabilidade em amostras sem tratamento térmico apresentaram comportamento similar ao da liga comercial, indicando que a usinabilidade do material praticamente não foi afetada com a mudança na composição química. Após a etapa de tratamento térmico, as ligas obtidas apresentaram valores de esforços de corte superiores aos do material comercial. Os resultados dos testes de desgaste abrasivo em amostras tratadas termicamente apresentaram menores valores de coeficientes de atrito e perda de massa em todos os casos em relação ao material comercial. Este comportamento é devido aos benefícios introduzidos pelo tratamento térmico executado e pela adição de alguns elementos resistentes ao desgaste na forma de carbonetos de Nb e Ti/W. Os resultados dos esforços de corte apresentaram boa concordância com os resultados dos ensaios de desgaste. Os materiais produzidos apresentam-se como potenciais candidatos para substituir, com vantagens, inserto para assento de válvula a base de Fe-Co em motores de combustão interna a gasolina. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:49:52Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:11Z (GMT). No. of bitstreams: 0 / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:49:52Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:03:11Z (GMT). No. of bitstreams: 0 / O objetivo do presente trabalho foi à obtenção de aço rápido tipo AISI M2 pela técnica da conformação por “spray” e a avaliação deste, quanto ao seu desempenho quando aplicado como ferramenta de usinagem. O material obtido foi laminado a quente com razões de redução de 50 e 72%, resultando em placas a partir das quais foram confeccionadas pastilhas intercambiáveis para realização de testes de usinagem. O desempenho das pastilhas obtidas a partir do material conformado por “spray” e laminado a quente, foi confrontado com o de pastilhas confeccionadas a partir de material processado convencionalmente (lingotamento convencional), e com material processado pela técnica da metalurgia do pó (MP). As variações do material obtido por conformação por “spray”, assim como também os demais materiais, foram caracterizados química, física, mecânica e microestruturalmente. Adicionalmente, foram realizados testes de usinagem para avaliação de desempenho do material quando submetido a condições reais de trabalho. Os resultados da caracterização dos materiais evidenciaram o potencial da técnica de conformação por “spray”, em possibilitar a obtenção de materiais com boas características e propriedades. Para as condições de processamento, conformação mecânica e de tratamentos térmicos aplicados neste trabalho, às análises dos resultados dos testes de usinagem revelaram um comportamento muito próximo entre todos os materiais de ferramenta utilizados. Procedendo a uma análise mais refinada dos resultados dos testes de usinagem contínua, foi verificado um desempenho ligeiramente superior para o material obtido pela técnica da metalurgia do pó (MP), seguido pelo material obtido pela técnica da metalurgia convencional, o qual apresentou uma vantagem mínima em relação ao obtido pela técnica da conformação por “spray” e laminado com redução de 72%. Por fim, o desempenho inferior ficou por conta do material obtido pela técnica da conformação por “spray” e laminado com redução de 50%, o qual apresentou os maiores valores de desgaste. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Alkali (A) and alkaline earth (AE) metals can form carbides and intercalated graphites with carbon. The carbides mostly represent acetylides which are salt-like compounds composed of C22− dumbbell anions and metal cations. Both the acetylide carbides and intercalated graphites are technologically important. Superconductivity has been observed in several intercalated graphites such as KC8 and CaC6. Li intercalated graphites are a major ingredient in Li ion batteries. CaC2 is an important commodity for producing acetylene and the fertilizer CaCN2. In spite of the extensive research on A–C and AE–C compounds, phase diagrams are largely unknown. The thermodynamic and kinetic properties of both carbides and intercalalated graphites are discussed controversially. Recent computational studies indicated that well-known carbides, like CaC2 and BaC2, are thermodynamically unstable. Additionally, computational studies predicted that acetylide carbides will generally form novel polymeric carbides (polycarbides) at high pressures. This thesis is intended to check the validity of theoretical predictions and to shed light on the complicated phase diagrams of the Li–C and the Ca–C systems. The Li–C and the Ca–C systems were investigated using well-controllable metal hydride–graphite reactions. Concerning the Li–C system, relative stabilities of the metastable lithium graphite intercalation compounds (Li-GICs) of stages I, IIa, IIb, III, IV and Id were studied close to the competing formation of the thermodynamically stable Li2C2. The stage IIa showed distinguished thermal stability. The phase Id showed thermodynamic stability and hence, was included in the Li–C phase diagram. In the Ca–C system, results from CaH2–graphite reactions indicate compositional variations between polymorphs I, II and III. The formation of CaC2  I was favored  only  at  1100  ◦C or  higher  temperature  and  with  excess calcium, which speculates phase I as carbon deficient CaC2−δ . To explore the potential existence of polycarbides, the acetylide carbides Li2C2 and CaC2 were investigated under various pressure and temperature conditions, employing diamond anvil cells for in situ studies and multi anvil techniques for large volume synthesis. The products were characterized by a combination of diffraction and spectroscopy techniques. For both Li2C2 and CaC2, a pressure induced structural transformation was observed at relatively low pressures (10–15 GPa), which was followed by an irreversible amorphization at higher pressures (25–30 GPa). For Li2C2 the structure of the high pressure phase prior to amorphization could be elucidated. The ground state with an antifluorite Immm structure (coordination number (CN) for C22− dumbbells = 8) transforms to a phase with an anticotunnite Pnma structure (CN for C22− dumbbells = 9). Polycarbides, as predicted from theory, could not be obtained. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.</p>

No description available.

No description available.

Des revêtements céramiques sont obtenus par un procédé de dépôt chimique en phase vapeur assisté par injection liquide pulsée (DLICVD) de précurseurs organométalliques. Des dépôts de carbure de chrome (CrCx) sont élaborés dans un réacteur tubulaire à paroi chaude à partir d’une solution de bis(benzène) chrome dans du toluène pour des températures de 475 °C et sous pression partielle d’azote (pression totale 50 Torr). Une couche d’accroche pouvant être nécessaire pour revêtir des pièces métalliques, tels des aciers et alliages, par un revêtement céramique non-oxyde de type CrCx, des couches de chrome métallique (Cr) et des carbures mixtes Cr-Si-C ont également été élaborées par ce procédé DLICVD. Ainsi, l’ajout d’un additif à base de chlore ou de soufre (par exemple l’hexachlorobenzène ou le thiophénol) dans la solution BBC/toluène permet la déposition de films de chrome métallique (Cr) à 475 °C. De plus, l’utilisation d’une solution de précurseur contenant simultanément du Si et du Cr tel que le tetrakis(trimethylsilylmethyl)chromium dans du toluène mène au dépôt d’un carbure mixte Cr-Si-C pouvant jouer le rôle d’interphase dans des assemblage céramique-métal. Des films de carbure de silicium (SiC) sont obtenus à partir de deux précurseurs (1,3 disilabutane et polysilyléthylène) injectés purs ou en solution également dans du toluène. Les dépôts sont faits dans une gamme de température comprise entre 700 et 800 °C, sous pression partielle d’azote (pression totale 50 Torr). Les films obtenus sont des films amorphes de SiC contenant une faible quantité d’hydrogène (provenant du mécanisme de décomposition des précurseurs) : a-SiC:H. Les films sont stœchiométriques dans le cas de l’injection de précurseur pur, et quasi stœchiométrique lorsque les précurseurs sont dilués dans du toluène. Les films amorphes tels que déposés deviennent nanocristallins en présentant la structure cubique du SiC après recuit sous vide à 1000 °C. L’influence du solvant (toluène) sur la composition, la morphologie et la vitesse de croissance des dépôts est discutée en fonction des systèmes chimiques étudiés et des conditions expérimentales, en particulier les conditions locales dans le réacteur DLICVD telles que les gradients de température et de concentration. Des films de carbure de hafnium (HfC) sont également élaborés par le même procédé à partir d’une solution de bis(cyclopentadiényl)diméthyl hafnium dans du toluène après avoir testé plusieurs précurseurs. Une température de 750 °C est utilisée et l’utilité d’une pression partielle de dihydrogène dans le gaz vecteur azote est démontrée (pression totale 50 Torr, 423 sccm de N2 et 77 sccm de H2). Tels que déposés, ces films sont riches en carbone (C-rich HfCx) et ont une structure quasi-amorphe. Ils deviennent nanocristallins après recuit sous vide à 1000 °C. Enfin, la mise en œuvre de films multicouches céramiques par DLICVD à paroi chaude est mise en évidence par l’élaboration de revêtements multicouches HfC/SiC à 750 °C, sous pression partielle d’un mélange de gaz vecteur N2/H2. Le contrôle du procédé permet une nano structuration de ces revêtements multicouches jusqu’à une bi-période de 100 nm (empilement de 100 couches d’environ 50 nm chacune). La stabilité thermique de ces architectures et des tests préliminaires de résistance à l’oxydation à haute température des films de SiC et HfC/SiC sont discutés. / Ceramic coatings are made from metalorganic precursors by a chemical vapour deposition process assisted by pulsed liquid injection (DLICVD). Chromium carbide (CrCx) films are grown in a tubular hot wall reactor from a solution of bis(benzene)chromium in toluene under partial pressure of nitrogen at 475 °C (total pressure set at 50 Torr). Bonding layers are useful on metallic components, such as steels and alloys, with non-oxide ceramic films such as CrCx, to that purpose metallic chromium (Cr) and mixed carbides Cr-Si-C have been made by DLICVD. Furthermore, adding a chlorinated or sulfur based additive (e.g. hexachlorobenzene or thiophenol) in the BBC/toluene solution allows depositing metallic chromium (Cr) at 475 °C. Moreover, using a precursor containing Si and Cr as tetrakis(trimethylsilylmethyl)-chromium in toluene leads to the deposition of Cr-Si-C mixed carbide. Silicon carbide films are made from two precursors (1,3-disilabutane and polysilylethylene) that have been injected either pure or diluted in toluene. A temperature range of 700 to 800 °C has been used under a partial pressure of nitrogen (total pressure of 50 Torr). SiC films are amorphous and contain a small quantity of hydrogen (hydrogen comes from precursor pyrolysis mechanism): a-SiC:H. Films are stoichiometric when pure precursors are injected, and quasi stoichiometric when precursors are diluted in toluene. As deposited coatings are amorphous and become nanocristalline (cubic SiC structure) after annealing at 1000 °C under vacuum. The influences of the solvent (toluene) on the composition, morphology and growth rate are discussed as a function of the chemical system and experimental conditions, in particular reactor gradient conditions such as temperature and precursors concentration in gas phase. Hafnium carbide films are also made using a solution of bis(cyclopentadiényl)diméthyl hafnium in toluene by the same process. Temperature is set to 750 °C and hydrogen partial pressure has been shown useful (total pressure of 50 Torr, 423 sccm of N2 and 77 sccm of H2). As-deposited films are C-rich HfCx and quasi amorphous. They become nanocristalline after annealing at 1000 °C under vacuum. Finally, ceramics multilayer HfC/SiC coatings were deposited by DLICVD at 750 °C under a partial pressure of a mixture of N2/H2. The process allows a good control of the multilayer nanostructure. Thermal stability and high temperature oxidation preliminary tests on SiC and HfC/SiC films are discussed.

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy.‘ Johannesburg, August 2017 / Fe-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. / MT2018

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters of Science in Engineering, 2016 / This research is focused on optimizing the surface integrity of Ti-6Al-4V using advanced carbide inserts and minimum quantity lubrication (MQL). Experiments were designed to machine twenty Ti-6Al-4V blocks under dry and MQL lubricating conditions using innovative cemented carbide inserts produced by Liquid Phase Sintering (LPS) and Spark Plasma Sintering (SPS). The cutting speed, feed rate, and depth of cut, were considered as machining parameters, while the cutting force, temperature, tool wear, surface roughness and residual stress were considered as performance characteristics. The results obtained for surface roughness and residual stress measurements were used to analyze the surface integrity of the machined Ti-6Al-4V samples. The influence of sintering techniques and milling parameters on performance characteristics and surface integrity were investigated from experimental results obtained. Discrepancy in behavioral trends with respect to lubricating conditions was also investigated. The analyses of milling results showed that cutting speed played a major role in the optimization of surface integrity of the Ti-6Al-4V work pieces followed by the depth of cut. The results also showed that the LPS inserts performed better than the SPS inserts due to their better mechanical properties (higher fracture toughness (KIC) and transverse rupture strength). The application of MQL was also observed to significantly reduce milling temperatures resulting in better surface integrity. However, an optimized surface integrity of the Ti-6Al-4V samples was achieved during MQL milling with the 10Co-L insert at a low finishing speed of 75m/minute and a reduced depth of cut of 0.5mm.