Revestimentos finos de carboneto de nióbio (NbC) puro e dopados com níquel (Ni) foram obtidos mediante a técnica de deposição reativa por magnetron sputtering, utilizando metano (CH4) como fonte de carbono (C). O filme de NbC usado como referência foi depositado aplicando uma potência de 2500 W ao alvo de Nb e, os revestimentos de NbxNiyCz foram depositados diminuindo a potência aplicada ao alvo de Nb e aumentando a potência aplicada ao alvo de Nb-Ni, dando origem à seis revestimentos com teores de Ni crescentes. As caracterizações microestrutural e estrutural dos revestimentos de NbC e NbxCyNiz foram realizadas por meio das técnicas de difração de raios-X (DRX), Espectroscopia Fotoeletrônica de Raios X (XPS), Espectroscopia Raman, Microscopia Eletrônica de Transmissão (MET) e Microscopia Eletrônica de Varredura (MEV). As propriedades mecânicas dos revestimentos foram estudas mediante a técnica de nanoindentação instrumentada, com o intuito de avaliar a dureza (H) e o módulo de elasticidade (E). A adesão dos revestimentos ao substrato foi avaliada usando ensaios Rockwell C e esclerometria linear instrumentada. A estabilidade térmica dos revestimentos foi realizada em forno com atmosfera controlada em temperaturas de 600 °C e 800 °C por 2h. Finalmente, a resistência à oxidação dos revestimentos foi estudada por meio de ensaios de Termogravimetria (TGA - \"Thermogravimetric Analysis\") de aquecimento contínuo e isotérmicos. Os resultados de adesão obtidos mostraram boa aderência (modo de falha HF1) dos filmes de NbC e NbxNiyCz ao substrato de aço AISI M2, nas condições como recém depositado e revenido a 600 °C, indicando que a deposição do gradiente de intercamadas de Cr, CrC e do gradiente CrC / NbC foi efetiva evitando falhas adesivas. A adição de Ni na estrutura dos revestimentos de NbC promoveu a formação de estruturas nanocompósitas, composta de nanocristalitos de NbC e NiCx. Adicionalmente, a introdução de níquel causou um aumento na dureza nos revestimentos como recém depositados, aumentando de 17 para 25 GPa para teores de Ni de 0 para 13 at. %, respectivamente, e, na resistência à oxidação sobre o revestimento puro de NbC, de 380 °C para 480 °C nos revestimentos com níquel. Finalmente, as análises de estabilidade térmica permitiram observar que os precipitados de NiCx se decompõem durante os tratamentos de recozimento a 600 e 800 °C, o que promoveu um aumento nos valores de dureza e módulo de Young para todos os revestimentos, atribuído ao aumento da cristalinidade dos revestimentos. / Niobium carbide (NbC) coatings doped with Nickel (Ni) were deposited by reactive DC - magnetron sputtering using methane (CH4) as carbon (C) source. Reference NbC coating was deposited with a total power of 2500 W and NbxNiyCz coatings were deposited by decreasing the power applied to the Nb target and increasing the power applied to the Nb-Ni target, giving rise to coatings with increasing Ni content. Structural and microstructural characterizations of NbC and NbxNiyCz coatings were performed using X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy, Transmission Electron Microscopy (MET) and Scanning Electron Microscopy (MEV). Mechanical properties of the coatings were studied using the instrumented nanoindentation technique, in order to evaluate the Hardness (H) and Elastic modulus (E). The adhesion between coatings and substrate was evaluated using Rockwell C test and instrumented linear scratch tests. The tests for studying the thermal stability of the coatings were carried out in a controlled atmosphere chamber furnace at temperatures of 600 °C and 800 °C for 2h. Finally, the oxidation resistance of the coatings was studied by means of Thermogravimetric Analysis (TGA) tests of continuous and isothermal heating. The NbC and NbxNiyCz films in the as-deposited condition and annealed at 600 °C, showed good adhesion (failure mode HF1) to the AISI M2 steel substrate, indicating that the adhesion interlayer of the Cr, CrC and a gradient CrC/NbC layer was effective in avoiding adhesive failures. The increasing of Ni content in the structure of NbC coatings promoted the formation of nanocomposite structures, composed of a mixture of NbC and NiCx nanocrystallites. Additionally, the introduction of nickel allows increasing the hardness for the coatings in the as-deposited condition, from 17 to 25 GPa for Ni contents from 0 to 13 at. %, respectively, and, improving the oxidation resistance over the pure NbC coating, from 380 °C to 480 °C for the Ni-rich coatings. Finally, the thermal stability analyses showed that the NiCx precipitate decompose during the annealing treatments at 600 °C and 800 °C, which promoted an increase in the hardness and Young\'s modulus values for all coatings. These behaviors were attributed to the increase of crystallinity of the coatings.

Transition metal carbides (TMCs) are of increasing interest for catalytic processes. Their performance and stability under common oxidative conditions in catalytic reactions are crucial for several applications, including catalysis and electrochemical reactions. In this work, we report a detailed XPS study of the interactions of stoichiometric NbC and TaC surfaces with common oxidizing agents like O2 and H2O, which are important media in many chemical processes. Experimental results showed that NbC reacts with O2 to produce Nb sub-oxrides, while TaC is inert to O2 exposure. TaC surfaces are more sensitive to H2O vapor, with a greater surface oxidation and hydroxylation. Atmospheric oxidation of NbC and TaC was also studied, and results showed that both films oxidized yielding to the formation of Nb2O5 and Ta2O5, hydroxylated/ oxide carbon species, and some adventurous carbon build-up. TMCs are catalytically active in many reactions, especially those involving electrochemical nitrogen reduction reactions (NRR) to ammonia. Experimental and DFT calculations were used to provide insight on how carbide surface structures change electrochemically and how that evolution relates to NRR activity. Results showed that NbC has NRR activity at pH 3.2 after immersion in 0.3 M NaOH, leaving niobium suboxides. However, photoemission data showed that the Nb2O5 overlayer is restored after polarization to -1.3 V vs. Ag/AgCl, inhibiting NRR activity. TaC, on the other hand, is inactive for NRR at potentials more positive than -1.0 V, as NaOH treatment fails to remove the Ta2O5 surface layer induced by ambient exposure. The study also found that the formation and stabilization of intermediate oxidation states on the surface of transition metal ions are crucial for N≡N bond activation and NRR activity.

Neste trabalho foi investigado o processamento e propriedades mecânicas e tribológicas do compósito cerâmico Al2O3- NbC com o objetivo de desenvolver um compósito cerâmico com melhores propriedades do que a alumina pura. Como material para comparação foi utlizado a alumina que é uma cerâmica tradicionalmente aplicada onde é necessária elevada resistência mecânica e ao desgaste. A composição Al2O3-0,5%wtY2O3-20%wtNbC, que origina o compósito, passou por moagem e mistura em moinho tipo atrittor e secagem em evaporador rotativo. O pó obtido foi caracterizado quanto a distribuição granulométrica e morfologia. A alumina seguiu a rota tradicional de moagem e mistura em moinho de bolas, secagem em spray dryer; e mesma caracterização realizada para o compósito. Foi realizado estudo da sinterização em dilatômetro para os dois materiais com intuito de determinar as condições ideais de sinterização. Por meio de prensagem uniaxial seguida de prensagem isostática a frio foram confeccionadas amostras na forma de discos e pinos. Os materiais densos foram obtidos por sinterização normal sendo que a alumina ao ar e o compósito em atmosfera de argônio. As fases formadas no dois materiais foram determinadas por difração de raios X. As microestruturas foram estudas em microscópio eletrônico de varredura nas superfícies polidas e atacadas. Entre as propriedades mecânicas foram medidas o módulo elástico, a dureza e a tenacidade à fratura por meio de impressões Vickers. O compósito desenvolvido apresentou valores superiores em todas estas propriedades em relação a alumina. O estudo do comportamento tribológico foi realizado por meio do deslizamento de pinos, com extremidade cônica, de alumina e do compósito cerâmico sobre discos de alumina. Os ensaios foram realizados com a velocidade de 0,4m/s e com carregamento, por meio de peso morto, de 10N sob diferentes níveis de umidade relativa ( 26,8; 48 e 76,3%). O coeficiente de atrito cinético médio, assim como a taxa de desgaste, diminuiu em todos os ensaios com o aumento da umidade relativa, para os dois materiais. Este comportamento foi relacionado com a formação de uma camada na interface de contato entre as superfícies do pino e do disco de hidróxido de alumínio. O compósito demostrou maior resistência ao desgaste em todas as condições tribológicas estudadas. O compósito cerâmico desenvolvido claramente possui boas perspectivas como um novo material cerâmico em importantes aplicações técnicas. / In the present work processing, and mechanical and tribological properties of a ceramic composite Al2O3-NbC were investigated in order to develop a ceramic material with superior properties. Alumina was chosen as a reference since it is a traditional ceramic material for applications where elevated mechanical properties and wear resistance are required. The composition Al2O3 - 0.5 wt%Y2O3-20 wt%NbC was prepared by attrition milling followed by drying in a rotaevaporator. The resulting powder mixture was characterized for granulometry and powder particles morphology. Alumina was processed according to the traditional route of ball milling followed by passing through a spray drier, and the processed powder was then characterized the same way as the composite. Dilatometry was accomplished for both materials in order to define the ideal sintering conditions. Samples with the shape of discs and pins were compacted by uniaxial pressing followed by cold isostatic pressing. Densification was achieved by sintering in air and in argon for alumina and the composite, respectively. Phase composition of sintered materials was studied by X-ray diffraction (XRD). Microstructure was investigated by means of scanning electron microscopy (SEM) on polished and etched surfaces. Materials were also characterized for a number of mechanical properties, in particular for Young modulus, hardness, and fracture toughness by Vickers indentation method. The developed composite exhibited superior mechanical properties as compared to alumina. Tribological behavior was investigated by means of a sliding pin on disk test with pins of a conical shape prepared both from alumina and the composite, and an alumina disk. Tests were performed with the sliding speed of 0.4 m/s and 10N load under varying humidity conditions (26.8; 48, and 76.3%). Both the mean coefficient of friction and the wear rate diminished in all tests with the increase of humidity for both materials. Such behavior was attributed to a aluminum hydroxide layer formation between the pin and the disk surfaces. The developed composite exhibited better wear resistance under all tribologic conditions studied. The developed ceramic composite obviously has good perspectives as a new material for a variety of important technical applications.

Made available in DSpace on 2014-12-17T14:06:56Z (GMT). No. of bitstreams: 1 MarcianoF.pdf: 4534279 bytes, checksum: c57f1201db2e0c16d64c992932b57fc3 (MD5) Previous issue date: 2007-09-28 / Metal powder sintering appears to be promising option to achieve new physical and mechanical properties combining raw material with new processing improvements. It interest over many years and continue to gain wide industrial application. Stainless steel is a widely accepted material because high corrosion resistance. However stainless steels have poor sinterability and poor wear resistance due to their low hardness. Metal matrix composite (MMC) combining soft metallic matrix reinforced with carbides or oxides has attracted considerable attention for researchers to improve density and hardness in the bulk material. This thesis focuses on processing 316L stainless steel by addition of 3% wt niobium carbide to control grain growth and improve densification and hardness. The starting powder were water atomized stainless steel manufactured for H?gan?s (D 50 = 95.0 μm) and NbC produced in the UFRN and supplied by Aesar Alpha Johnson Matthey Company with medium crystallite size 16.39 nm and 80.35 nm respectively. Samples with addition up to 3% of each NbC were mixed and mechanically milled by 3 routes. The route1 (R1) milled in planetary by 2 hours. The routes 2 (R2) and 3 (R3) milled in a conventional mill by 24 and 48 hours. Each milled samples and pure sample were cold compacted uniaxially in a cylindrical steel die (? 5 .0 mm) at 700 MPa, carried out in a vacuum furnace, heated at 1290?C, heating rate 20?C stand by 30 and 60 minutes. The samples containing NbC present higher densities and hardness than those without reinforcement. The results show that nanosized NbC particles precipitate on grain boundary. Thus, promote densification eliminating pores, control grain growth and increase the hardness values / O presente trabalho apresenta uma contribui??o ao estudo da sinteriza??o s?lida de um a?o inoxid?vel 316L, com o objetivo de aumentar a sua densidade e dureza atrav?s da inclus?o de part?culas nanom?tricas de Carbeto de Ni?bio - NbC. O a?o inoxid?vel 316L ? uma liga largamente utilizada pela sua propriedade de alta resist?ncia ? corros?o. Contudo, sua aplica??o ? limitada pela baixa resist?ncia ao desgaste, conseq??ncia da sua baixa dureza. Al?m disso, apresenta baixa sinterabilidade e n?o pode ser endurecido pelos m?todos tradicionais de tratamentos t?rmicos, devido a sua estrutura austen?tica, c?bica de face centrada, estabilizada principalmente pela presen?a do N?quel. Os materiais de partida empregados neste trabalho foram o a?o inoxid?vel, austen?tico 316L atomizado a ?gua, com tamanho de part?culas (D50) equivalente a 95μm, e duas partidas diferentes de NbC, com tamanhos m?dios de cristalitos de 16,39 nm e 80,35 nm. Amostras de a?os adicionadas com 3% em peso de NbC (cada amostra com carbetos de partidas diferentes), seguiram rotas diferenciadas de moagem mec?nica. A rota 1 (R1) em um planet?rio por uma hora, a rota 2 (R2) e rota 3 (R3), em moinho convencional por 24 e 48 horas respectivamente. Cada uma das amostras resultantes, assim como amostras do a?o puro foram compactados a 700 MPa, a frio, sem nenhum aditivo, uniaxialmente, em uma matriz cil?ndrica de 5 mm de di?metro, em quantidade calculada para ter uma altura m?dia final do compactado de 5 mm. Posteriormente, foram sinterizadas em forno a v?cuo, em temperatura de at? 1290? C com incremento de 10 ?C por minuto, sendo mantidas neste patamar por 30 ou 60 minutos e resfriadas ? temperatura ambiente. As amostras sinterizadas foram submetidas aos ensaios para a medi??o da densidade e da micro-dureza. As amostras contendo o refor?o de NbC apresentaram maiores valores de densidade e um aumento significativo na sua dureza. As an?lises complementares no microsc?pio ?tico, no microsc?pio eletr?nico de varredura e no difrat?metro de raios-X, mostram que o NbC, na forma processada, contribuiu com o aumento da dureza, pela densifica??o, pela sua pr?pria dureza e pelo controle do crescimento dos gr?os da matriz met?lica, segregando-se nos seus contornos

Transition metal carbides have great industrial importance with a wide area of applications. Unlike many ceramic materials which can be produced from raw materials found in nature, the refractory carbides generally do not exist in the natural state. Synthesis of these carbides is costly and exacting. Sputtered coatings of the refractory metal carbides are of great interest for applications where hard wear-resistant materials are desired. Understanding how the experimental conditions affect the microstructure and properties in reactive sputtering deposition process is still an area of intense research activity. Reactively sputtered zirconium carbide thin films were grown on (100) silicon substrate and the influence of substrate temperature on the properties of the films were investigated. The substrate temperature was varied from ambient to 500°C and partial pressures of the sputter gas and reactive gas (argon and methane) were optimised to obtain crystalline films. Structural characteristics showed that the films exhibit nanocomposite structure consisting of ZrC nanocrystallites embedded in amorphous carbon typically at lower growth temperature (TS < 300°C), and at higher growth temperatures film were highly textured. In addition, Films deposited at 325 °C showed a distinct increase in FWHM which had considerable effect on the mechanical properties of the film. Maximum hardness of 24.8 GPa was seen at 325ºC. The changes in atomic bonding structures, their relative fractions with respect to substrate temperature were discussed. We also report superhard nanocrystalline nanocomposite NbC thin film deposited on Si (100) under 500˚C growth temperature via reactive magnetron sputtering. The pronounced nano hardness and modulus value of 42 GPa and 267 GPa at 40/60 C/Nb ratio were found to be strongly dependent on the grain size and higher percentage of carbide content. HRTEM studies further confirm the formation of nanocomposite structure with nanocrystalline grains embedded in amorphous matrix. The influence of vapour incidence angle (α= 0˚ to 75˚) on optimized ZrC and NbC thin films were investigated by depositing films in Oblique angle deposition geometry (OAD). The anisotropic growth rate of crystallographic planes and the mechanism of development of micro structural features in OAD of carbide films have been investigated. XRD and pole figure measurements indicated that the films grown at higher growth temperatures (800°C) exhibited higher degree of preferred orientation coupled with larger crystallite size whereas the films deposited at room temperature displayed random polycrystalline nature. The strong increase in porosity with increase in deposition angle with distinctly separated nanometer sized columns resulted in lowering of hardness and reduced modulus value. The film with zero incidence angle exhibited a maximum hardness and reduced modulus of 28 GPa and 223 GPa respectively. On the other hand, NbC films deposited with OAD, remained to be polycrystalline in nature with less intense peaks and also exhibited loss of preferential orientation indicating lower crystal quality with increase in vapor deposition angle. It is apparent that variation in crystallographic texture coupled with sculptured nanostructures are solely material dependent properties. Nano metric modulated ZrC/NbC superlattice multilayer structure performance has been evaluated for structural stability and hardness enhancement. Multilayers present superlattice effect in XRD patterns, which are attributed to the precise periodical stacking of crystalline monolayers also confirmed by cross section FESEM. X-ray photoelectron spectroscopy depth profile analysis was performed to get information on chemical composition of modulated layers and also to get an insight on the interface region. Hardness and modulus value of 43.2 GPa and 272 GPa was observed which is higher than individual monolayers response to mechanical loading. The enhanced hardness is possibly due to the inhibition of dislocation motion along the interface and also due to strain effects at the interface.

Neste trabalho foi investigado o processamento e propriedades mecânicas e tribológicas do compósito cerâmico Al2O3- NbC com o objetivo de desenvolver um compósito cerâmico com melhores propriedades do que a alumina pura. Como material para comparação foi utlizado a alumina que é uma cerâmica tradicionalmente aplicada onde é necessária elevada resistência mecânica e ao desgaste. A composição Al2O3-0,5%wtY2O3-20%wtNbC, que origina o compósito, passou por moagem e mistura em moinho tipo atrittor e secagem em evaporador rotativo. O pó obtido foi caracterizado quanto a distribuição granulométrica e morfologia. A alumina seguiu a rota tradicional de moagem e mistura em moinho de bolas, secagem em spray dryer; e mesma caracterização realizada para o compósito. Foi realizado estudo da sinterização em dilatômetro para os dois materiais com intuito de determinar as condições ideais de sinterização. Por meio de prensagem uniaxial seguida de prensagem isostática a frio foram confeccionadas amostras na forma de discos e pinos. Os materiais densos foram obtidos por sinterização normal sendo que a alumina ao ar e o compósito em atmosfera de argônio. As fases formadas no dois materiais foram determinadas por difração de raios X. As microestruturas foram estudas em microscópio eletrônico de varredura nas superfícies polidas e atacadas. Entre as propriedades mecânicas foram medidas o módulo elástico, a dureza e a tenacidade à fratura por meio de impressões Vickers. O compósito desenvolvido apresentou valores superiores em todas estas propriedades em relação a alumina. O estudo do comportamento tribológico foi realizado por meio do deslizamento de pinos, com extremidade cônica, de alumina e do compósito cerâmico sobre discos de alumina. Os ensaios foram realizados com a velocidade de 0,4m/s e com carregamento, por meio de peso morto, de 10N sob diferentes níveis de umidade relativa ( 26,8; 48 e 76,3%). O coeficiente de atrito cinético médio, assim como a taxa de desgaste, diminuiu em todos os ensaios com o aumento da umidade relativa, para os dois materiais. Este comportamento foi relacionado com a formação de uma camada na interface de contato entre as superfícies do pino e do disco de hidróxido de alumínio. O compósito demostrou maior resistência ao desgaste em todas as condições tribológicas estudadas. O compósito cerâmico desenvolvido claramente possui boas perspectivas como um novo material cerâmico em importantes aplicações técnicas. / In the present work processing, and mechanical and tribological properties of a ceramic composite Al2O3-NbC were investigated in order to develop a ceramic material with superior properties. Alumina was chosen as a reference since it is a traditional ceramic material for applications where elevated mechanical properties and wear resistance are required. The composition Al2O3 - 0.5 wt%Y2O3-20 wt%NbC was prepared by attrition milling followed by drying in a rotaevaporator. The resulting powder mixture was characterized for granulometry and powder particles morphology. Alumina was processed according to the traditional route of ball milling followed by passing through a spray drier, and the processed powder was then characterized the same way as the composite. Dilatometry was accomplished for both materials in order to define the ideal sintering conditions. Samples with the shape of discs and pins were compacted by uniaxial pressing followed by cold isostatic pressing. Densification was achieved by sintering in air and in argon for alumina and the composite, respectively. Phase composition of sintered materials was studied by X-ray diffraction (XRD). Microstructure was investigated by means of scanning electron microscopy (SEM) on polished and etched surfaces. Materials were also characterized for a number of mechanical properties, in particular for Young modulus, hardness, and fracture toughness by Vickers indentation method. The developed composite exhibited superior mechanical properties as compared to alumina. Tribological behavior was investigated by means of a sliding pin on disk test with pins of a conical shape prepared both from alumina and the composite, and an alumina disk. Tests were performed with the sliding speed of 0.4 m/s and 10N load under varying humidity conditions (26.8; 48, and 76.3%). Both the mean coefficient of friction and the wear rate diminished in all tests with the increase of humidity for both materials. Such behavior was attributed to a aluminum hydroxide layer formation between the pin and the disk surfaces. The developed composite exhibited better wear resistance under all tribologic conditions studied. The developed ceramic composite obviously has good perspectives as a new material for a variety of important technical applications.

La durabilité des piles à combustible à membrane échangeuse de proton (PEMFC) est un des verrous technologiques majeurs qui freinent leurs implantations sur le marché. Ces travaux de thèse s’inscrivent dans ce contexte en proposant l’élaboration de matériaux en carbure de niobium comme support de catalyseur pour remplacer les supports carbonés actuellement utilisés dans les cathodes de PEMFC. Notre démarche est d’associer cette composition à différentes morphologies contrôlées pour développer des matériaux conducteurs, présentant une porosité adaptée et chimiquement plus stable que le carbone qui se corrode dans les conditions de fonctionnement des PEMFC. Ainsi trois voies de synthèse basées sur des techniques variées (filage électrostatique, synthèse hydrothermal avec agent structurant) ont été étudiées aboutissant à trois types de morphologie : des poudres nanostructurées, des tissus nanofibreux et des nanotubes aux parois poreuses. Après leurs caractérisations structurales et morphologiques approfondies, ces supports ont été catalysés avec des nanoparticules de platine synthétisées par une méthode polyol assisté par micro-onde. La finalité de ce projet est d’évaluer les performances électrochimiques relatives à la réaction de réduction de l’oxygène de ces supports catalysés pour mettre en avant leurs exceptionnelles stabilités comparées à un support catalysé de référence (Pt/C) sans perte significative d’activité catalytique. / One pivotal issue to be overcome for the widespread adoption of Proton exchange membrane fuel cells (PEMFC) is the stability overtime. In this context, This PhD project focuses on the elaboration of niobium carbide based electrocatalyst supports for the PEMFC cathode to replace the conventional carbon based supports that notoriously suffer from corrosion in fuel cell operating conditions. The approach is to associate this alternative chemical composition with controlled morphologies in order to design electronically conductive and chemically stable materials with the appropriate porosity. Three different syntheses involving hydrothermal template synthesis or electrospinning have been developed leading to three different morphologies: nanostructured powders with high surface area, self-standing nanofibrous mats, and nanotubes with porous walls. These various supports have been catalysed by deposition of platinum nanoparticles synthesised by a microwave-assisted polyol method, and they have been characterised for their chemical and structural composition, morphology, and electrochemical properties. This work demonstrates that the Pt loaded NbC supports feature a greater electrochemical stability than a commercial Pt/C reference and similar electrocatalytic activities towards the oxygen reduction reaction.

Made available in DSpace on 2014-12-17T14:07:10Z (GMT). No. of bitstreams: 1 JoseFSJ_TESE.pdf: 4970258 bytes, checksum: fcd6ec64fc7d0cdced502c9b8b4dd5f9 (MD5) Previous issue date: 2012-11-01 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The 15Kh2MFA steel is a kind of Cr-Mo-V family steels and can be used in turbines for energy generation, pressure vessels, nuclear reactors or applications where the range of temperature that the material works is between 250 to 450?C. To improve the properties of these steels increasing the service temperature and the thermal stability is add a second particle phase. These particles can be oxides, carbides, nitrites or even solid solution of some chemical elements. On this way, this work aim to study the effect of addition of 3wt% of niobium carbide in the metallic matrix of 15Kh2MFA steel. Powder metallurgy was the route employed to produce this metallic matrix composite. Two different milling conditions were performed. Condition 1: milling of pure 15Kh2MFA steel and condition 2: milling of 15Kh2MFA steel with addition of niobium carbide. A high energy milling was carried out during 5 hours. Then, these two powders were sintered in a vacuum furnace (10-4torr) at 1150 and 1250?C during 60 minutes. After sintering the samples were normalized at 950?C per 3 minutes followed by air cooling to obtain a desired microstructure. Results show that the addition of niobium carbide helps to mill faster the particles during the milling when compared with that steel without carbide. At the sintering, the niobium carbide helps to sinter increasing the density of the samples reaching a maximum density of 7.86g/cm?, better than the melted steel as received that was 7,81g/cm?. In spite this good densification, after normalizing, the niobium carbide don t contributed to increase the microhardness. The best microhardness obtained to the steel with niobium carbide was 156HV and to pure 15Kh2MFA steel was 212HV. It happened due when the niobium carbide is added to the steel a pearlitic structure was formed, and the steel without niobium carbide submitted to the same conditions reached a bainitic structure / O a?o 15Kh2MFA, da fam?lia dos a?os CrMoV, pode ser utilizado em turbinas para gera??o de energia, vasos de press?o, reatores nuclear ou aplica??es, onde o material ? submetido a temperaturas de servi?o entre 250 e 450?C. Uma forma de melhorar as propriedades do a?o, para que ele trabalhe a temperaturas mais altas ou que se torne mais est?vel ? adicionar part?culas de segunda fase na sua matriz. Estas part?culas podem estar na forma de ?xidos, carbetos, nitretos ou at? mesmo em solu??o s?lida quando alguns elementos qu?micos s?o adicionados ao material. Neste contexto, este trabalho objetiva estudar o efeito da adi??o de 3% de carbeto de ni?bio na matriz met?lica do a?o 15Kh2MFA. Para isto a metalurgia do p? foi a rota empregada para a produ??o deste comp?sito de matriz met?lica. Para tal, duas moagens distintas foram realizadas. A primeira com o a?o 15Kh2MFA e a segunda com o a?o 15Kh2MFA com adi??o de 3% de carbeto de ni?bio. A moagem de alta energia foi realizada durante 5 horas. Em seguida, os dois p?s produzidos foram sinterizados em um forno a v?cuo (10-4torr) a temperaturas de 1150?C e 1250?C durante 60 minutos. Ap?s a sinteriza??o as amostras foram submetidas ao tratamento t?rmico de normaliza??o a 950?C. Os resultados mostraram que a adi??o do carbeto de ni?bio ajuda o processo de cominui??o das part?culas, quando comparado com o a?o sem o carbeto de ni?bio. O carbeto de ni?bio tem um papel fundamental na densifica??o das amostras durante a sinteriza??o, levando a densidade 7,86g/cm?, que ? maior do que a densidade do a?o fundido recebido que era de 7,81g/cm?. Apesar desta boa densifica??o, ap?s a normaliza??o, o NbC n?o contribuiu de forma significativa para aumento da dureza, onde a melhor dureza obtida para o a?o com NbC foi de 156HV e para o a?o puro foi de 212HV. Isto se deve ao fato de que, quando o NbC foi adicionado ao a?o, formou-se uma estrutura perl?tica, enquanto que, com o a?o sem adi??o de NbC, submetido as mesmas condi??es, obteve-se uma estrutura bain?tica

Made available in DSpace on 2014-12-17T14:07:15Z (GMT). No. of bitstreams: 1 MariaVMS_Parcial.pdf: 533267 bytes, checksum: dfba6ad9f2b0c2ad0d26dac46721fabf (MD5) Previous issue date: 2013-10-31 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The refractory metal carbides have proven important in the development of engineering materials due to their properties such as high hardness, high melting point, high thermal conductivity and high chemical stability. The niobium carbide presents these characteristics. The compounds of niobium impregnated with copper also have excellent dielectric and magnetic properties, and furthermore, the Cu doping increases the catalytic activity in the oxidation processes of hydrogen. This study aimed to the synthesis of nanostructured materials CuNbC and niobium and copper oxide from precursor tris(oxalate) oxiniobate ammonium hydrate through gas-solid and solid-solid reaction, respectively. Both reactions were carried out at low temperature (1000?C) and short reaction time (2 hours). The niobium carbide was produced with 5 % and 11% of copper, and the niobium oxide with 5% of copper. The materials were characterized by X-Ray Diffraction (XRD), Rietveld refinement, Scanning Electron Microscopy (SEM), X-Ray Fluorescence Spectroscopy (XRF), infrared spectroscopy (IR), thermogravimetric (TG) and differential thermal analysis (DTA , BET and particle size Laser. From the XRD analysis and Rietveld refinement of CuNbC with S = 1.23, we observed the formation of niobium carbide and metallic copper with cubic structure. For the synthesis of mixed oxide made of niobium and copper, the formation of two distinct phases was observed: CuNb2O6 and Nb2O5, although the latter was present in small amounts / Os carbetos de metais refrat?rios t?m se revelado importantes no desenvolvimento de materiais de engenharia devido as suas propriedades, tais como: alta dureza, alto ponto de fus?o, alta condutividade t?rmica e alta estabilidade qu?mica. O carbeto de ni?bio apresenta essas caracter?sticas. Os compostos de ni?bio impregnados com cobre tamb?m possuem excelentes propriedades diel?tricas e magn?ticas e, al?m disso, a dopagem com Cu aumenta a atividade catal?tica em processos de oxida??o de hidrog?nio. Este trabalho teve como objetivo a s?ntese dos materiais CuNbC e ?xido de ni?bio e cobre nanoestruturados a partir do precursor tris(oxalato)oxiniobato de am?nio hidratado, atrav?s de rea??o g?s-s?lido e s?lido-s?lido, respectivamente. Para ambos, as rea??es foram realizadas a baixa temperatura (1000?C) e curto tempo de rea??o (2 horas). O carbeto de ni?bio foi produzido com 5% e 11% de cobre e o ?xido de ni?bio e cobre com 5% de cobre. Os materiais obtidos foram caracterizados atrav?s dos ensaios de Difra??o de Raios X (DRX), Refinamento Rietveld, Microscopia Eletr?nica de Varredura (MEV), Espectroscopia por Fluoresc?ncia de Raios-X (FRX), Espectroscopia de Infravermelho (IV), Termogravim?trica (TG), An?lise Termodiferencial (DTA), BET e granulometria a Laser. A partir das an?lises de DRX e do refinamento Reitiveld para o CuNbC com S= 1,23, observou-se a forma??o do carbeto de ni?bio e cobre puro com estrutura c?bica. Na s?ntese realizada do ?xido misto de ni?bio e cobre correu a forma??o de duas fases distintas: CuNb2O6 e Nb2O5, embora a ?ltima tenha sido formada em pequena quantidade / 2020-01-01