In the current work, two different coatings, nitrocarburised (CN) and titanium carbonitride (TiCN) on M2 grade high speed tool steel, were prepared by commercial diffusion and physical vapour deposition (PVD) techniques, respectively. Properties of the coating were characterised using a variety of techniques such as Glow-Discharge Optical Emission Spectrometry (GD-OES) and Scanning Electron Microscopy (SEM). Three non-commercial, oil-based lubricants with simplified formulations were used for this study. A tribological test was developed in which two nominally geometrically-identical crossed cylinders slide over each other under selected test conditions. This test was used to evaluate the effectiveness of a pre-applied lubricant film and a surface coating for various conditions of sliding wear. Engineered surface coatings can significantly improve wear resistance of the tool surface but their sliding wear performances strongly depend on the type of coating and lubricant combination used. These coating-lubricant interactions can also have a very strong effect on the useful life of the lubricant in a tribological system. Better performance of lubricants during the sliding wear testing was achieved hen used with the nitrocarburised (CN) coating. To understand the nature of the interactions and their possible effects on the coating-lubricant system, several surface analysis techniques were used. The molecular level investigation of Fourier Transform Infrared Spectroscopy (FTIR) revealed that oxidative degradation occurred in all used oil-based lubricants during the sliding wear test but the degradation behaviour of oil-based lubricants varied with the coating-lubricant system and the wear conditions. The main differences in the carbonyl oxidation region of the FTIR spectra (1900-1600 cm-1) between different coating-lubricant systems may relate to the effective lifetime of the lubricant during the sliding wear test. Secondary Ion Mass Spectrometry (SIMS) depth profiling shows that the CN coating has the highest lubricant absorbability among the tested tool surfaces. Diffusion of chlorine (C1), hydrogen (H) and oxygen (O) into the surface of subsurface of the tool suggested that strong interactions occurred between lubricant and tool surface during the sliding wear test. The possible effects of the interactions on the performance of whole tribological system are also discussed. The study of Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) indicated that the envelope of hydrocarbons (CmHn) of oil lubricant in the positive TOF-SIMS spectra shifted to lower mass fragment after the sliding wear testing due to the breakage of long-chain hydrocarbons to short-chain ones during the degradation of lubricant. The shift of the mass fragment range of the hydrocarbon (CmHn) envelope caries with the type of both tool surface and lubricant, again confirming that variation in the performance of the tool-lubricant system relates to the changes in surface chemistry due to tribochemical interactions at the tool-lubricant interface under sliding wear conditions. The sliding wear conditions resulted in changes not only in topography of the tool surface due to mechanical interactions, as outlined in Chapter 5, but also in surface chemistry due to tribochemical interactions, as discussed in Chapters 6 and 7.

Three samples of Ti(C,N) were fabricated with 40 vol.% Ni3Al: Ti(C0.3N07), Ti(C0.5N0.5) and Ti(C0.7N0.3), as well as TiC with 10, 20, 30 and 40 vol.% Ni3Al binder addition by means of melt infiltration and sintering. Each sample was evaluated for density and microstructure before being placed in a flat cell for electrochemical testing. Open circuit potential was evaluated, followed by the application of a cathodic potential, whereby the response was tracked using Corrware corrosion software throughout the duration of potentiodynamic testing. Following corrosion testing, each sample was reevaluated for changes in microstructure and chemical composition. Ti(C,N) samples were found to have adequate resistance to corrosion, with increased resistance with increasing carbon content, however these samples demonstrated a greater frequency of breakdown and repassivation, suggesting a greater susceptibility to corrosion, despite the initial improved resistance. SEM imaging demonstrated significant crevice corrosion throughout. TiC-cermets demonstrated similar results in terms of SEM evaluation of microstructure. TiC-cermets with the lowest binder content (10 vol.% Ni3Al) demonstrated greater initial resistance to corrosion but also had the greatest potential for breakdown.

O desenvolvimento de eletrocatalisadores eletricamente condutores com alta atividade para a reação de redução de oxigênio (ORR - Oxygen Reduction Reaction) e desprendimento de oxigênio (OER - Oxygen Evolution Reaction) é de extrema importância e interesse para dispositivos de eletro-conversão de energia, como as células a combustível e eletrolisadores, que operam tanto em meio alcalino quanto ácido. Em meio alcalino, é possível o uso de metais não nobres e, assim, são viáveis para o uso em larga escala. Em meio ácido, é necessário o uso de materiais estáveis, uma vez que eles são expostos a um ambiente extremamente corrosivo e à altos potenciais, principalmente durante o processo de liga/desliga do dispositivo. Diante disso, este trabalho foi dividido em três linhas de pesquisa: Parte I - estudos de eletrocatalisadores bifuncionais para a ORR e OER em meio alcalino, sendo eles compostos por espinélios de manganês-cobalto em combinação com nanopartículas de ouro (MnCo2O4/Au). Parte II - estudos de eletrocatalisadores alternativos para a ORR em eletrólito ácido, onde foram considerados carbetos e nitretos de molibdênio (Mo2C-MoN) e, oxinitretos de tântalo (Ta-ON). Parte III - estudo de suportes alternativos ao carbono para a ORR em eletrólito ácido, sendo eles compostos por carbonitretos de tântalo e titânio (Ta-CN e Ti-CN). Os resultados da Parte I para MnCo2O4/Au mostraram que houve um aumento significativo na atividade de MnCo2O4 com a adição de ouro para ambas as reações. Foi possível observar que a combinação de nanopartículas condutoras (ouro) com nanopartículas ativas, mas não condutoras (MnCo2O4), é promissora para o desenvolvimento de eletrocatalisadores ativos para uso como eletrodos de oxigênio. Quanto a Parte II, os materiais compostos por Mo2C-MoN foram obtidos por meio da inserção de carbono e nitrogênio com tratamento térmico, na presença de carbono Vulcan e NH3, em alta temperatura. O material nomeado como MoN + Mo2C (molibdato) foi o que apresentou maior atividade catalítica, o que pôde ser atribuído ao menor tamanho de cristalito, maior quantidade da sua fase MoN e ao efeito sinérgico entre MoN e Mo2C, facilitando a ORR em comparação ao nitreto e carbeto de molibdênio puros. Nesta mesma linha de pesquisa, oxinitretos de tântalo foram sintetizados utilizando ureia como fonte de nitrogênio. Foi observado que Ti-Ta-ON apresentou maior atividade catalítica quando comparado aos demais eletrocatalisadores. Já na Parte III, os resultados para carbonitreto de titânio como suporte para a platina (Pt/Ti-CN) mostraram que, além da sua atividade para a ORR ser semelhante à platina suportada em carbono (Pt/C), ele também se mostrou mais estável que Pt/C após a realização de testes de estabilidade. / The development of conductive electrocatalysts with high activity for the oxygen reduction and evolution reactions (ORR and OER) is of extremely importance for devices that electroconvert energy, such as fuel cells and electrolizers, which work in alkaline and acid media. A substantial amount of metals can be employed in alkaline electrolytes once the latter do not require the use of noble metals. The acid medium asks for stable materials, since they are exposed to a high oxidative environment and potentials during the start-up/shutdown events of the device. On the base of these facts, this research work has been divided into three parts: Part I - bifunctional electrocatalysts studies for the ORR and OER in alkaline electrolyte, the materials were composed of spinel manganese-cobalt oxide combined with gold nanoparticles (MnCo2O4/Au). Part II - studies of alternative electrocatalysts for the ORR in acid electrolyte, which included molybdenum carbides and nitrides (Mo2C-MoN), and tantalum oxynitrides (Ta-ON). Part III - alternative supports to the carbon for the ORR in acid electrolyte, which included tantalum and titanium carbonitrides (Ta-CN and Ti-CN). The results for MnCo2O4/Au, in Part I, showed that the addition of gold on the surface of the oxide improved the latter activity for both reactions. The combination of conductive nanoparticles (gold) with active, but non-conductive, nanoparticles (MnCo2O4) seems promising for the development of active electrocatalysts for the ORR and OER. In Part II, the materials composed of Mo2C-MoN were synthesized through carbon and nitrogen insertion, in a high temperature heat treatment, in the presence of Vulcan carbon and NH3. Among the gotten materials, the so called MoN + Mo2C (molybdate) showed the better electrocatalytic activity for the ORR, which could be attributed to its smaller crystallite size and the greater amount of its MoN phase, along with the synergistic effect between MoN and Mo2C. In this way, tantalum oxynitrides materials were obtained via a urea synthesis. The catalyst referred to as Ti-Ta-ON showed the better ORR activity among all the others studied oxynitrides materials. In Part III, besides the activity for the ORR of platinum supported on titanium carbonitride (Pt/Ti-CN) was similar to the activity of platinum supported on carbon (Pt/C), Pt/Ti-CN was also more stable than the latter, after the stability tests.

O desenvolvimento de eletrocatalisadores eletricamente condutores com alta atividade para a reação de redução de oxigênio (ORR - Oxygen Reduction Reaction) e desprendimento de oxigênio (OER - Oxygen Evolution Reaction) é de extrema importância e interesse para dispositivos de eletro-conversão de energia, como as células a combustível e eletrolisadores, que operam tanto em meio alcalino quanto ácido. Em meio alcalino, é possível o uso de metais não nobres e, assim, são viáveis para o uso em larga escala. Em meio ácido, é necessário o uso de materiais estáveis, uma vez que eles são expostos a um ambiente extremamente corrosivo e à altos potenciais, principalmente durante o processo de liga/desliga do dispositivo. Diante disso, este trabalho foi dividido em três linhas de pesquisa: Parte I - estudos de eletrocatalisadores bifuncionais para a ORR e OER em meio alcalino, sendo eles compostos por espinélios de manganês-cobalto em combinação com nanopartículas de ouro (MnCo2O4/Au). Parte II - estudos de eletrocatalisadores alternativos para a ORR em eletrólito ácido, onde foram considerados carbetos e nitretos de molibdênio (Mo2C-MoN) e, oxinitretos de tântalo (Ta-ON). Parte III - estudo de suportes alternativos ao carbono para a ORR em eletrólito ácido, sendo eles compostos por carbonitretos de tântalo e titânio (Ta-CN e Ti-CN). Os resultados da Parte I para MnCo2O4/Au mostraram que houve um aumento significativo na atividade de MnCo2O4 com a adição de ouro para ambas as reações. Foi possível observar que a combinação de nanopartículas condutoras (ouro) com nanopartículas ativas, mas não condutoras (MnCo2O4), é promissora para o desenvolvimento de eletrocatalisadores ativos para uso como eletrodos de oxigênio. Quanto a Parte II, os materiais compostos por Mo2C-MoN foram obtidos por meio da inserção de carbono e nitrogênio com tratamento térmico, na presença de carbono Vulcan e NH3, em alta temperatura. O material nomeado como MoN + Mo2C (molibdato) foi o que apresentou maior atividade catalítica, o que pôde ser atribuído ao menor tamanho de cristalito, maior quantidade da sua fase MoN e ao efeito sinérgico entre MoN e Mo2C, facilitando a ORR em comparação ao nitreto e carbeto de molibdênio puros. Nesta mesma linha de pesquisa, oxinitretos de tântalo foram sintetizados utilizando ureia como fonte de nitrogênio. Foi observado que Ti-Ta-ON apresentou maior atividade catalítica quando comparado aos demais eletrocatalisadores. Já na Parte III, os resultados para carbonitreto de titânio como suporte para a platina (Pt/Ti-CN) mostraram que, além da sua atividade para a ORR ser semelhante à platina suportada em carbono (Pt/C), ele também se mostrou mais estável que Pt/C após a realização de testes de estabilidade. / The development of conductive electrocatalysts with high activity for the oxygen reduction and evolution reactions (ORR and OER) is of extremely importance for devices that electroconvert energy, such as fuel cells and electrolizers, which work in alkaline and acid media. A substantial amount of metals can be employed in alkaline electrolytes once the latter do not require the use of noble metals. The acid medium asks for stable materials, since they are exposed to a high oxidative environment and potentials during the start-up/shutdown events of the device. On the base of these facts, this research work has been divided into three parts: Part I - bifunctional electrocatalysts studies for the ORR and OER in alkaline electrolyte, the materials were composed of spinel manganese-cobalt oxide combined with gold nanoparticles (MnCo2O4/Au). Part II - studies of alternative electrocatalysts for the ORR in acid electrolyte, which included molybdenum carbides and nitrides (Mo2C-MoN), and tantalum oxynitrides (Ta-ON). Part III - alternative supports to the carbon for the ORR in acid electrolyte, which included tantalum and titanium carbonitrides (Ta-CN and Ti-CN). The results for MnCo2O4/Au, in Part I, showed that the addition of gold on the surface of the oxide improved the latter activity for both reactions. The combination of conductive nanoparticles (gold) with active, but non-conductive, nanoparticles (MnCo2O4) seems promising for the development of active electrocatalysts for the ORR and OER. In Part II, the materials composed of Mo2C-MoN were synthesized through carbon and nitrogen insertion, in a high temperature heat treatment, in the presence of Vulcan carbon and NH3. Among the gotten materials, the so called MoN + Mo2C (molybdate) showed the better electrocatalytic activity for the ORR, which could be attributed to its smaller crystallite size and the greater amount of its MoN phase, along with the synergistic effect between MoN and Mo2C. In this way, tantalum oxynitrides materials were obtained via a urea synthesis. The catalyst referred to as Ti-Ta-ON showed the better ORR activity among all the others studied oxynitrides materials. In Part III, besides the activity for the ORR of platinum supported on titanium carbonitride (Pt/Ti-CN) was similar to the activity of platinum supported on carbon (Pt/C), Pt/Ti-CN was also more stable than the latter, after the stability tests.

Orientadores: Maria Helena Robert, Elisa Maria Ruiz Navas / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-19T00:27:43Z (GMT). No. of bitstreams: 1 BravoSalazar_JaimeAlejandro_D.pdf: 9695376 bytes, checksum: d35ebfbcaf1dac8c6665392b7d784d23 (MD5) Previous issue date: 2007 / Resumo: Este trabalho estuda o processo de fabricação de compósitos de matriz de alumínio AA6061 reforçado com TiCN por metalurgia do pó, envolvendo as etapas de mistura de pós, compactação uniaxial e sinterização com fase líquida. Para efeitos de comparação foram produzidos e caracterizados compactados da liga AA6061 sem adição de reforços. Foram investigados os parâmetros de processo: teores de reforço (5% e 10% massa), teor de aditivos Pb e Sn (0,1, 0,15, 0,2 e 0,4% massa), pressão de compactação (400, 600 e 800 MPa), tempos (15, 30, 45 e 60 min) e temperatura de sinterização (590, 600, 610 e 620 ºC). Em cada etapa do processo foram caracterizados os produtos (mistura de pós e compactados); o produto final obtido, após sinterização, foi caracterizado com relação à sua microestrutura, propriedades físicas (densificação e variação dimensional) e mecânicas (resistência à flexão e dureza). Os resultados obtidos mostraram uma grande eficiência do processo na obtenção de compósitos; a adição do teor de reforço de 5%TiCN foi eficiente na promoção de rupturas das camadas de óxidos do pó da liga de alumínio compactado à pressão de 400 MPa, auxiliando a sinterização por difusão da fase líquida formada a partir da fusão de Al+Mg2Si, melhorando a densificação e diminuindo a variação dimensional dos produtos sinterizados. Do ponto de vista metalúrgico, os materiais compósitos obtidos apresentaram microestruturas homogêneas, com uma boa distribuição dos reforços na matriz e relativa diminuição de poros. A adição de Pb e Sn promovem maior eficiência de ativação de mecanismos de sinterização; para compactados produzidos à pressão de 800 MPa, a adição de 0,1% desses elementos já apresentou significativa influência na sinterização. Com relação às propriedades mecânicas e físicas observou-se que a adição de TiCN aumentou quase no dobro de seus valores obtidos quando são comparados com a liga AA6061 / Abstract: This work investigates the process of production of composites of the alloy AA6061 reinforced with TiCN particles, by powder metallurgy involving the steps: conventional mixture of powders, compaction by uniaxial cold pressing and sintering with formation of a liquid phase. For comparative analysis it was also produced sintered AA6061 without addition of reinforcements. The following processing parameters were studied: reinforcing particles content (5 and 10 wt%); content of trace elements Pb and Sn (0.1, 0.15, 0.2 0.4 wt%); compaction pressure (from 400, 600 and 800 MPa); time and temperature of sintering (15, 30, 45, 60 min and 590, 600, 610, 620 oC). In each step products were characterized (powder mixture and green compacts); the final sintered product was characterized related to microstructure, physical (densification and dimensional changes) and mechanical (hardness and bending strength) properties. Obtained results showed high efficiency of the applied process to produce reliable composite products; the addition of 5 wt% TiCN was efficient to promote fracture of the oxide layer in the aluminum particles surface during pressing. At sintering temperatures liquid phase is formed by Al+Mg2Si melting and is distributed among particles through the fractures of the oxide layer, improving the material densification and its mechanical properties. Microstructures obtained showed homogeneous distribution of TiCN and reduced porosity, whereas AA6061 alloy microstructure showed higher porosity. Addition of Pb and Sn promoted higher efficiency of sintering mechanisms in compacts submitted to high pressures, leading to enhanced physical and mechanical properties in those materials. / Doutorado / Materiais e Processos de Fabricação / Doutor em Engenharia Mecânica

Global warming and depletion in fossil fuels have forced the society to search for alternate, clean sustainable energy sources. An obvious solution to the aforesaid problem lies in electrochemical energy storage systems like fuel cells and batteries. The desirable properties attributed to these devices like quick response, long life cycle, high round trip efficiency, clean source, low maintenance etc. have made them very attractive as energy storage devices. Compared to many advanced battery chemistries like nickel-metal hydride and lithium - ion batteries, metal-air batteries show several advantages like high energy density, ease of operation etc. The notable characteristics of metal - air batteries are the open structure with oxygen gas accessed from ambient air in the cathode compartment. These batteries rely on oxygen reduction and oxygen evolution reactions during discharging and charging processes. The efficiency of these systems is determined by the kinetics of oxygen reduction reaction. Platinum is the most preferred catalyst for many electrochemical reactions. However, high cost and stability issues restrict the use of Pt and hence there is quest for the development of stable, durable and active electrocatalysts for various redox reactions. The present thesis is directed towards exploring the electrocatalytic aspects of titanium carbonitride. TiCN, a fascinating material, possesses many favorable properties such as extreme hardness, high melting point, good thermal and electrical conductivity. Its metal-like conductivity and extreme corrosion resistance prompted us to use this material for various electrochemical studies. The work function as well as the bonding in the material can be tuned by varying the composition of carbon and nitrogen in the crystal lattice. The current study explores the versatility of TiCN as electrocatalyst in aqueous and non-aqueous media. One dimensional TiC0.7N0.3 nanowires are prepared by simple one step solvothermal method without use of any template and are characterized using various physicochemical techniques. The 1D nanostructures are of several µm size length and 40 ± 15 nm diameter (figure 1). Orientation followed by attachment of the primary particles results in the growth along a particular plane (figure 2). (a) (b) (c) Figure 1. (a) SEM images of TiC0.7N0.3 nanowires (b) TEM image and (c) High resolution TEM image showing the lattice fringes. (a) (b) (d) Figure 2. Bright field TEM images obtained at different time scales of reaction. (a) 0 h; (b) 12 h; (c) 72 h and (d) 144 h. The next aspect of the thesis discusses the electrochemical performance of TiC0.7N0.3 especially for oxygen reduction. Electrochemical oxygen reduction reaction (ORR) reveals that the nanowires possess high activity for ORR and involves four electron process leading to water as the product. The catalyst effectively converts oxygen to water with an efficiency of 85%. A comparison of the activity of different (C/N) compositions of TiCN is shown in figure 3. The composition TiC0.7N0.3 shows the maximum activity for the reaction. The catalyst is also very selective for ORR in presence of methanol and thus cross-over issue in fuel cells can be effectively addressed. Density functional theory (DFT) calculations also lead to the same composition as the best for electrocatalysis, supporting the experimental observations. Figure 3. Linear sweep voltammetric curves observed for different compositions of titanium carbonitride towards ORR. The next chapter deals with the use of TiC0.7N0.3 as air cathode for aqueous metal - air batteries. The batteries show remarkable performance in the gel- and in liquid- based electrolytes for zinc - air and magnesium - air batteries. A partial potassium salt of polyacrylic acid (PAAK) is used as the polymer to form a gel electrolyte. The cell is found to perform very well even at very high current densities in the gel electrolyte (figures 4 and 5). Figure 4 Photographs of different components of the gel - based zinc - air battery. (a) (b) Figure 5. a) Discharge curves at different current densities of 5, 20, 50 and 100 mA/cm2 for zinc-air system with TiC0.7N0.3 cathode b) Charge – discharge cycles at 50 mA/cm2 for the three electrode configuration with TiC0.7N0.3 nanowire for ORR and IrO2 for OER and Zn electrode (2h. cycle period). Similarly, the catalytic activity of TiC0.7N0.3 has also been explored in non-aqueous electrolyte. The material acts as a bifunctional catalyst for oxygen in non- aqueous medium as well. It shows a stable performance for more than 100 cycles with high reversibility for ORR and OER (figure 6). Li-O2 battery fabricated with a non-aqueous gel- based electrolyte yields very good output. (a) (b) (c) Figure 6. Galvanostatic charge –discharge cycles. (a) at 1 mA/cm2 (b) specific capacity as a function of no. of cycles (c) photographs of PAN-based gel polymer electrolyte. Another reaction of interest in non –aqueous medium is I-/I3-. redox couple. TiC0.7N0.3 nanowires show small peak to peak separation, low charge transfer resistance and hence high activity. The catalyst is used as a counter electrode in dye sensitized a solar cell that shows efficiencies similar to that of Pt, state of the art catalyst (figure 7). (a) (b) (c) Figure 7 (a) Cyclic voltammograms for I-/I3 - redox species on TiC0.7N0.3 nanowires (red), TiC0.7N0.3 particle (black) and Pt (blue). (b) Photocurrent density - voltage characteristics for DSSCs with different counter electrodes. TiC0.7N0.3 nanowire (black), TiC0.7N0.3 particle (blue), Pt (red). (c) Photograph of a sample cell. (a) (b) (c) (d) Figure 8 a) Comparison ORR activity for (i) NiTiO3(black), (ii) N-rGO (red), (iii) NiTiO3 – N-rGO (green) and (iv) Pt/C (blue) (b) Linear sweep voltammograms for OER observed on NiTiO3 – N-rGO composite (black), NiTiO3 (brown), N-rGO (blue), glassy carbon (red) in 0.5 M KOH. (c) Galvanostatic discharge curves of NiTiO3 – N-rGO as air electrode (d) Charge – discharge cycle at 5 mA/cm2 for the rechargeable battery with 10 min. cycle period. The last part of the thesis discusses about a ceramic oxide, nickel titanate. The electrocatalytic studies of the material towards ORR and OER reveal that the catalyst shows remarkable performance as a bifunctional electrode. A gel - based zinc - air battery fabricated with nickel titanate – reduced graphene oxide composite shows exceptional performance of 1000 charge-discharge cycles in the rechargeable mode (figure 8). Of course, the primary battery configuration works very well too The thesis contains seven chapters on the aspects mentioned above with summary and future perspectives given as the last chapter. An appendix based on TiN nanotubes and supercapacitor studies is given at the end.