Philosophiae Doctor - PhD / Titanium oxynitride has attracted research interest for the fact that it is a bioactive non-toxic material. It is suitable for surface coating of biomaterials and in other applications such as anti-reflective coatings, while oxygen-rich titanium oxynitride has been applied in thin film resistors and photocatalysis. Two common types of titanium oxynitrides are TiOₓNᵧ. and TiO₂-ₓNᵧ. In this work, titanium mixed metals oxynitrides (Ti-TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂) were synthesized for the first time using ball milling (BM) and annealing processes. Their structural, morphological, surface, optical, and gas sensing properties were studied in detail. Structural analyses showed that upon milling a pure TiO₂ phase, tetragonal to orthorhombic phase transformation occurred. However, when milling TiO₂ mixed with Mg, Sn and Ti no evidence of the transformation was observed. Furthermore, scanning electron microscopy, transmission electron microscopy and atomic force microscopy showed that the milling process promotes particle refinement. The gas sensing analyses also demonstrated that the sensing response of the TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂ materials improved upon milling. Moreover, the Mg-TiO₂ showed improved sensing compared to pure TiO₂ due to incorporation of Mg, which might have resulted in a decrease of charge carrier concentration. The Mg-TiO₂ sensing materials showed fast response-recovery time of ~32 s and ~48 s, respectively, as well as high selectivity to NH₃ gas compared to other gases (H₂, and CH₄). In addition, the improved response observed for the milled samples is due to increased surface area and pore diameter, providing more active sites for the target gas and allowing more gas adsorption with an increase in point defects related to oxygen vacancies (Vo), which are the most favorable adsorption sites for oxygen species and thus can enhance the possibility of interaction with gas molecules. A combination of photoluminescence, x-ray photoelectron spectroscopy, vibrating sample magnetometer and sensing analyses demonstrated that a direct relation exists between the magnetization, sensing and the relative occupancy of the Vo present on the surface of TiO₂ nanoparticles. Therefore, based on these finding we conclude that the milling process promotes particle refinement, resulting in an increased BET surface and partial breaking of Ti–O bonds on the TiO₂ surface layer, which results in the formation of oxygen vacancies in the TiO₂ lattice, therefore anticipating improved sensing response. / National Research Foundation

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第17795号 / エネ博第278号 / 新制||エネ||58(附属図書館) / 30602 / 京都大学大学院エネルギー科学研究科エネルギー社会・環境科学専攻 / (主査)教授 石原 慶一, 教授 東野 達, 准教授 奥村 英之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Cette thèse est consacrée à l'étude des métaux-N-H des matériaux pour le stockage d'hydrogène de solide. Le but est de caractériser la synthèse mechanochemical, structurelle et les propriétés d'hydrogénation de Li-N-H, Li-Mg-N-H et des systèmes Li-Mg-B-N-H. Premièrement, l'assimilation hydrogène pendant mechanochemistry de Li3N sous 9 MPA de H2 a été analysée au moyen de l'absorption solide-à-gaz in situ et la Diffraction de Radiographie d'ex-situ (XRD) des mesures. Deux étapes de H-sorption menant à une assimilation hydrogène globale de 9.8wt le % ont été obtenus. La première étape de réaction comprend la transformation de polymorphe-li3n (S.G.P6/mmm) dans li3n (S.G.P63/mmc) métastable la phase et la réaction du dernier avec l'hydrogène pour former lithium imide :-li3n + H2 Li2NH + LiH. La deuxième étape absorbant est lithium imide des convertis à lithium amide / This thesis is dedicated to the study of novel metal-N-H materials for solid state hydrogen storage. The aim is to characterize the mechanochemical synthesis, structural and hydrogenation properties of Li-N-H, Li-Mg-N-H and Li-Mg-B-N-H systems. Firstly, hydrogen uptake during mechanochemistry of Li3N under 9 MPa of H2 has been analyzed by means of in-situ solid-gas absorption and ex-situ X-Ray Diffraction (XRD) measurements. Two H-sorption steps leading to an overall hydrogen uptake of 9.8wt% was obtained. The first reaction step comprises the transformation of polymorph -Li3N (S.G.P6/mmm) into -Li3N (S.G.P63/mmc) metastable phase and the reaction of the latter with hydrogen to form lithium imide: -Li3N + H2 Li2NH + LiH. The second absorption step is lithium imide converts to lithium amide following the reaction scheme Li2NH + H2 LiNH2 + LiH. The assessment of reaction paths in this system as well as of the appraisal of the underlying reaction mechanisms was under taken. Secondly, reactive ball milling (RBM) under H2 of Li3N and Mg powder with a molar ratio of 2:1 was taken on to destabilize Li-N-H system and accelerate its sorption kinetics. The onset dehydrogenation temperature of the as-milled 2Li3N+Mg mixture was detected at 125°C, which is about 75°C lower than that of the Li-N-H system. The structural and phases evolution of the Li-Mg-N-H system during both the synthesis and subsequent hydrogenation/dehydrogenation cycling were characterized by combined analysis of in-situ XRD and neutron powder diffraction (NPD) measurements. It was found step wised for the both processes depending on mainly the temperature and hydrogen pressure to the system. Finally, the effect of the addition of Co-based compounds, lithium borohydides and the combination of them to Li-Mg-N-H system were systematically investigated by XRD, scanning electron microscopy (SEM), fourier transform infra-red (FTIR), differential scanning calorimetry (DSC) and hydrogen storage properties measurements with the aim to overcome the kinetic barriers and further decrease the dehydrogenation temperature. The Li-Mg-B-N-H/3wt% ZrCoH3 composite synthesized by RBM has the best hydrogen storage properties. It is shown that the activation energy was decreased and the N-H bonds were weakened, which could be the main reasons for improving the hydrogen storage properties of Li-Mg-N-H system

Thermoelectric materials are playing a larger role in the global effort to develop diverse, efficient, and sustainable energy technologies: primarily through power-generating thermoelectric modules. The principal components of thermoelectric modules are solid-state thermoelectric materials – typically heavily doped semiconductors – that convert heat directly into electricity. However, this conversion efficiency is too low to supplant traditional energy technologies – severely limiting the distribution of clean and sustainable thermoelectric energy technologies. Efforts to enhance thermoelectric efficiency, which have been underway for decades, have been slow to realize appreciable gains in thermoelectric efficiency. However, a key advance in improving efficiency – the New Paradigm in thermoelectric material research – has been the development of thermoelectric nanocomposites. Thermoelectric nanocomposites show improved efficiency; however, they are often synthesized from highly toxic elements via energetically intense and costly synthesis procedures. Therefore, this research focuses on the discovery and development of a novel procedure for synthesizing thermoelectric nanocomposites – attrition enhanced nanocomposite synthesis – from open cage-like skutterudite-based materials. With further optimization, high-performance power-generating thermoelectric materials can be produced via this technique. Therefore, attrition-enhanced nanocomposite synthesis may play a small, though instrumental, role in achieving sustainable electrical power. / Graduation date: 2012 / Access restricted to the OSU Community at author's request from Jan. 6, 2012 - Jan. 6, 2013

Les poudres d'aluminium sont depuis longtemps utilisées comme additifs dans de nombreuses formulations pour la pyrotechnie ou les propergols. Introduites en quantité relativement faible dans les propergols des fusées, elles permettent d'accroître significativement les vitesses de combustion. Cependant, les mécanismes à l'origine de cette meilleure réactivité restent encore mal compris et vont bien au-delà d'une simple explication en terme d'augmentation de l'état de division. Au cours de cette thèse nous avons élaboré des poudres d'aluminium par broyage à haute énergie, technique mécanique peu couteuse qui pourrait être une alternative aux procédés de fabrication actuels. Nous montrons que cette méthode permet l'obtention de particules micrométriques ou nanométriques suivant les conditions de broyage. Les particules sont polycristallines et présentent une morphologie en plaquette. Les poudres nanométriques obtenues présentent une réactivité comparable voire même supérieure aux nanopoudres sphériques actuellement élaborées par électro-explosion de fil ou voie plasma. Nous montrons que cette bonne réactivité est lié à la morphologie des particules ainsi qu'à la microstructure de la couche d'alumine native qui passive l'aluminium. / Aluminium nanopowders have been using for a long time as additives in many formulations for propergol or in pyrotechnics. Introduced in small quantity in rocket propergol they allow to increase the combustion rate. However, the reactivity of aluminium particles is not really understood and not just linked to the size reduction effect. All along this study, we have elaborated aluminium powders using high energy mechanical milling, a mechanical technique with a low cost which could be an alternative process of powders fabrication. We prove that this method allows elaboration of micro and nano-particles as a function of the mechanical parameters. The particles are polycristallines with flake morphology. The obtained nanopowders have a similar reactivity, or better than spherical nanopowders obtained by wire electrical explosion or plasma. We proove that this good reactivity is linked to the particles morphology, as well as also to the microstructure of the native alumina core shell.

Lignin, the second most abundant biopolymer on earth, is potentially a replaceable source for bulky fuels and chemical feedstocks. There have been numerous reports on methods for the oxidative cleavage of β-O-4 linkages but relatively few reports of how those methods affect other linkages that are present in lignin. We investigated how the β-1 and β-5 linkages respond under oxidative conditions proposed for lignin deconstruction based on their effect on β-O-4 linkages. Mechanochemical treatment of lignin can greatly improve the yield of monomer products and we applied a mechanochemical approach, using powerful ring-and-puck milling to promote lignin degradation. Along with similar production of monomers in a much shorter period than what we observed in previous ball-milling process, much more unexpected reactions were taking place during the current mechanochemical process. Lignin is a promising feedstock for epoxy resins since lignin-derived aromatic monomers usually bear hydroxyl and carboxyl groups. We are working on utilizing these mono-aromatic compounds and highly-functionalized-lignin as precursors for preparation of epoxy thermosets. We are interested in investigating the properties of thermosets by utilizing the actual isolated monomer streams from raw lignin. We expect to observe attractive thermal and mechanical properties from these lignin-derived epoxy thermosets compare to that of the commercialized but currently limited-used BPA-based epoxy resins.

A study was carried out on the selection of processing condition that would yield Mg-Ti with most favourable hydrogenation properties. Processing routes under consideration were / mechanical milling under inert atmosphere, reactive milling i.e. milling under hydrogen atmosphere, ECAP (equal channel angular pressing) and thermal plasma synthesis. Structure resulting from each of these processing routes was characterized with respect to size reduction, coherently diffracting volume and the distribution of Ti catalyst. Mechanical milling yielded a particulate structure made up of large Mg agglomerates with embedded Ti fragments with a uniform distribution. Mg agglomerates have sizes larger than 100 &micro / m which arises as a result of a balance between cold welding process and ductile fracture. Repeated folding of Mg particles entraps Ti fragments inside the Mg agglomerates resulting in a very uniform distribution. Coherently diffracting volumes measured by X-ray Rietveld analysis have small sizes ca. 26 nm which implies that the agglomerates typically comprise 1011 crystallites. Mechanical milling under hydrogen, i.e. reactive milling, led to drastic reduction in particle size. Mg and Ti convert to MgH2 and TiH2 which are milled efficiently due to their brittleness resulting in particle sizes of sub-micron range. Hydrogenation experiments carried out on Mg-10 vol % Ti milled under argon yields enthalpy and entropy values of -76.74 kJ/mol-H2 and -138.64 J/K.mol-H2 for absorption and 66.54 kJ/mol H2 and 120.12 J/K.mol H2 for desorption, respectively. For 1 bar of hydrogen pressure, this corresponds to a hydrogen release temperature of 280 &deg / C. This value is not far off the lowest desorption temperature reported for powder processed Mg based alloys. ECAP processing is a bulk process where the powders, consolidated in the first pass, have limited contact with atmosphere. This process which can be repeated many times lead to structural evolution similar to that of milling, but for efficient mixing of phases it was necessary to employ multi-pass deformation. An advantage of ECAP deformation is strain hardening of the consolidated powders which has improved milling ability. Based on this, a new route was proposed for the processing of ductile hydrogen storage alloys. This involves several passes of ECAP deformation carried out in open atmosphere and a final milling operation of short duration under inert atmosphere. The plasma processing yields Mg particles of extremely small size. Evaporation of Mg-Ti powder mixture and the subsequent condensation process yield Mg particles which are less than 100 nm. Ti particles, under the current experimental condition used, have irregular size distribution but some could be quite small, i.e. in the order of a few tens of nanometers. Of the four processing routes, it was concluded that both reactive milling and thermal plasma processing are well suited for the production of hydrogen storage alloys. Reactive milling yield particles in submicron range and plasma processing seems to be capable of yielding nanosize Mg particles which, potentially, could be decorated with even smaller Ti particles.

O presente estudo está dividido em duas partes. A primeira parte está relacionada à preparação de pós de Cr3C2-25(Ni20Cr) nanoestruturados através do processo de moagem de alta energia, bem como à caracterização dos pós moídos e no estado como recebido. A análise dos dados obtidos nesta etapa do trabalho foi feita utilizando-se uma abordagem essencialmente teórica. A segunda parte deste estudo refere-se à produção e caracterização de revestimentos preparados com os pós de Cr3C2-25(Ni20Cr) nanoestruturados e como recebido. O comportamento destes revestimentos sob erosão-oxidação em alta temperatura foi comparado com base em uma abordagem de caráter mais tecnológico. O tamanho médio de cristalito do pó de Cr3C2-25(Ni20Cr) decresceu rapidamente de 145 nm para 50 nm nos estágios iniciais de moagem e, posteriormente, com o aumento do tempo de moagem, decresceu mais lentamente até atingir um estado estacionário para um tamanho de cristalito em torno de 10 nm. Este estado estacionário corresponde ao início do processo de recuperação dinâmica. A máxima deformação da rede cristalina (δ = 1,17%) foi observada para pós moídos por 16 horas, caracterizando um tamanho crítico de cristalito da ordem de 28 nm. Por outro lado, o parâmetro de rede atingiu um mínimo para pós moídos por 16 horas. Após atingir o tamanho crítico de cristalito, a densidade de discordâncias praticamente não mais varia (estado estacionário) e toda deformação plástica posteriormente introduzida no material é acomodada através de eventos que ocorrem nos contornos de grão, particularmente por meio do processo designado deslizamento de contorno de grão (grain boundary sliding). A energia de deformação armazenada na rede cristalina dos pós de Cr3C2-25(Ni20Cr) moídos com diferentes tempos de moagem foi determinada por meio de medidas da variação de entalpia. Estes resultados indicaram que a máxima variação de entalpia (ΔH = 722 mcal) também ocorreu para pós moídos por 16 horas. Analogamente, a máxima variação do calor específico (ΔCp = 0,278 cal/gK) ocorreu para pós moídos por 16 horas. As seguintes propriedades mecânicas dos revestimentos de Cr3C2-25(Ni20Cr), preparados utilizando-se o processo HVOF de aspersão térmica, foram determinadas: microdureza Vickers, módulo de Young e tenacidade à fratura. As propriedades dos revestimentos preparados com os pós nanoestruturados e como recebido foram comparadas. A dureza e o módulo de Young dos revestimentos preparados com os pós nanoestruturados foram aproximadamente 26% maiores que aqueles preparados com os pós como recebido. A tenacidade à fratura dos revestimentos nanoestruturados foi aproximadamente 36% maior do que o verificado para os revestimentos produzidos com pós no estado como recebido. A resistência à erosão-oxidação do revestimento produzido com o pó nanoestruturado foi em torno de 52% maior do que a do revestimento preparado com o pó no estado como recebido, a 800ºC. Ambos os revestimentos mostraram um aumento da taxa de erosão-oxidação para temperaturas acima de 450ºC. / This study is divided in two parts. The first part is about the preparation of nanostructured Cr3C2-25(Ni20Cr) powders by high energy milling followed by characterization of the milled and the as received powder. Analyses of some of the data obtained were done using a theoretical approach. The second part of this study is about the preparation and characterization of coatings prepared with the nanostructured as well as the as received Cr3C2-25(Ni20Cr) powders. The high temperature erosion-oxidation (E-O) behavior of the coatings prepared with the two types of powders has been compared based on a technological approach. The average crystallite size of the Cr3C2-25(Ni20Cr) powder decreased rapidly from 145 nm to 50 nm in the initial stages of milling and thereafter decreased slowly to a steady state value of around 10 nm with further increase in milling time. This steady state corresponds to the beginning of a dynamic recovery process. The maximum lattice strain (δ = 1,17%) was observed in powders milled for 16 hours, and this powders critical crystallite size was 28 nm. In contrast, the lattice parameter attained a minimum for powders milled for 16 hours. Upon reaching the critical crystallite size, the dislocation density attained a steady state regime and all plastic deformation introduced in the material there after was in the form of events occurring at the grain boundaries, due mainly to grain boundary sliding. The deformation energy stored in the crystal lattice of the Cr3C2-25(Ni20Cr) powders milled for different times was determined from enthalpy variation measurements. These results indicated that the maximum enthalpy variation (ΔH = 722 mcal) also occurred for powders milled for 16 hours. In a similar manner, the maximum specific heat variation (ΔCp = 0,278 cal/gK) occurred for powders milled for 16 hours. The following mechanical properties of Cr3C2-25(Ni20Cr) coatings prepared using the HVOF thermal spray process were determined: Vickers micro-hardness, the Young Modulus and the fracture toughness. The properties of the coatings prepared with the nanostructured and the as received powders were compared. The hardness and Young Modulus of the coatings prepared with nanostructured powders were approximately 26% higher than that of the coatings prepared with as received powders. The fracture toughness of the nanostructured coating was 36% higher. The erosion-oxidation resistance of the coating produced with the nanostructured powder was around 52% higher than that of the coating prepared with the as received powders at 800 ºC. The E-O wastage of both types of coatings increased with temperature beyond 450 ºC.

In this thesis, the effect of mechanical deformation on structure, thermal stability and hardness of a single-phase spray-deposited quasicrystalline alloy with composition Al62.5Cu25Fe12.5 has been investigated in detail. The purpose of the investigation was to study the effect of mechanical milling at different milling speeds (which approximately scale with the milling intensity) on mechanically-induced phase transformations during milling and on the phase evolution during subsequent heating. The results of the milling experiments indicate that, irrespective of the milling speeds used, mechanical milling of Al62.5Cu25Fe12.5 quasicrystals leads to the formation of a disordered CsCl-type ß phase with grain size of about 10 – 20 nm. The analysis of the kinetics of the QC–to–ß phase transformation reveals that the milling intensity has a considerable effect on the characteristics of the transformation. The increase of the milling speed considerably shortens the incubation time needed to start the QC–to–ß phase transformation. Also, the overall transformation is much faster for milling at high speeds. The QC–to–ß phase transformation starts when the grain size of the quasicrystals is reduced to about 10 nm irrespective of the milling speed used and clearly indicates that a critical grain size of the quasicrystals for initiating the transformation exists. On the other hand, no critical value of lattice strain was found for the QC–to–ß transformation. This indicates that the phase transformation is controlled by the local length scale (i.e. the grain size) and by the corresponding grain boundaries rather than by the energy stored in the lattice. Energetic considerations obtained through a simple model based on the mass and velocity of the milling balls reveal that the energy needed for the QC–to–ß transformation increases with increasing the milling speed, that is, the energetic efficiency of the process decreases with increasing the milling intensity. This indicates that part the extra energy supplied during milling at high intensities is not used to induce the phase transformation but it is dissipated by heat. During heating, the milled powder displays a multi-step thermal behavior characterized by the grain growth of the disordered ß phase at low temperatures, followed, at higher temperatures, by its transformation into the original icosahedral quasicrystalline phase. The transformation is gradual and the quasicrystals and the disordered ß phase coexist over a temperature interval of more than 250 K. The phase transformations occurring during milling and subsequent annealing have a remarkable effect on the hardness, which can be tuned within a wide range of values (7–9.6 GPa) as a function of the volume fraction of the different phases. This suggests that a composite material with optimized mechanical properties can be produced by an appropriate thermo-mechanical treatment. The quasicrystals milled at a very low speed show a transition between Hall-Petch to inverse Hall-Petch behavior at a grain size of about 40 nm, which represents the critical value for grain size softening of the present Al62.5Cu25Fe12.5 quasicrystals. This behavior may be attributed to the complexity of the quasicrystalline structure and to its peculiar deformation mechanism at room temperature (i.e. shear banding), where meta-dislocation-assisted deformation is almost absent. In order to analyze the effectiveness of the Al62.5Cu25Fe12.5 quasicrystals as reinforcing agent in metal matrix composites, Al-based composites were synthesized by hot extrusion of elemental Al blended with different amounts of Al62.5Cu25Fe12.5 quasicrystalline particles. The work was focused on two specific aspects: evaluation of the mechanical properties through room temperature compression tests and modeling of the resulting properties. The addition of the quasicrystalline reinforcement is very effective for improving the room temperature mechanical properties of pure Al. The compressive strength increases from 155 MPa for pure Al to 330 and 407 MPa for the composites with 20 and 40 vol.% of reinforcement, respectively, reaching an ultimate strain of 55 % and 20 % before fracture occurs. These results indicate that the addition of the QC reinforcement leads to composite materials with compressive strengths exceeding that of pure Al by a factor of 2 – 2.5, while retaining appreciable plastic deformation. The mechanical properties of the composites have been modeled by taking into account the combined effect of load bearing, dislocation strengthening and matrix ligament size effects. The calculations are in very good agreement with the experimental results and reveal that the reduction of the matrix ligament size, which results in a similar strengthening effect as that observed for grain refinement, is the main strengthening mechanism in the current composites. Finally, the interfacial reaction between the Al matrix and the QC reinforcement has been used to further enhance the strength of the composites through the formation of a new microstructure consisting of the Al matrix reinforced with Al7Cu2Fe w-phase particles. The optimization of the structure-property relationship was done through the systematic variation of the processing temperature during consolidation. The mechanical behavior of these transformation-strengthened composites is remarkably improved compared to the parent material. The yield strength of the composites significantly increases as the Al + QC -> ω transformation progresses from 195 MPa for the sample reinforced only with QC particles to 400 MPa for the material where the Al + QC -> ω reaction is complete. These results clearly demonstrate that powder metallurgy, i.e. powder synthesis by ball milling followed by consolidation into bulk specimens, is an attractive processing route for the production of novel and innovative lightweight composites characterized by high strength combined with considerable plastic deformation. In addition, these findings indicate that the mechanical behavior of Al-based composites reinforced with Al62.5Cu25Fe12.5 quasicrystalline particles can be tuned within a wide range of strength and plasticity depending on the volume fraction of the reinforcement as well as on the extent of the interfacial reaction between Al matrix and QC reinforcing particles.

O presente estudo está dividido em duas partes. A primeira parte está relacionada à preparação de pós de Cr3C2-25(Ni20Cr) nanoestruturados através do processo de moagem de alta energia, bem como à caracterização dos pós moídos e no estado como recebido. A análise dos dados obtidos nesta etapa do trabalho foi feita utilizando-se uma abordagem essencialmente teórica. A segunda parte deste estudo refere-se à produção e caracterização de revestimentos preparados com os pós de Cr3C2-25(Ni20Cr) nanoestruturados e como recebido. O comportamento destes revestimentos sob erosão-oxidação em alta temperatura foi comparado com base em uma abordagem de caráter mais tecnológico. O tamanho médio de cristalito do pó de Cr3C2-25(Ni20Cr) decresceu rapidamente de 145 nm para 50 nm nos estágios iniciais de moagem e, posteriormente, com o aumento do tempo de moagem, decresceu mais lentamente até atingir um estado estacionário para um tamanho de cristalito em torno de 10 nm. Este estado estacionário corresponde ao início do processo de recuperação dinâmica. A máxima deformação da rede cristalina (δ = 1,17%) foi observada para pós moídos por 16 horas, caracterizando um tamanho crítico de cristalito da ordem de 28 nm. Por outro lado, o parâmetro de rede atingiu um mínimo para pós moídos por 16 horas. Após atingir o tamanho crítico de cristalito, a densidade de discordâncias praticamente não mais varia (estado estacionário) e toda deformação plástica posteriormente introduzida no material é acomodada através de eventos que ocorrem nos contornos de grão, particularmente por meio do processo designado deslizamento de contorno de grão (grain boundary sliding). A energia de deformação armazenada na rede cristalina dos pós de Cr3C2-25(Ni20Cr) moídos com diferentes tempos de moagem foi determinada por meio de medidas da variação de entalpia. Estes resultados indicaram que a máxima variação de entalpia (ΔH = 722 mcal) também ocorreu para pós moídos por 16 horas. Analogamente, a máxima variação do calor específico (ΔCp = 0,278 cal/gK) ocorreu para pós moídos por 16 horas. As seguintes propriedades mecânicas dos revestimentos de Cr3C2-25(Ni20Cr), preparados utilizando-se o processo HVOF de aspersão térmica, foram determinadas: microdureza Vickers, módulo de Young e tenacidade à fratura. As propriedades dos revestimentos preparados com os pós nanoestruturados e como recebido foram comparadas. A dureza e o módulo de Young dos revestimentos preparados com os pós nanoestruturados foram aproximadamente 26% maiores que aqueles preparados com os pós como recebido. A tenacidade à fratura dos revestimentos nanoestruturados foi aproximadamente 36% maior do que o verificado para os revestimentos produzidos com pós no estado como recebido. A resistência à erosão-oxidação do revestimento produzido com o pó nanoestruturado foi em torno de 52% maior do que a do revestimento preparado com o pó no estado como recebido, a 800ºC. Ambos os revestimentos mostraram um aumento da taxa de erosão-oxidação para temperaturas acima de 450ºC. / This study is divided in two parts. The first part is about the preparation of nanostructured Cr3C2-25(Ni20Cr) powders by high energy milling followed by characterization of the milled and the as received powder. Analyses of some of the data obtained were done using a theoretical approach. The second part of this study is about the preparation and characterization of coatings prepared with the nanostructured as well as the as received Cr3C2-25(Ni20Cr) powders. The high temperature erosion-oxidation (E-O) behavior of the coatings prepared with the two types of powders has been compared based on a technological approach. The average crystallite size of the Cr3C2-25(Ni20Cr) powder decreased rapidly from 145 nm to 50 nm in the initial stages of milling and thereafter decreased slowly to a steady state value of around 10 nm with further increase in milling time. This steady state corresponds to the beginning of a dynamic recovery process. The maximum lattice strain (δ = 1,17%) was observed in powders milled for 16 hours, and this powders critical crystallite size was 28 nm. In contrast, the lattice parameter attained a minimum for powders milled for 16 hours. Upon reaching the critical crystallite size, the dislocation density attained a steady state regime and all plastic deformation introduced in the material there after was in the form of events occurring at the grain boundaries, due mainly to grain boundary sliding. The deformation energy stored in the crystal lattice of the Cr3C2-25(Ni20Cr) powders milled for different times was determined from enthalpy variation measurements. These results indicated that the maximum enthalpy variation (ΔH = 722 mcal) also occurred for powders milled for 16 hours. In a similar manner, the maximum specific heat variation (ΔCp = 0,278 cal/gK) occurred for powders milled for 16 hours. The following mechanical properties of Cr3C2-25(Ni20Cr) coatings prepared using the HVOF thermal spray process were determined: Vickers micro-hardness, the Young Modulus and the fracture toughness. The properties of the coatings prepared with the nanostructured and the as received powders were compared. The hardness and Young Modulus of the coatings prepared with nanostructured powders were approximately 26% higher than that of the coatings prepared with as received powders. The fracture toughness of the nanostructured coating was 36% higher. The erosion-oxidation resistance of the coating produced with the nanostructured powder was around 52% higher than that of the coating prepared with the as received powders at 800 ºC. The E-O wastage of both types of coatings increased with temperature beyond 450 ºC.