Nous présentons une étude expérimentale de la dynamique d'une sphère près d'une paroi texturée dans un fluide visqueux. Le déplacement de la sphère est mesuré par interférométrie laser à haute fréquence. La texture consiste en un réseau de micro-piliers, dont on fait varier les paramètres géométriques (hauteur et concentration) sur une large gamme, afin d'étudier leur influence sur la dynamique d'approche, de collision et de rebond de la sphère. A petit nombre de Reynolds, la vitesse d'approce de la sphère est augmentée comparativement au cas d'une paroi lisse, en raison de la modification de la force de lubrification par la texture. Cette modification est décrite par plusieurs modèles hydrodynamiques. Une longueur effective de glissement au-dessus du réseau de piliers est mesurée. A nombre de Stokes modéré, l'inertie de la sphère n'est pas négligeable, et elle peut se coller ou rebondir sur les piliers. La dynamique de collage présente différents régimes qui sont modélisés en tenant compte de la modification de traînée par la texture. Le nombre de Stokes critique pour la transition de rebond est modifié par la texture. La dynamique des micro-rebonds est caractérisée. Enfin, la dynamique de contact sphère-piliers est décrite par une loi de Hertz modifiée prenant en compte la géométrie de la texture. / We investigate experimentally the dynamics of a sphere near a textured wall in a viscous fluid. High frequency laser interferometry is used to measure small displacements of the sphere. The texture is comprised of arrays of micro-pillars, whose geometrical parameters are systematically varied in order to study their influence of the dynamics of approach, collision and rebound of the sphere. Under creeping flow conditions, the sphere velocity near the textured wall is increased compared to the case of a smooth wall, as a result of the modification of the lubrication force due to the texture. We describe this modification using several hydrodynamic models. An effective slip length over the pillar array is measured. At moderate Stokes number the sphere inertia is non-negligible and the sphere can either stick or bounce off the wall. The sticking dynamics exhibit several regimes which can be described by taking into account the modification of the lubrication force by the texture. The micro-pillars modify the critical Stokes number for the bouncing transition. The micro-rebounds are characterized. Finally, the contact dynamics between the sphere and the pillars are described by a Hertz law modified by the texture geometry.

In past studies, several techniques have been employed to create microscopic features on relatively simple surfaces. Of these, lithography-based techniques have proven effective at manufacturing large fields of deterministic microasperities and microcavities on planar and cylindrical substrates. The present study focuses on adapting UV-lithography to a more complex substrate. Machined from stainless steel, a conical mold insert introduces an interesting geometry designed for the injection molding of radial lip seal elastomer. The distinct shape of this mold insert poises unique challenges to a conventional lithography procedure. Spray application is investigated as a feasible means to deposit layers of photoresist on the surface. An appropriate masking element is designed and created to facilitate transfer of a particular pattern via UV exposure. A clamping technique is implemented to align and secure the photomask. These techniques are incorporated into a three-day process, and results are obtained through optical microscopy and light interferometry. By applying Design of Experiments (DOE) and Analysis of Variance (ANOVA), significant process variables are indentified. Based on these findings, refinements to the process are enabled and future considerations are made evident.

Tato disertační práce se zabývá vlivem mělkých mikro-textur na tření a tloušťku filmu v mazaných nekonformních kontaktech za extrémních podmínek a za podmínek hladovění kontaktu. Měření byla realizována na tribometru v konfiguraci ball-on-disk. Kontakt byl pozorován pomocí vysokorychlostní kamery. Pro stanovení součinitele tření byl využit snímač krouticího momentu. V této studii byly popsány dva typy mikrotextur – mikrovtisky a příčné mikrodrážky. Výsledky naznačují, že za podmínek hladovění vedou mikrovtisky ke snížení tření a to díky nárůstu tloušťky mazacího filmu. Mechanismus doplňování mikrovtisků čerstvým mazivem je pravděpodobně způsoben kapilárními jevy ve vstupní oblasti. Třecí plochy s příčnými mikrodrážkami, jejichž délka byla menší než průměr Hertzova kontaktu, potom obecně vykazovaly lepší tribologické parametry ve srovnání s hladkými povrchy. Příčné mikrodrážky vedly k výraznému nárůstu tloušťky mazacího filmu za podmínek hladovění i za extrémních provozních podmínek (protisměrný pohyb). Numerické simulace přechodových jevů příčných mikrodrážek ukázaly dobrou shodu s experimentálními výsledky.

A geração de novas fontes de energia limpa, segura e renovável por meio da fusão nuclear envolve importantes desafios tecnológicos, dentre eles a pesquisa, caracterização e a fabricação de materiais avançados para os futuros reatores de fusão nuclear. Os aços ferrítico-martensíticos de reduzida atividade radioativa, em especial a liga Eurofer-97, destacam-se por apresentar uma combinação única de propriedades para esta aplicação. O objetivo desta Tese de Doutorado é avaliar a estabilidade microestrutural deste aço recozido numa ampla faixa de temperaturas. Cálculos termodinâmicos e testes de dilatometria foram usados para determinar as temperaturas de transformação de fase. A estabilidade microestrutural foi estudada por meio de recozimentos isotérmicos entre 200 e 1350oC após laminação a frio com reduções de 40, 70, 80 e 90%. A avaliação da estabilidade mecânica do aço Eurofer-97 foi realizada por meio de medidas de dureza Vickers. As principais técnicas utilizadas para caracterização microestrutural foram microscopias eletrônica de varredura e de transmissão, tomografia por sonda atômica e medidas de magnetização DC. Tanto a textura como a microtextura foram determinadas por meio de medidas de difração de raios X e de elétrons retroespalhados (EBSD). Recuperação, recristalização primária e crescimento anormal de grão ocorrem neste material recozido abaixo de 800oC. Acima desta temperatura, a transformação martensítica ocorre alterando bastante a micoroestrutura. A cinética de crescimento anormal de grão é alterada pela quantidade de redução a frio previamente aplicada. A hipótese proposta para explicar o crescimento anormal de grãos neste material baseia-se principalmente na vantagem de tamanho adquirida pelos núcleos de recristalização primária com diferenças de orientação médias superiores a 45º em relação aos vizinhos. Neste caso, o crescimento anormal de grão é responsável por fortalecer as componentes {111} e {111}, {001} e {110}. Acima de 800oC a transformação martensítica prevalece elevando a dureza Vickers e randomizando a textura deste aço. As características do produto transformado dependem tanto da temperatura de austenitização quanto do tamanho incial do grão ferrítico. Os dados de composição química das partículas estáveis após recozimento em temperaturas inferiores a 800oC foram usados para validar os resultados dos cálculos termodinâmicos obtidos via Thermo-Calc. / Clean, safe, and renewable energy sources such as nuclear fusion comprise important technological challenges, including research, characterization and manufacture of advanced materials for future fusion reactors. Modified ferritic-martensitic steels with reduced radioactive activity (RAFM), especially Eurofer-97 steel, are among worldwide references in the nuclear field for their unique properties. The scope of this Thesis is to evaluate the microstructural (thermal) stability in ferritic-martensitic Eurofer-97 after annealing within a wide range of temperatures. Themodinamic calculations as well as dilatometric tests were used to determine the main phase transformation temperatures. The microstructural stability of this steel was followed by isothermal annealing between 200 and 1350°C after cold rolling to 40, 70, 80 and 90% reductions in thickness. The mechanical stability in the Eurofer-97 was assessed by Vickers microhardness measurements. Representative samples for each metallurgical condition were characterized by scanning electron microscopy, transmission electron microscopy, atom probe tomography, and DC-magnetization tests. Both texture and microtexture were evaluated by X-ray diffraction and electron backscattered diffraction (EBSD) techniques. Recovery, primary recrystallization, and abnormal grain growth (secondary recrystallization) processes have been observed at temperatures below 800°C. The amount of abnormally grown grains depends on the amount of previous cold rolling. The hypothesis for the most probable mechanism responsible for abnormal grain growth is based on the advantage size acquired by nuclei with misorientations above 45º surrounding their neighboring grains, even in regions where primary recrystallization was incomplete. The texture developed after abnormal grain growth has components belonging to ?- and ?-fibers with predominance of {111}, {111}, {100} e {110} components. The martensite transformation takes place when this steel is annealed above 800°C causing an increase of hardness, significant changes in microstructure, and texture weakening. The martensitic sructure depends very much on both austenitization temperature and initial austenitic grain size. The results of chemical analyses of stable particles present in samples annealed below 800oC were used to validate the thermodynamic calculations provided by Thermo-Calc.

A geração de novas fontes de energia limpa, segura e renovável por meio da fusão nuclear envolve importantes desafios tecnológicos, dentre eles a pesquisa, caracterização e a fabricação de materiais avançados para os futuros reatores de fusão nuclear. Os aços ferrítico-martensíticos de reduzida atividade radioativa, em especial a liga Eurofer-97, destacam-se por apresentar uma combinação única de propriedades para esta aplicação. O objetivo desta Tese de Doutorado é avaliar a estabilidade microestrutural deste aço recozido numa ampla faixa de temperaturas. Cálculos termodinâmicos e testes de dilatometria foram usados para determinar as temperaturas de transformação de fase. A estabilidade microestrutural foi estudada por meio de recozimentos isotérmicos entre 200 e 1350oC após laminação a frio com reduções de 40, 70, 80 e 90%. A avaliação da estabilidade mecânica do aço Eurofer-97 foi realizada por meio de medidas de dureza Vickers. As principais técnicas utilizadas para caracterização microestrutural foram microscopias eletrônica de varredura e de transmissão, tomografia por sonda atômica e medidas de magnetização DC. Tanto a textura como a microtextura foram determinadas por meio de medidas de difração de raios X e de elétrons retroespalhados (EBSD). Recuperação, recristalização primária e crescimento anormal de grão ocorrem neste material recozido abaixo de 800oC. Acima desta temperatura, a transformação martensítica ocorre alterando bastante a micoroestrutura. A cinética de crescimento anormal de grão é alterada pela quantidade de redução a frio previamente aplicada. A hipótese proposta para explicar o crescimento anormal de grãos neste material baseia-se principalmente na vantagem de tamanho adquirida pelos núcleos de recristalização primária com diferenças de orientação médias superiores a 45º em relação aos vizinhos. Neste caso, o crescimento anormal de grão é responsável por fortalecer as componentes {111} e {111}, {001} e {110}. Acima de 800oC a transformação martensítica prevalece elevando a dureza Vickers e randomizando a textura deste aço. As características do produto transformado dependem tanto da temperatura de austenitização quanto do tamanho incial do grão ferrítico. Os dados de composição química das partículas estáveis após recozimento em temperaturas inferiores a 800oC foram usados para validar os resultados dos cálculos termodinâmicos obtidos via Thermo-Calc. / Clean, safe, and renewable energy sources such as nuclear fusion comprise important technological challenges, including research, characterization and manufacture of advanced materials for future fusion reactors. Modified ferritic-martensitic steels with reduced radioactive activity (RAFM), especially Eurofer-97 steel, are among worldwide references in the nuclear field for their unique properties. The scope of this Thesis is to evaluate the microstructural (thermal) stability in ferritic-martensitic Eurofer-97 after annealing within a wide range of temperatures. Themodinamic calculations as well as dilatometric tests were used to determine the main phase transformation temperatures. The microstructural stability of this steel was followed by isothermal annealing between 200 and 1350°C after cold rolling to 40, 70, 80 and 90% reductions in thickness. The mechanical stability in the Eurofer-97 was assessed by Vickers microhardness measurements. Representative samples for each metallurgical condition were characterized by scanning electron microscopy, transmission electron microscopy, atom probe tomography, and DC-magnetization tests. Both texture and microtexture were evaluated by X-ray diffraction and electron backscattered diffraction (EBSD) techniques. Recovery, primary recrystallization, and abnormal grain growth (secondary recrystallization) processes have been observed at temperatures below 800°C. The amount of abnormally grown grains depends on the amount of previous cold rolling. The hypothesis for the most probable mechanism responsible for abnormal grain growth is based on the advantage size acquired by nuclei with misorientations above 45º surrounding their neighboring grains, even in regions where primary recrystallization was incomplete. The texture developed after abnormal grain growth has components belonging to ?- and ?-fibers with predominance of {111}, {111}, {100} e {110} components. The martensite transformation takes place when this steel is annealed above 800°C causing an increase of hardness, significant changes in microstructure, and texture weakening. The martensitic sructure depends very much on both austenitization temperature and initial austenitic grain size. The results of chemical analyses of stable particles present in samples annealed below 800oC were used to validate the thermodynamic calculations provided by Thermo-Calc.