While the free electron model can often be surprisingly successful in describing properties of solids, there are plenty of materials in which interactions between electrons are too significant to be neglected. These strongly correlated systems sometimes exhibit rather unexpected, unusual and useful phenomena, understanding of which is one of the aims of condensed matter physics. Heat capacity measurements of paramagnetic YFe$_{2}$Ge$_{2}$ give a Sommerfeld coefficient of about 100 mJ mol$^{−1}$ K$^{−2}$, which is about an order of magnitude higher than the value predicted by band structure calculations. This suggests the existence of strong electronic correlations in the compound, potentially due to proximity to an antiferromagnetic quantum critical point (QCP). Existence of the latter is also indicated by the non-Fermi liquid T$^{3/2}$ behaviour of the low temperature resistivity. Below 1.8 K a superconducting phase develops in the material, making it a rare case of a non-pnictide and non-chalcogenide iron based superconductor with the 1-2-2 structure. This thesis describes growth and study of a new generation of high quality YFe$_{2}$Ge$_{2}$ samples with residual resistance ratios reaching 200. Measurements of resistivity, heat capacity and magnetic susceptibility confirm the intrinsic and bulk character of the superconductivity, which is also argued to be of an unconventional nature. In order to test the hypothesis of the nearby QCP, resistance measurements under high pressure of up to 35 kbar have been conducted. Pressure dependence of the critical temperature of the superconductivity has been found to be rather weak. μSR measurements have been performed, but provided limited information due to sample inhomogeneity resulting in a broad distribution of the critical temperature. While the superconductivity is the result of an effective attraction between electrons, under different circumstances the electronic properties of a system can instead be dictated by the Coulomb repulsion. This is the case for another transition metal based compound NiS$_{2}$, which is a Mott insulator. Applying hydrostatic pressure of about 30 kbar brings the material across the Mott metal-insulator transition (MIT) into the metallic phase. We have used the tunnel diode oscillator (TDO) technique to measure quantum oscillations in the metallised state of NiS$_{2}$, making it possible to track the evolution of the principal Fermi surface and the associated effective mass as a function of pressure. New results are presented which access a wider pressure range than previous studies and provide strong evidence that the effective carrier mass diverges close to the Mott MIT, as expected within the Brinkman-Rice scenario and predicted in dynamical mean field theory calculations. Quantum oscillations have been measured at pressures as close to the insulating phase as 33 kbar and as high as 97 kbar. In addition to providing a valuable insight into the mechanism of the Mott MIT, this study has also demonstrated the potential of the TDO technique for studying materials at high pressures.

Les écoulements rampants, recirculants et instationnaires d’un fluide viscoplastique autour d’un cylindre ont été étudiés.Numériquement, les morphologies des écoulements, la localisation des zones rigides, les champs de contraintes et pression autour du cylindre ainsi que le coefficient de traînée, ont été déterminés sur un large domaine des nombres de Reynolds et d’Oldroyd.Expérimentalement, les fluides étudiés sont des gels de polymère Carbopol®. Le comportement élastoviscoplastique de ces gels a été modélisé par une loi d’Herschel-Bulkley adaptée. Le montage expérimental conçu et réalisé a été validé par l'étude de l'écoulement d'un fluide newtonien autour d'un cylindre et la mise en place d’une procédure adaptée pour les fluides à seuil.On a pu constater l'influence des conditions d’interface avec l’apparition d’une morphologie de lâchers de tourbillons simultanés et symétriques. / Creeping, recirculating and unsteady flows around a cylinder for yield stress fluid were studied.Numerically, morphologies of the flows, the location of the unyielded zones, the pressure and stress fields around the cylinder and the drag coefficient were determined over a wide range of Reynolds and Oldroyd numbers.Experimentally, fluids studied are polymer gels Carbopol®. The elastoviscoplastic behavior of these gels was modeled by a Herschel-Bulkley adapted law. The experimental setup was designed and validated by studying the flow of a newtonian fluid around a cylinder. An appropriate procedure for the viscoplastic fluid was implemented.We observe the influence of interface conditions with the appearance of a morphology showing simultaneous and symmetrical vortex shedding.

Made available in DSpace on 2014-12-17T15:01:32Z (GMT). No. of bitstreams: 1 KarenPBA_DISSERT.pdf: 3887883 bytes, checksum: 3ebc4a634530a0a4a6493a962284d14f (MD5) Previous issue date: 2013-10-18 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The bio-oil obtained from the pyrolysis of biomass has appeared as inter-esting alternative to replace fossil fuels. The aim of this work is to evaluate the influence of temperature on the yield of products originating from the pyrolysis process of the powder obtained from the dried twigs of avel?s (Euphorbia tirucalli), using a rotating cylinder reactor in laboratory scale. The biomass was treated and characterized by: CHNS, moisture, volatiles, fixed carbon and ashes, as well as evaluation of lignin, cellulose and hemicellulose, besides other instrumental techniques such as: FTIR, TG/DTG, DRX, FRX and MEV. The activation energy was evaluated in non-isothemichal mode with heating rates of 5 and 10 oC/min. The obtained results showed biomass as feedstock with potential for biofuel production, because presents a high organic matter content (78,3%) and fixed-carbon (7,11%). The activation energy required for the degradation of biomass ranged between 232,92 392,84 kJ/mol, in the temperature range studied and heating rate of 5 and 10?C/min. In the pyrolysis process, the influence of the reaction temperature was studied (350-520 ? C), keeping constant the other variables, such as, the flow rate of carrier gas, the centrifugal speed for the bio-oil condensationa, the biomass flow and the rotation of the reactor. The maximum yield of bio-oil was obtained in the temperature of 450?C. In this temperature, the results achieved where: content of bio-oil 8,12%; char 32,7%; non-condensed gas 35,4%; losts 23,8%; gross calorific value 3,43MJ/kg; pH 4,93 and viscosity 1,5cP. The chromatographic analysis of the bio-oil produced under these conditions shows mainly the presence of phenol (17,71%), methylciclopentenone (10,56%) and dimethylciclopentenone (7,76%) / O bio-?leo obtido da pir?lise da biomassa tem aparecido como alternativa interessante para substitui??o dos combust?veis f?sseis. O objetivo deste trabalho ? avaliar a influ?ncia da temperatura no rendimento dos produtos originados do processo de pir?lise do p? obtido dos galhos secos do avel?s (Euphorbia tirucalli), utilizando um reator de cilindro rotativo em escala de laborat?rio. A biomassa foi tratada e caracterizada por: CHNS, umidade, materiais vol?teis, cinzas e carbono fixo, bem como, avalia??o dos teores de lignina, celulose e hemicelulose, al?m de outras t?cnicas instrumentais, tais como: FTIR, TG/DTG, DRX, FRX e MEV. A energia de ativa??o foi avaliada no regime n?o isot?rmico com taxas de aquecimento de 5 e 10 oC/min. Os resultados obtidos mostraram a biomassa como mat?ria prima com potencial para produ??o de biocombust?veis, pois apresenta alto teor de mat?ria org?nica (78,3%) e carbono fixo (7,11%). A energia de ativa??o exigida para degrada??o da biomassa variou entre 232,92 392,84 kJ/mol, no intervalo de temperatura da rea??o estudado e taxa de aquecimento de 5 e 10oC/min. No processo de pir?lise, estudou-se a influ?ncia da temperatura da rea??o (350-520 ?C), mantendo-se constantes as demais vari?veis, ou seja, a vaz?o do g?s de arraste , a velocidade de centrifuga??o para condensa??o do bio-?leo, a vaz?o de biomassa e a rota??o do reator. O rendimento m?ximo em bio-?leo foi obtido na temperatura de 450?C. Nessa temperatura, os resultados alcan?ados foram: teor de bio?leo de 8,12%; carv?o 32,7%; fase gasosa n?o condensada 35,4%; perdas 23,8%; poder calor?fico superior 3,43MJ/kg; pH 4,93; viscosidade 1,5cP. A an?lise cromatogr?fica do bio-?leo produzido nessas condi??es mostra a presen?a, principalmente, de fenol (17,71%), metilciclopentenona (10,56%) e dimetilciclopentenona (7,76%)

Made available in DSpace on 2014-12-17T15:01:35Z (GMT). No. of bitstreams: 1 KarenPBA_DISSERT.pdf: 3887883 bytes, checksum: 3ebc4a634530a0a4a6493a962284d14f (MD5) Previous issue date: 2013-10-18 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The bio-oil obtained from the pyrolysis of biomass has appeared as inter-esting alternative to replace fossil fuels. The aim of this work is to evaluate the influence of temperature on the yield of products originating from the pyrolysis process of the powder obtained from the dried twigs of avel?s (Euphorbia tirucalli), using a rotating cylinder reactor in laboratory scale. The biomass was treated and characterized by: CHNS, moisture, volatiles, fixed carbon and ashes, as well as evaluation of lignin, cellulose and hemicellulose, besides other instrumental techniques such as: FTIR, TG/DTG, DRX, FRX and MEV. The activation energy was evaluated in non-isothemichal mode with heating rates of 5 and 10 oC/min. The obtained results showed biomass as feedstock with potential for biofuel production, because presents a high organic matter content (78,3%) and fixed-carbon (7,11%). The activation energy required for the degradation of biomass ranged between 232,92 392,84 kJ/mol, in the temperature range studied and heating rate of 5 and 10?C/min. In the pyrolysis process, the influence of the reaction temperature was studied (350-520 ? C), keeping constant the other variables, such as, the flow rate of carrier gas, the centrifugal speed for the bio-oil condensationa, the biomass flow and the rotation of the reactor. The maximum yield of bio-oil was obtained in the temperature of 450?C. In this temperature, the results achieved where: content of bio-oil 8,12%; char 32,7%; non-condensed gas 35,4%; losts 23,8%; gross calorific value 3,43MJ/kg; pH 4,93 and viscosity 1,5cP. The chromatographic analysis of the bio-oil produced under these conditions shows mainly the presence of phenol (17,71%), methylciclopentenone (10,56%) and dimethylciclopentenone (7,76%) / O bio-?leo obtido da pir?lise da biomassa tem aparecido como alternativa interessante para substitui??o dos combust?veis f?sseis. O objetivo deste trabalho ? avaliar a influ?ncia da temperatura no rendimento dos produtos originados do processo de pir?lise do p? obtido dos galhos secos do avel?s (Euphorbia tirucalli), utilizando um reator de cilindro rotativo em escala de laborat?rio. A biomassa foi tratada e caracterizada por: CHNS, umidade, materiais vol?teis, cinzas e carbono fixo, bem como, avalia??o dos teores de lignina, celulose e hemicelulose, al?m de outras t?cnicas instrumentais, tais como: FTIR, TG/DTG, DRX, FRX e MEV. A energia de ativa??o foi avaliada no regime n?o isot?rmico com taxas de aquecimento de 5 e 10 oC/min. Os resultados obtidos mostraram a biomassa como mat?ria prima com potencial para produ??o de biocombust?veis, pois apresenta alto teor de mat?ria org?nica (78,3%) e carbono fixo (7,11%). A energia de ativa??o exigida para degrada??o da biomassa variou entre 232,92 392,84 kJ/mol, no intervalo de temperatura da rea??o estudado e taxa de aquecimento de 5 e 10oC/min. No processo de pir?lise, estudou-se a influ?ncia da temperatura da rea??o (350-520 ?C), mantendo-se constantes as demais vari?veis, ou seja, a vaz?o do g?s de arraste , a velocidade de centrifuga??o para condensa??o do bio-?leo, a vaz?o de biomassa e a rota??o do reator. O rendimento m?ximo em bio-?leo foi obtido na temperatura de 450?C. Nessa temperatura, os resultados alcan?ados foram: teor de bio?leo de 8,12%; carv?o 32,7%; fase gasosa n?o condensada 35,4%; perdas 23,8%; poder calor?fico superior 3,43MJ/kg; pH 4,93; viscosidade 1,5cP. A an?lise cromatogr?fica do bio-?leo produzido nessas condi??es mostra a presen?a, principalmente, de fenol (17,71%), metilciclopentenona (10,56%) e dimetilciclopentenona (7,76%)

A crescente demanda de diminuição das emissões e redução do consumo dos motores de combustão interna exige a melhoria dos métodos para diagnose, em tempo real, e para melhor controle do processo de combustão. Portanto, é desejável determinar a relação ar-combustível sobre uma extensa faixa de condições de operação para obter um melhor controle do motor. Este trabalho apresenta a aplicação do Método dos Momentos para obtenção de um modelo de estimação da relação ar-combustível, através do sinal de pressão no cilindro, em um motor ciclo Otto a etanol. O modelo obtido permitirá o desenvolvimento de novos sistemas de controle utilizando como estratégia a pressão no cilindro. / The increasing demands for low emission and low fuel consumption in internal combustion engines require improved methods for diagnosis, in real-time and best possible control of the combustion process. Therefore, determining air-fuel ratio over a wide range of engine operating conditions is desirable for better engine control. This work presents the Moment Method application for obtaining air-fuel ratio estimation model, by cylinder pressure sign, in Otto cycle engine powered by ethanol. The obtained model will allow the development of new control systems, for engine powered alcohol, using as strategy the cylinder pressure.

Em experimentos com um cilindro flexível com rigidez a flexão ortotrópica, proporcionando razão entre frequências naturais longitudinal e transversal muito próximas a quatro (fx1fy1/=4,08), Fujarra et al (2001) [17] revelaram um ramo de resposta em VIV em alta velocidade. Tal ramo de respostas estável, excitado por perturbações a montante, era caracterizado por amplitudes transversais ao escoamento com ordem de 1 diâmetro e componentes de alta frequência no espectro de energia, estendendo-se por faixa de velocidade reduzida 12 < U* < 20 e esvaindo-se a partir da emergência de oscilações no segundo modo de vibração. Almejando aprofundar as investigações acerca deste intrigante achado experimental, uma nova campanha investigativa foi planejada e executada. Quatro cilindros flexíveis com distintas razões de rigidez ortotrópica foram construídos em resina polimérica, com enrijecedores internos de seções retangulares feitos de alumínio, resultando um conjunto com frequências naturais, correspondentes a modos longitudinais e transversais, com razões relativas: 4:1, 3:1, 2:1 e 1:1. Os cilindros foram verticalmente fixados a uma célula de carga 3D. Uma série de ensaios foi conduzida, sob velocidade de escoamento crescente e decrescente. As vibrações dos cilindros flexíveis foram capturadas através de um sistema óptico subaquático, posicionado a jusante, que rastreava a posição de alvos constituídos por fitas refletivas afixadas ao longo do comprimento do cilindro. Clássicas técnicas de análise espectral, decomposição modal no domínio do tempo e a Transformada de Hilbert-Huang foram utilizadas de modo encadeado e recursivo com o fim de determinar as respostas de vibração, em amplitude e frequências dominantes, em função da velocidade reduzida. O ramo de resposta em altas velocidades reduzidas foi recuperado nos experimentos com os quatro cilindros. Análise comparativa sistemática permitiu concluir que o ramo de resposta em tela se deve ao lock-in de modos longitudinais, que são excitados em velocidades reduzidas tão mais altas quanto maior for a razão de frequências considerada. Empregando-se, então, os deslocamentos e velocidades modais medidos em formulação de Mineto (2013) [23], foi realizada estimativa de potência elétrica a ser extraída pelo emprego de finos filmes de material piezelétrico depositados sobre as lâminas enrijecedoras internas aos cilindros. / A previous work by Fujarra et al. (2001) revealed a \"high speed mode branch\" in cantilevered flexible cylinders subjected to VIV. That work studied a flexible cylinder with orthotropic bending stiffness in a recirculating water channel, by setting the ratio between the first natural frequencies, in the in-line and cross-wise directions to the flow, very close to four (fx1fy1/=4,08). Such a new stable response branch, triggered by upstream hydrodynamic perturbations, was characterized by amplitudes, perpendicular to the flow, of order of one diameter and by high frequencies components in the energy spectrum. The branch extends from reduced velocity circa 12 up to 20, being extinguished by the onset of the second vibration mode lock-in. Aiming at investigating further this intriguing experimental finding, a new campaign has been planned and performed. Four flexible cylinders with distinct orthotropic bending ratios were built, by molding thin aluminum stiffeners, of rectangular cross section, inside the cylinders made of a flexible resin. The respective natural frequencies ratio, between in-line and cross-wise bending modes, was set to 4:1, 3:1, 2:1 and 1:1. The flexible cylinders were vertically clamped to a 3D load cell. A series of experimental runs were carried out, under ascending and descending flow speeds. The flexible cylinder vibration was captured trough an underwater optical system, placed downstream, by tracking 3D Cartesian coordinates of reflective strips placed all along the span, at equi-spaced cross sections. Besides standard spectral analysis techniques, modal decomposition in time domain and the Hilbert-Huang Transform have been recursively used together to determine amplitude and dominant response frequencies. The \"high speed mode branch\" revealed for all four cylinders tested. Systematic comparative analysis allowed us to conclude that such a response branch is caused by the lock-in of longitudinal modes, excited at reduced velocities ranges that increase with the frequency ratio considered. Finally, using measured modal responses in Mineto (2013) [23] formulation, the electric power that would be extracted by the use of thin piezoelectric layers deposited over the internal stiffeners blades was estimated.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work we are treated in detail, three subjects of mathematics that relate to each other: Plane Geometry, Geometry and Spatial Revolution Solid. In this approach, we prioritized the calculation of the area of the lateral surfaces and full of Prism, Pyramid, Cylinder, Cone and Sphere, and the calculation of its volumes in the latter, using the principle of the deduction Cavalieri their formulas. In the study of Revolution Solids, we highlight the theorems of Pappus, used to derive the formulas of surface areas and volumes of cylinder, cone and revolution sphere. / Neste trabalho são tratados, de forma detalhada, três temas da Matemática que se relacionam entre si: Geometria Plana, Geometria Espacial e Sólidos de Revolução. Nessa abordagem, priorizou-se o cálculo da área das superfícies lateral e total do Prisma, da Pirâmide, do Cilindro, do Cone e da Esfera, bem como o cálculo de seus volumes, neste último, utilizando-se o princípio de Cavalieri na dedução de suas fórmulas. No estudo dos Sólidos de Revolução, destacam-se os Teoremas de Pappus, usados para deduzir as fórmulas das áreas das superfícies e dos volumes do Cilindro, do Cone e da Esfera de revolução.

Esta tese apresenta o estudo da transição para um escoamento tridimensional da esteira de um cilindro oscilando. Esta é a transição secundária do escoamento ao redor de um cilindro, sendo a primária a própria esteira de von Kármán. A investigação é realizada na mesma faixa de número de Reynolds (Re) que ocorre a transição da esteira de um cilindro fixo: 200 Re 400. O estudo envolve simulações numéricas diretas bi- e tridimensionais do escoamento incompressível ao redor de um cilindro oscilando usando o método dos elementos espectrais. A transição também é analisada através do estudo de estabilidade linear do escoamento. O método de Gaston Floquet é adequado para a análise de estabilidade a perturbações tridimensionais devido à periodicidade característica da esteira de von Kármán. Além disto, o método é mais geral e não é aplicado somente a equações autônomas com soluções periódicas: ele também pode ser usado em análises cujo campo base é estacionário como a análise da transição primária da esteira de um cilindro. Mostra-se que a transição da esteira para tridimensionalidade é influenciada pela oscilação do cilindro. As oscilações podem atrasar a transição quando em amplitudes baixas e com escoamento a um número de Reynolds até 260. Em outros casos, a transição é similar à observada no escoamento ao redor um cilindro fixo. Além disso, quando há mudança de padrões de desprendimento de vórtices devido às oscilações, desencadeiam-se modos instáveis diferentes dos observados na esteira de um cilindro fixo. A comparação dos resultados da análise de estabilidade de Floquet com simulações numéricas diretas e experimentos publicados mostra na maioria dos casos que, apesar de ser uma análise de estabilidade linear, muitas características da análise persistem além do limiar de estabilidade. As situações com discrepâncias são identificadas nesta tese, como padrão de desprendimento diferentes em simulações bi- e tridimensionais na mesma situação. As causa das discrepâncias são estudadas através das simulações numéricas diretas e considerações sobre o sistema linearizado e a abrangência da análise de estabilidade. Em adição, mostra-se que há um limiar de amplitude de oscilação acima do qual a dinâmica da esteira, e portanto a transição secundária, é afetada. Abaixo deste limiar, retoma-se o comportamento do escoamento ao redor de um cilindro fixo. / This thesis presents the investigation of the transition to a three-dimensional flow in the wake of an oscillating circular cylinder. This is the secondary transition of the flow around a circular cylinder, while the primary transition leads to the von Kármán wake. The investigative work is performed in the Reynolds number (Re) range which occurs the secondary transition of the wake o a circular cylinder: 200 Re 400. The study consists of two- and three-dimensional direct numerical simulations of the incompressible flow around an oscillating circular cylinder by means of the spectral/hp method. The transition is also analyzed through linear stability study of the flow. The Gaston Floquet method is an appropriate method for this linear stability analysis to three-dimensional perturbations due to the typical periodicity of the von Kármán wake. Furthermore, the Floquet method is of a general kind and it is not specifically applied to autonomous equation with periodic solutions: it can be employed in analysis with a stationary base flow like the primary transition of the wake of a circular cylinder. The thesis shows that the transition to a three-dimensional wake is affected by the cylinder oscillation. The oscillations can delay the transition when they have low amplitudes and the flow has Reynolds number below 260. In other situations, the transition is similar to that observed in the flow of a fixed circular cylinder. In addition, when there is a change in the vortex shedding pattern due to the oscillatory motion, the observed unstable modes are different than those in a wake of a fixed circular cylinder. The comparison of the results from the Floquet stabilty analysis with the ones from direct numerical simulations and published experiments shows that in most cases, despite the linearization for the stability analysis, many features of the analysis persist beyond the instability threshold. The discrepant features observed are specified in this thesis, e.g. vortex shedding patterns different in two- and three-dimensional simulations with the same parameters. The cause of the discrepancies are studied through direct numerical simulations and careful considerations on the linearized system and the range of the stability analysis. In addition, this thesis shows that there is a threshold amplitude of oscillation for which the wake dynamics, therefore the secondary transition, is affected. Below this threshold, the wake behaves the same way as in the flow around a fixed circular cylinder.

Level sets are extensively used for the visualization of scalar fields. The Reeb graph of a scalar function tracks the evolution of the topology of its level sets. It is obtained by mapping each connected component of a level set to a point. The Reeb graph and its loop-free version called the contour tree serve as an effective user interface for selecting meaningful level sets and for designing transfer functions for volume rendering. It also finds several other applications in the field of scientific visualization. In this thesis, we focus on designing algorithms for efficiently computing the Reeb graph of scalar functions and using the Reeb graph for effective visualization of scientific data. We have developed three algorithms to compute the Reeb graph of PL functions defined over manifolds and non-manifolds in any dimension. The first algorithm efficiently tracks the connected components of the level set and has the best known theoretical bound on the running time. The second algorithm, utilizes an alternate definition of Reeb graphs using cylinder maps, is simple to implement and efficient in practice. The third algorithm aggressively employs the efficient contour tree algorithm and is efficient both theoretically, in terms of the worst case running time, and practically, in terms of performance on real-world data. This algorithm has the best performance among existing methods and computes the Reeb graph at least an order of magnitude faster than other generic algorithms. We describe a scheme for controlled simplification of the Reeb graph and two different graph layout schemes that help in the effective presentation of Reeb graphs for visual analysis of scalar fields. We also employ the Reeb graph in four different applications – surface segmentation, spatially-aware transfer function design, visualization of interval volumes, and interactive exploration of time-varying data. Finally, we introduce the notion of topological saliency that captures the relative importance of a topological feature with respect to other features in its local neighborhood. We integrate topological saliency with Reeb graph based methods and demonstrate its application to visual analysis of features.

Le moteur à combustion, utilisé dans l’automobile est en perpétuelle évolution pour des raisons économiques et écologiques. Pour parvenir à de faibles consommations de carburant et émissions polluantes, l’un des axes étudiés est la réduction des pertes mécaniques par frottement du moteur qui constituent 15 à 20% de la consommation totale d’énergie du moteur. 50% de ces frottements proviennent des contacts Segment-Piston- Chemise et de la liaison Maneton-Bielle-Coussinet. De ce fait la compréhension de la tribologie de ce contact, l’optimisation de la lubrification et le vieillissement des lubrifiants deviennent primordiaux. L’objectif de cette thèse est d’étudier les mécanismes de lubrification de ces contacts segment/chemise et maneton/coussinet, et en particulier de comprendre l’influence du couplage additif, surface et matériau dans les différentes régimes de lubrification. Il s’agira de répondre à la question : comment maîtriser le frottement et l’usure en contrôlant la chimie du lubrifiant, la topographie et le matériau ? La démarche expérimentale choisie s’appuie sur l’analyse du comportement tribologique de deux additifs modificateurs de frottement sans cendres d’une part, puis sur l’impact du matériau et enfin sur l’influence de la topographie d’autre part. L’analyse de la cinétique d’évolution du coefficient de frottement et des traces d’usure en régime limite ont notamment permis d’identifier les mécanismes de réduction de frottement induits par les deux modificateurs étudiés. / The combustion engine used in automotive industry is constantly changing for economic and ecological reasons. To achieve low fuel consumption and pollutants emissions, one of research axes studied is the reduction of the mechanical friction loss of the motor which constitute 15-20 % of the total energy consumption of the engine. 50% of these come from friction contacts cylinder /piston rings and conrod bearing. Thereby understanding of tribology of the contact, optimizing lubrication and lubricants aging become paramount. The objective of this thesis is to study the mechanisms of lubrication of these contact (piston ring/cylinder and conrod bearing), and in particular to understand the influence of the additive coupling surface and material in the different lubrication regimes. This will answer the question : how to control the friction and wear by controlling the chemistry of the lubricant, the topography and the material ? The experimental approach chosen is based on the analysis of the tribological behavior of two friction modifiers additives ashless at first time, and on the impact of material and finally the influence of topography on the second time. XPS Analysis of the evolution of the coefficient of friction and wear track under boundary regime have enabled to identify mechanisms to reduce friction induced by the two modifiers studied.