Le développement de nouvelles technologies permettant d’obtenir des procédés de fabrication novateurs, de même que l’utilisation de nouveaux matériaux et lubrifiants, ont rendu possible une amélioration globale de l’efficacité des systèmes mécaniques et la résolution de nombreuses défaillances. Néanmoins, certains types de détérioration comme le grippage restent difficiles à appréhender car ils résultent de nombreux paramètres connexes tels que le régime de lubrification, la température du contact et le coefficient de frottement. Le grippage est un phénomène qui apparait de manière brutale et qui se caractérise par des arrachements et des transferts de matières entre les surfaces en contact pouvant aller jusqu’à la destruction complète du système. Ce dernier est généralement associé à une mauvaise dissipation des calories entrainant des températures de contact élevées. De nombreux critères basés sur la dissipation d’énergie dans le contact ont été établis sans pour autant que l’un d’eux ne soit unanimement reconnu par la communauté scientifique. La première partie de cette étude a consisté à caractériser le comportement en frottement d’un couple matériau-lubrifiant grâce à l’utilisation d’une machine bi-disque. En parallèle, une modélisation thermique de ce banc d’essais a été réalisée en utilisant la méthode des réseaux thermiques. Les corrélations entre les formules analytiques et les mesures expérimentales ont permis d’aboutir à l’établissement d’une loi de frottement liant conditions de contact et température de masse des éprouvettes. Il a aussi pu être démontré que la température de masse des disques pouvait être considérablement différente de la température d’injection du lubrifiant en fonction des conditions opératoires. Ce résultat permet une analyse nouvelle des critères de grippage dont la plupart assimilent la température des éprouvettes à celle d’injection du lubrifiant. Malgré cela, les différents essais de grippage réalisés montrent que l’unique considération de la température de contact pour établir un critère de grippage s’avère insuffisante. / The development of new technologies, which allow innovative manufacturing processes, as well as the use of new materials and lubricants have led to an overall improvement of mechanical systems efficiency and reliability. However some failures, like scuffing, remain difficult to understand since they depend on many related parameters such as the lubrication regime, the contact temperature and the friction coefficient. Scuffing is a critical damage that appears suddenly and which is characterised by local welds and scratches between the sliding surfaces. It can lead to complete destruction of the mechanical system. This phenomenon is in general related to poor heat dissipation and overheating resulting in high contact temperature. Numerous studies were conducted in order to establish a scuffing criterion based on energetic approach. But none of them is unanimously recognized by the scientific community. The first part of this study consisted in characterizing the frictional behaviour of a given couple of lubricant and material thanks to the use of a twin-disc machine. In a second part, a thermal modelling of this test bench has been realised using the thermal network methodology. Correlations between analytical formulas and experimental measurements allowed to establish a friction law relating contact conditions and disc bulk temperature. It has also been demonstrated that the disc bulk temperature could be considerably different from the lubricant injection temperature depending on the operating conditions. This result allows a new interpretation of existing scuffing criteria, which for most of them consider the disc temperature to equal the oil temperature. Despite this analysis, the scuffing experiments performed reveal that accounting solely for the contact temperature is not sufficient to establish a reliable scuffing criterion.

Orientador: Fernando de Azevedo Silva / Resumo: As uniões parafusadas possuem uma grande aplicação na indústria, mas quando se pensa na utilização de parafusos existe uma impressão de ser um elemento de simples dimensionamento e de fácil aplicação. No entanto, o desenvolvimento de uniões por parafusos possui uma complexa sistemática de cálculo e sua montagem requer um cuidado para proporcionar o torque correto e consequentemente a pré-carga ideal. O intuito do trabalho foi a análise da pré-carga de uniões parafusadas a partir do torque de aperto. Para isso foi desenvolvido um torquímetro instrumentado com extensômetros para a aquisição do torque aplicado, foi desenvolvida também uma célula de carga para a aquisição da pré-carga aplicada na junta parafusada. Ambos os sinais foram aquisitados instantaneamente e a partir destes sinais foram feitas as análises das uniões parafusadas. Foram analisadas três condições diferentes de lubrificação para o parafuso e porca, estas foram: com lubrificação, sem lubrificação e lubrificação seca (grafite). Foram analisadas também duas condições de aplicação de torque, estas foram: torque pela porca e torque pelo parafuso. Portanto, foram analisados seis grupos diferentes de uniões parafusadas. Para cada um dos grupos foram aplicados 3 torqueamentos para cada conjunto de parafuso, porca e arruela, simulando três situações de montagem e desmontagem. Ao final foram feitas considerações sobre a influência das diferentes condições de lubrificação no torque aplicado e na pré-carga gerada na junt... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Bolted joints are broadly used in the industry, but when it comes to their use there is a false impression of being a simple component, of easy dimensioning and easy application. However, designing bolted joints requires a complex calculus systematic and its assembly requires attention in order to apply the correct torque and consequently the ideal pre-load. The purpose of this dissertation was to analyze the pre-load of bolted joints from the tightening torque. Therefore, it was developed a torque wrench with strain gauges to measure the applied torque, and also developed a load cell to collect the pre-load onto the bolted joint. Both signals were instantaneous collected, and from this data, analysis were made over the bolted joints. Three different lubrication conditions for bolt and nut were analyzed: with and without lubrication and dry lubrication (graphite). Two torque application conditions were analyzed: the torque onto the nut and onto the bolt. Therefore, six different groups of bolted joints were analyzed. For each group of bolt, nut and washer, it was applied three torques simulating three assembly and disassembly operations. At the end, considerations were made on the influence of the different lubrication conditions on the applied torque and the generated pre-load onto the bolted joint, and the influence of successive torques on a bolted joint. Torque and friction coefficient tables were generated for all the above mentioned six groups / Mestre

L’instrumentation de plus en plus poussée des systèmes mécaniques (aéronautique, automobile,…) impose une utilisation croissante des connecteurs électriques. Cependant, leur environnement de fonctionnement (sollicitations chimiques et vibratoires) peut entrainer une dégradation plus ou moins sévère des contacts électriques limitant ainsi le passage du courant. Pour limiter cette dégradation et assurer la stabilité des connexions, des revêtements d’or sont couramment appliqués au niveau des contacts. Cependant, la conjecture économique et le coût très élevé de l’or nécessite de trouver une alternative moins chère. Parmi les métaux conducteurs, l’argent est aujourd’hui le meilleur candidat. L’objectif de cette thèse est donc d’étudier la réponse électrique et l’endommagement de dépôts argent soumis à des sollicitations de fretting. Pour cela, ces travaux de recherche ont été abordés selon trois axes. Le premier axe a permis une étude complète d’un contact homogène argent/argent afin d’identifier les mécanismes de dégradation responsables de la rupture électrique aussi bien en fretting qu’en glissement alterné. Il a aussi été possible, par une approche énergétique, de mettre en place un modèle prédictif permettant d’extrapoler les durées de vie du contact selon différents paramètres de chargement. Une étude complémentaire a également montré l’impact d’une atmosphère corrosive à base de soufre sur les contacts électriques en argent. Le second axe a permis, quant à lui, d’étudier le comportement tribologique et électrique de nouveaux matériaux à base d’argent développés dans le but de remplacer les dépôts dorés. L’analyse de ces contacts homogènes a permis de mettre en évidence les mécanismes de dégradation et les comportements mécaniques des contacts soumis à des environnements humides. Dans le dernier axe, une étude a été menée sur ces mêmes matériaux à base d’argent mais en configuration hétérogène contre un dépôt d’or afin d’identifier le comportement tribologique et électrique de ces contacts quand ils sont composés par des matériaux avec des propriétés similaires ou opposées. / Advanced instrumentation in mechanical systems (aeronautical, automobile etc…) goes hand in hand with an ever increased use of electrical connectors. However, the unfavorable operating environment (chemical attack and vibrational loads) causes more or less severe degradation of electrical contacts, which in turn perturbs their electrical conductivity. Gold plating is usually applied in electrical contacts in order to limit damage and to ensure connector stability. However, economic constraints and the high cost of gold require cheaper alternatives. Amongst conductive metals, silver is the best candidate. Hence, the purpose of this PhD project is to investigate the electrical response and the degradation of silver coatings when subjected to fretting loadings. The study is divided into three main research axes. The first axis consists in realizing a complete study of a homogeneous silver/silver contact in order to identify the degradation mechanisms that are responsible for the electrical failure, both in fretting loadings and reciprocating sliding. It was possible to formalize a predictive model, using an energy density approach, allowing to extrapolate the lifetime of the contact as a function of various loading parameters. A complementary study also showed the impact of a corrosive sulfur atmosphere on these electric contacts. As part of the second research axis, an investigation of the tribological and electrical behavior of novel silver-based materials, solely synthesized as a gold replacement, was performed. The analysis of these homogeneous contacts allowed to explain the degradation mechanism and the mechanical behavior of these contacts when subjected to a wet environment. In the last research axis a study was led on the same silver-based materials but in a heterogenous configuration against a gold coating in order to identify the tribological and electrical behavior of these contacts when composed by materials with similar or opposite properties.

Analisa o comportamento tribológico do pneumático. Discute o coeficiente de atrito do pneu, a influência do pavimento e os avanços na modelagem. Apresenta uma metodologia para estimativa do desgaste de pneus pelo método dos elementos finitos. Usa o conceito de trabalho de abrasão. Considera nas condições de contorno do modelo de pneu o efeito do camber e do ângulo de deriva. Investiga a interação vibracional entre o pneu e a suspensão como causa de desgaste irregular. Utiliza modelagem de ônibus rodoviário por multicorpos com eixo flexível. Emprega técnica tempo-freqüência para análise do acoplamento modal entre pneu e suspensão. Propõe novos modelos para estudo do desgaste em pneus analisando-se um modelo completo de veículo pelo método dos elementos finitos. Sugere análises de sensibilidade considerando os parâmetros de regulagem da suspensão e condições operacionais dos componentes. Propõe análises estocásticas da especificação do pneu para otimização do sistema pneu-suspensão. / The tire tribological behavior is analyzed. The friction coefficient of rubbers is presented, and its inherent modeling difficulties regarding the operational condition dependence during measurements are discussed. The influence of the pavement roughness and the advances in friction modeling are presented. A predictive methodology to evaluate the tread wear using finite element method and the concept of frictional energy was used. Camber, lateral forces and slip angles are taken into account as boundary conditions for the tire simulations in steady state. The vibrational interaction between tire and suspension concerning irregular wear on front axle truck tires was investigated. A multibody bus model with flexible front axle was used for modal analysis purposes. A time-frequency methodology was applied to identify modal vibrations of the tire and suspension assemblage. A new simulation model for the tire wear was proposed intending to analyze the whole vehicle with under the finite element method. Sensitivity analysis of the vehicle suspension setup and operational conditions of components was suggested. Stochastic analysis of tire specification is recommended to optimize the tire-suspension system.

O ensaio de desgaste micro-abrasivo por esfera rotativa vem conquistando elevada aceitação em universidades e centros de pesquisa, sendo amplamente adotado em estudos envolvendo desgaste abrasivo de materiais. Dois modos de desgaste abrasivo podem ser observados neste tipo de ensaio: rolamento resulta quando as partículas abrasivas rolam sobre a superfície do corpo-de-prova, enquanto riscamento é observado quando as partículas abrasivas deslizam sobre o mesmo; o tipo do modo de desgaste abrasivo apresenta uma significante influência sobre o comportamento de um sistema tribológico. Diversos trabalhos envolvendo coeficiente de atrito durante ensaios de desgaste abrasivo estão disponíveis na literatura, mas somente uma pequena parcela dedicaram-se ao estudo do coeficiente de atrito desenvolvido em ensaios de desgaste micro-abrasivo conduzidos por esfera rotativa. Adicionalmente, pesquisas preliminares reportaram que os resultados são dependentes da variação de pressão, ocasionada pela condução de ensaios sob condições de força normal constante. Logo, o propósito desta Tese de Doutorado é pesquisar a relação entre coeficiente de atrito e modos de desgaste abrasivo em ensaios desgaste micro-abrasivo por esfera rotativa, em condições de força normal constante e pressão constante. Ensaios ball-cratering foram conduzidos com esferas de aço AISI 52100 e um corpo-de-prova de aço-ferramenta AISI H10. A pasta abrasiva foi preparada com partículas de carbeto de silício (SiC) preto (tamanho médio de partícula de 3 m) e água destilada. Diferentes valores de força normal constante e pressão constante foram definidos para os experimentos. As forças normal (N) e tangencial (T) foram monitoradas continuamente durante os ensaios e a relação entre T/N foi calculada para fornecer uma indicação do coeficiente de atrito atuante no sistema tribológico esfera / partículas abrasivas / corpo-de-prova. Em todos os casos, análises por Microscopia Óptica das crateras de desgaste revelaram somente a presença de desgaste abrasivo por riscamento. Entretanto, observações mais detalhadas, conduzidas por Microscopia Eletrônica de Varredura, indicaram que diferentes níveis desgaste abrasivo por rolamento atuaram ao longo dos riscos, fenômeno nomeado de micro-rolling abrasion (microrolamento). Além disso, os resultados obtidos mostraram, também, que: i) a distância de deslizamento apresenta significante influência sobre a transição entre os modos de desgaste abrasivo, ii) para os valores de força normal constante e pressão constante adotados, o coeficiente de atrito manteve-se, aproximadamente, na mesma faixa de valores e iii) o coeficiente de atrito é independente da taxa de desgaste. / The micro-scale abrasive wear test by rotative ball has gained large acceptance in universities and research centers, being widely used in studies on the abrasive wear of materials. Two wear modes are usually observed in this type of test: rolling abrasion results when the abrasive particles roll on the surface of the tested specimen, while grooving abrasion is observed when the abrasive particles slide; the type of wear mode has a significant effect on the overall behaviour of a tribological system. Several works on the friction coefficient during abrasive wear tests are available in the literature, but only a few were dedicated to the friction coefficient in micro-abrasive wear tests conducted with rotating ball. Additionally, recent works have identified that results may also be affected by the change in contact pressure that occurs when tests are conducted with constant applied force. Thus, the purpose of this work is to study the relationship between friction coefficient and abrasive wear modes in ball-cratering wear tests conducted at constant normal force and constant pressure. Micro-scale abrasive wear tests were conducted with a ball of AISI 52100 steel and a specimen of AISI H10 tool steel. The abrasive slurry was prepared with black silicon carbide (SiC) particles (average particle size of 3 m) and distilled water. Two constant normal force values and two constant pressure values were selected for the tests. The tangential and normal loads were monitored throughout the tests and their ratio was calculated to provide an indication of the friction coefficient. In all cases, optical microscopy analysis of the worn craters revelated only the presence of grooving abrasion. However, a more detailed analysis conducted by SEM has indicated that different degrees of rolling abrasion have also occurred along the grooves. The results have also shown that: i) the sliding distance presents an important role on the wear mode transition, ii) for the selected values of constant normal force and constant pressure, the friction coefficient presented, approximately, the same range of values and ii) the friction coefficient was independent of the wear rate.

Two failure mechanisms in a double lap joint are investigated. Analytical models of net-section and gross-section failure modes are proposed to describe these mechanisms. The effects of lamping force, interference fit, maximum axial load and WDCP on fatigue performance of the joint are included in the models. The effect of WDCP is assumed to give a reduction in friction coefficient. Three types of stress reduction factors are proposed in the net-section failure model to account for these parameters. The stress reduction factors modify stress range that is used in crack growth calculation. If there are no effects of these parameters, the stress reduction factors are equal to one. Two types of fretting stress are introduced in gross-section failure model to describe either sliding contact or incipient sliding contact on faying surface. The fretting stress is combined with body stress to modify stress range. The net-section failure model predicts that fatigue life is increasing as interference fit, clamping force and friction coefficient increase. The gross-section failure model predicts that fatigue life is decreasing as clamping force and friction coefficient increase. Both models predict that fatigue life is decreasing as maximum axial load increases. Transition of the failure mode occurs earlier as friction coefficient and interference fit increase, while it is delayed as maximum axial load increases. A transition parameter is proposed to establish a relationship between the four main parameters. The transition parameter is expressed in a polynomial equation. It gives an optimum combination of the four main parameters in order to achieve relatively higher fatigue life by having gross-section failure mode. Finite element analysis and fatigue testing are performed to validate the models. The finite element and the analytical models show that stress concentration factor at the edge of the hole is decreasing as clamping force increases. The rate of decrease of stress concentration factor is increasing as friction coefficient increases. While stress concentration factor on the faying surface is increasing as clamping force and friction coefficient increase. Fatigue testing reveals that the fatigue life of the joint is in good agreement with the predicted fatigue life of the proposed models.

The aim of this thesis was to investigate roller ski characteristics; classical and freestyle roller skis’ rolling resistance coefficients (μR) and classical style roller skis’ static friction coefficients (μS), and to study the influence of different μR and μS on cross-country skiers’ performance and both physiological and biomechanical indices. The aim was also to study differences in skiing economy and efficiency between recreational skiers, female and male junior and senior elite cross-country skiers.The experiments showed that during a time period of 30 minutes of rolling on a treadmill (warm-up), μR decreased significantly (p<0.05) to about 60-65 % and 70-75 % of its initial value for freestyle and classical roller skis respectively. Also, there was a significant influence of normal force on μR, while different velocities and inclinations of the treadmill only resulted in small changes in μR.The study of the influence on physiological variables of a ~50 % change in μR showed that during submaximal steady rate exercise, external power, oxygen uptake, heart rate and blood lactate were significantly changed, while there were non-significant or only small changes to cycle rate, cycle length and ratings of perceived exertion. Incremental maximal tests showed that time to exhaustion was significantly changed and this occurred without a change in maximal power, maximal oxygen uptake, maximal heart rate and blood lactate, and that the influence on ratings of perceived exertion was non-significant or small.The study of classical style roller skis μS showed values that were five to eight times more than the values of μS reported from on-snow skiing with grip-waxed cross-country skis.The subsequent physiological and biomechanical experiments with different μS showed a significantly lower skiing economy (~14 % higher v̇O2), higher heart rate, lower propulsive forces coming from the legs and shorter time to exhaustion (~30 %) when using a different type of roller ski with a μS similar to on-snow skiing, while there was no difference between tests when using different pairs of roller skis with a (similar) higher μS.The part of the thesis which focused on skiing economy and efficiency as a function of skill, age and gender, showed that the elite cross-country skiers had better skiing economy and higher gross efficiency (5-18 %) compared with the recreational skiers, and the senior elite had better economy and higher efficiency (4-5 %) than their junior counterparts, while no differences could be found between the genders. / Syftet med denna avhandling var att undersöka fristils- och klassiska rullskidors rullmotståndskoefficienter (μR) och klassiska rullskidors statiska friktionskoefficienter (μS) samt effekter av olika μR och μS på längdskidåkares prestation vid rullskidåkning på rullande band. Syftet var även att undersöka s.k. åkekonomi och mekanisk verkningsgrad mellan motionärer och kvinnliga och manliga junior- och seniorlängdskidåkare på elitnivå.Experimenten visade att under en period av 30 minuters kontinuerligt rullande, på rullande band, så sjönk μR signifikant (p<0.05) till 60-65 % och 70-75 % av initiala värden, för fristils- respektive klassiska rullskidor. Undersökandet av olika normalkrafter, hastigheter och lutningars påverkan på μR resulterade i en signifikant, negativ korrelation för μR som funktion av normalkraft, medan olika hastigheter och lutningar endast medförde små förändringar av μR.Studien som undersökte fysiologiska effekter av olika μR visade, vid submaximala konstanta arbetsbelastningar, att yttre effekt, syreupptagning, hjärtfrekvens och blodlaktat förändrades signifikant vid ~50 % förändring av μR. Försökspersonernas frekvens och sträcka per frekvens samt skattning av upplevd ansträngning resulterade dock i mestadels icke signifikanta eller små förändringar. Protokollen med successivt ökande arbetsbelastning (maxtest) resulterade i signifikant förändrad tid till utmattning, vid ~50 % förändring av μR. Detta inträffade utan signifikant skillnad i maximal syreupptagning, hjärtfrekvens och blodlaktat, vilket även mestadels gällde för skattning av upplevd ansträngning.Experimenten som undersökte klassiska rullskidors μS visade att dessa erhöll värden som är fem till åtta gånger högre än vad som rapporterats från studier av μS på snö med fästvallade skidor.Den efterföljande studien som undersökte fysiologiska och biomekaniska influenser av olika μS visade, vid submaximala konstanta arbetsbelastningar, att åkekonomin försämrades (~14 % högre syreförbrukning), hjärtfrekvensen ökade, den framåtdrivande kraften från benen på rullskidorna minskade samt att det blev kortare tid till utmattning (~30 %), vid maxtest, när skidåkarna använde rullskidor med en μS i likhet med vad som rapporterats för skidåkning på snö. För arbetsförsöken med olika rullskidor av olika fabrikat med en högre, och likartad, μS förelåg ingen skillnad i de undersökta variablerna.Studien som undersökte åkekonomi och mekanisk verkningsgrad som funktion av prestationsnivå, ålder och kön, visade att elitskidåkarna hade bättre åkekonomi och verkningsgrad (5-18 %) i jämförelse med motionärerna, att seniorerna hade bättre åkekonomi och verkningsgrad (4-5 %) än juniorerna och att ingen skillnad kunde konstateras mellan könen.

CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / O trabalho tem como objetivo contribuir ao entendimento do efeito das caraterísticas superficiais no desgaste abrasivo com e sem efeito corrosivo. Utilizou-se o equipamento de micro-abrasão-corrosão desenvolvido no Laboratório de Tribologia e Materiais (LTM). Foi utilizado nos testes: amostras de aço inoxidável austenítico AISI 304, partículas abrasivas de sílica e velocidade de rotação de 150 rpm. Para a parte corrosiva foi usada solução eletrolítica com 1N de H2SO4. Para a influência da topografia de superfície do corpo de prova o aço AISI 304 foi preparado em lixas de carbeto de silício de granulometria #80 e #4000, e com disposições de lixamento perpendicular e paralelo com respeito ao sentido de rotação da esfera. Esferas de zircônia (Ø 25,4 mm) foram usadas como contra-corpos. Para avaliar a influência da natureza e evolução topográfica da superfície do contra-corpo usaram-se cinco contra-corpos esféricos (Ø 25,4 mm): um cerâmico (nitreto de silício, Si3N4), um metálico (aço AISI 52100) e três polímeros termoplásticos (polipropileno, PP, poliacetal, POM, e poliamida 6,6, PA 6,6); para os testes de micro-abrasão-corrosão foram usados um cerâmico (Si3N4) e um polímero termoplástico (PP). Foi acompanhada a evolução topográfica ao longo dos testes nos contra-corpos por meio de interferometria e perfilometría. O deslizamento das partículas abrasivas foi predominante nas calotas geradas em todos os testes realizados. A topografia de superfície do corpo e contra-corpo mostrou ter relação com a taxa de desgaste na micro-abrasão e micro-abrasão-corrosão. Topografia de superfície com parâmetros de rugosidade maiores apresentam maiores taxas de desgaste, tendo maior sensibilidade na micro-abrasão-corrosão. Atribuiu-se que, com maiores valores dos parâmetros de rugosidade, consegue-se maior efetividade no arraste e participação de partículas abrasivas no contato. / The aim of this work is to contribute to the understanding of the effect of surface characteristics on abrasive wear with and without corrosive effect. The micro-abrasion-corrosion apparatus developed at the Tribology and Materials Laboratory (LTM) was used. The following tests were used: AISI 304 austenitic stainless steel test specimens (body), silica abrasive particles and rotational speed of 150 rpm. For the corrosive analysis, an 1N H2SO4 electrolytic solution was used. For the influence of the surface topography of the specimen, the AISI 304 steel was grinded using silicon carbide (SiC) (#80 and # 4000), and tested with perpendicular and parallel arrangements with respect to the direction of rotation of the sphere. Zirconia balls (Ø 25.4 mm) were used as counter bodies. In order to analyze the influence of the nature and topographic evolution of the counter-body surface, five spherical counter bodies (Ø 25.4 mm) were used in the micro-abrasion tests: one ceramic (silicon nitride, Si3N4), one metal (AISI steel 52100) and three thermoplastic polymers (polypropylene, PP, polyacetal, POM and polyamide 6.6, PA 6.6); For the micro-abrasion-corrosion tests one ceramic (silicon nitride) and one thermoplastic polymer (PP) were used. The topographic evolution of the counter bodies was monitored along the tests through interferometry and profilometry. The grooving of the abrasive particles was predominant in the wear scars generated in all tests performed. The surface topographies of the body and counter-body showed to have relation with the wear rate in the micro-abrasion and micro-abrasion-corrosion tests. Higher roughness parameters induced higher wear rates, and show greater sensitivity in micro-abrasion-corrosion. It was attributed that higher values of the roughness parameters achieved greater effectiveness in the drag and participation of abrasive particles in the contact. / Tese (Doutorado)

Le développement des lubrifiants est un véritable levier pour les constructeurs automobiles et les pétroliers pour minimiser les pertes d'énergie dues aux frottements dans les moteurs. La viscosité est le principal paramètre sur lequel les formulateurs peuvent jouer pour abaisser les frottements. Cette thèse porte sur l'étude du rôle et des actions des additifs améliorants de viscosité (AVI) dans les lubrifiants moteur. L'objectif de cette thèse est de corréler l'étude portée à l'échelle moléculaire aux propriétés macroscopiques ainsi qu'aux performances des lubrifiants moteur pour pouvoir orienter les formulateurs vers une ou des molécules cibles qui permettront de répondre le mieux possible à ces problématiques. Ce travail est consacré à des poly-diène-styrène hydrogénés (P-diène-SH) et des copolymères éthylène propylène (EPC). Dans un premier temps, des études structurales des polymères AVI et rhéologiques des solutions ont permis de mettre en évidence l'influence des différentes chimies et structures des polymères sur leurs propriétés rhéologiques. Cette étude a porté sur une large gamme de températures (-20 à +135°C), taux de cisaillement (0 à 107s-1) et concentrations permettant de caractériser la configuration des polymères dans des conditions correspondant à l’application. Dans un second temps, ces propriétés rhéologiques ont été corrélées avec les coefficients de frottement et les épaisseurs de film centrales dans un contact de type sphère/plan en tribologie.Enfin, la capacité des chaînes de ces polymères à se rompre sous la contrainte d’un fort taux de cisaillement en écoulement pulsé ou continu a été étudiée et reliée à la configuration des polymères AVI en solution ainsi que leurs structures. L’ensemble de ces données a été utilisé pour proposer une structure chimique et une architecture présentant des propriétés AVI, une bonne résistance mécanique et un comportement tribologique satisfaisant. / The development of lubricants is a real challenge for the automotive and the petroleum industries to reduce the energy losses in engines due to frictions. The viscosity is the main parameter that the lubricant formulators can vary to reduce the frictions. This PhD deals with the role and mode of actions of viscosity modifier polymers (VM polymers) in lubricants.The purpose of this PhD is to correlate the study at the molecular scale with the macroscopic properties and the lubricant performances in order to orient the formulators towards one or several target molecules which could present the best properties.This work consisted in characterizing hydrogenated Poly-diene-styrene (P-diene-SH) and ethylene-propylene-copolymers (EPC).First, studies on the structure of VM polymers and on rheological properties of the solutions have shown the influence of the different chemical structures and polymer architectures on their rheological properties. This study was carried out on a large scale of temperatures (-20 à +135°C), shear rates (0 à 107s-1) and concentrations leading to characterizations under conditions corresponding to practical conditions.Secondly, these rheological properties were correlated with frictions coefficients and the film thickness in a sphere/plate contact in tribology.Finally, the ability of the polymer chains to break when a high shear rate was applied in a continuous or pulsed flow was studied and related to the chains configurations of the VM polymer and their structure.All these data were further used to propose a chemical structure and a polymer architecture leading to viscosity improved properties, a good resistance to degradation and a satisfactory behavior in tribology.

La méthode de refroidissement basée sur le principe thermosiphon présente un grand intérêt en raison de sa simplicité, de sa nature passive et de son coût faible. Elle est adoptée pour le refroidissement à 4,5 K de l’aimant supraconducteur du détecteur de particules CMS auprès du LHC en construction au CERN à Genève. Le travail présenté dans cette thèse étudie expérimentalement les propriétés thermiques et hydrodynamiques d’un écoulement d’He I diphasique en circulation naturelle. Le dispositif expérimental utilisé consiste en une boucle thermosiphon monobranche composée principalement d’un séparateur de phases, d’un tube descendant et d’une section d’essai. Les expériences ont été réalisées en faisant varier plusieurs paramètres tels que le diamètre des sections d’essai (10 mm ou 14 mm) et le flux de chaleur allant jusqu’à l’apparition de la crise d’ébullition. Ces expériences ont permis de déterminer les lois d’évolution des différentes grandeurs caractérisant l’écoulement (le débit massique de circulation, le débit massique vapeur, le titre massique, le coefficient de friction et le coefficient d’échange thermique) en fonction de la densité du flux de chaleur appliquée. Au regard des résultats obtenus, nous discutons la validité des différents modèles classiques existants dans la littérature. Nous montrons que le modèle homogène est le modèle le mieux adapté pour prédire les propriétés hydrodynamiques de ce type d’écoulement dans la gamme de titre massique 0?x?30%. De plus, nous proposons deux modèles pour la prédiction du coefficient de transfert de chaleur diphasique et la densité de flux de chaleur critique. Le premier considère que les effets de la convection forcée et de l’ébullition nucléée agissent simultanément et contribuent au transfert de chaleur. Le deuxième corrèle la densité de flux de chaleur critique mesurée en fonction du rapport altitude sur diamètre / The method of cooling based on the thermosiphon principle is of great interest because of its simplicity, its passivity and its low cost. It is adopted to cool down to 4,5 K the superconducting magnet of the CMS particles detector of the Large Hadron Collider (LHC) experiment under construction at CERN, Geneva. This work studies heat and mass transfer characteristics of two phase He I in a natural circulation loop. The experimental set-up consists of a thermosiphon single branch loop mainly composed of a phase separator, a downward tube, and a test section. The experiments were conducted with varying several parameters such as the diameter of the test section (10 mm or 14 mm) and the applied heat flux up to the appearance of the boiling crisis. These experiments have permitted to determine the laws of evolution of the various parameters characterizing the flow (circulation mass flow rate, vapour mass flow rate, vapour quality, friction coefficient, two phase heat transfer coefficient and the critical heat flux) as a function of the applied heat flux. On the base of the obtained results, we discuss the validity of the various existing models in the literature. We show that the homogeneous model is the best model to predict the hydrodynamical properties of this type of flow in the vapour quality range 0?x?30%. Moreover, we propose two models for the prediction of the two phase heat transfer coefficient and the density of the critical heat flux. The first one considers that the effects of the forced convection and nucleate boiling act simultaneously and contribute to heat transfer. The second one correlates the measured critical heat flux density with the ratio altitude to diameter