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The arrangement of the constituents of organic-composite friction materials is a key factor of their microstructure and thermal and mechanical properties which can influence braking performance. Among these constituents, fibres can present complex morphologies and different arrangements depending on their type and the process of manufacturing. Besides, synergistic effects acting between these constituents and the resulting properties are still not well investigated. This work relates to rock- wool used for brake friction materials, and for which the process can lead to various arrangements. The focus is on synergies between these fibre arrangements and the other material constituents, in ways that reveals the link between the resulting microstructural characteristics and properties of organic composite materials. To achieve these objectives, two simplified formulations are elaborated with two distinct arrangements of rock wool fibres. The friction materials are investigated in terms of microstructure, thermo-physical and mechanical properties. It is found that fibre arrangements affect carbonaceous particle distribution, porosity, and fibre-matrix adhesion. On one side, homogeneous distribution and regular size of fibre bundles results in a better connectedness of conductive particles and thus enhances thermal conductivity. On the other side, a regular fibre bundles repartition lead to a more homogeneous distribution of strain localizations and a softer mechanical response.

The manufacturing process of brake materials used for braking applications consists of a succession of steps among which the hot molding has a major impact on properties and performance of materials. In this paper, impact of hot molding temperature and duration on mechanical and thermal properties of friction materials developed with simplified formulation was investigated. Two different hot molding conditions were studied: condition 1 (low temperature associated to long duration) and condition 2 (high temperature associated to short duration). Braking behavior, thermo-mechanical phenomena and wear and friction mechanisms were also investigated. Results indicated that hot molding conditions did not significantly affect mechanical properties and tribological behavior, but they had impact on thermal properties (material molded according to condition 1, material A presented a higher thermal conductivity) and on wear mechanisms involved in the contact. In addition, results revealed that the studied hot molding conditions impacted thermal localization recorded during braking that was denser for the disc rubbed against material B (material molded according to condition 2).

The size and morphology of reinforcing fibres have a great influence on organic brake friction composite material properties and performance. This research aims to establish the link between friction material microstructure heterogeneity induced by rockwool fibre morphology and distribution and the resulting tribological behaviour. The adopted approach is based on simplified formulations designed to limit synergistic effects by reducing the number and size distribution of constituents. Two simplified materials are developed with different rockwool fibre size and morphology. The first material is elaborated with calibrated fibre balls, and the second one is performed with separated fibres. Friction and wear behaviour are correlated with thermal phenomena in order to reveal wear mechanisms and thus understand the link between microstructural characteristics and the resulting tribological behaviour. It was found that a regular size and distribution of rockwool fibre balls induce better tribological behaviour and enhance wear resistance. Indeed, a homogeneously distributed porosity, which is induced by fibre balls, favours the development and preservation of the load-bearing plateaus in the contact. This, consequently guarantees a stable friction and a reduced wear rate. Consequently, reducing microstructural heterogeneity, resulting from rockwool fibre morphology and distribution, improves the performance of composite friction material.

Braking is an energy dissipation mechanism used to restrict the movement of vehicles. Friction brakes may induce vibrations and noise. These effects constitute a major shortcoming related to the functioning of friction braking systems. Known as brake squeal, this phenomenon involves unstable vibrations induced by coupling modes between components in frictional contact leading to large amplitude vibrations. Despite significant progress in experimental techniques and numerical modeling, the origin of squeal occurrence remains misunderstood and is still a matter of debate. It is, however, commonly admitted that squeal is affected by many different factors on both micro and macro scales. In addition, a close correlation between wear and squeal occurrence in braking system has been reported. This study examines linking the change in the third-body layer with the occurrence of squeals in sliding dry contact. A simplified customized test rig was used with a transparent glass disc and an artificial alumina third-body. Results show that squeal occurrence is strongly linked to the densification and redistribution of the third-body, as well as internal flows in the interface.

The lack of knowledge on the link between the manufacturing process and performance constitutes a major issue in brake lining development. The manufacturing process of organic brake friction composite materials includes several steps (mixing, preforming, hot molding and post-curing), which define their final microstructure, properties and performances. This study focuses on the effect of mixing duration on the microstructure, properties and tribological behavior of organic friction composite materials. The adopted methodology is based on simplified formulations effective in limiting synergistic effects by reducing the number and size distribution of constituents. Two simplified materials are here developed according to the mixing duration of the constituent introduction sequence. The microstructural characteristics are studied using 2D and 3D analyses, and then correlated with the thermophysical and mechanical properties. Wear mechanisms and tribological behavior are studied in relation to the microstructure and properties of the materials. The results show the effect of mixing duration as regards particle distribution and fiber arrangement. The distribution and size of fiber entanglements contribute to the formation of carbonaceous particle clusters, which create bulk bridges improving thermal conductivity. Moreover, the arrangement of rock fibers affects density, porosity and thermo-physical properties. In addition, the mixing disrupts the cohesion of fiber bundles with the matrix, affecting compressive modulus and wear behavior. This microstructural defect also fosters abundant third-body source flow, which disturbs the tribological circuit and behavior. Porosities induced by fiber entanglements, having a large and irregular size and distribution on the frictional surface, result in low wear resistance and alter the frictional stability.

This paper relates to changes in the third body arrangement of a squealing pad-disc interface, triggered by altering the tribological circuit of particles. A sealed chamber placed around a friction experiment, allows to run-in the parts with dry air atmosphere, and then to switch from air to argon. Observations of surfaces worn in dry air match the literature. The major part of the surfaces is covered by a third-body layer containing oxidized metal, in the form of powder or compacted plateaus, the rest of the surfaces being native, non-oxidized material from the first bodies. The squealing sound is steady and has a well-defined spectral content. Two stages of sliding of 400 s each are then performed with changed atmosphere. The squealing sound becomes sporadic and its spectral content changes. On the pad's surface, the coverage of compacted third-body plateaus reduces, while more non-oxidized metal appears. A specific pattern of plateaus appears on top of graphite particles. The evolution of the vibrational behavior and of the third-body layer are both gradual and have not reached a steady state when the experiment is stopped. We propose a scenario involving a change in the balance between the source flows and the wear flow as an explanation of the observed tribological changes.

International audience; Aviation is one of the safest public transport means today. To reach such a performance, aircraft safety mainly relies on experience feedbacks and a set of constantly evolving rules which concern the flying products and operations. This also works for emergency landings or crash situations wherein the aircraft "belly" is directly in contact with the runway (Figure 1). For this purpose, a four years research project (PHYSAFE) funded by the French DGAC started in August 2015. Part of the research aims at experimentally studying and characterizing various phenomena which may have a noticeable influence on aircraft passengers' safety in case of emergency landing or crash. Among these experimental studies, the development of test means and facilities to characterize the dynamic wear behavior of aircraft primary structure materials once in contact with the ground was selected as being of common interest for aircraft and rotorcraft airframes. The part of the PhD work to be presented is notably focusing on the study and characterization of wear and heat phenomena, for metallic and composites aircraft structural materials (reference materials: Au2024, T700/M21) during emergency landing situations. It aims at estimating (through "pin on disc" tests [1]) the main phenomena and principles to be taken into account for an experimental protocol (test bench, specimens, instrumentation, etc.) dedicated to the study of wear and heat of materials in representative conditions, followed by a first comparison of metallic and composite reference materials performances. The methodology set up to partially answer the studied problematic, starts with pin-on-disc tests using a concrete pad and discs made of aluminum or composite material. The preliminary experimental design permits to observe the results of interactions between concrete and the materials like in an aircraft fuselage. A first identification of the tribological systems representing the studied contact, aims at defining the first bodies and the third body produced within the studied contacts. Once the tribological mechanisms identified (by post mortem and in-situ analysis), an estimated dissipated energy may be linked to those mechanisms through the writing of material and energy balances [2,3]. A future step of the work would concern the study of possible similitude rules (through non-dimensional numbers establishment relying on the Vaschy-Buckingham's theorem [4]), for a selection of identified wear and abrasion mechanisms, to check the possible extrapolation of experiments at a laboratory scale at full-scale level.; Les transports aériens sont de nos jours assez fiables en matière de sécurité et cela grâce aux avancées constamment faites dans le domaine. Pour continuer l'amélioration de cette sécurité, nombreuses sont les études faites autour différentes thématiques, dont le crash et les atterrissages d'urgence (situations où les aéronefs peuvent se retrouver sur le "ventre").Le projet de recherche (PHYSAFE) de la DGAC a ainsi été lancé à l'ONERA depuis 2015 pour une durée de 4 ans. Il a pour objectif principal d'apporter des éléments de compréhension du contact fuselage/piste via différentes études et notamment la conception d'un banc dédié à l'étude du frottement du fuselage sur la piste.La partie présentée des travaux de thèse, viendra rendre compte de l'étude et la caractérisation de l'usure et de l'échauffement produits dans le contact entre la piste (en béton) et les composants structuraux représentatif du fuselage (alliage d'aluminium Al2024/T3 et composite carbone epoxy T700/M21). Les travaux s'appuient sur des essais de type "pion/disque" qui permettront l'identification d'un système tribologique représentatif afin de pouvoir définir et caractériser le 3ème corps produit dans le contact.Une fois les mécanismes tribologiques identifiés (analyses post-mortem et in-situ), il s'agira d'écrire des équations de bilan de matière et d'énergie pour établir la corrélation entre les mécanismes tribologiques et la dissipation de l'énergie liée à la vitesse.L'étape suivante du travail consistera à étudier l'existence de règles de similitude (nombres adimensionnels définis via le théorème de Vaschy-Buckingham), pour vérifier, dans le cas de certains mécanismes tribologiques, la possibilité d'extrapoler les expériences faîtes en laboratoire à une échelle supérieure.

International audience; A brass-free and a brass-containing model friction material were investigated for their frictional behavior at temperatures increasing from room temperature (RT) up to 400 °C, and back. This research highlights the effect of some material constituents on the friction recorded under sliding against stainless steel at pre-selected and well characterized areas. Sliding specifically onto rock fibers increases friction in comparison to sliding onto the other constituents inclusive brass particles. On recording a mean friction across areas containing the different constituents of both model friction materials, no difference was noticed between the brass-free and brass-containing one up to 400 °C. However, at 400 °C that mean friction drastically decreases on both composites. Thermal stability and thermo-mechanical behavior of the individual constituents allowed to explain the evolution of friction during temperature cycling. Degradation of the organic constituents present in both model friction materials appeared as the factor determining friction.

Friction materials used for brake lining are highly heterogeneous composites for which the link between formulation, the resulting material properties, and performances is not well understood. Their heterogeneity is induced by the variety of ingredients (morphology, size, properties etc.) and the manufacturing process which includes a succession of steps: mixing, preforming, hot molding, and post-curing. Among these steps, mixing have a great impact on the material microstructure in terms of ingredient distribution and, therefore, on its mechanical properties. However, mastering the mixing process is very difficult, since it is still based on the empirical experience of manufacturers. In this study, a new method and methodology of mixing state evaluation of friction material constituents was developed. First, it consists on studying constituents’ physical properties permitting to facilitate the investigation of mixing state evolution. This investigation includes two steps: binary and multi-constituent mixture study. This work suggests a non-time-consuming image analysis method using two statistical coefficients to evaluate the mixing state, which are Kurtosis and Coefficient of Variation (C.V). The latter enable to describe the mixing quality and state evolution at the surface of the mixing volume as well as to evaluate the mixing time.

International audience; Following the ban of its use in brake linings, asbestos has been replaced by various materials based on industrial feedback and trial and error, but without any real understanding of how it affects contact during braking. As copper too will be prohibited in a few years' time, we set out to study the influence of one of its alloys, namely brass. A simplified brass-containing material was developed for this purpose. This material has an anisotropic microstructure and consequently exhibits anisotropic mechanical behavior and increased thermal diffusivity. It displays good tribological behavior at moderate and high temperatures. A surface analysis shows that the contact area is very effective at trapping third bodies and developing secondary plateaus, which provide good conditions for stabilizing friction to a high degree, even at high temperatures.

International audience; The use of dynamic seals to reduce the rotor/stator dynamic clearance in jet engine compressor stages leads to a higher rubbing occurrence between each blade and the coated inside of the casing. This article describes the development of a test rig capable to investigate forces and wear at the dynamic blade/seal interaction, in conjunction with blade kinetics. Testing conditions are consistent with those of low-pressure compressor stages of jet engines: high-speed rubbing occurs between a TA6V blade substitute and an aluminium-silicon/boron nitride abradable seal. The platform is instrumented to allow a dynamic measurement of forces and displacements as well as high-speed imaging of the blade/seal interaction zone throughout the experiment. The experiments showed that the blade incursion speed and penetration depth in the abradable seal both affect the amplitude and frequency of blade vibration. The amount and severity of blade incursions into the abradable seal have an impact on seal wear type and intensity, which can in turn increase blade excitation.

International audience; Cracking by thermal fatigue is one of the major problems encountered during the operation of numerous parts such as exhaust manifolds or brake discs. One way to improve their lifespan is to develop new materials solutions. Estimating thermo-mechanical fatigue resistance of new materials can be performed with a specific test machine developed by CTIF, associating a multi-position wheel, an induction heating and a cooling by air blowing. Test consists in measuring the number of thermal cycles corresponding to the first crack appearance and to the complete failure of a specific versatile sample. Two examples of studies performed with this machine are shown hereafter. First tests were performed to compare the thermo-mechanical fatigue resistance of different grades of ferritic spheroidal graphite cast iron. These materials are an interesting alternative to alloyed steels or nickel-alloyed cast irons, for the constitution of exhaust manifolds and turbine housings working in the upper-middle temperatures (up to 950 °C). The compared materials are SG irons alloyed with silicon (with or without molybdenum), and aluminium. Truck brakes dissipate several megajoules of energy every few seconds, which leads to high thermal stresses in the rubbing discs. Therefore, truck brake discs are mainly damaged by thermal fatigue cracking. One improvement way is to evolve disc material with the aim of increasing their thermal fatigue resistance while keeping the braking performance. For the thermal fatigue tests being as close as possible to real brake conditions, a specific geometry of sample and inductor was developed, to reproduce the thermal gradients observed during tests on real brake discs. The test parameters were then chosen after a coupled numerical-experimental study on full-scale braking. Behaviour of different materials under cycling loading (microstructural changes and cracking) is compared with regard to the results obtained during braking tests.

International audience; Aiming to provide a better understanding of thermal phenomena occurring in a sliding contact under tribological solicitation, a numerical model of pad-on-disc tribometer has been proposed. This study deals with an inverse problem concerning the identification of the heat exchange coefficient “h”. The method used allows the sequential estimation of the thermal boundary conditions by minimizing an error function between numerical and experimental temperature values. Coupled with the identification of the heat flux partition coefficient, the proposed model is validated.

International audience; Industrial brake lining materials are composite with complex formulations consisting of multiple constituents. Resulting from the fabrication process, the morphology and distribution of the constituents have significant influences on the future properties and braking performance. In this study, an in-depth analysis ranging from the microscale to the macroscale was performed to assess the relationships between the microstructure, the mechanical properties and the braking performance of an industrial brake lining material formulated for heavy vehicles. It was observed that the manufacturing process had different effects on the morphology and size of constituents and on their distribution in the phenolic binder. The morphologies of large organic particles such as rubber and graphite were affected by the mixing procedure, contrary to those of fibres and mineral particles. A transverse anisotropy consistent with fibre orientation due to cold preforming and hot moulding was observed. The microstructure displayed a strong local heterogeneity right up to the mesoscopic scale at which friction and wear mechanisms typically occur. The mechanical properties were analysed with regard to the heterogeneity of the microstructure to determine the scale at which these properties could be considered to be associated with a homogenised behaviour. The rubbing surface after braking showed that load-bearing localisation depends on the nature, morphology and orientation of constituents but that this heterogeneity can be of interest with regard to the braking ability.

International audience; This paper focuses on the study of the relationships between brake disc surface temperatures and disc distortion for various high-energy stop-braking conditions. An original thermal metrology method combining an infrared camera and a fibre-optic two-colour pyrometer was used to record the spatial and temporal variation in disc surface temperature during braking. Disc distortion was investigated in situ by means of a high-frequency displacement sensor. In addition, an optical trigger kept track of disc revolutions and enabled the synchronization of the IR camera, two-colour pyrometer and displacement sensor measurements. This experimental set-up was successfully used to determine the surface temperature and investigate thermal localization and waviness distortion during braking. The results were correlated with each other in relation to the level of energy dissipation. It was shown that the highest temperature was reached in the hot spot regions at an early stage of stop-braking. By contrast, the greatest disc distortion appeared much later, during the last stage of stop-braking.

International audience; In hot forging processes, wear is the predominant factor in the operating lives of dies. To test die materials, studies on friction and wear devices are commonly conducted in laboratories using different configurations of tribometers. In this study, a specific tribometer was designed to study the tribological problems of rubbing surfaces in the case of hot forging process. A conformal contact is chosen with a pad-disc configuration. Tested sample materials are made, respectively, from X40CrMoV5-1 steel and brass. The technical difficulties (heating, lubrication, alignment) and the technological solutions are presented. The instrumentation of the tribometer (force, temperature, contact resistance) is described. To evaluate the new device, three tests are conducted at different temperatures and loading conditions. Results show a satisfaction compliance of the contact between the pad and the disc and a metal transfer covers the pad in the presence of some abrasive grooves.

International audience; In order to preserve contact situations occurring during a tribological test, an experiment was performed to investigate the pin–disc interface while keeping the pin pressed against the disc. The pin was made from a commercial brake pad material and the disc from gray cast iron. Thus the experiment simulated a moderate brake application. After fixation of the friction-couple, macro- and micro-cross-sections were prepared by conventional metallography and focused ion beam technique, respectively. The amount of third body between the two first bodies and its structure and chemical composition was determined by microscopic and micro-analytical techniques including SEM, FIB, TEM and EDX. Furthermore, the size and structure of wear particles ejected from the contact area during the test was examined. Although testing conditions were moderate in respect to energy input and corresponding temperature rise, at least the main features of the third body structure are comparable to features observed after severe braking conditions. The observations confirm that during braking the first bodies are separated by a third body which is mainly responsible for the friction response. Lacking evidence of third body films after braking with certain pad materials can be attributed to film loss after braking due to weak adherence to the disc surface.

In the field of railway braking, improving the performances of friction brake systems requires better knowledge of the local tribological behaviour of pad–disc contact under braking conditions. After analysing the tribological triplet involved in railway braking, this paper focuses on the development of laboratory tests for studying local physical phenomena caused by friction in braking. The aim is to generate at reduced scale similar contact conditions to those at full-scale. An original braking tribometer is presented whose design is based on similitude rules between reduced-scale and full-scale. As it is difficult to reproduce representative full-scale thermal loading generated by friction at reduced-scale, compromises are needed and the choice of experimental parameters, based on a non-dimensional conversion rule, is discussed. The results obtained at reduced-scale are presented and comparison with results available at full-scale shows that the reduced-scale tests are very representative.

International audience; Glass fibers constitute a good candidate for brake-lining performance improvement. Their contribution is investigated within a reduced formulation, based on a limited number of components, specifically developed and efficient in braking situations.Friction and wear mechanisms and tribological behavior are analyzed in relation with the composite microstructure as well as material properties, considering two different braking-test protocols designed with regard to brake-induced thermal loadings. Infrared thermography is used to monitor thermal phenomena occurring during braking tests.It is shown that addition of glass fibers, in the form of bundles containing tens of fibers, increases thermal properties, enhances normal-compression modulus and allows decreasing wear. Fibers orientation appears to be a key factor with regard to the load-bearing mechanisms, allowing the development of a compacted third body at the rubbing interface when oriented normal to the surface. Conversely, fibers oriented in the sliding plane emerge from the rubbed surface and are not able to improve the load-bearing capacity. Addition of glass fibers helps to reduce thermal localizations, in consistence with higher thermal effusivity allowing better heat absorption and higher normal compressive modulus reducing the anisotropy and improving the pressure distribution.

International audience; Abradable materials are used as inner coatings in aeronautical compressors. In the event of blade–casing interaction, the abradable coating accommodates the incursion of the blade tip in order to protect the blade and the casing from severe damage. In this study, an experiment was conducted in conditions representative of the full-scale situation in terms of velocity, temperature and the dynamical characteristics of blades. The mechanisms of incursion accommodation were investigated based on dynamical data and a post mortem analysis of the coating and the wear debris. First, dynamical blade tip incursion was estimated by considering blade vibrations and coating waviness. The tip incursion was found to be significantly different from the measured displacement of the blade foot. Next, a post mortem analysis of the coating was performed at the surface and sub-surface. Four mechanisms were identified, namely debris release, surface shearing, compaction and reversible deformation. Care was taken to provide orders of magnitude for the different mechanisms relating to residual thickness loss of the coating.

A dispersion-strengthened copper (DSC) has been studied as an alternative material for railway brake discs, to be used in severe railway braking applications. Compared to standard steel, the DSC shows a more homogeneous thermal field at the friction surface. This also applies in depth, both avoiding hot spots. This thermal behaviour implies a lower wear and a very interesting friction coefficient evolution without any “anti-fade” phenomenon.

International audience; Abradable seals are used in aeronautical compressors to minimize the blade–casing clearance while ensuring the integrity of the mechanical parts in the invent of contact. Considering the tight clearance, in-flight blade–seal interactions may occur and have to be taken into account when designing abradable seals. The blade–seal interacting force is difficult to access but constitutes a key feature to understand the phenomena and to develop numerical models. The present paper aims to present an experimental methodology to estimate the blade–seal interacting force from indirect measurements. The methodology has been applied to a short interaction produced on a dedicated test rig in conditions representative of the full-scale configuration. The obtained force was correlated with the wear profile of the abradable seal.

International audience; The ratio of two forces acting respectively perpendicular and normal to a contact surface of two bodies, the coefficient of friction, is widely used in engineering and science depicting the friction resistance of materials sliding over one another. Ruled by the so-called Amontons-Coulomb friction laws (independence from the load, the contact area and the sliding speed), this dimensionless quantity appears to be convenient for engineering and relatively easy to determine. Nevertheless, the use of tabulated friction coefficients becomes somewhat an issue to predict friction behavior of mechanical systems. The system dependence of friction is sometimes ignored, leading to misapplication. Moreover, the fundamental origins of sliding resistance are not as clear and care should be taken when attributing a fundamental significance to the friction coefficient. This paper aims to clarify findings on friction Charles Augustin Coulomb did and that have been used for hundred of years. At the demand of the French Académie Royale des Sciences calling for rules and accurate data to design machines, Coulomb developed an experimental study of friction of woods and metals used for rubbing parts in machinery. Alongside his findings, he was aware of limitations of Amontons' findings. He demonstrated by experimentation the dependence of friction on the normal load, on the area of contact or on the sliding speed. In his day, he proposed some physical interpretations in agreement with his findings. He explained friction was originating from a variety of sources which led to various friction behaviors.

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International audience; Minimizing the clearance between turbofan blades and the surrounding casing is a key factor to achieving compressor efficiency. The deposition of an abradable coating on casings is one of the technologies used to reduce this blade-casing clearance and ensure blade integrity in the event of blade-casing contact. Aircraft in-service condi-tions may lead to interactions between the blade tip and the coated casing, during which wear of the abradable coating, blade dynamics and interacting force are critical yet little-understood issues. In order to study blade/abradable-coating interactions of a few tens of milliseconds, experiments were conducted on a dedicated test rig. The experi-mental data were analyzed with the aim of determining the friction-induced vibrational modes of the blade. This involved a time-frequency analysis of the experimental blade strain using the Continuous Wavelet Transform, combined with a modal analysis of the blade. The latter was carried out with two kinds of kinematic boundary conditions at the blade tip: free and modified, by imposing contact with the abradable coating. The interaction data show that the blade vibration modes identified during interactions cor-respond to the free boundary condition, due to the transitional nature of the phenomena and the very short duration of contacts. The properties of the Continous Wavelet Trans-form were then used to identify the occurence of blade-coating contact. Two kinds of blade/abradable-coating interactions were identified: bouncing of the blade over short time periods associated with loss of abradable material, and isolated contacts capable of amplifying the blade vibrations without causing significant wear of the abradable coat-ing. The results obtained were corroborated by high-speed imaging of the interactions.

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International audience; The interaction between microstructure, mechanical, and frictional properties of a commercial brake lining material (BLM) was investigated in order to correlate them to braking performance. For this purpose, a Scanning Electron Microscope (SEM) with energy dispersive X-ray (EDX) mapping and spectrum were used to identify and analyze different constituents. The mechanical properties were determined using compression test. Relevant physical properties (density and porosity) were determined using standard test methods. The friction coefficient and wear behavior of the friction material on contact with the grey cast iron disc were established using a pad-on disc tribometer. The results have shown that the brake lining material contains phenol resin such as the matrix and other various ingredients, including silica, rock and mineral filler reinforcement, barium sulfate and carbon-rich particles as filler and brass particles as friction modifier. It had a varied amount and size up to 1 mm for brass particles. The density and porosity were 1.8 g cm−3 and 7%, respectively. The investigated material exhibited excellent mechanical properties in the normal solicitation direction. The average friction coefficient was about 0.65, whereas the friction coefficient was stable. The different actions of various ingredients in terms of their effects on the friction and wear behavior of the BLM could be related to their different bonding strengths with the resin matrix and their different abilities to form friction films (third-body layer) on the surfaces of the material and transfer films on the counterpart cast iron surface in relation to the surface temperature evolution and mechanical properties.

Brake pad materials for automobile applications are basically polymer matrix composites. Various reinforcing constituents used in brake pads are organic, metallic and ceramic fillers which play among others an important role on the mechanical and thermal properties, and the wear resistance at high temperature. Friction and wear depend on various parameters such as the micro-chemical structure of the pad and of the metallic counter-face, the rotation speed, the pressure, and the contact surface temperature (M.G. Jacko 1983). This latter parameter can be locally as high as 600 up to 1.500 ∘ C depending on the brake type (M.G. Jacko 1983; Blau 2001). Thermal models have been developed to study interface effects at contacting surfaces (Majcherczak, Dufrenoy et al. 2007). Frictional energy can be dissipated through different mechanisms such as oxidation, rise in temperature, formation of wear particles, entropy changes associated to viscoelastic and viscoplastic deformation, and noise generation (Eddoumy, Addiego et al. 2011). Studies of friction brake show that more than 95% of the dissipated energy is transformed into heat (Kasem, Thevenet et al.; Majcherczak, Dufrenoy et al. 2007). Thermal analysis is therefore a primordial step in the study of brake systems since it provides thermo-mechanical properties (Majcherczak, Dufrenoy et al. 2007). The influence of the addition of metallic fibers on the performance of organic friction composites has been investigated using friction tests (Qu, Zhang et al. 2004). Benefits or limitations of the different fibers have been reported, however the issues of thermo-mechanical properties or effect of temperature on friction and wear behavior were not yet investigated (Bijwe, Kumar et al. 2008). No effort was done to correlate the thermo-mechanical and thermal properties with the friction and wear behavior. An important prerequisite is to get a good understanding on how brake materials behave. However, a link between thermal transitions and tribological behavior occurring at different temperatures is still missing.

Les materiaux de friction sont des systemes complexes de par leur formulation et leur procede de fabrication. Leur formulation peut renfermer jusque 40 constituants introduits de maniere empirique sans reelle comprehension de leur role ni des synergies entre constituants. Leur formulation comprend une succession d’etape dont l’impact sur le materiau et sur son comportement en frottement est mal maitrise. Malgre les evolutions constantes des normes environnementales conduisent a des modifications parfois profondes des materiaux, ces derniers doivent maintenir leur performance et garantir leur role securitaire. Afin de maintenir cet objectif, il est primordial de comprendre les liens entre formulation, procede et comportement du materiau. Dans cet objectif, le developpement d’une formulation simplifiee est propose : le nombre de ces constituants est reduit et son procede d’elaboration connu et maitrise. Il s’agit ensuite d’utiliser le materiau de friction a formulation simplifiee pour l’etude du role des constituants et de l’impact du procede de fabrication sur les proprietes des materiaux et leur performance en frottement et usure.

Squealing is a major matter of concern for environmental issues and the raised expectations for superior performances in braking systems. Squealing, as well as other types of noise nuisances, contributes to noise pollution and leads to additional expenses for phonic isolation. Brake squeal noise, which is defined as noise at frequency higher than 1000 Hz, occurs if the system has very high amplitude of mechanical vibration with sound pressure level above 80 dB. It involves instable vibrations, which are quite well understood as a result of mode couplings between the rubbing parts. These mode couplings are allowed by modal frequency shifts induced by the changes of sliding contact conditions. Nonetheless, the origin of squeal occurrence remains misunderstood as many parameters have to be taken into account, making it virtually impossible to predict when a squeal noise will appear or disappear. The strong coupling between structure and material, the continual change of the materials surfaces (chemistry and morphology), the influence of external parameters (temperature, pressure, hygrometry) all play a role in squealing. As a consequence, research on squealing is empirical by nature. Following this, one of the main goals in braking sciences is to determine the set of conditions on the parameters that provoke the start or the end of a squealing noise.

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International audience; Abstract This work focuses on the study of transient phenomena, in particular the non-uniformity and space-time variation of friction forces and surface temperature of a brake disc during stop-braking. Friction tests were conducted on a braking tribometer. The friction forces in the contact were measured using a 3D piezoelectric sensor, while the disc surface temperature was investigated by means of a high frequency fibre-optic two-colour pyrometer. An optical lap-top device was used to keep track of disc revolutions, and an original programme was written to plot the space-time variations of the measured parameters. This new original approach helps better understand the coupling between thermal and tribological phenomena occurring during braking.

International audience; Premature failure is the major problem encountered in the operation of truck brake discs. The present study was carried out with the aim of developing brake discs with increased lifespans, so as to improve the safety and reliability of these components. To this end, two aspects, namely design and material, were investigated. In order to understand the different damage modes, we studied the behaviour of a grey cast iron brake disc combined with two semi-metallic pads under extreme conditions. A coupled numerical-experimental approach enabled us to determine the critical thermomechanical loadings associated with braking-induced metallurgical phase transformations.

International audience; It is well known that truck brakes dissipate several megajoules of energy every few seconds, which leads to high thermal stresses in the rubbing parts. Therefore, premature failure by cracking of truck brake discs is a matter of major concern. Improving the design and material of brake discs may enhance braking performance. This study focuses on the latter aspect and was carried out with the aim of developing new material solutions for increasing disc lifespan. To do so, braking experiments were conducted on a specially designed braking tribometer. The brake pads that were used were made from a commercial brake lining material. Two advanced cast irons with different graphite morphology were studied in comparison with the lamellar grey cast iron commonly used for brake disc. To verify the friction and thermal behaviour of the two cast irons, braking tests were carried out as a series of stop-brakings with increasing dissipated power and energy and as a series of slowdowns to achieve heat accumulation effects. Thermal phenomena were studied through bulk temperature measurements and infrared monitoring of the disc surface. Friction behaviour, braking performance and variations in thermal loading were analysed in relation to the level of energy dissipation. The two advanced cast irons and lamellar cast iron had equivalent braking performance and stored similar amounts of heat, according to their thermophysical properties. Observations of the rubbing surfaces indicated damage mechanisms affected by the graphite morphology. Less plastic deformation on the surface was observed with an interdendritic graphite.

International audience; Graphite is commonly used in brake pads. The use of graphite powder has the main goal of solid state lubrication and friction coefficient stabilization. In this article results on resin bonded brake pads with focus on noise performance and heat dissipation are presented. Experimental tests are based on model friction materials with a known formulation and a reduced number of components for a better identification of the role of the graphite type. Results clearly indicate that both noise performance and thermal conductivity are strongly affected by the type of graphite. Guidelines for the selection of graphite types for optimized friction materials are given.

International audience; The main difficulties of brake disc infrared temperature measurements are the unknown of disc surface emissivity, non-uniform and time-varying, and the high rotation speed and deceleration of the disc, which makes it difficult to follow the special distribution of the temperature. To provide information on the emissivity variation during braking, brake disc temperature and emissivity were investigated by an original optic-fiber two-color pyrometer combined with an infrared camera, which allows monitoring the true friction areas. In addition, an optical top-tour was used to identify disc revolutions. The evolutions of the surface temperature and emissivity on successive disc revolutions were successfully determined. It was showed that circumferential thermal gradients form on the surface and maintain their angular position, they can, however, win or lose in intensity with braking progress

International audience; This article presents an investigation of the temperature changes in dry sliding contact for braking applications. An original metrology method was developed using a special pad with a calcium fluoride 'window' in its centre. As calcium fluoride is transparent to infrared radiation (up to 92% in 1-5 μm spectra), it allows access to the disc surface during friction for estimating its temperature using a two-colour pyrometer. Using this set-up, the surface temperature of the disc was successfully determined during friction. In addition, the temperature in the contact area was compared to that measured immediately outside the contact area; the difference between them proved to be very minimal (

International audience; The heat generated between friction couple during high speed braking induces thermal distortion in the disc and leads to appearance of hot spots. The nature and extent of intensity of this phenomenon are highly influenced by the mechanical and thermal properties of materials. In the present work, three non-asbestos organic (NAO) friction composites with three metallic particulate fillers (iron, copper and brass) were developed. They were evaluated on a specially designed tribometer for enabling the studies of hot spots and thermal localization phenomenon under braking conditions. Overall, composite with iron powder proved best by showing least tendency for hot-spot generation.

International audience; Thermal localisation (TL) phenomenon is defined as localisation of high temperature as consequences of high strain gradients developed in the bulk of materials of a tribo-pair. High extent of localisation of thermal gradient is rated as a non-desirable parameter for a good friction material because it affects both the surfaces leading to thermal crazing, cracks and fatigue, especially of the disc apart from affecting wear. In the present work, four composites with identical parent composition (containing ingredients of all the four categories) but differing in the average particle size of natural graphite (21, 41, 71 and 137 μm) were developed and evaluated on specially designed tribometer for enabling the studies of TL phenomenon. Two series of testing modes viz. inertia mode and rpm-mode were selected to bring out the correlation of selected graphite and their tendency to produce hot bands, hot spots and related phenomena on the counter-face. Overall, it was observed that the composites with lower particle size of graphite rendered better results than that with higher particle size. The study of hot-spot appearance and the TL behavior of tribo-pair are discussed in details in the paper.

International audience; In the hot forging industry, die life is an important process factor because of the cost involved in lost production, replacement of die blocks and operative handling of the dies. There is still no consensus, however, on the type of wear affecting dies or the dominant mechanisms for die failure, which varies from one situation to another. This metallographic study of a failed industrial hot forging die used to forge gas cylinder valves has indicated various failure modes. Although plastic deformation and thermal fatigue are usually quoted as the main causes of damage, oxidative and abrasive wear, fatigue cracking and chipping appeared to bet he most important in this study. Feedback coupling of fatigue and wear effects are detected. Detailed scanning electron microscopy observations and energy-dispersive X-ray and optical profilometry analysis suggest that these failures might very depending on their localisation on the die surfaces and show a complex mechanisms related to the variation of process parameters.

Cet article traite du comportement en frottement et a l'usure de l'acier X40CrMoV5-1 oxyde, sous faibles vitesse et distance de glissement, sans recouvrement afin d'etudier l'influence des oxydes sur les mecanismes de frottement et d'usure. Une caracterisation de la croissance de couches d'oxydes est menee en microscopie electronique a balayage avec sonde EDS et en profilometrie interferometrique. Des essais de frottement sont realises sur un tribometre en configuration pion disque. Les resultats montrent une transition entre une usure severe caracterisee par des mecanismes d'abrasion et d'arrachement de particules a une usure douce associee a l'activation d'un circuit tribologique dans lequel les oxydes contribuent a la formation d'un troisieme corps protecteur.