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The quality of friction stir welding joints is intimately related to the correct mixing of the stirred material. The material flow is strongly dependent on the plastic behavior of the welded alloy. For this reason, the friction stir weldability depends on the structure, microstructure, and chemical composition of the base material. In this work, in-plane forces and acoustic emission signals were monitored while welding two heat-treatable aluminum alloys. The force evolutions suggested possible continuous and intermittent material flow during friction stir welding depending on the welding parameters. The differences observed in the in-plane forces were corroborated by acoustic emission, confirming the modification in the material flow phenomenology. Therefore, differences observed in aluminum alloys’ friction stir weldability are due to the plastic behavior at high temperature and medium-high strain rate. The higher the deformability of the aluminum alloys, the wider the weldability window in friction stir welding because of continuous material flow in an extended range of process parameters.

The tribological behavior of tungsten carbide–cobalt materials is influenced by the cobalt content and the WC grains size. The main wear mechanisms in these materials are cobalt depletion, intergranular cracks, and WC grain cleavages. More specifically, coarse WC grains favor the apparition of transgranular cracks during sliding friction tests. A promising way to access in real-time blinded tribological contacts is the technique of acoustic emission (AE). This study clearly identifies transgranular cracks in AE signals. The AE energy and frequency of this mechanism were experimentally associated with the size of the transgranular cracks. A mechanical model based on the classical beam theory and harmonic motion equations confirms these relations. The AE centroid period (i.e., inverse of the centroid frequency) increases linearly with the size of the transgranular cracks. The AE energy increases linearly with the cube of the transgranular cracks length.

The tribological behavior of a steel/PEKK ball-on-flat contact was studied regarding the load, the velocity and the sliding distance in reciprocating or unidirectional motion. The friction measurements were synchronized with an acoustic emission (AE) device. The results show a behavior change associated with an adiabatic effect on the polymer when the mechanical energy input increases. Two interfacial mechanisms are also characterized: Schallamarch ridges and longitudinal ploughings. These mechanisms are related to two AE frequencies of 0.2 MHz and 1 MHz. The discussion on these two AE populations brings a stronger analysis and completes the friction and the wear mechanisms results.

International audience; The wear behavior of conventional and graded polycrystalline diamond compact cutters was studied using a vertical lathe with mortar counterfaces. Different cutters were considered regarding to their diamond grain size and the high pressure and the high temperature conditions of the manufacturing process (HPHT). On the base of these cutters, a cobalt graduation process was performed on the WC–Co substrates by reactive imbibition. A quality factor developed in previous studies was calculated to evaluate cutters wear performances. The results showed that a controlled HPHT process can act on the wear resistance certainly by improving the diamond grain boundary cohesion. Unexpectedly, the diamond granulometry appeared to be a secondary factor influencing the wear resistance. The reactive imbibition clearly increased the wear resistance, even for cutters with coarse diamond grains (i.e. potentially impact resistant). Finally, a third body approach describes that the quartz particles detached from the mortar rock realizes abrasive scratches on the cutters wear flat. When the wear flat reaches the substrate, the formation of voids in the contact, associated with the trapping of abrasive particles, rises the wear kinetic. As a secondary mechanism, Raman spectroscopy measurements highlighted tribological transformed structures by the formation of graphite and amorphous carbon on the diamond worn surfaces.

The tribological behavior of commercial roller cone bit inserts was studied by using a rotary tribometer and abrasive alumina counterfaces. Three cemented carbide WC–Co inserts were selected with different cobalt content and WC grain size distribution. During tests, a nominal load was set at 264 N, the velocity at 0.5 m ⋅ s− 1 and the test time at 1 h. The experimental measurements were performed using load, torque, displacement sensors, an acoustic emission sensor and four thermocouples. These measurements showed that the friction coefficient and the mean contact temperature decrease with the cemented carbide's cobalt content. The acoustic emission energy displayed a clear dependence with the mean WC grain size and the WC–Co fracture toughness. It was also found that the insert's wear is proportional to the load but not to the time (or distance). Eventually, a third body approach clearly showed that the inserts and the counterfaces contribute to form an interfacial abrasive paste. The stability, the composition and the cohesion of this paste govern the tribological behavior of the WC–Co/alumina contact.

International audience; The contact behavior of an abradable coating (Al-Si 6%) and a labyrinth seal tooth (stainless steel) in a turbo-engine application was studied as a function of the incursion depth parameter, during labyrinth seal/abradable interaction. A controlled and a gradual increase of the labyrinth seal incursion (by step of 50 μm) is performed to obtain the chronological contact evolution under severe operating tribological conditions. The labyrinth seal/abradable contact experiments were conducted on a dedicated test rig able to reach high contact speeds from 0 to 130 m.s−1. To complete contact forces measurement during tests, a suitable instrumentation (acoustic emission sensor, accelerometer, thermocouples, etc.) is developed and coupled as close as possible of the interaction area. The experimental results from the both severe tribological conditions are presented by an analysis of signals recorded during contact tests. Macrographic and micrographic rub-groove observations of post tests samples, coupled with recorded signals from the contact complete the Al-Si 6% behavior study. A wear process description using the third body approach has been proposed to sum up the whole tribological results. Two different varieties of particles production have been identified; a ductile and an adhering layer on the rub-groove bottom and pulverulent fine powder particles, thus providing two different kind of third body and two different material flows.

Extreme working conditions affect material used as friction components in transportation field: they rapidly reach their limits and critical parts require to be regularly replaced. Alternative solutions withstanding higher operating conditions imply to find innovative materials. Stellite matrix composites including various solid lubricants, WS2 and h-BN, able to admitextreme conditions were developed using a Spark Plasma Sintering technique, which makes possible the formation of new microstructures out of reach by conventional means. Sliding tests were conducted using a pin-on-disc tribometer in air at 450°C, with a velocity of 0,25 m/s and various normal load ranged from 2.5 to 40 N. Influence of solid lubricant content and sensitivity to test parameters were studied in terms of friction and wear responses of the contacting materials. Friction properties are equivalent to Stellite ones and sometimes lesseffective. A reduction of wear is quantified for many composites, and the best behavior is observed for those that contain WS2. In agreement with the third body approach, interpretations are proposed to describe the interphase dynamics within the contact.

Titanium alloys are well known to present poor sliding behaviour and high wear values. Various coatings (soft thick coatings and thin hard coatings) and treatments have been tested to prevent such an occurrence under fretting conditions at high frequency of displacement (100 Hz). An original test apparatus, using an open-loop system, has been performed to directly display the phenomenon of seizure. No seizure was recorded at low load (6 N), while, at higher load (10 N), all samples undergo a more or less early seizure. The total sliding distance D 0 proved to be a pertinent parameter to study the seizure resistance. Furthermore, the results highlight that D 0 is linked to the total energy dissipated in the contact, E dt , and reveal two distinct behaviours at low load, which suggest two distinct dissipating mechanisms of energy. The first trend can be connected with the plastic deformation and the trapping process of debris within the contact zone occurring on soft coatings. The second trend can be related to the higher debris ejection observed on hard samples. So, soft thick coating satisfies most of the chosen criteria except those of wear. In contrast, thin and hard coatings are not sufficient to totally protect the substrate but they are already able to efficiently reduce wear.

This work focuses on the acoustic emission signals related to three different tribological systems: a rotary sliding contact between WC-Co pins against alumina flat counterfaces, a fretting contact between alumina pins against alumina flat counterfaces and a reciprocating sliding flat on flat contact between thermoplastic polyurethanes (TPU) and a steel counterface. This document relates dependences observed between tribological behaviors and variations of acoustic emission signals. Therefore, a third body approach is used to explain these correlations and to highlight the aspect of nature and associated energy of acoustic emission sources.

This work attempts to link tribological and adhesion phenomena in fretting conditions. Fretting experiments were carried out in three environmental conditions (air atmosphere, demineralized water and kerosene) and with various couples of industrial materials. Contacting materials and liquids were chosen mainly for their surface properties, in order to have a wide range of surface free energies. The fretting device included a pin-on-plane configuration. The pin (or slider) was fixed and the plane was the moving substrate. The total distance (d T ) covered by the moving substrate was chosen as the tribological parameter representative of the history of an experiment. The distance d T was then compared to surface parameters such as Dupre's work of adhesion between the two substrates in contact (w 12 ) and with three bodies (w 132 ), the index 3 being the liquid medium. In these conditions, d T = f(w 12 ) and d T = f(w 132 ) variations were found to be linear, thus showing a clear link between friction and adhesion.

The tribological behavior of a thermoplastic polyurethanes (TPU) sliding against a non-polished steel counterface was studied as a function of load, velocity and temperature under dry conditions. Thermomechanical and chemical analyses were carried out on the TPU using DSC, FTIR and DMTA devices prior to the sliding tests. Contact changes were followed using a speed camera and an acoustic emission device. A simple thermal numerical simulation, to calculate temperature rise generated by friction, completes these analyses. To characterize wear performances, a wear rate was determined by considering the linear dependence between the wear volume and the product of the normal load by the travel distance (Archard model). Finally, wear mechanisms of TPU involve both abrasion scratches and adhesion patterns (Schallamach ridges). To conclude, a wear process description using the third body approach was proposed to gather the whole tribological results.

Increasing attention is being paid to carbon-fibre/SiC-matrix composites (C/SiC) for their friction and wear performance because the carbon-fibre reinforcement enables a significant reduction in the coefficient of friction through self-lubricating properties. This study was performed by using a C/SiC composite at high-speed (15 m s−1). Two distinct types of tribological behaviour, as a function of the rubbing pin features (100Cr6 or alumina), were found. Analysis of both the surface degradation and velocity adaptation mechanisms has made it possible to establish the major role of debris trapped in the contact zone. The wear debris from the 100Cr6-C/SiC pair is a heterogeneous layer adhering strongly to the friction track. Consequently, the velocity adaptation is a direct effect of the shearing of the transfer layers and fracturing of bulk materials, resulting in severe wear. In contrast, with an alumina-C/SiC pair, wear particles are finely ground owing to the fact that they remain for a long time in the contact zone. The velocity adaptation here mainly occurs through debris rolling and shearing within the powder bed, thus resulting in reduced wear.

Materials used as friction components in transportation field are subjected to extreme working conditions: they rapidly reach their structural limits and critical parts require to be regularly replaced. Alternative solutions withstanding higher operating conditions imply to find innovative materials. Steel matrix composites including various solid lubricants, WS2 and h-BN, able to support high temperatures were developed using a Spark Plasma Sintering technique, which makes possible the formation of new microstructures out of reach by conventional means. Sliding tests were conducted using a pin-on-disc tribometer in air at 450°C, with a normal load of 15 N and various velocities ranged from 0.1 to 1.5 m/s. Influence of solid lubricant content and sensitivity to test parameters were studied in terms of friction and wear responses of the contacting materials. Test results reveal an improvement of friction properties for composites containing highest WS2contents. A reduction of wear is quantified for all composites, and the best behaviour is observed for those that contain WS2. In agreement with the third body approach, interpretations are proposed to describe the interphase dynamics within the contact.

Relative movements of low amplitudes between two materials in contact are generally reproduced on fretting devices with imposed displacement or imposed tangential force. The damage kinetics observed (cracking, wear) is established under such conditions. In this article, a fretting device working under free displacement is used to characterize the damages generated by seizure and wear. The conditions of seizure are analyzed from the total sliding distance and the discussion is focused on a correlation established with Dupre's work of adhesion. The wear behavior of materials has been characterized from an energetic wear coefficient taking into account the wear volume of contact, the total sliding distance and the dissipated energy.

A new tribometer was designed to study the reactivity of rubbing surfaces under controlled environmental conditions. The contacting samples are isolated from the atmosphere by a chamber under secondary vacuum (10 −6 mbar) where reactive gasses can be injected. The sample chamber can also be heated up to 900 °C by a radiative furnace. The sliding velocity can be varied from 0.05 to 1.5 m/s and the applied load from 5 to 100 N (contact pressure ranged between 130 MPa and 1.3 GPa). The instrumentation of the tribometer enables continuous measurement of the normal and tangential forces, vertical displacement of the contacting samples, temperature in the vicinity of the contact zone, partial pressure and gas composition in the test chamber and electrical contact resistance. The design difficulties have been exposed and the chosen technological solutions are presented. A test has been carried out to validate the tribological device.

Transparent plastics are promising materials for optical applications but they are unfortunately not scratch resistant. This damage leads to a loss of optical properties. Coating of these plastics by a hard material seems to be a means to overcome the observed disadvantages. This goal was achieved using both polysiloxanes and colloidal particles of silica. Tribological experiments have been carried out to better understand the surface modifications due to a sliding friction. The surface damage exhibits cracking and coating removal by abrasion, depending on the silica content of the film. With this respect, films prepared using a colloidal silica solution exhibit the best tribologic properties. With a low silica content the crack extension is severe. With too high a silica content the film adhesion is not good enough and the coating may be removed by the friction. Three successive stages of surface degradation have been distinguished : (1) damage initiation by cracking, (2) particle detachment and (3) debris circulation (accumulation or elimination), in close agreement with the third body approach. The adhesion and the residual stresses of the coating play a major role during the two first stages. The last stage of degradation is mainly related to the behaviour of the debris which acts as a protective screen or an abrasive body depending on the location of the debris in the contact zone.

Titanium alloys are well known to present poor sliding behaviour and high wear values. Various coatings and treatments have been tested to prevent such an occurrence under fretting conditions at high frequency of displacement (100 Hz). An original test apparatus, using an open-loop system instead of a classical imposed displacement simulator, has been performed to directly display the phenomenon of seizure, defined as the stopping of the relative motion between the contacting elements. A classification of the tested coatings has been proposed on the basis of their capacity to maintain full or partial sliding conditions, to present low wear rates and to prevent seizure.

Transparent plastics are not scratch resistant. The damage leads to a loss of optical properties. Coatings prepared using either tetraethoxysilane or colloidal silica particles embedded in “glymo” is a way to avoid these disadvantages. Tribological experiments are carried out to better understand the surface modification due to a sliding friction. It is shown that the wear track is not directly related to usual mechanical properties such as Young's modulus and the hardness of the coating. The different stages leading to material loss are discussed in term of particle removal and debris circulation (accumulation or elimination) through the friction track. The mechanical properties of the film combined with the film to substrate adhesion are expected to play an important role as it can be deduced from results obtained as a function of the coating composition.