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The tribological behavior of a steel/glass ball-on-flat contact was studied by synchronizing the friction measurements with an acoustic emission device and a vision system. The results highlight two distinct friction regimes identified with low and high friction values. Their transition is characterized by a modification of acoustic emission signals. In addition, two main damage and wear mechanisms are identified: the creation and propagation of Hertzian cracks visible on the glass surface and the constitution of an interfacial layer of debris. The different accommodation mechanisms, activated successively or simultaneously, are identified for acoustic emission frequencies between 300 and 700 kHz. Eventually, this approach allows a real-time wear mechanisms identification and gives better insights about acoustic emission signals in relation to tribological systems.

This review proposes to carry out a state-of-the-art associated with micro-arc oxidation. Firstly, the different aspects of the growth mechanisms of the oxides are detailed. Then, the formation of micro-arcs and the case of soft-spark treatment are discussed. Then, the electrolytic reactions involved in the layer construction are outlined. We focused on the influence of aluminium alloys on the appearance of the coating and its characteristics before considering the electrolyte formulation. We have concentrated some of our efforts on silicate-based electrolytes, mainly used in research and industry. The importance of electrical parameters in layer formation is detailed later. The main factors studied in the literature are the current source, current density, treatment frequency and duration, and duty cycle. We have also noted the different phase compositions identified in the literature. Finally, since the process is particularly advantageous for protecting the surfaces of aluminium parts against wear, we conclude this review by presenting work on the tribological properties of this coating. In this final section, we highlight the work on the wear-reducing properties and tribological mechanisms identified in the literature. Particular attention is paid to the relationship between the nature of the substrates used, the role of the electrolyte and the counterpart choice on the friction and wear results.

Optimising the performance of materials requires, among other things, the characterisation of residual stresses during the design stage. Raman spectroscopy offers access to these residual stresses at the micrometre scale when this inelastic light scattering is active in these materials. In this case, the relationship between the Raman mode shift and the pressure must be known. High-pressure cells with diamond anvils or bending instruments coupled to Raman spectrometers are habitually used to determine this relationship. In this article, we propose a new method that involves a Hertzian contact to obtain this relationship. A device that compresses an alumina ball against a transparent glass plane is connected to a Raman spectrometer. Under these conditions, the contact pressure can be as high as 1.5 GPa. The contact between the glass plane and the ball is observed through a diaphragm. Several hundred Raman spectra are recorded depending on the contact diameter. The spectral profiles obtained represent the shift in the Raman modes of alumina and glass along the contact diameter. Hertz’s theory accurately describes the pressure profile as a function of position for elastic materials. Therefore, the contact diameter can be measured by fitting the spectral profile with a function identical to the Hertz profile. We then deduce the maximum pressure. Next, the calculated pressure profile along the contact diameter is correlated with the spectral profile. We obtain a pressure dependence of the Raman mode with a coefficient equal to 2.07 cm−1/GPa for the Eg modes of alumina at 417 cm−1, which is in good agreement with the literature. In the case of glass, we refine the measurement of the Q3 mode shift at 1096 cm−1 in the studied pressure range compared to the literature. We find a coefficient of 4.31 cm−1/GPa. This work on static contacts opens up promising prospects for investigations into dynamic contacts in tribology.

Secondary air systems of civil aircraft engines require labyrinth seals with a minimum gap clearance for optimal functioning. In the case of high speed contact during the engine running-in period, an abradable material is deposited on the stationary part of the seal to limit the damage of the rotating shaft, which is made of a titanium alloy. Such situations are potentially critical for the seal; hence, the present study aims to observe the material behaviour during these contact conditions and to establish the tribological circuit of a third body through the interface. A high-speed contact test rig was developed to recreate contact conditions occurring in an aircraft engine. Two contact configurations occurring in different locations of the engine, with different materials and surface areas were explored. Thermal and mechanical instrumentation were used in each test. The influence of the contact geometry and the test conditions show that material flows through the contact determine the life cycle of the contact (by establishing a balance between the source, internal and material flow) and allows for the control of the thermomechanical constraints in a high-speed contact.

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.

This paper deals with the development of an original apparatus called TRIBOLUMEN designed specifically for friction experiments on transparent materials. The friction measurement is synchronized with an acoustic emission (AE) sensor and the device is also equipped with a high-speed camera offering a direct view at the interface to gain a deeper understanding of tribological mechanisms. The TRIBOLUMEN device is in ball-on-flat contact configuration with a range of strokes from 5 to 500 &micro, m and an oscillation frequency from 5 to 600 Hz. The experiments showed that this device has an adequate rigidity and can detect subtle friction modifications of the oscillating contacts. The observation of a steel-on-glass contact in real-time highlighted the initiation of Hertzian cracks followed by the formation of debris in the contact. Using the synchronous measurement, these mechanisms were clearly associated with different stages in the friction measurement and in the AE signals, which permitted to identify the AE signature of Hertzian cracks.

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.

Most fretting studies analyze material׳s responses using criteria directly issued from characteristic parameters of the fretting loops (loop aperture, dissipated energy, contact stiffness…). The present study concerns the influence of the device, first element of the tribological triplet, on the gross slip response of a ball-on-flat contact. The role of the static and dynamic device stiffness (vibratory analysis), of the control system mode (imposed displacement or imposed actuator force) and of the material type (ductile or brittle) on the loop shape is analyzed in order to distinguish the respective contributions of devices and materials.

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.

Civil aircraft engines present a wide range of labyrinth seals to ensure a good airtightness between the different components of the secondary air system. An increase in efficiency requires lower clearances gaps. As a consequence, brief contacts between rotating and stationary parts may occur especially during the engine running-in period. Such events can cause critical situations (seizure…) depending on the working conditions. In this paper, experimental simulations by means of a high-speed contact test device (76 m s−1) was developed to precisely recreate the friction conditions occurring in a turboshaft labyrinth seal and to better understand the material behavior in such tribological cases. This device was instrumented to carry out mechanical (axial and tangential forces and torque) and thermal measurements (IR camera and pyrometer). An experimental campaign was carried to study the contact between a Ti6Al4V rotor and an abradable coating of Al-Si polyester. Presented results show the complex interactions that strongly depend on the way the worn material behaves in the contact area. Local interaction dynamics are analysed with regards to mechanical and thermal measurements with different rotating speeds, incursion depths, and interaction speeds.

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.

In general, there is no available tool which can help engineers and researchers to choose optimal materials for friction pairs. This article proposes a dual approach for the choice of materials and coatings. First, in order to select the initial materials, a selection matrix helps to rank a reduced number of solutions to a tribological problem with the aim of building the most credible and viable experimental campaign. Then, this experimental phase is necessary for final selection taking into account tribological properties. The final step involves experimental validation on a prototype and on the real device. This methodology was applied on the complex geometry of an air compressor under severe friction conditions. Technical specifications are defined by a functional analysis of the tribological system. Then, the selection matrix is created on the basis of empirical rules and bibliographic data, including predetermined material/coating properties, process considerations, and tribological features, in accordance with the functional analysis. As an example, four potential solutions were tested: diamondlike carbon (DLC) and polytetrafluoroethylene (PTFE) coatings on 15-5PH stainless steel and two composites, reinforced PTFE and polyetheretherketone (PEEK). Experimental results were then compared to expected values from the specifications. The performance of each solution was highlighted by a graphic radar representation. The selection matrix gave the DLC coatings as one of the best solutions, and experimental tests confirmed this choice while allowing to refine the preselected solutions. This result shows that the selection matrix gives a reliable choice of optimal solutions.

International audience; A new high-speed test rig was designed to simulate the interactions between labyrinth seals and abradable coatings in similar turbo-engine operating conditions. To determine a solution for turbo-engine efficiency enhancement, we investigated the clearance reduction between the rotary parts in air systems, the successive starts and stops, the thermal expansion and the vibrations that might cause direct rub interactions between a rotary seal, known as a labyrinth seal, and a turbo-engine housing coated with a sacrificial abradable material. High interaction speeds from 0 to 130 m s−1 were obtained using a 5-axis milling machine fitted with a unique magnetic bearings spindle developed specifically for the study. The purpose of this paper is to study the interaction phenomena between an abradable material (Al-Si 6%) and a nickel alloy (Alloy 718) to obtain a first contact assessment under different turbo-engine operating conditions. The experimental results are first presented by visual observations of the posttest samples, as specified by accurate profile measurements. A quantitative approach to the interaction forces recorded during the tests and micrographic observations complete the preliminary study. This work provides new basic data for a preliminary study of the interaction between labyrinth seal teeth tips and abradable coatings in turbo-engine applications.

International audience; To enhance the efficiency of a turbo engine, one solution is reducing the clearance between the rotary parts in the secondary air system. This clearance reduction causes direct interactions in the secondary air system of a turbo engine when a rotary seal, called a labyrinth seal, rubs against the turbo engine casing as a result of successive starts and stops, thermal expansions and vibrations. To protect sealing systems from severe damage, abradable coatings are used on the inner periphery of the casing. The purpose of the present paper is to study the labyrinth-abradable interaction during high speed contacts through a detailed tribological characterization. The labyrinth-abradable interaction experiments were conducted on a dedicated test rig that was able to reproduce representative turbo engine operating conditions. A complete tribological analysis based on a third body approach and on accommodation flows was investigated using high speed imaging of the interaction. A schematic description of the interaction, with the addition of images extracted from recorded videos, is proposed to define two types of third body formation and their evolution during labyrinth-abradable interactions. Finally, the labyrinth-abradable interaction life cycle was used as a basis to discuss the coating subject to a labyrinth tip speed increase.

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.

International audience; This paper presents ananalysis of the excavation forces involved in the cutting action and in the frictionof a cutter/rock contact.A vertical lathe-type device provided data on the forces applied on a single cutterunder dry excavation conditions.A cutting device was used to perform cutting experiments with unwornand worn cutters.A tribometer wa sused to perform friction tests on the cutters wear flat previouslyrealized with the vertical lathe.The experiments display results conform to theliterature as the non-dependence of the cutting coefficient to the rock properties.Then, this study focuses on the cuttingforces and explains that these forces include a component of rocks hearing and impact friction.Theimpact friction is induced by the interaction between ejected rock particles and the cutting active area.The evidence of this impact friction is then brought by the formation of an eroded zone on the cuttingactive area

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.

The quasi-static assemblies used in industrial applications are subjected to microsliding due to vibrations of variable amplitudes related to mechanical or aerodynamic instabilities. The titanium alloys used in such structures present extensive wear and must be protected by coatings. This article is devoted to the fretting behaviour of a coated Ti–6Al–4V alloy under free (un-imposed) displacement. The coating is made of a cupronickel alloy (CuNiIn or CuNiSi) topped by a polymer bonded solid lubricant MoS 2 . A fretting map was established to highlight the boundary between wear and seizure. An energetic coefficient of wear considering the global wear volume of the contact and the tribological parameters such as sliding distance and dissipated energy was established. Various morphologies of the third body observed in the contact were analyzed in terms of cohesion, ductility and adhesion. They were correlated to the tribological behaviour of the studied materials.

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.

International audience; The tribological behaviour of innovative graded cemented carbide inserts were studied by using a rotary tribometer and abrasive alumina counterfaces. This work completes the study made on commercial inserts with homogeneous cobalt content. Inserts with three types of graduation processes were considered: inserts with borides WCoB phases, imbibed inserts and inserts combining both processes (i.e. inserts with reactive imbibition). Physicochemical and mechanical measurements show that the WCoB phases increase the hardness towards the active surface and the imbibition increases the insert core fracture toughness. The wear tests indicate that the boride phases lower the friction coefficient. In addition, as for the commercial inserts, cemented carbide volumes with higher cobalt content also reduce the friction coefficient. Concerning the wear results, the boride phases improve the abrasion resistance. By applying a third body approach, the WCoB phases limit the introduction of cobalt binder in the source flow, the cohesion of alumina particles in the internal flow and the formation of an abrasive paste in the contact. The imbibition process, where the cobalt migration is controlled, does not affect the wear resistance by avoiding a cobalt enrichment of the cemented carbide near the active surface.

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.

International audience; This article proposes a simulation of cutting forces in broaching of fir tree slots. Every step needed to build an in-house fir tree broaching simulation is presented. This simulation is based on the discretization of each tooth of the broach, computation of elementary cutting forces of each discretized section, and finally the summation of each elementary cutting force into a global axes system. This work also presents orthogonal cutting experiments to implement realistic specific cutting forces into the model. The simulation obtained is compared with the real force signals of a fir tree broach to evaluate the quality of the simulation. At the end, the application to industrial context is developed.

International audience; The quality of innovating PDC (Polycrystalline Diamond Compact) bits materials needs to be determined with accuracy by measuring cutting efficiency and wear rate, both related to the overall mechanical properties. An original approach is developed to encompass cutting efficiency and wear contribution to the overall sample quality. Therefore, a lathe-type test device was used to abrade specific samples from various manufacturers. Post-experiment analyzes are based on models establishing coupled relationships between cutting and friction stresses related to the drag bits excavation mechanism. These models are implemented in order to evaluate cutting efficiency and to estimate wear of the diamond insert. Phase analysis by XRD and finite element simulations were performed to explain the role of physicochemical parameters on the calculated quality factor values. Four main properties of PDC material were studied to explain quality results obtained in this study: cobalt content in samples that characterizes hardness/fracture toughness compromise, undesired phase as tungsten carbide weakening diamond structure, diamond grains sizes and residual stresses distribution affecting abrasion resistance.

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.

En tribologie, un moyen de progresser efficacement dans la comprehension des phenomenes de couplage mecanique/thermique/environnemental sur la reactivite au niveau des surfaces et des interfaces est l'experimentation ou les conditions de contact sont parfaitement controlees (en termes de pression partielle d'oxygene, d'humidite ou sous vide, et de temperature). Pour cela, un tribometre environnemental a ete developpe. L'objet de cet article est d'exposer les solutions technologiques adoptees, de presenter l'instrumentation (capteurs de force, de deplacement, de pression, de temperature, de resistance de contact, analyseur de gaz in situ), et de decrire les performances de ce prototype.

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.