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International audience; During friction stir lap joint welding, hooks are systematically formed at the interface of dissimilar Al2024/Ti-6Al-4V plates. Various hooks dimensions and shapes are observed according to the welding parameters (welding speed, rotational speed and double-pass). Based on digital image correlation (DIC) during tensile shear tests, it is found that hooks features govern the fracture behaviours and, hence, the mechanical properties of the overall welds. In particular, three fracture behaviours are obtained: at the interface, in the Al2024 and across the Ti-6Al-4V plate, which was never reported in dissimilar Al/Ti studies. The aim of this work is (i) to study the influence of FSW parameters on the mechanisms of hooks formation and (ii) to understand the role of both AS and RS hooks on the mechanical behaviour of the welds, by the means of two new approaches. First, hooks are investigated by X-ray tomography which provided some innovative insight into the material flow (3D viewing). Based on these qualitative and quantitative results, hooks are used as tracers and two new scenarios of AS and RS generation are proposed. Then, the contribution of both hooks on the stress distribution is clearly revealed by some calculations of the stresses reached around AS and RS hooks. Contrary to RS hook, AS hook is sharp and, hence, the stress reached at the top of AS hook is characterized by a stress concentration. The results showed that the fracture behaviours are greatly influenced by hooks size and the formation of microstructural defects inside the Ti-6Al-4V, which depend on the welding parameters. In conclusions, this work revealed the relationship between the achievable parameters, the resulting hooks dimensions and the fracture behaviours. In addition, based on the understanding of the material flow, some ways of optimization of the mechanical properties are proposed.

WOM 2019 - 22nd International Conference on Wear of Materials, 14-18 April 2019, Hyatt Regency Miami, Miami, FL, USA; International audience; This paper examines plastic mechanisms that lead to damage to the sub-surfaces of ductile metallic contact areas and, by extension, to the creation of wear particles. The word “damage” is used here as a generic term to designate all topological, morphological and/or microstructural modifications to the surfaces that are the consequence of interfacial shear stress under friction. During friction, the accommodation mechanisms of plastic deformation by the M3 mode in the contact area can be different, depending on the microstructures of the alloy and the first sites (bulk materials).For example, for tempered martensitic steels under frictional stresses, plasticity strain occurs in Tribologically Transformed Surfaces (TTS) under dislocation gliding. The creation of new dislocations and the rearrangement of all these dislocations into a “bamboo”-type structure leads to a decrease in hardness, or softening of the steel.Otherwise, depending on the stacking fault energy in alloy microstructures, plasticity may occur through mechanisms involving either perfect dislocation gliding and/or partial dislocation gliding. In addition to hardening, the mobility and morphology of dislocations are also related to stacking fault energy, which can promote phase transformation leading to shear strain in TTS. These shear strain mechanisms have a great influence on the internal flow of wear particles and on the formation of the “Third-body layers”. These findings are given in a review of tribological results of measurements carried out on a wrought X38CrMoV5 grade steel and on cobalt-base thick coatings, using tribometers, under various loadings, test temperatures and sliding speeds. The cobalt-base thick coatings are deposited on steel by several processes which modify the nominal chemical composition of the cobalt alloy and make it possible to study the influence of the iron content on shear strain under friction stresses.

WOM 2019 - 22nd International Conference on Wear of Materials, 14-18 April 2019, Hyatt Regency Miami, Miami, FL, USA; International audience; This paper deals with the influence of the mechanisms of plasticity on the evolution of the friction coefficient in cobalt-based hardfacings. Particularly depending on the alloying element content and so on the stacking fault energy, plasticity in Co-based alloys may occur through mechanisms involving either perfect dislocations gliding and/or partial dislocation gliding. As the coalescence of stacking faults by partial dislocations promotes the phase transformation of the Co, this study focus on the impact of this phase transformation on the evolution of the friction coefficient.Stellite 21 (Co-27Cr-5Mo-0.25C) hardfacings deposited on a steel substrate by two different processes, namely metal inert gas (MIG) and laser, are studied. The tribological properties are evaluated with a ring on disc tribometer under high load (800 daN) and continuous sliding (5 mm/s) at room and high temperatures (450 °C and 600 °C). The wear volume is characterized by confocal microscope. From micro-hardness measurements and SEM observations the work-hardening and the plastic strain of the Tribological Transformations of Surfaces are identified in relationship with chemical analysis (by EDS). Moreover X-rays diffraction and EBSD reveal the crystal structure evolution.As-deposited Co-based hardfacings have a face-centered cubic (FCC) structure. Meanwhile depending on the process of deposition on the steel substrate, the nominal composition of the Stellite 21 can be modified due to dilution effect. So, the iron content is found to be higher in the MIG hardfacing than in the laser one, leading to different mechanisms of plasticity, respectively by perfect dislocations gliding and by phase transformation (FCC to hexagonal closed-packed (HCP)). Moreover, a significant influence of the phase transformation on the friction coefficient has been evidenced: without phase transformation the cobalt remains in the FCC structure and the friction coefficient is stable during the test, while a decrease of its value occurs during the FCC to HCP phase transformation.

International audience; In this study, the high temperature wear behavior of hot forming tool steel grades is investigated by successive sliding of a pre-alloyed Usibor1500P (R) strip heated at high temperature. Experimental tests are performed at high temperature on an instrumented Deep-Drawing Process Simulator (DDPS). This laboratory pilot is employed to rank different steel grades used as tool materials in the hot-stamping process. The wear damage of the tool (die radius) is characterized by profilometry and SEM observations, and three quantitative criteria are determined from 2D profile measurements to assess adhesive and abrasive wear. Under examined conditions at high temperature, a predominant transfer mechanism is observed, while abrasive wear appears as minor damage. When the surface hardness of the tool material is not great enough, the sub-surface of the die radius can exhibit a plastic shear deformation of about 10 gm in depth. This leads to emission of wear debris coming from the cumulated cyclic plastic deformation of the sub-surface. In contrast, for high surface hardness, the adhesive wear rapidly reaches an asymptotic state.

High-chromium white cast irons are commonly used in applications requiring excellent abrasion resistance, as in the mining and mineral ore processing industry. Their excellent abrasion resistance is mainly due to their solidification microstructures. In this study, 15 experimental high-chromium white cast alloys containing different chromium and molybdenum contents and a ratio Cr/C of 10 are examined. The wear experiments were carried out on a pin-on-disc tribometer at 700°C. Microstructure has the most important role in dictating friction. It was observed that the wear loss was insignificant compared with that obtained at room temperature. The mean friction coefficient was also lower than that observed at room temperature sliding wear. Copyright © 2012 John Wiley & Sons, Ltd.

16th International Conference on Wear of Materials 15-17 April 2007 • Montreal, Canada; International audience; Improvement of hot rolling process involves the lengthening of rolling campaigns. This can be achieved through the wear resistance improvement of work roll grades. The wear damage observed in service on the hot working rolls is the result of a complex interaction between oxidation, friction and thermal fatigue. An approach is to assess the coupled effect of oxidation and friction on the overall tribological behaviour at high temperature. A first step consists in a good knowledge of the material behaviour against each stress. Tribological behaviour of roll grades at high temperature is indeed strongly affected by the nature, the thickness and the thermo‐mechanical properties of their oxide scales involved into the contact. All these parameters depend on roll grade microstructure. An experimental methodology has also been performed to assess the influence of the roll grade microstructure on their tribological behaviour at high temperature. It constitutes the subject of the present paper. Results have focussed on high temperature friction tests and on the analysis of oxide morphologies (through TGA tests and environmental SEM, ESEM, observations). Nevertheless this approach does not aim to reproduce strictly rolling conditions, but only to analyse the chemical and mechanical response of roll grades regarding thermal and mechanical stresses involved in hot rolling. Material investigations have shown heterogeneous oxidation according to the different microstructural states and the environmental testing conditions. A particular scale roughness, depending on grades and oxidation conditions, and influencing the tribological behaviour, has been brought out.

20th International Wear of Materials Conference (WOM), Toronto, CANADA, APR 12-16, 2015; International audience; Aeronautic forging dies are subjected to very high loads and temperatures for a long contact time between the pre-heated parts and dies. Cobalt-based hardfacings are commonly deposited on dies and their main wear mechanism is large plastic deformation of the die radii. This paper deals with the wear damage mechanisms of three different cobalt-based hardfacings: Stellite 21 deposited by a MIG process, Stellite 21 and Stellite 6 deposited by a LASER process. The tribological tests are carried out on a high load Ring on Disc tribometer at room temperature. The postmortem investigations are undertaken by SEM observations, micro-hardness measurements as well as by X-ray diffraction analyses. Results show that the increase of the hardness, in order to improve the wear behaviour, can be achieved by a higher carbon content and by a lesser iron dilution that depends on the deposition process. A very important work-hardening, up to 90%, is also observed under sliding conditions and a relationship is established between the increase of the micro-hardness and the plastic strain level. Two different plastic strain mechanisms are observed. For high (MIG) or low (LASER) iron dilution levels, the plastic strain causes respectively a reorientation of grains or a FCC to HCP phase transformation; the latter being associated with a lower friction coefficient.

International audience; In a hot strip mill, the contact established between the hot strip and the work rolls in the first times of running has to be oxide on oxide to allow the strip to be pulled in the roll bite. The oxide scale formed on the roll is submitted to thermo-mechanical stresses and grows up. From a critical thickness, the scale spalls and causes some superficial damage to the rolls and to the strip. For the roll manufacturers as well as for the steel makers, it is essential to understand the influence of the creation and the growth of such a scale on friction in order to control the antagonist superficial damage and consequently to reduce the running cost of the mill. The present work aims to study the interaction between the oxides formed on a work roll grade and the coefficient of friction established with a strip steel usually rolled by this roll grade. A high temperature tribometer was set up in a pin-on-disc configuration. A previous part of study showed that friction was dependent on the nature of antagonist materials and the thermal transfer. We observed the establishment of a running-in period in the case of a metal-on-oxide initial contact between the pin and the disc which corresponds to the creation of an oxide layer on the pin. The mechanisms that allowed the formation of this scale were determined. SEM observations in conjunction with EDS analysis, both inside and outside the contact area on both antagonists, led to the development of a phenomenological model explaining the creation and the movement of oxide debris in the contact.

In machining, it is clearly noticed that the cutting tool wear influences the cutting process. However, it is difficult with experimental methods to study the effects of tool wear on several machining variables. Thus, in the literature, some earlier studies are performed separately on the effect of tool flank wear and crater wear on cutting process variables (such as cutting forces and temperature). Furthermore when the workpiece material adheres in cutting tool, it affects considerably the heat transfer phenomena. Accordingly, in this work the finite element analysis (FEA) is performed to investigate the influence of combination of tool flank and crater wear on the local or global variables such as cutting forces, tool temperature, chip formation on the one hand and the effects of the oxidized adhesion layer considered as oxide (Fe 2 O 3 /Fe 3 O 4 /FeO) on the heat transfer in cutting insert on the other hand. In this investigation, an uncoated cutting insert WC–6Co and medium carbon steel grade AISI 1045 are used. The factorial experimental design technique with three parameters (cutting speed Vc, flank wear land VB, crater wear depth KT) is used for the first investigation without adhesion layer. Then, only linear investigation is performed. The analysis has shown the influence of the different configurations of the tool wear geometry on the local or global cutting process variables, mainly on temperature and cutting. The simulation’s results show also, the highly influence of the oxidized adhesion layer (oxide Fe 2 O 3 /Fe 3 O 4 /FeO) on the heat transfer.

Issu de : ICTMP 2014 - 6th International Conference on Tribology in Manufacturing Processes & joining by plastic deformation, Darmstadt, GERMANY, June 22-24, 2014; International audience; High temperature forging tools are highly damaged by wear shearing under cyclic loading which reduces the life of tools. In real conditions, depending upon the tool areas, the level of wear can change. The surfaces of tools can be treated by cobalt-based hardfacing using different welding processes. This study focuses on tribological damages of Stellite 21 deposited by MIG process. Wear tests are carried out at room and high temperature on a ring on disc tribometer under high normal load. Different testing conditions are examined. The surface plastic strain due to the friction shear stresses is demonstrated by different methods like SEM observations, micro-hardness measurements and XRD analyses. More particularly, it is shown that the initial (200) crystallographic preferred orientation due to the welding process is modified into (111) crystallographic preferred orientation due to shear stresses regardless the loading. Moreover, a relationship between the gradient of the plastic shear strain at the friction subsurface and the level of the micro-hardness measurement has been established. In regard of the results, when the shear strain exceeds a threshold, the micro-hardness measurement is stabilised. The ultimate value could be induced by the stacking fault energy that is considered as weak in Stellites.

SCIE Article le 06/02/14; International audience; This original study investigates the damages of WC-6Co uncoated carbide tools during dry turning of AISI 1045 steel at mean and high speeds. The different wear micromechanisms are explained on the basis of different microstructural observations and analyses made by different techniques: (i) optical microscopy (OM) at macro-scale, (ii) scanning electron microscopy (SEM), with back-scattered electron imaging (BSE) at micro-scale, (iii) energy dispersive spectroscopy (EDS), X ray mapping with wavelength dispersive spectroscopy (WDS) for the chemical analyses and (iv) temperature evolution during machining. We noted that at conventional cutting speed Vc ≤ 250 m/min, normal cutting tool wear types (adhesion, abrasion and built up edge) are clearly observed. However, for cutting speed Vc > 250 m/min a severe wear is observed because the behavior of the WC-6Co grade completely changes due to a severe thermomechanical loading. Through all SEM micrographs, it is observed that this severe wear consists of several steps as: excessive deformation of WC-6Co bulk material and binder phase (Co), deformation and intragranular microcracking of WC, WC grain fragmentation and production of WC fragments in the tool/chip contact. Thus, the WC fragments accumulated at the tool/chip interface cause abrasion phenomena and pullout WC from tool surface. WC fragments contribute also to the microcutting and microploughing of chips, which lead to form a transferred layer at the tool rake face. Finally, based on the observations of the different wear micromechanisms, a scenario of WC-6Co damages is proposed through to a phenomenological model

International audience; This paper presents the tribological characteristics of PACVD diamond coatings on titanium alloys. The coating morphology and quality have been investigated by SEM and Raman spectroscopy, and the surface roughness has been determined by AFM measurements. Friction and wear characteristics have been studied with a pin-on-disk tribometer as a function of the applied load, the sliding speed and the counterface material. For diamond-on-diamond contact, the dynamic friction coefficient stabilizes at a value of 0.05-0.08 after the surface polishing of diamond crystal tips. For non-diamond coated pins, a surface running in time at low sliding speed is necessary to avoid a coating spalling on the disk. After this step, the sliding speed can be increased to 1 m s(-1) with a friction coefficient equal to or lower than 0.1. The wear of the diamond-coated disk generally cannot be measured.

International audience; A methodology is developed in this article to assess the wear damage occurring on a die radius in a deep-drawing process. Profilometric and topographic measurements were performed with a white-light-source confocal microscope on the rubbed part of the die radius, using a repeatable procedure. Four new quantitative damage criteria were defined from 2D profiles, to quantify and determine the degree of regularity of both adhesive and abrasive damage. This method allows different tool steels to be classified in terms of their adhesive and abrasive wear behavior, with fairly good repeatability and reliability.

17th International Conference on Wear of Materials, Las Vegas, NV, APR 19‐23, 2009Extrait de : WOM 09 / sous la dir. de K.C. LUDEMA et S.J. SHAFFER; International audience; High‐chromium white cast irons are commonly used in applications requiring excellent abrasion resistance, as in the mining and mineral ore processing industry. Their excellent abrasion resistance is mainly due to their solidification microstructures. During their solidification, high levels of chromium (16‐32%) lead to the formation of a high‐volume fraction of eutectic M(7)C(3)‐carbides, which may or may not be associated with primary carbides in a heterogeneous austenitic/martensitic dendritic structure. Generally, in common white high‐chromium cast iron, the molybdenum content is less than 3 wt.% (by weight) so as to avoid a perlitic transformation. It is reported that by addition of molybdenum in quantities of more than 3 wt.%, new carbides (M(2)C, M(6)C) are formed which greatly increase the high‐temperature wear resistance. In this paper, 15 high‐chromium white cast alloys containing different chromium contents (16 wt.%Cr to 32 wt.%Cr) and molybdenum (Mo free to 9 wt.%Mo) are examined. For each alloy, the chemical composition and volume fraction of carbides and matrix have previously been determined. The matrix microstructure and the type of carbides depend on the relative contents of molybdenum and chromium. The wear experiments are carried out on a pin‐on‐disc tribometer at room temperature. The pin is made of cast iron. The wear mechanisms are observed by scanning electron microscopy (SEM). It is observed that the pin height loss, the evolution of friction versus time curves and the mean friction coefficient are largely dictated by the matrix microstructure. The carbides volume fraction and the macroscopic hardness both play only a minor role. The pin height loss is significant for a single‐phased matrix and the mean friction coefficient is high. When the matrix is multiphased, the pin height loss tends towards zero and the coefficient of friction is lower. Detailed SEM observations and analysis of the evolution of the friction versus time curves indicate the substantial contribution of the large carbides in friction contact.

17th International Conference on Wear of Materials, Las Vegas, NV, APR 19-23, 2009; International audience; In machining processes, surface roughness and dimensional accuracy of machined parts depend on tool wear. Cemented carbide based cutting tools remain widely used in machining processes for their wear resistance. The aim of this paper is to study wear mechanisms of a WC-Co cutting tool grade under tribological conditions. Dry friction experiments are carried out on a high-speed pin-on-disc tribometer considering WC-Co pins against steel discs made of an AISI 1045 grade. Furthermore, two type-K thermocouples are embedded in WC-Co pins in order to estimate the contact temperature during sliding tests. A large sliding speed range is considered: from 60 up to 600 m/min. Results deal with wear rate versus tribological conditions, evolution of friction coefficient and temperature versus sliding speed and evolution of wear mechanisms. WC-Co tribological pins exhibit different wear mechanisms: abrasion, adhesion, transgranular WC micro-cracking and WC/WC debonding. Circulation of debris in the friction contact depends on sliding speed and on test duration. The evolution of surface temperature versus friction coefficient is studied too. Furthermore, a particular attention is paid on the relationship between the thermal energy in the pin and the mechanical energy in the contact. The thermal energy is calculated from the temperature values of K-thermocouples and the mechanical energy dissipated in the pin is calculated from friction coefficient evolutions. Relationships are established through modelling approaches.

An approach is proposed for the evaluation of surface fatigue damage of hot forming tools that undergo severe thermo mechanical loading. Fatigue crack propagation in a hot work tool steel X38CrMoV5-47HRC is investigated using single-edge cracked tension specimens with 3 different thicknesses (2.5, 1, 0.6 mm) and two R-values. The stress intensity factor is evaluated with ABAQUS®. Paris curves are established for the crack propagation experiments.

International audience; An approach is proposed for the evaluation of surface fatigue damage of hot forming tools that undergo severe thermo mechanical loading. Fatigue crack propagation in a hot work tool steel X38CrMoV5‐47HRC is investigated using single‐edge cracked tension specimens with 3 different thicknesses (2.5, 1, 0.6 mm) and two R‐values. The stress intensity factor is evaluated with ABAQUS (R). Paris curves are established for the crack propagation experiments.

Issu de : WOM 09 - 17th International Conference on Wear of Materials 19-23 April 2009 • Las Vegas, USA; International audience; Given the specific micro‐structure of some steel grades, under tribological conditions the sub‐surface material or sub‐layers of sliding bodies are prone to cumulative cyclic plastic deformation, leading to the formation and emission of wear debris. In the present paper, a new wear model based on a cyclic ratchetting‐type plastic deformation of subsurface material is proposed. It is considered that the debris is formed and the wear‐loss occurs when the accumulated plastic deformation at sub‐surface exceeds ``a critical strain'' or ``rupture limit''. The model takes into account the number of cycles or test duration, a characteristic thickness of the sub‐layer dependent on tribological conditions and material properties, the shear rupture ductility and an average plastic strain increment. The average plastic strain increment is estimated by numerical simulation of pin‐on‐disc friction. A very close correlation is found between the predicted and experimental wear heights versus time and/or versus the number of cycles. The wear investigations were carried out on a high‐temperature pin‐on‐disc tribometer under dry friction conditions. Experiments were performed under constant load, speed and disc temperature for different durations. The steel grade involved was a tempered martensitic tool steel X38CrMoV5 (AISI H11). Wear mechanisms were investigated by Scanning Electron Microscopy (SEM) observations in surface and cross‐section.

International audience; Cemented carbide based cutting tools remain widely used in machining processes for their wear resistance. Knowledge and modelling of wear processes are of prior importance to get models to predict and increase the cutting tool service life. The present paper deals with some results of a research work that study relationships between wear and temperature in the case of pin-on-disc tribological experiments. Topic of the paper is focussed on the estimation of the pin/disc contact temperature by coupling experimental measurements and computational methods. Friction experiments are conducted with WC-6Co pins against steel discs made of an AISI 1045 grade. Furthermore, WC-6Co pins are instrumented with two type-K thermocouples. A large sliding velocity range is considered in the study: from 100m/min up to 600m/min. The present paper is focussed on the 600m/min velocity. During these tribological tests, tangential forces and thermocouple temperatures are measured and monitored. From these values, the heat flux in the WC-6Co pin is estimated by two different ways. On one hand by considering the unidirectional Fourier law, on the other hand by estimating the heat partition coefficient [1] between the disc and the pin. The heat partition coefficient is determined from a physical approach based on the consideration of the mechanical power dissipated in the contact. In both cases, the heat transfer in the pin is then modelled by finite element methods. It is necessary to perform numerical analysis to estimate the pin/disc interface temperature because of the impossibility to measure it directly during friction tests. Results of experiments and of numerical simulations are compared.

16th International Conference on Wear of Materials 15-17 April 2007 • Montreal, Canada; International audience; The wear rate of sliding materials seems impossible to predict. It is clearly a function of substrate material, but we are learning that many surface events are also important. In particular, studies on the last 20 years have shown that substances develop between two sliding bodies have a strong influence on both friction and wear. These substances had become known as "third bodies". Whereas third bodies have been proven to be important, they have not been completely characterized. The third bodies have been studied, but it appears that the geometries of the sliding pairs are also important. This paper describes some experiments in which the composition of third-body materials are controlled by temperatures and in which the durability of third-body films are influenced by the geometry of contact between sliding bodies.

Issu de : WOM 05 - 15th International Conference on Wear of Materials; International audience; The tribology behaviour of the die radius, in the deep-drawing process, results from surface interactions between the metal sheet and the tool under contact pressure. Friction, degradation of the sheet, surface quality and tool wear are a result of this interaction. This article aims to study degradations of the radius portion of a die in the deep-drawing process. The study on an industrial press is complex, so we developed our own test facility, an experimental device, which represents a deep-drawing process-simulator. It allows friction between the metal sheet and a die radius. It can dissociate the diverse mechanical effects exerted on the die radius by enabling a study of the component conditions found in deep-drawing (effect of the blankholder, of the pulling forces, the sliding rate, etc.). An experimental study was conducted on a low-carbon steel sheet and a X160CrMoV12 steel die radius. It revealed that the die radius surface was degraded by ploughing and transfer mechanisms. The distribution of these degradations on the die radius was localised in two areas but varied in intensity depending on the exit angle between the sheet and the die radius. The die radius friction coefficient has been modelled with respect to the mechanical characteristics of the deep-drawing process-simulator. This model leads to a correlation between the friction coefficient and the degradation evolutions on the die radius.

International audience; A significant part of the energy in forging is used to break the interfacial junctions due to friction between the tool and the workpiece. The life of hot‐forging tools is usually limited by complex interactive mechanisms under cyclic loading such as abrasive, adhesive and scaling wear, thermal and mechanical fatigue, and plastic deformation. This contribution deals with the wear mechanisms of the tempered martensitic X38CrMoV5 steel (AISI HI 1) under high‐temperature and dry‐sliding wear. The investigations are carried out with high‐temperature pin‐on‐disc tests. The pin is cut from bars of X38CrMoV5 steel treated at 42 and 47 HRC. The disc is made of common steel (AISI 1018, XC18). Temperature of the disc ranges from 20 to 950 degreesC. Before the test starts, the disc is first pre‐heated for 1 h. The experiments are performed under constant load and velocity. The friction coefficient decreases quasi‐linearly with the rising disc temperature up to 800 degreesC. Over this temperature, it decreases drastically for the 42 HRC steel but remains linear For the 47 HRC steel. Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) investigations have revealed that wear is essentially due to abrasion, plastic deformation and fatigue. Set of cracks due to contact rolling fatigue is observed on the pin and the disc. Those cracks are located on the transferred scale on the pin and on the oxide scale of the disc wear track. The cross‐section observations of the pin have revealed a plastically deformed zone beneath the surface. In this sub‐surface layer, the tempered martensitic microstructure seems to be more aligned due to friction and the plastic deformation.

13th International Conference on Wear of Materials, VANCOUVER, CANADA, APR 22-26, 2001; International audience; In this study an experimental methodology to study surface wear of injection moulds on a full scale polymer injection moulding machine is developed. A comparative study of four different tool materials for mould cavities is carried out in situ. The polymer injection conditions (geometry cavities, machine settings, polymer) are chosen in order to accelerate the degradation of the mould cavity surface. The reinforced polycarbonate resin Lexan 341R-739, containing 40% of weight of short glass fibres, well known for their abrasive character, was injected with jetting and normal injection conditions. Jetting conditions implies high pressure and velocity of the injected resin. The die wear is discussed in relation to the polymer flow conditions and the die materials.

13th International Conference on Wear of Materials, VANCOUVER, CANADA, APR 22-26, 2001; International audience; in hot forming processes, the interface tool/product is important for the quality of the finished product. In hot rolling, the scale formed on the roll material plays an essential role. As soon as the contact oxide-oxide is established between the roll and the slab, friction allows the process to start. However, the oxide scale will continue to grow under the cyclic action of hot contacts and water cooling, and will be subjected to thermo-mechanical stresses. Beyond a certain critical thickness, the oxidized surface layer of the cylinders has not sufficient mechanical strength to withstand the shear stresses. The wear and friction behavior of the oxides appearing on the surface of the hot working rolls is not well known. The influence of these oxides on the friction and, consequently, the quality of the products of finishing mills, seems very significant. So, in this study, we investigate the evolution of the friction coefficient and the wear, according to the growth, the nature and the thickness of the formed scale. We use a high temperature pin on disc tribometer. The pin consists of material ``rolls'' while the disc consists of the slab. The pin is instrumented with thermocouples in order to couple the friction coefficient measurements with the thermal gradient in the pin and the surface temperature and the formed oxides, Then, the characterization of the surfaces is done by scanning electronic microscopy (SEM) and EDS analyses. We use the method of sin(2) Psi to evaluate residual stresses of oxide and correlate these data with shear stress behavior.

The tribo-characteristics of metal forming at high temperatures have not yet been well understood due to the complex nature of thermal, microstructural, interaction, and process parameters. This is a review paper on the effects of temperature, coating, and lubrication to the tribological characteristics in hot forming as well as the tribometers for different metal forming processes at elevated temperatures mainly based on the experimental work. The tribological behaviors of oxides in hot forming, such as rolling and stamping, were reviewed and presented. Some commonly used surface coatings and lubricants in hot forming were given. Many types of tribometer were selected and presented and some of them provided a great potential to characterize friction and wear at elevated temperatures. Nevertheless, more testing conditions should be further investigated by developing new tribometers. Eventually, experimental results obtained from reliable tribometers could be used in theory and model developments for different forming processes and materials at high temperatures. The review also showed the great potential in further investigations and innovation in tribology.