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Flat samples of WC–Co hardmetals with 6–12 wt%Co were surface finished by grinding, polishing and wire-electro-discharge machining (EDM). Comparative dry reciprocating sliding experiments against WC–6 wt%Co pins were performed using a Plint TE77 tribometer. Tribological characteristics were recorded online. Wear surfaces were characterized by surface scanning topography and scanning electron microscopy. Wire-EDM’ed samples exhibited higher friction and wear compared to ground and polished equivalents. This trend was correlated to X-ray diffraction measurements revealing tensile residual surface stresses in WC after wire-EDM contrary to compressive surface stresses after grinding and polishing. However, finer executed EDM reduces friction and wear significantly.

Based on the well-known pin-on-disc test rig, a new test setup has been developed for online measuring of the wear and friction behavior of polymer matrix composites. Contrasting with a traditional friction and wear test rig, a steel pin and composite disc are presently used to study the influence of wear debris and fiber orientation. This test setup has a main influence on the results. The polymer materials, a thermoset polyester, and a thermoplastic polyphenylene sulfide were reinforced with glass and carbon fibers, respectively. The results show two different promising aspects of ‘‘new’’ materials for wear and friction applications and more specific to different machine components, namely selflubricating and self-protecting ability. The self-lubricating ability exists from creating a polymer film on the surface which reduces the coefficient of friction, and the self-protecting behavior is because of the creation of a protecting carbon film on the surface. The self-lubricating ability depends on the test parameters, while the self-protecting ability was due to the material and the structure. POLYM. COMPOS., 30:932–940, 2009. a 2008 Society of Plastics Engineers

Polyimides are used in dry sliding applications due to their high strength and thermal stability. Friction and wear behaviour of sintered polyimide (SP) and thermoplastic polyimides (TP) are investigated for steel counterface temperatures between 100 and 260 °C. Afterwards, the tribophysical phenomena in the sliding interface are investigated by calorimetric and thermogravimetric analysis of the wear debris and Raman spectroscopy of the worn polymer surfaces. Spectroscopic analysis of SP shows that C N C bonds in the main molecular structure are progressively stretched along the sliding direction with a reorganisation between axial and transverse orientation at 180 °C. Stretching of the C O and C O C side groups depends on the applied load. For TP there is degradation of the imide part under high temperatures while the sliding surface visually colours from transparent to opaque yellow. The observed chemical reaction of TP under sliding can be attributed to ring opening by chain scission induced by sliding and cross-linking between the adjacent polymer chains. From thermal analysis, a reversible endotherm reaction at 180 °C might be attributed to dehydratation of polyimides. This reaction depends slightly on the applied sliding temperature and is less prominent for 100 °C and the most prominent for the 260 °C sliding temperature, suggesting further imidisation under sliding for sintered polyimides. Thermoplastic polyimides show a clear shift in secondary glass temperature towards higher temperatures and a decrease of the melting peak and recrystallisation peak intensities, suggesting the formation of cross-links in the amorphous phase.

Polymer composites with graphite additives are often used to control sliding properties. The lubricating action of graphite, however, strongly depends on the working conditions of normal load and sliding velocity. Sintered polyimide with 15 wt.-% graphite powders is worn under 50 to 200 N normal loads and 0.3 to 1.2 m/s sliding velocities in line contact with a steel counterface. The tribological properties of graphite are unfavourable under low sliding velocity and high normal loads, causing high friction and wear rates. The transfer film then mainly consists of rough graphite flakes, and large wear debris particles indicate overload, as the loading capacity of graphite is limited. Graphite powders reduce coefficients of friction from 0.45 to 0.12 under medium to high sliding velocities and normal loads. The transfer films are rendered smooth by mixing polyimide and graphite fillers. Stabilisation in friction and wear as a function of normal load and sliding velocity (pv-value) coincides with levelling of the maximum polymer surface temperature of 115 °C. The favourable action for graphite under sliding, i.e. mechanical shear, and the improved thermal conductivity of the polymer bulk, control the sliding mechanisms of polymer composites under certain sliding conditions.

The acoustic emission (AE) technique was used for condition monitoring of pultruded glass/polyester composites subjected in abrasive wear. Three wear mechanisms were recognized by means of wavelet and cluster analyses of the AE data. We were able to associate these mechanisms with fiber breakage, debonding and a hybrid type based on the results of pattern recognition, previously performed for the signals recorded during tensile tests. For the latter tests, the temporal order of appearance of the different damage mechanisms allows to draw conclusions more easily about the correlation with the AE signals. A number of AE features were selected for classification using parameter-less self-organized mapping (PLSOM), which is a type of neural network that is not bound to the naturally subjective learning rate, neighborhood function and their annealing with the training progress.

Polyimide surfaces after macroscopic wear at 80 to 260°C are studied by atomic force microscopy to give additional insight in the tribophysical and -chemical processes during sliding. Three sliding regimes are distinguished with hydrolysis resulting in rough surfaces, imidisation resulting in orientation of polymer molecules and melting resulting in short-range arrangements.

The tribological behaviour of a polymer composite is compared during small-scale and large-scale sliding tests and it is observed that test results strongly depend on the fibre orientation and test configuration. Different wear mechanisms are evaluated by optical microscopy and finite element modelling in relation to a real application of polyester/polyester discs as bearing elements.

Polymer wear debris particles undergo a thermal and mechanical (shear) cycle since their generation and therefore contain information on the friction and wear processes, while it is often difficult to draw quantitative data from them relating to transitions in tribological behaviour. Results from thermal DTA/TGA analysis and morphological pattern spectra of debris are presented and related to a transition at 180°C sliding temperatures own to hydrolysis and imidisation.

Pultruded glass fibre reinforced polyester where used to investigated the frictional behaviour of the 45° fibre orientation. Therefore, on a rebuild Pin-on-disc test rig, using composite discs and steel pins, the frictional behaviour of these materials is investigated. Due to the indicated fibre orientation, and a simple trigger mechanism, it was possible to distinguish the friction force for different fibre orientations. The classical know orientations, parallel and perpendicular to the direction of sliding provided expected results, for both cases. The ~45° orientation, which is extremely important regarding filament wounded bearings, behaved in both cases similar -- regarding friction force-- in an unexpected way. The friction force of the 45° orientation was higher in all cases, and the amount was similar to the difference between the parallel and perpendicular case. These findings yield the conclusion that the 45° orientations cannot be neglected in frictional studies, and the behaviour of weft-warp structures also determines performance.

Polymer matrix composites are widely used as bearing materials for heavy load applications. Still fundamental knowledge about the wear mechanisms of these materials and the evolution in time of these mechanisms is lacking. Currently these mechanisms are only analyzed by post mortem analysis. The Laboratory Soete, based on the well-known pin-on-disc test rig, has developed a new test-setup. Instead of the standard composite specimen and steel disc, a rotating composite disc and steel pin is used to be able to have a visible composite wear track. Standard wear and friction measurements will be further combined with vibration measurements. The vibration measurements give valuable information about the pin-disc contact. The combination of all these measurements should yield valuable information on the active wear mechanisms, the occurrence of fiber and matrix fracture, fiber pull out, generated frictional heat, formation mechanisms of wear particles, and the interaction between all these effects.

After 11,300 h of operation, one of the axles of the reduction gearbox of the hoist mechanism of a dockside crane failed due to the propagation of a fatigue crack. The axle contained two gears, the largest of which was mounted swith a key, while the smallest gear was an integral part of the axle. On both ends of the axle a bearing was mounted: one cylindrical bearing and one spherical roller bearing (locating bearing). The axle was made from 17CrNiMo6 steel, and the machined gear teeth were case carburised. The failure of the axle occurred in the middle between both bearings of the axle, on a non-contacting part of the gear machined on the axle. During visual inspection striations were observed on the fracture surface and three separate crack initiation zones could be observed on three neighbouring gear teeth. The ratio of final fracture versus fatigue fracture was low, indicating a low nominal stress on the axle. The three initiation sites were investigated with optical microscopy and SEM, and clear indications of a ductile overload fracture were found. In an etched longitudinal section of one of the gear teeth where initiation took place several cracks could be observed in the hardened case of the tooth. The hardness at the initiation site was found to be 777 HV, while the hardness at the final fracture (diametric position relative to the initiation of the crack) was found to be lower, namely 722 HV. It was concluded that at the initiation site during the case carburisation quenching cracks were formed, which then propagated through the whole section of the axle under relatively low operation load.

Wear and failure mechanisms of polyoxymethylene homopolymer (POM-H) loaded above its yield strength are studied on test samples with a 22500 mm 2 sliding area at 8–150 MPa contact pressures. Test results are compared to small-scale cylinder-on-plate tests. Plastification of the sliding surfaces at high loads is favourable for low friction, while different wear mechanisms compared to small-scale testing are induced. Small-scale tests show a transition from mild adhesive/abrasive wear to severe wear due to softening, which is characterised by the formation of shear lips. Softening of large-scale sliding surfaces does not cause overload but it contributes to stable wear rates. Overload of large-scale samples is characterised by the transition from softening to melting and degradation. The dimensional stability of polymer elements is influenced by creep and it is verified that deformation of small-scale samples and large-scale samples loaded at 8 MPa is recovered after sliding, while it remains as permanent deformation for large-scale tests at 16–150 MPa. The wear transitions are further analysed by optical microscopy and available temperature models. The flash temperature concept can be applied for small-scale tests and large-scale tests up to 8 MPa. Calculated flash temperatures indicate softening and are in agreement with visual observations of the polymer surfaces. Flash temperatures for large-scale tests at 16–150 MPa indicate melting and degradation that was not visually observed on the polymer surfaces. The bulk temperature model prevails during large-scale sliding and only indicates melting at 150 MPa. Thermal analysis of the worn polymer surfaces confirms that crystallisation happened during small-scale sliding and large-scale sliding up to 55 MPa, occurring between 120 and 150 °C. Thermo-oxidative degradation above 200 °C is evidenced at 150 MPa.

Based on the well-known pin-on-disc test rig, a new test setup for online measuring of wear and friction behaviour of polymer matrix composites has been developed. In contrast to a traditional friction-and-wear test rig, a steel pin and composite disc are used for studying the influence of wear debris and fibre orientation. During sliding, a thin adhesive film is possibly formed on the wear track of a composite disc, consisting of wear debris that is squeezed under the steel pin and that finally smoothens onto the composite surface. By optical microscopy, it was observed that most of the debris particles originate from the edges of the wear track. The thin film deforms continuously, with large and dark wear particles observed at the edge of the wear track. A lower coefficient of friction is achieved when the particles are re-adhered to the mating surface. The film formation mechanism depends on the normal force, sliding velocity, and bulk composite structure: because pultruded composite profiles are prese...

Wear mechanisms of glass fiber reinforced polyester composites subjected to sliding wear for loads ranging from 60 to 300 N at a constant speed (10 mm/s) are studied by scanning electron microscope (SEM). The friction and wear tests are carried out, in dry conditions, on a newly built pin-on-disc machine with a rotating composite disc and fixed steel pin. The composite disc is cut out of pultruded plates, revealing a specific structure. The classical laws of fiber orientation and their relation to friction can be proven in one single test, while the development of a thin film layer lowers the overall coefficient of friction, with almost 20%. Further on there is a clear relation between the coefficient of friction and the material structure. Also the importance of the fiber orientation and the relation to the wear of the pin sample are clearly shown. The experimental results have shown that there is an existence of noticeable features on the worn surfaces. The use of SEM images in the study of this film formation and in the wear mechanisms of these materials gives an indication of the importance of shear forces on the wear behavior of these materials.

Polyimide cylinders are slid under 50 N normal load and 0.3 m/s sliding velocity against carbon steel ( R a =0.2 and 0.05 μm), high-alloy steel ( R a =0.05 μm), diamond-like carbon (DLC, R a =0.05 μm) and diamond-like nanocomposite (DLN, R a =0.05 μm). Only for a limited range of test parameters, the friction of polyimide/DLN is lower than for polyimide/steel, while polyimide shows higher wear rates after sliding against DLN compared to steel counterfaces. The DLN coating shows slight wear scratches, although less severe than on DLC-coatings that are worn through thermal degradation. Therefore, also friction against DLC-coatings is high and unstable. Calculated bulk temperatures for steel and DLN under mild sliding conditions remain below the polyimide transition temperature of 180 °C so that other surface characteristics explain low friction on DLN counterfaces, as surface energy, structural compatibility and transfer behaviour. Friction is initially determined through adhesion and it is demonstrated that higher surface energy provides higher friction. After certain sliding time, different polyimide transfer on each counterface governs the tribological performance. Polyimide and amorphous DLC structures are characterised by C–C bonds, showing high structural compatibility and easy adherence of wear debris on the coating. However, it consists of plate-like transfer particles that act as abrasives and deteriorate the polyimide wear resistance. In sliding experiments with high-alloy steel, wear debris is washed out of the contact zone without formation of a transfer film. Transfer consists of island-like particles for smooth carbon steel and it forms a more homogeneous transfer film on rough carbon steel. The latter thick and protective film is favourable for low wear rates; however, it causes higher friction than smooth counterfaces.

Pure and internal PTFE-lubricated polyesters are slid against steel and stainless steel on large-scale and small-scale tribological test rigs under normal loads between 8 and 150 MPa and temperatures of 80-200 °C. The polymer sliding surfaces are analysed by Raman spectra according to a two-phase system of trans (oriented) and gauche (non-oriented) structures. Comparing the spectra of pure and internal lubricated polyesters reveals a clear band at 713 cm -1 correlated to PTFE depositions. For small-scale tests, the crystalline content and overall orientation is larger (40-50%) while no significant increase in the PTFE-related band is observed. A PTFE/polyester film is transferred onto the steel counterface without back-transfer to the polymer surface. Large-scale polymer surfaces have a lower orientation (30-40%) and contradictory to small-scale samples, PTFE lubricants are observed on the polymer surface. The formation of a PTFE film on the polyester sliding surface is then favourable for low friction and stabilization in wear rates. Orientation and wear effects depend on the movability of wear debris and plastification of the surface, illustrated by Raman spectroscopy.

Polyimides are used in friction and wear applications because of their high strength and thermal stability up to 310 (short term) or 450 °C (long term). After sintering, secondary transition temperatures do not occur up to the degradation point. The tribological characteristics of sintered polyimides are presently investigated for counterface bulk temperatures between 100 and 260 °C. Through the high stiffness and lack of softening, transfer mechanisms are different than those of traditional engineering polymers. No transfer film was observed at low temperatures, resulting in high friction (μ > 0.30). For bulk temperatures above 180 °C a sudden drop in friction was found towards μ < 0.30, in accordance with the formation of a non-homogeneous transfer film consisting of plate-like particles. Wear rates are minimum for a bulk temperature of 140 °C and progressively increase at higher temperatures. The evaluation of two temperature models for calculating bulk and flash temperatures indicates that the wear transition corresponds to flash temperatures above 180 °C. Thermal analysis reveals that the 180 °C transition temperature is explained by dehydration effects. The orientation of the polymer chains at the sliding surface is investigated by post-mortem Raman spectroscopy, showing that the C–N–C bonds in the molecular structure are progressively stretched along the sliding direction with higher sliding temperatures. However, at 180 °C there is a reorganisation between the axial and the transverse orientation.

Polymers are frequently used in tribological applications because of their self-lubricating ability and loadability. However, most research on their friction and wear mechanisms is performed on small-scale test samples under relatively low normal loads. The present paper presents test results that are obtained on large-scale test samples under extremely high loads (190–3380 kN, corresponding to 8–150 MPa), providing more accurate design information for construction of e.g. sea-lock or crane guides. Performance and failure modes in overload conditions are investigated. Pure polyethyleneterephtalate (PET) shows high friction and unstable wear due to stick-slip. Filled with polytetrafluouroethylene (PTFE), friction is reduced and becomes stable even when loaded above the yield strength. The plastic deformation of the sliding surface under overload conditions allows for the formation of a transfer film on the counterface, resulting in a decrease of specific wear rate at higher loads. A model is presented for correction of wear measurements that contains both real material loss and deformation due to creep and thermal expansion. For the latter, temperatures as measured from frictional heating are corrected and evaluated according to known temperature models. Compared to small-scale cylinder-on-plate and bloc-on-ring tests, friction coefficients measured on large-scale tests are much lower due to large polymer transfer, while wear rates are much higher. The availability of PTFE and favourable plastication of the sliding surface on large-scale tests result in the build up of a coherent lubrication transfer film.

On a pin-on-disc test rig online measurements of the wear and friction of steel sliding against carbon fiber reinforced PolyPhenylene Sulfide were done. Instead of the standard set-up, a rotating composite disc and steel pin are used. The frictional behavior of this material pair results in a friction coefficient of 0.33, while a carbide film is formed in the wear track. This results in a lowering of the frictional behavior. The wear rate is rather low, but when the wear track is covered with a carbide film suddenly the wear rate raises. This is not due to the wear of the composite material but only as a result of the start of terrible wear of the steel counter face. Moreover, the wear of the pin is strongly related to the wear track. The pin is flattened at the sides of the formed wear track, but in contact with the carbide film there is less wear, resulting in a pin with two flat sides, making contact with the original surface, and a rig in the middle of the pin following the roundness of the wear track. The frictional behavior is strongly dependent on the weft-warp direction.

The Ghent Knee Rig was built in 2006 for studying the biomechanical behavior of post-mortem human knees. To validate this test rig the patellofemoral contact pressures and areas were investigated in 3 post-mortem knees tested under the same circumstances and compared to results in literature. To load the quadriceps, the vastus intermedius and rectus femoris were separated and clamped together. The pulling cable was aligned according to the shaft of the femur to keep the Q-angle at physiological values. A pressure film was inserted in the patellofemoral joint to measure the patellofemoral contact area and pressure. The results follow the general accepted trends of patellofemoral contact during knee flexion and extension; when the patella enters the trochlear groove at approximately 20 degrees of knee flexion, the intra articular contact pressure and area start to build up and the contact area on the patella shifts from distal to proximal. Though working with cadaveric specimens remains a simulation of in vivo situations with well known limitations, the test rig shows a good repeatability and reliability. The next stage of this research project is a comparison of normal with pathological knees.

The Ghent Knee Rig was built in 2006 for studying the biomechanical behavior of postmortem human knees. To validate this test rig the patellofemoral contact pressures and areas were investigated in 3 post-mortem knees tested under the same circumstances and compared to results in literature. To load the quadriceps, the vastus intermedius and rectus femoris were separated and clamped together. The pulling cable was aligned according to the shaft of the femur to keep the Qangle at physiological values. A pressure film was inserted in the patellofemoral joint to measure the patellofemoral contact area and pressure. The results follow the general accepted trends of patellofemoral contact during knee flexion and extension; when the patella enters the trochlear groove at approximately 20 degrees of knee flexion, the intra articular contact pressure and area start to build up and the contact area on the patella shifts from distal to proximal. Though working with cadaveric specimens remains a simulation of in vivo situations with well known limitations, the test rig shows a good repeatability and reliability. The next stage of this research project is a comparison of normal with pathological knees

A test rig for the evaluation of the biomechanical behavior of post mortem human knees during squatting has been constructed based on the accepted Oxford knee rig. Post mortem human knees were used to simulate their biomechanical behavior during a squatting movement.

The aim of this study was to investigate the variation of channel bed roughness in two rivers, as important parameter in hydraulic modelling especially with regard to flood control. The universities of Ghent (UG) and Antwerp (UA) are conducting scientific research in the river Aa in Belgium and the Biebrza river in Poland in order to better understand the phenomena involved and to come to a more accurate determination of the different parameters influencing flow. In this paper, the determination of the roughness coefficient n from the Manning equation is used. This coefficient is not easy to determine and is varying constantly. It is influenced by the meandering character of the river, the bed material and the average grain size, the channel bed forms, the channel obstructions, the geometry changes between sections and the vegetation in the channel. Furthermore, due to these parameters, the roughness of the channel is not equally distributed over the channel, the banks and the floodplains. So, using literature data does not always lead to satisfactory results, due to the different situation in the field (Werner et al. J Hydrol 314:139157, 2005). Therefore, measurements are necessary to determine the variation of the Manning coefficient. The Manning coefficient is a function of the discharge, but will also vary over the time due to the mentioned influences. In a multidisciplinary research project on the fundamental exchange processes in river ecosystems, hydraulic measurements were performed on a regular base in the river Aa. During these measurement campaigns, velocity and discharge measurements were carried out in multiple cross-sections. Once a month, the discharge and the water levels were measured at the upstream and the downstream end of the test stretch. On the river Biebrza, similar intensive measurement campaigns took place along a 6 km stretch in the upstream part of the river. An accurate determination of the Manning coefficient according a seasonal variation is an important tool in hydraulic modelling.

A test rig for studying the biomechanical behaviour of post-mortem human knees is built. For loading the quadriceps tendon a special clamping device is constructed, so the forces up to 3000 N could easily be transferred to the tendon. Several post-mortem human knees used in this study are tested to collect data about the general biomechanics of knee joint during squatting. The forces in the quadriceps tendon and in the tibia are measured continuously; the flexion angle is also measured. On this modular test rig, the contact pressures of the patellofemoral joint are measured as well. Therefore a thin pressure film is used. Not only are measured the contact pressures, but the changing contact area is also visualized and measured. The forces and rotations measured in the joint and in the contact area allowed us to obtain perfectly reproducible values as well as a positive relation between contact area, contact position and working direction of the quadriceps tendon.

The Laboratory Soete developed a new test setup, based on the well-known pin-on-disc test rig. Instead of the standard composite specimen and steel disc, a rotating composite disc and a steel pin is presently used to have a visible wear track. Other measurement techniques (Acoustic Emission, vibrations, temperature) can be used on the test rig. Continuous monitoring of the wear track combined with standard wear and friction measurements, give results of the current state of the wear track. Fourier frequency analysis (FFT) of these signals gives an indication of the change in condition and contact geometry of the resulting pin-disc combination. A high-speed camera will be used to acquire digital images of the worn composite surface. These online measurements yield to the gradual evolution in damage of the composite specimens.

On a Pin-on-Disc test rig with composite disc and steel pin tribological experiments were done on pultruded glass fiber reinforced polymer matrix composites plates. The wear and frictional behavior strongly depends on the structure. Also the normal load plays an important role in the frictional behavior, which is of greater importance than the speed. The formation of a thin polymer film onto the wear track results in a lowering of the coefficient of friction with 20%.

Tribological properties for polymers are mainly experimentally determined, while few standards are available and variation in data is often large. Polyimide is slid against steel on a cylinder-on-plate tribotester at 50–200 N and 0.3 m/s. There is a statistical variation of ±7% on dynamic friction, ±10% on static friction, ±8% on wear rates from weight loss and ±12% from wear rates from dimensional measurements, or even higher at high normal loads. Different parameters influencing statistical repeatability are discussed. Friction for polymers shows higher variation and wear rates show lower variation than steel/steel pairs due to visco-elastic deformation that has a contribution to friction but reduces stress concentrations. It is experimentally verified that the visco-elastic deformation of polymers in line contact is higher than calculated from theoretical models. The role of recoverable and permanent deformation is verified and there is a difference in deformation at 50–100 N and 150–200 N revealed from static loading tests, creep tests and wear measurements. The limit between running-in and steady-state coincides with the stabilization in contact pressures after 250 μm diameter reduction. Besides test rig design, a variation in counterface roughness seems the second most important influence.

This article summarizes the efforts done for using acoustic emission as wear mechanisms monitoring technique for wear testing in a pin-on-disc setup. The basic characteristics of the mechanisms were previously determined via controlled tensile testing (fiber breakage, debonding …). The knowledge of these basic characteristics then resulted in an easier classification of wear mechanisms related to wear testing of pultruded glass fiber reinforced polyester.

Present study investigates changes in the conformation of functional groups at the polymer sliding surface by postmortem Raman spectroscopy, thermomechanical analysis and SEM/EDX. Two case studies on sintered polyimide (SP) and thermoplastic polyethylene-terephtalate (PET) are presented, indicating the influences of sliding temperature, counterface type and internal lubricants (PET/PTFE) on some characteristic absorption bands in the Raman spectrum of the sliding surfaces.Copyright © 2005 by ASME