Predicting the polyethylene wear rate in pin-on-disc experiments in the context of prosthetic hip implants: deriving a data-driven model using machine learning methods.

Pin-on-disc (PoD) experiments are widely used to quantify and rank wear of different material couples for prosthetic hip implant bearings. However, polyethylene wear results obtained from different PoD experiments are sometimes difficult to compare, which potentially leaves information inaccessible. We use machine learning methods to implement several data-driven models, and subsequently validate them by quantifying the prediction error with respect to published experimental data. A data-driven model can supplement results from PoD wear experiments, and enables predicting polyethylene wear of new PoD experiments based on its operating parameters. It also reveals the relative contribution of individual PoD operating parameters to the resulting polyethylene wear, thus informing design of experiments, and potentially reducing the need for time consuming PoD wear measurements.

Effect of cast-iron disc thickness on the reliability of tribological data from pin-on-disc test: Case study of brake friction materials.

For braking system, rubbing between disc and brake pad generates heat, physical transformation on the contact, and activates many complex solicitations. Therefore, researchers used reduced scale testing to evaluate tribological and thermomechanical performance of friction composite. The pin-on-disc configuration of the tribometer is considered the better solution to reproduce these generated transformation and friction evolution. However, if the pin geometry is chospaden based on representative elementary volume (REV) approach, no research has investigated and justified the choice of the disc geometry and thickness. In this research work, an examination of the critical disc thickness is discussed, highlighting its substantial impact on the friction-wear behavior during brake application. For that, four disc thicknesses were tested, varying on thickness values (10, 15, 20, and 22 mm). Through this experiment, the friction coefficient evolution as function of braking numbers and sliding duration was studied while maintaining pressure and sliding velocity constant. Wear and temperature rise are identified for each disc thickness situation. A 3D thermal model is employed also to simulate the heat conduction and dissipation in brake system for various disc thickness. The temperature predictions are carried out using Ansys software. Experimental investigation presented the effect of the thickness on the friction stability, particularly at high temperature. Numerical results highlight the effect of reducing the thickness of the disc on the kinetic temperature rise through a rapid increase in disc temperature. The thickness modification used for the brake disc enhances the resulted thermal response and tribological brake pad performance, and consequently enhance the reliability of tribometer outcomes.

The Effect of Sliding Speed on the Tribological Properties of Ceramic Materials.

Ceramics are considered to be candidate materials for galvanising pot bearing materials due to their excellent corrosion resistance in many molten metals. Galvanising pot roll bearings must have excellent wear resistance, and, therefore, it is important to understand the wear behaviour of prospective bearing materials. This study investigates the friction- and wear-resistant properties of select ceramics, namely, pure hBN, BN M26, AlN-BN, Macor, 3YSZ, Al2O3 and Si3N4. The ceramics were tested at different sliding speeds using a pin-on-disc device equipped with SiC pins. The lowest coefficient of friction (COF) achieved was below 0.1, and it was measured for pure hBN at a 3.14 m/min sliding speed. However, a wear scar analysis showed that the BN grades suffered from severe wear. The highest wear rate was obtained for BN M26 at a 9.42 m/min sliding speed and was equal to 17.1 × 10-6 mm3 N-1 m-1. It was shown that the wear coefficient of the tested ceramics varied exponentially with hardness. The lowest wear was observed on the 3YSZ, Al2O3 and Si3N4 ceramics, which showed no volume loss, and, for this reason, they can be potentially used as bearing materials in continuous galvanising lines.

Testing Passenger Car Brake Pad Exploitation Time's Impact on the Values of the Coefficient of Friction and Abrasive Wear Rate Using a Pin-on-Disc Method.

The braking system is one of the most important components in any motor vehicle. Its proper function in emergency situations may save road users' lives. Today, as vehicles have more and more power at their disposal, leading to increased acceleration and maximum speed, the issue of effective braking is particularly important. It must also be noted that brakes are used in harsh conditions (water and salt, especially during winter), and must provide appropriate durability (on average, circa 30,000 km). For these reasons, many institutions conduct research aimed, among other things, at minimizing fading. However, this study looked into a different matter, focusing on how the operating conditions mentioned above, including the lifespan of brakes, impact the tribological properties of the friction pair. To achieve this, samples from brake pads were obtained (both brand new and used). Next, using a pin-on-disc tribological test, it was shown that the pads have lower coefficients of friction and abrasive wear rates. The results indicated that both parameters change in a manner that is dependent on how long the brake system has been in use.

Effects of Automotive Test Parameters on Dry Friction Fiber-Reinforced Clutch Facing Surface Microgeometry and Wear-Part 2.

Coefficient of friction values, wear and surface roughness differences are revealed using pin-on-disc test apparatus examinations under three pv loads, where samples are cut from a reference, unused, and several differently aged and dimensioned, used, dry friction fiber-reinforced hybrid composite clutch facings. Tests are characterized by surface activation energy and separated into Trend 1, 'clutch killer', and 2, 'moderate', groups from our previous study. The results reveal that acceptable, 0.41-0.58, coefficient of friction values among Trend 1 specimens cannot be reached during high pv tests, though the -0.19--0.11 difference of minimum and maximum pv results disappears when activation energy reaches 179 MJ. The maximum pv friction coefficient can decrease by up to 30% at working diameter due to clutch killer test circumstances, as 179 MJ surface activation energy is applied, while by moderate tests such losses can only be detected close to 2000 MJ energy values among small-sized facings. Besides that, Trend 2 specific wear values are the third of trend 1 results at inner diameter specimens. Compared to reference facing values, specific wear results at working diameter under maximum pv decrease by 47-100%, while increasing specific wear during lifetime can only be detected at the inner diameter of facings enduring clutch killer tests or that are small-sized facings. Among Trend 1 radial and tangential Ra delta results, inner diameter samples provide more decreasing surface roughness data, while by Trend 2 values, the opposite relation is detected. Apart from the effects of activation energy, mileage and driver profile, facing size and friction diameter influence is also revealed.

Mechanical and Tribological Characterization of WC-Co and WC-AISI 304 Composites by a Newly Developed Equipment.

Tungsten carbide-based composites are, in many cases, the materials of choice in applications requiring high wear resistance. In the present research work, the mechanical characterization of the WC-Co and WC-AISI 304 composites was carried out, with evaluation of the hardness and fracture toughness and tribological characterization of the composites that included the study of friction and wear rate coefficient through unlubricated sliding tests according to the Pin-on-Disc test method. It was possible to correlate the effect of the different binding phases on the mechanical and tribological properties of WC-based composites, and it can be concluded that the system composed by the tribological pair WC-AISI304/100Cr6 was the one that showed the lowest coefficient of friction while the tribological pair WC-Co/Al2O3 was the one that showed the lowest wear rate coefficient.

Effect of Functional Fillers on Tribological Characteristics of Acrylonitrile Butadiene Rubber after High-Pressure Hydrogen Exposures.

In a high-pressure hydrogen environment, the sealing rubber material is swelled by hydrogen, and the mechanical and tribological properties are reduced, causing various problems in the sealing performance. The focus of this study was the effect of the filler type and content on the tribological characteristics of rubber after exposure to high-pressure hydrogen. Acrylonitrile butadiene rubber specimens were exposed to high-pressure hydrogen at 96.6 MPa, and the change in the amount of wear with time after exposure was observed. The wear test was performed using a pin-on-disc ball tip to measure the amount of wear before and after hydrogen exposure of the materials under fixed revolutions per minute and normal load. Scanning electron microscopy was used to observe the wear track and cross section of the specimen to examine the changes in the wear mechanism after hydrogen exposure and to analyze the wear mechanism for each filler. The results of this study are expected to contribute to the evaluation of the tribological properties of the sealing materials used in hydrogen environments.

Effects of Microstructure Modification by Friction Surfacing on Wear Behavior of Al Alloys with Different Si Contents.

In this work, Al alloys with 6.6%, 10.4%, and 14.6% Si were deposited as thick coatings by Friction Surfacing (FS), resulting in grain refinement and spheroidization of needle-shaped eutectic Si phase. Lubricated sliding wear tests were performed on a pin-on-disc tribometer using Al-Si alloys in as-cast and FS processed states as pins and 42CrMo4 steel discs. The chemical composition of the worn surfaces was analyzed by X-ray photoelectron spectroscopy (XPS). The wear mechanisms were studied by scanning electron microscopy (SEM) and focused ion beam (FIB), and the wear was evaluated by measuring the weight loss of the samples. For the hypoeutectic alloys, spheroidization of the Si phase particles in particular leads to a significant improvement in wear resistance. The needle-shaped Si phase in as-cast state fractures during the wear test and small fragments easily detach from the surface. The spherical Si phase particles in the FS state also break away from the surface, but to a smaller extent. No reduction in wear due to FS was observed for the hypereutectic alloy. Here, large bulky primary Si phase particles are already present in the as-cast state and do not change significantly during FS, providing high wear resistance in both material states. This study highlights the mechanisms and limitations of improved wear resistance of Si-rich Al alloys deposited as thick coatings by Friction Surfacing.

Precise control of operating conditions in tribotesting with respect to trace humidity and contact temperature.

Research in tribology are often connected to tribosystems operating in specific environments, where climate chambers are needed for tribotesting to resemble the environmental conditions in the real application. Although the effect of humidity on the tribological performance of many materials and lubricants is evident, many studies are conducted without sufficient systems to accurately monitor and control the humidity level throughout testing. In this paper, a humidity controlling system was developed to enable continuous monitoring and precise control of the humidity at trace moisture levels. The climate controller was validated in a tri-pin-on-disc tribometer with excellent performance and can be fitted to most climate chambers. To further improve the control of operating conditions during tribotesting, a thermodynamic simulation of the contact temperature was developed.•The developed climate controller is a simple and cost-effective method to accurately monitor and control the humidity in a climate chamber at trace moisture levels.•The portable design of the humidity controller enables use with most climate chambers and enclosed tribometers.•To have better control over the temperature in the sliding interface during testing, a thermodynamic simulation method was used to estimate contact temperature between sliding bodies from near-contact temperature measurements and the measured friction forces.

Stribeck Curve of Magnetorheological Fluid within Pin-on-Disc Configuration: An Experimental Investigation.

The paper focuses on the coefficient of friction (COF) of a magnetorheological fluid (MRF) in the wide range of working conditions across all the lubrication regimes-boundary, mixed, elastohydrodynamic (EHD), and hydrodynamic (HD) lubrication, specifically focused on the common working area of MR damper. The coefficient of friction was measured for MR fluids from Lord company with concentrations of 22, 32, and 40 vol. % of iron particles at temperatures 40 and 80 °C. The results were compared with a reference fluid, a synthetic liquid hydrocarbon PAO4 used as a carrier fluid of MRF. The results show that at boundary regime and temperature 40 °C all the fluids exhibit similar COF of 0.11-0.13. Differences can be found in the EHD regime, where the MR fluid COF is significantly higher (0.08) in comparison with PAO4 (0.04). The COF of MR fluid in the HD regime rose very steeply in comparison with PAO4. The effect of particle concentration is significant in the HD regime.

Surface Damaging of Brass and Steel Pins when Sliding over Nitrided Samples Cut by Finishing and Roughing EDM Conditions.

In the forging industry, surface quality and surface treatments of dies are crucial parameters to extend their life. These components are usually machined by milling or by Electrical Discharge Machining (EDM), and the final surface roughness depends on the machining techniques and operational conditions used in its fabrication. After milling, a nitriding treatment is widely applied to extend its service life. Nevertheless, no scientific report that informs about nitriding after EDM has been found. Accordingly, this work focuses on the wear and friction behavior of pins made of brass and medium carbon steel sliding over AISI H13 discs, made by wire EDM in the conditions of finishing and roughing. The discs are plasma nitride, and their effect on the wear during pin-on-disc tests is evaluated. In this sense, the analysis of the surface damage for the different pins will help us to understand the service life and wear evolution of the forging dies. The results show that plasma nitride reduces the friction and prevents the degradation of the pin, independently of the material of the pin, when sliding over finishing and roughing EDM conditions.

Comparison of the Friction and Wear Characteristics between Copper and Paper Based Friction Materials.

Copper-based friction material (CFM) and paper-based friction material (PFM) are the two most commonly used clutch friction materials. The friction and wear characteristics of these two kinds of friction materials under dry conditions were investigated by the pin-on-disc test over a broad range of applied loads, rotating speeds and ambient temperatures. Before experiments, the running-in test was conducted to stabilize the coefficient of friction (COF) and wear amount of the test samples. After experiments, the metallographic micrographs of the tested samples were presented to investigate the wear mechanisms. Experimental results showed that both the COF and wear depth of the CFM are much greater than these of PFM. The COF of CFM decreases with the increase of applied load, and changes slightly with the variation of rotating speed, whereas it increases first and then decreases with the increase of ambient temperature. However, the COF of PFM decreases dramatically with the increase of the rotating speed and ambient temperature, while it remains stable at first and then decreases slowly as the applied load increases. Additionally, under such three working conditions, the wear depth of CFM changes linearly, while the wear depth of PFM varies greatly. This study can be used as a guide for selecting friction materials for clutches with different applications.

Sliding Wear Behavior of UNS R56400 Titanium Alloy Samples Thermally Oxidized by Laser.

Wear of elements subjected to friction and sliding is among the main causes of low tribological performance and short lifetime of strategic materials such as titanium alloys. These types of alloys are widely used in different areas such as aerospace and the biomechanics industry. In this sense, surface modification treatments allow for the overcoming of limitations and improvement of features and properties. In the case of titanium alloys, improvements in the main weaknesses of these materials can be obtained. Laser texturing of UNS R56400 (Ti6Al4V) alloy, according to Unified Numbering System designation, surface layers in a non-protective atmosphere produces an increase of the oxides, especially of titanium dioxide (TiO2) species. The presence of oxides in the alloy results in color tonality variations as well as hardness increases. In addition, specific roughness topographies may be produced by the track of laser beam irradiation. In this research, thermochemical oxidation of UNS R56400 alloy has been developed through laser texturing, using scan speed of the beam (Vs) as the process control variable, and its influence on the sliding wear behavior was analyzed. For this purpose, using pin on disc tribological tests, wear was evaluated from the friction coefficient, and wear mechanisms involved in the process were analyzed. Combined studies of wear mechanisms and the friction coefficient verified that by means of specific surface treatments, an increase in the wear resistance of this type of alloys is generated. The most advantageous results for the improvement of tribological behavior have been detected in textured surfaces using a Vs of 150 mm/s, resulting in a decrease in the friction coefficient values by approximately 20%.

Wear Behavior and Microstructure of Mg-Sn Alloy Processed by Equal Channel Angular Extrusion.

Mg-5wt.% Sn alloy is often used in portable electronic devices and automobiles. In this study, mechanical properties of Mg-5wt.% Sn alloy processed by Equal Channel Angular Extrusion (ECAE) were characterized. More precisely, its hardness and wear behavior were measured using Vickers hardness test and a pin-on-disc wear test. The microstructures of ECAE-processed Mg-Sn alloys were investigated by scanning electron microscope and X-ray diffraction. ECAE process refined the grain sizes of the Mg-Sn alloy from 117.6 µm (as-cast) to 88.0 µm (one pass), 49.5 µm (two passes) and 24.4 µm (four passes), respectively. Meanwhile, the hardness of the alloy improved significantly. The maximum wear resistance achieved in the present work was around 73.77 m/mm³, which was obtained from the Mg-Sn alloy treated with a one-pass ECAE process with a grain size of 88.0 µm. The wear resistance improvement was caused by the grain size refinement and the precipitate of the second phase, Mg2Sn against the oxidation of the processed alloy. The as-cast Mg-Sn alloy with the larger grain size, i.e., 117.6 µm, underwent wear mechanisms, mainly adhesive wear and abrasive wear. In ECAE-processed Mg-Sn alloy, high internal energy occurred due to the high dislocation density and the stress field produced by the plastic deformation, which led to an increased oxidation rate of the processed alloy during sliding. Therefore, the oxidative wear and a three-body abrasive wear in which the oxide debris acted as the three-body abrasive components became the dominant factors in the wear behavior, and as a result, reduced the wear resistance in the multi-pass ECAE-processed alloy.

Tribochemistry of Bismuth and Bismuth Salts for Solid Lubrication.

One of the main trends in the past decades is the reduction of wastage and the replacement of toxic compounds in industrial processes. Some soft metallic particles can be used as nontoxic solid lubricants in high-temperature processes. The behavior of bismuth metal particles, bismuth sulfide (Bi2S3), bismuth sulfate (Bi2(SO4)3), and bismuth oxide (Bi2O3) as powder lubricants was studied in a range of temperatures up to 580 °C. The mechanical behavior was examined using a high-temperature pin-on-disc setup, with which the friction force between two flat-contact surfaces was recorded. The bismuth-lubricated surfaces showed low coefficients of friction (µ ≈ 0.08) below 200 °C. Above the melting temperature of the metal powder at 271 °C, a layer of bismuth oxide developed and the friction coefficient increased. Bismuth oxide showed higher friction coefficients at all temperatures. Bismuth sulfide exhibited partial oxidation upon heating but the friction coefficient decreased to µ ≈ 0.15 above 500 °C, with the formation of bismuth oxide-sulfate, while some bismuth sulfate remained. All surfaces were studied by X-ray diffraction (XRD), confocal microscopy, high-resolution scanning electron microscopy (HR-SEM), and energy-dispersive X-ray spectroscopy (EDS). This study reveals how the partial oxidation of bismuth compounds at high temperatures affects their lubrication properties, depending on the nature of the bismuth compound.

The biochemical characteristics of wear testing lubricants affect polyethylene wear in orthopaedic pin-on-disc testing.

Lubricant protein concentration is known to affect crosslinked polyethylene wear in in vitro testing; however, the biochemical nature of these lubricants may also have a significant effect on wear and dictate its clinical relevance. A modified approach to pin-on-disc testing was implemented to explore the effect of four biochemically different lubricants on the wear of two types of crosslinked polyethylene materials (XLK™ and Marathon™; DePuy Synthes, Warsaw, IN, USA). XLK was associated with higher wear rates than Marathon. In comparison to lubricants containing deionized water, lubricants containing phosphate buffered saline solution and hyaluronic acid increased osmolality by up to 1.2 times and thermal stability by up to 1.4 times. This biochemical change reduced wear by up to 12.5 times. Wear rates for XLK and Marathon differed by a factor of 3.2 using lubricants with phosphate buffered saline solution as the dilutive media, but only 2.0 for lubricants with deionized water. Interestingly, varying the concentration of hyaluronic acid did not have a significant effect on wear, and differences between XLK and Marathon wear rates were not found to be statistically significant when hyaluronic acid was added to the lubricant. The findings of this study showed that increasing the osmolality and thermal stability of lubricants to more clinical levels decreased wear; however, the effect of hyaluronic acid on wear may not be apparent in simplistic pin-on-disc testing. It was suggested that phosphate buffered saline solution be used as the dilutive media of choice in order to better differentiate the ranking of materials while maintaining some clinical relevance.

Wear testing of crosslinked polyethylene: wear rate variability and microbial contamination.

The wear performance of two types of crosslinked polyethylene (Marathon™ and XLK™, DePuy Synthes Inc., Warsaw, IN) was evaluated in a pin-on-disc wear tester, a hip wear simulator, and a knee wear simulator. Sodium azide was used as the microbial inhibitor in the calf serum-based lubricant. In the pin-on-disc wear tester, the Marathon wear rate of 5.33±0.54mm(3)/Mc was significantly lower (p=0.002) than the wear rate of 6.43±0.60mm(3)/Mc for XLK. Inversely, the Marathon wear rate of 15.07±1.03mm(3)/Mc from the hip wear simulator was 2.2-times greater than the XLK wear rate of 6.71±1.03mm(3)/Mc from the knee wear simulator. Differences in implant design, conformity, GUR type, and kinematic test conditions were suggested to account for the difference between the wear rates generated in the different types of wear testing apparati. In all wear tests, sodium azide was ineffective at inhibiting microbial growth in the lubricant. Eight different organisms were identified in the lubricant samples from the wear tests, which suggested the necessity of using an alternative, more effective microbial inhibitor. Careful sample preparation and thorough cleaning has shown to improve the consistency of the wear results. The wear rates generated in the hip and knee wear simulators closely reflected the wear behaviour of Marathon and XLK reported in published data that were tested under similar conditions.