The potential lubrication mechanisms for articular cartilage remain as complex as the material itself. Many studies have examined systematically the effects of sliding velocity on the frictional response of cartilage, but a systematic analysis of probe curvature and contact area remains largely uncharted. Here, we explore this potential effect in three ways: a constant force experiment, a constant pressure experiment, and a nonconstant pressure and force experiment. Hard, impermeable spheres are used as probes and with sizes varied between 1.6 mm and 25.4 mm, allowing a sweep of physiologically relevant pressures. The aim is to effectively isolate the influence of probe curvature on the friction measurements. Articular cartilage samples were collected from horses at necropsy from the left and right distal and medial aspects of the radius. Each test was run for 5 min at a reciprocating sliding speed of 1 mm/s. Smaller probe diameters (and curvature) consistently resulted in high measured coefficients of friction of cartilage. Two lubrication regimes are observed during testing: an area-dependent regime at lower areas and an area-independent regime at higher areas. A power law (-1/3 to -1/2) relationship between friction coefficient and probe diameter is observed. Therefore, measurements made using different probe sizes will most likely result in different measurements of the friction behavior, and great care should be given when comparing these results.

This study investigates the effect of accelerated aging on extraction forces of Sn-coated lead-free electrical connectors. As electrical connectors are an integral component of commercial electronics, as well as the widespread shift toward lead-free electronics due to the hazardous effects of lead, understanding the mechanical properties of these connectors is essential. Of particular interest is the extraction forces which serve to maintain the integrity of electrical contacts. Despite extensive research on aging effects, limited work has been conducted as to its effects on extraction forces in lead-free electrical connectors.The goal of this work is to develop a better understanding of the effects of extended aging on the extraction forces of Sn-coated electrical connectors. Extraction (de-mating) force testing of multiple aged sample sets were performed to assess age dependent differences in extraction forces. To simulate long-term aging, samples were subjected to accelerated aging in a high-temperature oven for varying durations. Profilometry measurements were also made of the blade contacting surfaces and the results discussed in conjunction with the observed extraction force results.

As the performance and efficiency requirements of electric vehicles (EVs) continue to expand, the demand for advanced driveline lubricants has grown exponentially. Unlike traditional internal combustion engine (ICE) vehicles, EVs experience unique challenges, including increased acceleration and deceleration rates, immediate torque delivery, higher operating speeds, and elevated drivetrain temperatures. Moreover, EV lubricants must endure exposure to damaging bearing currents, which can lead to morphological damage on bearing surfaces, such as electrical pitting. Addressing these challenges is critical to ensuring the longevity and reliability of EV components. This study aims to explore and validate innovative lubricant solutions tailored explicitly for EV applications. This work provides experimental validation of the capabilities of silver (Ag) and different concentrations of magnetic iron-oxide nanoparticles (NPs) in reducing damage through reciprocating rolling ball-on-disk tests. Additionally, an electrically conductive carbon black lithium-thickened grease was tested under comparable conditions. The significance of this research lies in its potential to revolutionize the EV lubricant industry by offering a robust solution to a prevalent problem. Successful implementation of nanoparticleenhanced lubricants could lead to increased durability and efficiency of EV drivetrains, reducing maintenance costs and improving overall vehicle performance. This advancement aligns with the evolving demands of the EV market and sets a new standard for lubricant technology in electrified mobility. [DOI: 10.1115/1.4067489]

Surface roughness of additively manufactured metal parts has significant impacts on the part’s performance (e.g., fatigue resistance) and poses a significant roadblock to the wider adoption of metal additive manufacturing. This work aims to show that, in some cases, it is possible to estimate the depth of the deepest valley of an additively manufactured part with a relatively high accuracy using either only line measurements from a contact profilometer (and limited areal scans used for calibration) or areal scans from optical areal scanners of a considerably smaller area of the same specimen or even another specimen produced in the same batch under the same conditions. Both problems are approached by employing the block-maxima method from extreme value theory, whereby the underlying distribution of the depth of individual valleys is modeled with a Gumbel distribution. The experimental results from additively manufactured Ti-6Al-4V specimens demonstrate that the proposed methods can produce estimates that significantly outperform more straightforward benchmarks (e.g., simply using linear parameters for areal ones), and the proposed methods achieve a relatively accurate estimation with mean errors of 5–15

Electrical contact is fundamental to almost every aspect of modern industry, including the fast-growing electric vehicle industry. In metallic contacts in atmospheric conditions, most of the electrical current passes via the micro-junctions formed between two electrodes. The classic electrical contact theory predicts an infinite current density at the circular contact periphery. In the present work, we explore the influence of the dielectric breakdown of air outside the contact area on the electrical contact interface. Incorporating the discharging boundary condition governed by the modified Paschen law, we develop the numerical model as well as two sets of closed-form solutions for low applied voltage cases where two electrodes are in solid-solid contact and complete separation, respectively. For Hertzian contact, the present work theoretically proves that the ignorance of discharge can lead to a singular current density at the contact periphery and an overestimation of the electrical contact resistance. The current density monotonically increases along the radial direction to a finite value at the contact area periphery, followed by a monotonic drop within the discharge zone. The present study serves as a foundation for the modeling of discharging rough surface electrical contact and sheds light on the machine element surface damages caused by the electrical discharge machining.

Rolling element bearings are an integral component of electric vehicles, supporting radial and axial loads in powertrain components such as electric motor shafts and wheel bearings. Fast-switching inverters enable precise, variable control of motor performance at the cost of possible stray current leakage into mechanical components. These currents naturally seek to cross the insulating fluid film in rolling element bearings. In doing so, a destructive discharge or arc may form and cause irreversible damage to metallic bearing surfaces. A unique contribution of the work is that it provides a method to use the statistical height distribution to predict the likelihood of electrical breakdown and discharging. To predict film thickness it uses a closed-form elasto-hydrodynamic lubrication (EHL) models to present a semi-analytical model of this discharging phenomenon. Existing EHL models are modified for mixed lubrication and electrical contacts by incorporating a solid rough surface asperity contact model and a flow factor modified lubrication model. The model accounts for transient effects and considers changes in speed and other parameters during operation. The resulting model predicts the likelihood of surface damage and electrical properties of the bearings through the statistical asperity height above a critical value calculation. The damaged regions predicted by the model are in qualitative agreement with the experimental tests.

Rolling element bearings are an integral component of electric vehicles, supporting radial and axial loads in powertrain components, such as electric motor shafts and wheel bearings. Fast-switching inverters enable precise, variable control of motor performance at the cost of possible stray current leakage into mechanical components. These currents naturally seek to cross the insulating fluid film in rolling element bearings. In doing so, a destructive arc may form and cause irreversible damage to metallic bearing surfaces. A unique contribution of the work is that it provides a method to use the statistical height distribution to predict the likelihood of electrical breakdown and arcing. To predict film thickness, it uses a closed-form elasto-hydrodynamic lubrication (EHL) model to present a semi-analytical model of this arcing phenomenon. Existing EHL models are modified for mixed lubrication and electrical contacts by incorporating a solid rough surface asperity contact model and a flow factor-modified lubrication model. The model accounts for transient effects and considers changes in speed and other parameters during operation. The resulting model predicts the likelihood of surface damage and electrical resistance of the bearings through the statistical asperity height above a critical value calculation. The damaged regions predicted by the model are in qualitative agreement with the experimental tests.

The random process model, also known as the multi-asperity contact model or statistical model, is one of the dominant methodologies for analyzing the contact between nominally flat rough surfaces. In 1975, Bush, Gibson, and Thomas developed a complete random process model (known as the BGT model) assuming that solid-solid contacts occur on the summits of the asperities, which are semi-ellipsoids uniquely described by three random variables (the asperity height and two principal peak curvatures). The Hertzian elliptic contact theory states that the contact area and normal load are implicit functions of the random variables, which results in an original BGT model with a complex formulation. In this study, we used an adapted Hertzian elliptic contact theory to simplify the formulation of the BGT model. The relative contact area to normal load relation predicted by the simplified BGT model perfectly agrees with that of the original formulation. It is anticipated that rough surface contact models with complex asperity interactions can be effectively built under the framework of this simplified BGT model.

With a growing emphasis on renewable power generation and sustainable transportation, the popularity of electric vehicles (EVs) is expanding rapidly. As a result, ensuring the efficient and sustainable performance of gears and rolling element bearings within tribological contacts is imperative. EVs typically experience elevated component loads and drivetrain temperatures, but concerns also arise regarding damaging electric leakage currents that cause electrical arcing across mechanical surfaces. Preliminary research indicates that blending conductive nanoparticles into grease can mitigate pitting damage on mechanical contacts under electrical loads. This study employs a rolling ball on disk test under electro-tribological loads to compare the performance of nanoparticle-enriched greases with neat and fully formulated oils under various conditions. Rotational tests were initially run with limited pitting damage. However, electrical damage was evident when a reciprocating motion was used at different track lengths and speeds. When conductive nanoparticle greases were incorporated, damage was reduced. These results suggest that incorporating conductive nanoparticles in additive packages enhances performance and reduces surface damage, extending lubricant and bearing operating life. These findings benefit EV technology and hold promise for power generation and transformer applications where electrical discharges are common.

In electric vehicles and similar applications, leakage currents threaten lubricated bearing and gear interfaces, leading to significant surface damage such as micro-pitting. This damage is typically caused by electrical arc or plasma initiation, resulting in localized rapid temperature rises. Recent studies have highlighted the potential of oil-soluble ionic liquids (ILs) as promising antiwear (AW) additives. However, their effects on improving the electrically induced bearing damage are yet to be explored. Also, Ag nanoparticles have proved to be significantly effective in reducing electrically induced micropitting when added with grease. Further investigation is required to determine the effectiveness of Ag nanoparticles in lubricants without grease thickeners. Therefore, this research investigates the impact of ionic liquids and Ag nanoparticles as additives in PAO-based lubricants on electrically induced bearing damage. Reciprocating rolling sphere-on-disk tests were conducted under electro-tribological loads using E52100 steel samples. Surface analysis using optical spectroscopy and scanning electron microscopy (SEM) was carried out for quantifying surface damages.

Leakage currents accelerate surface degradation of metal contacts via small scale arcing across lubricating films, but recent observations suggest that metallic nanoparticle additives in lubricants may be useful to improve contact performance. These findings prompted a study that examined electrically induced surface pitting of steel contacts in the presence of several lubricating greases including some containing nanometer-sized colloidal silver (Ag) particles. Reciprocating rolling sphere-on-disk experiments were conducted under electro-tribological loads employing polyurea greases derived from mineral and synthetic base oils with and without additives. Friction forces and electrical resistance were monitored continuously during the tests; surface changes were characterized by means of optical spectroscopy, stylus profilometry, and scanning electron microscopy (SEM) including compositional analysis using energy dispersive spectroscopy (EDS). The observations demonstrate that surface pitting induced by arcing occurs mainly at the points were the rolling motion changes direction and that eroded metal is deposited along the wear grove. Micron-sized pits are formed which contain carbon and oxygen indicating that arcing causes decomposition of the hydrocarbon lubricants. Numerous findings indicate a significant inhibition of pitting is induced by the Ag nanoparticles; some greases containing other additives exhibit a similar, although less pronounced, effect.

As public attention is increasingly drawn toward more sustainable transportation methods, the popularity of electric vehicles (EVs) as part of the solution is rapidly expanding. Operating conditions within EVs can be severe compared to standard combustion powertrains, and the risk of electrical arcing across mechanical surfaces from electric leakage currents incites additional concern. This study employed a series of electro-tribological tests utilizing various moving patterns to improve understanding of the driving conditions for electrically induced bearing damage (EIBD). Rolling ball-on-disk tests were performed with different polyurea-thickened greases. Rotational tests were initially run at various speeds and test durations, but electrical damage was limited. However, electrical damage was unmistakable when a reciprocating motion was used at different track lengths and speeds. These results suggest that the conditions associated with the track length, such as the number of directional changes and speed-dependent film thickness, play a considerable role in forming electrical damage. This work provides critical insights into the mechanisms of EIBD in EVs and other electrical systems. It highlights the importance of understanding the operational conditions that contribute to EIBD, which can lead to improved designs and maintenance practices, ultimately enhancing the efficiency and lifespan of these systems.

The Editor-in-Chief and Editorial Board of the ASME Journal of Tribology would like to thank all of the reviewers for volunteering their expertise and time reviewing manuscripts in 2021. Serving as reviewers for the journal is a critical service necessary to maintain the quality of our publication and to provide the authors with a valuable peer review of their work. Below is a complete list of reviewers for 2021. We would also like to acknowledge three outstanding Reviewers of the Year.2021 Reviewers of the YearWassim Habchi — Lebanese American University, LebanonLi Chang — The University of Sydney, AustraliaHyun-Joon Kim — Kyungpook National University, South KoreaThe Reviewers of the Year Award is given to reviewers who have made an outstanding contribution to the journal in terms of the quantity, quality, and turnaround time of reviews completed during the past 12 months. The prize includes a Wall Plaque, 50 free downloads from the ASME Digital Collection, and a one year free subscription to the journal.

The flash temperature in the sliding frictional contact between micro-asperities has an important influence on the frictional characteristics of advanced functional ceramics. In this paper, the elastic sliding frictional contact of a three-dimensional micron/submicron scale asperity pair is considered. A three-dimensional finite element model (FEM) for fully coupled thermal-stress analysis of sliding contact of SiC/Al2O3 asperity pair is developed. An empirical correction factor for contact characteristics is obtained based on the FEM results. The FEM results show that, compared with the Hertz theoretical solution, the contact area becomes smaller and the contact pressure becomes larger in the case of sliding contact with large deformation. The flash temperature has a negative correlation with the composite radius of the asperity pair and a positive correlation with the interference depth and sliding speed. Using Hertz theory, a parabolic distributed heat source, the Fourier heat conduction law, and the newly proposed correction factor, a semi-analytical model of flash temperature during the elastic frictional sliding between two single asperities is established. The relative difference between the flash temperature predicted by the established semi-analytical model and the FEM model is less than 1.2%. The relative difference decreases with the increasing interference depth. This work is a valuable reference for studying the frictional heat-related issues of advanced ceramics.

Reflecting on the application and implications of this work [1], the authors realized that a few items should be discussed.Although the work is for elastic sliding contact, it could be adapted for elastic–plastic contact by employing an existing sliding elasto-plastic model [2,3]. These models are slightly different but originated at the same time. The paper by Jackson et al. provides equations to predict the tangential and normal forces during an elastic–plastic sliding. The model by Green [2] is based on a strain energy balance and is calibrated to finite element results found in a thesis by Moody [4]. It might be possible to adjust the factor given by Eq. (11) in the paper [1] for a maximum pressure predicted by these works. The maximum pressure for an elastic–plastic model will likely be lower than the elastic case, and the flash temperature will also be lower. Perhaps, the plastic deformation of asperities could effectively limit the magnitude of the flash temperature.Looking at the asperity scale, which this calculation is likely to be implemented for, the size of an asperity can vary in size by many orders of magnitude. Some asperities will be on the nanoscale and therefore molecular models might provide additional insight. For instance, in the work by Vadgama et al. considered a molecular dynamics model of a sliding interference between two copper asperities [5]. As the asperities interfere the temperature rose by 100's of Kelvin. Zhang et al. [6] studied the sliding contact of a hemispherical silicon carbide asperity pair, and found there is no clear relationship between forces and lattice orientation for silicon carbide when compared with Vadgama et al.'s work. However, these molecular models are also quite different from this case because they also include plasticity, adhesion, and effective friction between the surfaces. In addition, in a molecular dynamics model, a thermostat technique is often used that could eliminate or lessen the observed flash temperature.In addition, with high temperatures, the yield strength of the contact materials will decrease (i.e., thermal softening). If the temperature reaches the melting point of the material, then the yield strength effectively becomes nil [7]. This is effectively included in the molecular dynamics models previously mentioned. In molecular models, the temperature is atomic vibration that allows plastic displacement of atoms to occur under less force. This effect will result in the increase of plastic deformation and lowering of the flash temperature, which also appears to be indicated by this recent work that uses a meshless continuum mechanics model [8].The next step of these types of models would be to predict the flash temperature of a rough surface with multiple asperities in contact. Based on Green's function, Vick and Furey [9] and Coulibaly et al. [10] studied the calculating of temperature rises due to multiple, interacting contacts when a set of regularly arranged rectangular asperities sliding against a plane surface. Choudhry et al. [11] formulated a multiscale model of fractal surfaces that predicted the flash temperatures on asperities of different sizes. This work [11] also included the influence of wear which can effectively limit the magnitude of the flash temperature rise. However, it was modeled as a rough surface loaded against a flat surface and so did not address the asperity against asperity contact considered by the current work [1].In rough surface contact with sliding, it will also almost always be true that the surfaces change over time due to deformation and wear. Therefore, the geometry of the asperities will change and the flash temperatures will as well. In some moderately higher temperature conditions, the asperities on the surface will be smoothed out [12]. In more severe conditions, this could lead to thermo-elastic instabilities [13–15] and scuffing [16], where the surfaces often appear to be smeared [17] and could lead to welding of the surfaces. In summary, the flash temperature of sliding rough surfaces could in some conditions lead to a smoothing and perhaps improved performance, and in other conditions, a thermally induced failure. The current work [1] could help to improve these types of predictions, all there is still much additional work required.There are no conflicts of interest.No data, models, or code were generated or used for this paper.

Editorial Editorial: A Transformative Time for Tribology Accepted Manuscript Robert L. Jackson Robert L. Jackson Department of Mechanical Engineering Samuel Ginn College of Engineering Auburn, AL 36849 Email: jacksr7@auburn.edu Search for other works by this author on: This Site PubMed Google Scholar Author and Article Information Robert L. Jackson Department of Mechanical Engineering Samuel Ginn College of Engineering Auburn, AL 36849 Email: jacksr7@auburn.edu Contributed by the Tribology Division of ASME for publication in the Journal of Tribology. J. Tribol. 1-2 (2 pages) Paper No: TRIB-24-1036 https://doi.org/10.1115/1.4064758 Published Online: February 13, 2024 Article history Received: February 5, 2024 Revised: February 8, 2024 Accepted: February 9, 2024 Published: February 13, 2024

In many devices and applications, electrical contacts are exposed to vibrations, sliding, or rolling conditions and are prone to fretting-based degradation. Thus, lubricants are often employed in such contacts to reduce sliding wear and fretting corrosion. However, due to the non-conductive behavior of the lubricants with fluorocarbons and hydrocarbons, lubricants lead to a few adverse problems. Also, the fluid dynamics upon excitation, vibration, or sliding causes extended breaks or gaps in between the conducting surfaces. In reality, this can be noticed during vibrations occurring as a result of earthquakes or technical personnel maintenance. This could also have applications to electrified rolling element bearings. Factors such as surface roughness and fluid viscosity will determine the time taken for the two surfaces of the connectors to separate from a solid conductive contact. In this work, a coupled structural-fluid theoretical model is developed for evaluating such intermittent contact breaks/gaps when two metallic rough surfaces in contact are under vibrations. The model is capable of predicting the increase in the fluid film as well as the contact resistance change with time due to the possible connector vibration. The experimentally observed rocking vibration mode seen in connectors and the time-dependent squeeze film lubrication effect are also considered.

This work employees the modification of the Reynolds equation for computational efficiency by analyzing rough surfaces in the hydrodynamic flow regime through the use of flow factors. This analysis is aimed towards modeling the surface interactions and pressure variations across power cylinder components of an internal combustion engine, namely the piston ring and cylinder wall. The Patir and Cheng equations are for general surfaces that accounts for the overall effect of roughness and anisotropic structure. However, rough surfaces are not fully characterized by these two parameters and therefore the aim of this paper was to provide flow-factors specific to the cross-hatched cylinder liner surfaces and piston ring interface. These interacting surfaces were measured directly through the use of a profilometer. Average flow factors are derived via full deterministic solution of the Reynolds equation over rough surfaces to model the pressure and shear variations relative to smooth surfaces. These flow factors can then be used to consider the effect of roughness in lubrication problems without deterministically modeling roughness.

There is a need to support the biofuel sector by utilizing waste materials for its production and utilizing co -products through an integrated approach. Due to bio-oil's diverse composition, its applications for foams and resins syntheses are emerging, but most importantly, as biolubricants, product with increasing global demand. In this study, four hydrocarbon biolubricants (HBL) were produced via a hydrotreatment process. Two samples were produced using hydrothermal liquefaction (HTL) of algae (HAL) and sewage sludge (HSS). Additional samples included animal fat (poultry fat; HPF) and non-edible oil (carinata; HCA). All four samples were eval-uated for their tribological properties and compared with the mineral base oil (MBO). These potential bio-lubricants samples had viscosity indices (VI) ranging from 197 to 254, pour points (PP) from-10 degrees C to-20 degrees C, and Noack volatilities between 16% and 23%. The coefficient of friction (COF) for HAL and HSS was lower than MBO, HPF, and HCA, but the wear was higher than HPF. Large amounts of oxygenates and olefins imparted higher VI and lowered PP to HPF. Even though both HSS and HAL demonstrated higher amounts of paraffin, they exhibited lower thermo-oxidative stability, higher PP, and higher volatility than other samples. In the case of HAL and HSS, the aromatics could have played a more prominent role in determining the lubricating properties than the paraffin alone. The HPF had the lowest wear, highest VI, and lower PP but higher COF. The volatility and COF predominantly dependent on the cyclic structures, unsaturation, and heteroatoms. The results indicated that the hydrotreated bio-oil from HTL biocrude and waste precursors could be considered as eco-friendly hy-drocarbon biolubricant blend stock.