Residual stresses (RS) arise in a wide range of manufacturing processes, including additive manufacturing, welding, forming, grinding, and machining. Accurate characterization and prediction of RS are crucial for optimizing functional performance and structural integrity, as tensile stresses reduce fatigue strength while compressive stresses enhance it. Traditional finite element methods provide detailed insights into RS distributions but are computationally expensive for real-time use. To overcome this limitation, we propose a physics-informed neural network (PINN) framework that embeds the Prandtl-Reuss constitutive equations for elastoplasticity directly into the loss function, enabling mesh-free forward simulation of RS distribution and inverse identification of parameters under Hertzian contact loading. The inverse formulation simultaneously reconstructs stress fields and identifies key parameters, namely the effective friction coefficient and normalized load factor, from sparse data, addressing the nonuniqueness and instability of traditional inverse methods. Validation against high-fidelity Runge-Kutta-Gill reference solutions shows that residual stress prediction errors remain below 8% across a wide parameter range, while parameter identification errors converge to below 1%. The PINN predictions were compared with representative experimental trends for Ti-6Al-4V under burnishing and orthogonal cutting, confirming consistency across chip-generating and chipless processes. By enabling real-time parameter updates from minimal data, the proposed framework can accelerate the development of digital twins for manufacturing, supporting predictive modeling and process optimization. This advancement provides physics-based rapid RS analysis for critical applications, including bearing contacts and machining process optimization, significantly improving speed and usability over traditional approaches.
Laser-fabricated grid-like textures were introduced on Ti-6Al-4V flats to quantify their influence on fretting performance under variable normal loads (20-100 N) and a fixed displacement amplitude of 40 & micro;m. Using a ball-on-flat configuration (105 cycles, 100 Hz), the evolution of friction, system deformation, and maximum wear depth was correlated with high-resolution wear scar topography and debris distribution. In the gross slip regime (20-60 N), deeper and denser grids (approximate to 40 & micro;m depth, 140 & micro;m pitch) entrapped a compliant debris layer that reduced maximum wear depth by up to 35% compared with untextured controls, even after the central texture had been erased. The same textures shifted the partial slip threshold to lower loads (approximate to 60 N) by locally accommodating displacement and moderating shear. In near-stick conditions (80-100 N), however, deep grids promoted plastic indentation and mesa cracking, indicating a trade-off between debris retention and structural integrity. Stable friction coefficients were marginally lowered in gross slip but slightly elevated in partial slip because of increased adhesive resistance. System deformation saturated beyond 80 N; the deepest grid (T3) exhibited the lowest stabilized value in partial slip, corroborating its superior elastic accommodation. The results demonstrate that grid-like surface textures provide a robust route to mitigate fretting wear of Ti-6Al-4V provided groove depth and density are optimized to balance debris storage, load-bearing capacity, and resistance to texture fracture.
This study systematically investigates the tribological behavior of fine-grained isotropic graphite under synergistic variable conditions, addressing a critical knowledge gap regarding its performance in extreme conditions. While graphite is a promising high-temperature solid lubricant, its utility is limited by environmental dependencies. Our research reveals that the tribological performance of fine-grained isotropic graphite depends on the formation and stability of interfacial tribofilm rather than its bulk properties. Comprehensive reciprocating test against 440C stainless steel counter bodies explores friction and wear across varied temperatures (room temperature, 100 degrees C, and 300 degrees C), load, and velocities. Advanced characterization including X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy, optical profilometry, and Raman spectroscopy explains the underlying wear mechanisms. Performance paradox was identified under high-load conditions, failure at 100 degrees C, while low friction (& micro; approximate to 0.035) and low wear (0.718 mm(3)) performance was observed at 300 degrees C. Raman analysis confirmed that the 100 degrees C failure was due to mechanical refinement of graphite into highly disordered carbon debris following moisture desorption. The low wear and friction performance at 300 degrees C load ramp test was due to the formation of a stable ordered graphitic transfer film. However, under high-velocity conditions, frictional heat, a second distinct failure mechanism, causes tribo-oxidation, which degrades the protective film. These findings underscore that controlling the interfacial temperatures is the most important parameter for the sustained low-friction and wear performance. This work provides insights for optimizing graphite's application in demanding tribological environments.
This article investigates the transient flow behavior and distribution patterns of lubricating oil within high-speed bearings through in situ visualization. An experimental setup, comprising a transparent bearing and a visualization platform, was developed. Experiments were conducted under varied rotational speeds using two lubricants with different viscosities. A corresponding numerical model was established to simulate the lubrication flow field. The research results indicate that the two lubricants exhibit distinct transient flow characteristics inside the bearing. At lower speeds, spherical oil droplets form on the cage surface, which then deform, elongate, and are ejected. As speed increases, the oil transitions to finer filaments or accumulates on the outer ring, depending on the oil viscosity. The oil volume fraction (OVF) on the inner ring, cage, and balls decreases with increasing rotational speed. In contrast, the variation of OVF on the outer ring follows different patterns under ambient and high-temperature conditions, as well as with different lubricants. Overall, the effective oil volume retained inside the bearing cavity is relatively limited, ranging from approximately 0.5 mL to 7.8 mL. The findings of this study provide theoretical guidance for the design of oil-jet lubrication systems in high-speed bearings.
Contact temperature plays a crucial role in friction and wear behavior. In this study, a multiscale model is developed to directly calculate the contact temperature distribution in line contacts under boundary lubrication. By coupling macro-scale and micro-scale heat transfer models, more detailed contact temperature information can be obtained. First, experimental and analytical methods are employed to validate the effectiveness of the model. Furthermore, the effects of surface topography, roughness, and bulk temperature on contact temperature and heat transfer processes are analyzed by using the multiscale model and compared with single-scale model results. The simulation results indicate that rougher surfaces with steeper, sharper asperities concentrate the load on fewer contacts, producing smaller real contact areas and hotspots, whereas deeper valleys tend to mitigate these extreme pressures and temperatures. Moreover, the bulk temperature difference between friction pairs affects the contact temperature distribution. Finally, the correlations and limitations among different scuffing criteria of line contacts are analyzed and discussed.
This research studies the wear characteristics of various heat-treated Al7075/3 wt% carbon nanotubes (CNTs) metal matrix composites processed by the casting process. The wear characteristics of both untreated Al7075 alloy and Al7075/3 wt% CNTs composites under as-cast, solution-treated, and T6 heat-treated conditions were evaluated using a pin-on-disc apparatus in dry sliding conditions. The influence of process parameters such as heat treatment, applied load, and sliding velocity on wear-rate and friction coefficient was studied. The wear mechanisms under high-stress conditions were examined using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) of the worn surfaces and revealed that delamination, abrasion, oxidation, and adhesion were the dominant wear mechanisms. Different machine learning models, including random forest (RF), gradient boosting (GB), support vector machine (SVM), neural network (NN), XGBoost (XGB), and light gradient boosting machine (LGBM), have been trained for the tribological parameters prediction. For wear-rate prediction, the XGBoost showed the best overall performance with the lowest root-mean-square error (RMSE) of 0.082 and mean absolute error (MAE) of 0.065, along with the highest R-2 score of 99.1%, indicating excellent predictive accuracy and minimal error. The gradient boosting achieved the lowest RMSE of 0.013 and MAE of 0.010, along with the highest R-2 score of 97.6% for the average coefficient of friction.
This study focuses on the film-forming characteristics of the secondary lubricating medium in a water environment under different working conditions, and the roller-on-disc lubrication film test rig, along with the fluorescent approach, is used to directly observe the film formation of the secondary lubricating medium. Observations reveal that working conditions of higher load and higher speed can decrease the film-forming ability of the secondary lubricating medium but benefit the stability of the oil film thickness. The block-on-ring test rig and the confocal microscopy are used to prove the characteristics of friction and wear reduction of the injected secondary lubricating medium, and the flow condition of the secondary lubricating medium in the contact region of the block-on-ring test rig is then simulated by using the CFD model. This research facilitates the adoption of environmentally friendly lubricants as the secondary lubricating medium in engineering applications lubricated with pure water by providing essential data support.
To enhance the tribological performance and self-locking torque stability of the friction drive interface in traveling-wave ultrasonic motors, this study proposes a dual-surface enhancement strategy applied to both the rotor and stator. The rotor was coated with a 30 wt% polyetheretherketone/polytetrafluoroethylene (PEEK/PTFE) composite, while an AlCrN coating was deposited on the stator via physical vapor deposition. The tribological properties and motor output performance of various friction pairs were systematically evaluated using a custom-built tribometer and motor testing system. Results demonstrate that the PEEK/PTFE composite paired with AlCrN achieved an ultralow wear-rate of 9.97 & times; 10(-7) mm(3)/Nm, superior to that obtained when paired with phosphor bronze (1.85 & times; 10(-6) mm(3)/Nm). Moreover, the PEEK/PTFE-AlCrN combination exhibited the smallest fluctuation in self-locking torque, with a coefficient of variation (CV) of only 3.8%, considerably better than that of the uncoated phosphor bronze stator (CV = 14.3%). This study confirms that the synergistic application of PEEK/PTFE composites and AlCrN coatings effectively improves both wear resistance and torque stability at the ultrasonic motor drive interface, facilitated by the protective stator coating and the formation of a transfer film, thereby offering a novel material-matching solution for reliable motor design in precision driving applications.
As a critical component of the pantograph-catenary system, the surface physical characteristics of the carbon strip directly affect the wear mechanism of the friction pair and the current-carrying stability. In this study, a ring-on-block high-speed current-carrying tester was employed to investigate the correlation between the surface physical characteristics of the carbon strip (surface roughness) and its wear mechanisms under a sliding speed of 300 km/h, a current of 300 A, different normal loads, and varying test durations. The evolution of surface roughness was analyzed using white light interferometry (WLI). The results indicate that the surface roughness of carbon strips varies with both increasing normal load and prolonged test duration. Furthermore, the surface roughness of the carbon strips shows a negative correlation with current-carrying efficiency and a positive correlation with the wear-rate. Scanning electron microscopy (SEM) analysis reveals that the wear mechanism of the carbon strip varies under different loads. Notably, nodular protrusions appear on the surface of the carbon strip under low normal loads, which are formed by material transfer from the contact wire. Due to their high hardness, they significantly increase the friction coefficient and surface roughness of the carbon strip. The suppression of arc erosion pits and nodular protrusions can effectively improve the contact state of the friction pair, thereby enhancing the current-carrying efficiency of the pantograph-catenary system and the operational stability of the train.
Bearings used in hydrogen technology face significant lubrication challenges. The main difficulty stems from hydrogen molecules dissociating into atoms on the nascent wear sites and diffusing into steel, ultimately causing hydrogen embrittlement and failure of tribological components. Lubricant additives that rapidly form tribofilms, such as antiwear and extreme pressure additives, can suppress atomic hydrogen generation and permeation in steel, but the resulting tribofilms tend to increase friction. Nanol (TM), a sustainable copper-based nanoadditive capable of forming stable dispersions in oils, offers unique friction-modifying, antiwear, and thermal advantages. In this study, Nanol (TM) dispersed in a polyalphaolefin base oil was used to lubricate ball and roller bearings undergoing rolling contact fatigue tests under boundary lubrication conditions in a hydrogen environment. Beyond its ability to reduce friction by immediately decreasing the real-contact area between the moving parts, Nanol (TM) also reacts with nascent iron to form cohesive, low-friction tribofilms composed of copper and iron oleate on the wear track. These mechanisms, along with the properties of the chemically formed tribofilm, were key to lowering friction and extending the fatigue life of the bearings operating in hydrogen and under severe conditions.
The operational reliability of high-speed railways in cold, wet environments, such as high-altitude regions, presents a significant challenge, particularly concerning the durability of flash-butt-welded rails. This study investigates the reciprocating sliding wear behavior and mechanisms of U71Mn steel rail flash-welded joints under both water and ice-water conditions. This study comparatively analyzed three critical zones: the base metal, the as-welded joint, and the normalized joint. The results indicated that the ice-water environment more strongly influenced the friction and wear properties than the water-only environment. Although the friction coefficients generally decreased in the ice-water mixture, the wear damage was significantly more severe for all materials. Notably, the base metal experienced a disproportionately larger increase in the wear-rate than the welded joints, revealing that environmental degradation critically depends on the microstructure. Microscopic analysis identified that the predominant wear mechanisms under ice-water conditions were abrasive and oxidative wear, with the base metal being particularly susceptible to crack propagation at the surface. These findings clarify the wear mechanisms of rail joints in icy environments and provide a foundational basis for developing strategies to enhance the low-temperature wear resistance of high-speed railway welds.
As supercritical carbon dioxide (S-CO2) Brayton cycles advance toward higher power densities, the lubrication and dynamic characteristics of support bearings are critical to the stability of turbine rotor systems. This article presents a numerical approach that combines the full-variable frequency perturbation method with the equivalent coefficient method to predict the equivalent dynamic stiffness and damping coefficients of S-CO2 tilting-pad bearings. The method captures the evolution of dynamic coefficients over the entire frequency range and determines their high-frequency limiting characteristics through a limiting process analysis. The limiting perturbation pressure solution shows that the high-frequency limiting coefficients are explicitly governed by the steady-state density rho & strns; 0 and the compressibility term partial derivative p & strns; rho & strns; ( p & strns;0 , T & strns;0 ) , which distinguish them from conventional gas-lubricated bearings. Furthermore, the effects of pad inertia, pivot offset ratio, static load configuration, and ambient parameters on bearing dynamics are systematically analyzed. The results show that S-CO2 tilting-pad bearings exhibit a characteristic stiffness hardening and damping vanishing behavior at high frequencies. Pad inertia reduces direct stiffness and amplifies cross-coupled stiffness at high frequencies, while enhancing damping near synchronous frequencies. Moreover, the influence of ambient parameters on the dynamic characteristics strongly depends on the thermodynamic region, with the pseudo-critical and stable supercritical zones showing distinct trends. The proposed approach offers an effective framework for analyzing the rotor dynamics and stability of S-CO2 turbine systems.
Accurate measurement of ball screw preload, critical for performance optimization, remains a persistent challenge due to the limitations inherent in conventional methods. Based on the validated applicability of Stribeck theory to screw friction behavior, this study establishes a direct relationship between dynamic preload drag torque (DPDT) and preload force through the coefficient of friction (COF), thereby developing a comprehensive methodology for preload calculation. A novel Stribeck-based COF model was established through synchronous measurements under various conditions as a benchmark case. Decomposition of the total DPDT into Coulomb, viscous, and Stribeck-effect components reveals distinct regimes. Simplified empirical equations for preload were further derived through Taylor expansion. Validation demonstrates a mean error below 5% across operating conditions, outperforming traditional formulas (an error of more than 30%). It provides a robust methodology for high-accuracy preload measurement in double-nut ball screws, advancing the fundamental understanding of friction behavior in precision transmission systems and laying the foundation for developing universal models in subsequent research.
Classical theories state that the friction coefficient is independent of geometry, depending only on the real contact area determined by roughness. However, experimental evidence shows significant differences between static and dynamic friction coefficients. Recent models introduce an energetic theory for friction, analogous to the Griffith theory for fracture, resulting in a constant dynamic friction coefficient at heavy loads but a larger static coefficient at low loads. We show that, for power-law punches, the ratio of static to dynamic friction coefficients at low normal loads is higher for flatter profiles and, in principle, grows unbounded at zero load. This may explain the observed variability in friction ratios. The model's predictions align well with recent experimental results.
The effect of hydrogen bonding on the mechanical properties of lubricating greases (LGs) is examined. Four greases are prepared in an attempt to isolate the hydrogen bonding interactions within the LG system. Small amplitude oscillatory shear is first used to evaluate the viscoelastic moduli and crossover stress in the undisturbed state at varied temperatures. A 1-h full rotation shear is then applied to mechanically disrupt the LGs. The storage modulus is then tracked over time under varied temperatures to investigate modulus recovery. Our results show that hydrogen bonding within and between the thickener and oil plays a significant role in the mechanical properties in both the undisturbed and postdeformation states. Specifically, hydrogen bonding between the oil and the thickener increases the modulus and exhibits stronger temperature dependence. Hydrogen bonding within only the thickener increases the modulus with less temperature dependence. Overall, these results show that the interaction of functional groups between thickener-thickener and thickener-oil plays an important role in the mechanical properties and should be a design consideration.
Surface texture shapes and texturing methods were created to improve the tribological performance of mechanical systems. High-speed conventional micromachining in general and semihemispherical microdimples prepared through ball nose end milling in particular, have recently gained recognition as an effective method for improving tribological performance. The tribological behavior of semihemispherical microdimples created through ball nose end milling was investigated in this research at constant and accelerated sliding speeds. Experiments were conducted using an oscillating pin-on-disc arrangement with varying load (2 and 4 N), lubrication (0.2, 2, and 20 & micro;l), and temperature (50, 100, and 150 degrees C) to replicate the characteristics of the piston-liner contact in an internal combustion engine. The coefficient of friction decreases with an increase in lubrication and a drop in load, sliding speed, and temperature across all evaluated surfaces. D40 surfaces exhibited better efficiency throughout most tribological test settings, with a mean texture efficiency of 26.86% for the evaluated conditions. Analysis of texture efficacy for the variation of linear speed similar to the piston-ring-liner interface at higher load and temperature suggests a novel approach to study the variation of friction for the variation of lubrication regimes in reciprocating motion. Overall, it was found that the textured surfaces with semihemispherical microdimples of 240 & micro;m in diameter, 40 & micro;m in depth, and 10% area density, created by conventional micromachining, are suitable for piston applications.
The objective of the research presented in this article was to determine the influence of a micro-addition of boron on the abrasive wear resistance, as well as the strength and plastic properties of chromium-containing cast steel. The study compared a reference cast steel (Fe-C-Cr) with a cast steel of a similar chemical composition, but containing a micro-addition of boron (Fe-C-Cr-B). Abrasive wear resistance was assessed under laboratory conditions using soil masses with varying granulometric compositions, classified as light, medium, and heavy soils. The experimental results were preceded by an analysis of phase transformations occurring during continuous cooling, using dilatometric and metallographic studies, on the basis of which CCT diagrams were developed for both types of cast steel. Subsequently, for a selected heat treatment condition-quenching and tempering at 200 degrees C-hardness measurements, static tensile tests, Charpy impact tests, and fractographic analyses were carried out. The obtained results demonstrated that the cast steel containing a micro-addition of boron exhibits higher resistance to abrasive wear and improved strength properties, which result from a greater proportion of martensite in its microstructure, regardless of the cooling conditions. These findings indicate that the synergistic action of boron and chromium enables the production of cast steels with enhanced resistance to abrasive wear.
The current challenge with magnetorheological (MR) damper technology lies in its limited durability, primarily due to the abrasive nature of ferromagnetic particles. The critical component appears to be the seal, representing a soft contact. This comparative study focuses on testing the friction coefficient (COF) and wear loss of compliant contacts immersed in magnetorheological fluid. This article presents wear loss and the coefficient of friction for six selected sealing materials flooded by MR fluid. Furthermore, the effect of particle concentration on wear loss is also tested. Finally, the effect of long-term loading of MR fluid on tribological properties is presented. The results show that the polyurethane-based sealing material exhibited the lowest wear, while polytetrafluoroethylene (PTFE) exhibited the lowest COF. Polyurethane materials were found to be approximately 3.5 times more resistant to abrasion than nitrile butadiene rubber seals and 2.5 times more resistant than PTFE. The concentration of particles in MR fluids within the tested range (22-40 vol%) did not significantly affect abrasiveness. It was found that MR fluid subjected to long-term mechanical loading exhibited nearly a threefold increase in abrasiveness compared to new MR fluid. Consequently, it can be concluded that long-term mechanical loading of MR fluid has a substantial impact on its tribological properties. However, a more detailed study of tribological properties changes with long-term loading is the subject of further research.
The limiting load represents a pivotal design constraint for linear motion ball guides (LMBGs). Despite its significance, there is a paucity of analytical models capable of accurately predicting this limit. This article presents a novel analytical approach to calculate the limiting load of LMBGs. The mathematical model is established by coupling displacement compatibility, contact mechanics, and limiting load criteria. A numerical iterative solver is employed to obtain the solution. Validated by a case study on an industrial-grade LMBG, the proposed method proves to be both comprehensive and practical for the design and analysis of linear guides.
Agricultural waste-derived metal matrix composites (MMCs) provide sustainable and cost-effective alternatives to conventional ceramic-reinforced materials. This work provides a systematic assessment of the solid particle erosion behavior of AlSi10Mg composites reinforced with a silicon-based refractory compound (SiRC) derived from rice husk, synthesized via powder metallurgy, contributing to the development of high-performance engineering materials aligned with circular economy-driven manufacturing practices. The SiRC powder was first produced through controlled pyrolysis of rice husk, yielding highly crystalline particles (similar to 20 & micro;m) composed of cristobalite and quartz and subsequently used to fabricate AlSi10Mg composites containing 0, 3, 6, and 9 wt% reinforcement via powder metallurgy. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energydispersive spectroscopy (EDS) analyses confirmed uniform dispersion and phase stability. Incorporation of SiRC reduced composite density by up to 11% while increasing hardness progressively, achieving an overall improvement of approximately 24% at 9 wt% reinforcement. Erosion tests under varying impact velocities and impingement angles revealed that the 6 wt% SiRC composite exhibited superior resistance, attributed to an optimal balance between hardness and deformation resistance. Velocity exponent analysis (n approximate to 2-3, R-2 > 0.95) indicated ductile erosion behavior, while field emission scanning electron microscopy (FESEM) and surface roughness evaluations confirmed reduced material loss and mechanical embedding of erodent particles. Overall, SiRC incorporation enhanced mechanical integrity and erosion resistance, demonstrating the potential of these composites as sustainable materials for demanding industrial applications.
