Abstract 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 × 10−7 mm3/Nm, superior to that obtained when paired with phosphor bronze (1.85 × 10−6 mm3/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.

Abstract Healthy articular cartilage can sustain near-frictionless motion within the joint; in vivo friction coefficients fall well within the superlubricity regime (<0.01). However, despite nearly a century of study, the mechanisms underpinning these behaviors remain unclear. This uncertainty has been compounded by how cartilage's operating conditions have been defined and extended to explant testing approaches, resulting in drastically varied tribological responses. To address these discrepancies, we compared friction coefficients results from a comprehensive set of historical cartilage explant tribology studies, and from new data evaluating testing configuration (stationary contact area [SCA] vs convergent SCA [cSCA]) and lubricant choice (phosphate-buffered saline [PBS] vs. synovial fluid [SF]) on friction. These data demonstrate that the SCA, the most common testing configuration utilized, consistently reports friction greater than the migrating contact area (MCA) and cSCA configurations. Intriguingly, while the SCA and MCA are almost universally slid at sub-physiological speeds (∼1mm/s), the cSCA has routinely been tested at greater, and more physiologically consistent speeds (∼60mm/s). Nevertheless, SF drives noticeable reductions in friction within both SCA and cSCA studies. Importantly, particularly in vivo, only rapidly slid, SF-lubricated cSCA cartilage explants reliably demonstrate biofidelic friction (<0.005), indicating that hydrodynamic-related phenomena must not be discounted in cartilage lubrication. Collectively, these results underscore the need to further probe mechanisms of sustained cartilage superlubricity, a key finding only recently replicated via the cSCA configuration. Such knowledge will be crucial to understanding the true lubrication capacity of articular cartilage.

Abstract 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 °C, and 300 °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 °C, while low friction (µ ≈ 0.035) and low wear (0.718 mm3) performance was observed at 300 °C. Raman analysis confirmed that the 100 °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 °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.

Abstract Self-lubricant coatings are widely used in tribological applications, particularly under extreme conditions such as high speed, high load, and long-term operations. To evaluate the long-term performance of such coatings, a 12.5-hour reciprocating tribological study was conducted on a tungsten disulfide (WS2)-blended Inconel 625 (In625) coating. The coating was deposited on an SS304 substrate using an in-house-developed laser-assisted cold spray setup. Microstructural analysis revealed the presence of In625, WS2, Cr2S3, and tungsten (W) phases within the coating matrix. Initial tribological testing for 30 min demonstrated that the coating reduced the coefficient of friction (COF) to 0.31, nearly half that of the uncoated substrate (0.58). Subsequently, prolonged testing over 12.5 h confirmed that the COF of the coating remained stable, maintaining the same low value observed during the shorter test. Scanning electron microscopy and elemental mapping on the wear track indicated that the coating exhibited excellent adhesion, with no significant peeling off after 12.5 h of wear testing. Additionally, analysis of the wear counterpart showed the transfer of sulfide phases from the coating, facilitating the formation of a lubricating layer on both the coating and the counterpart. This transfer of lubricious phases was beneficial in achieving a low and stable COF. Furthermore, the wear-rate of the coating was significantly lower than that of the substrate (17 times), with the wear-rate decreasing over time. These results highlight the coating's potential for enhanced durability and performance in severe tribological applications.

Abstract 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.

Abstract 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 µl), and temperature (50, 100, and 150 °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 µm in diameter, 40 µm in depth, and 10% area density, created by conventional micromachining, are suitable for piston applications.

Abstract In an effort to acquire harder and more wear-resistant FeCoCrNiMn high-entropy alloys, the Cr3C2-reinforced FeCoCrNiMn HEA composite coatings were fabricated by laser cladding, and their microstructure, hardness, and tribological properties were characterized. The results indicate that the coatings are composed of face-centered cubic (FCC) matrix and Cr3C2 phase, and the addition of Cr3C2 enhances the microhardness, with values ranging from 371.1 to 559.2 HV0.5. When paired with Si3N4 ceramic, the average friction coefficient first decreases from 0.77 to 0.59, and then increases to 0.64. When coupled with 440C steel, the average coefficient of friction first decreases from 0.71 to 0.50, and then rises to 0.62. However, the addition of Cr3C2 carbide can significantly improve the wear resistance, and the wear-rate of the coatings paired with Si3N4 ceramic is always much lower than that paired with 440C steel. When paired with 440C steel, the minimum wear-rate of 1.44 × 10−5 mm3/Nm can be achieved. When paired with Si3N4 ceramic, the minimum wear-rate reaches 9.24 × 10−7 mm3/Nm (FeCoCrNiMn/20Cr3C2 coating), which is only 4.91% of the FeCoCrNiMn coating without Cr3C2 reinforcement. The superior wear resistance stems from a cooperative effect of Cr3C2 enhancement and tribo-oxidization. The main wear mechanism against 440C steel is abrasive and adhesive wear, and the main wear mechanism against Si3N4 ceramic shifts from abrasive-adhesive to oxidative wear as the Cr3C2 content increases.

Abstract 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 ρ¯0 and the compressibility term ∂p¯ρ¯(p¯0,T¯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.

Abstract 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.

Abstract It is crucial to further analyze the causes of hip and knee replacement failure to better enhance the success and minimize the shortcomings of joint replacement in patient outcomes. The purpose of this study is to collect samples of failed hip and knee orthopedic implants from surgeons and analyze the features of those implants to find possible reasons for implant failure so that these causes can be successfully prevented and/or mitigated. Twelve implants were collected and cleaned according to a standard protocol. The implants were analyzed using visual observation and an optical microscope, and initial reports are presented in this study. The preliminary findings suggest that a combination of factors, including material, design, patient, and surgical factors, may contribute to the failure of total hip and knee arthroplasties. Mechanical trauma to the implants may be a contributing factor to hip and knee implant failure, as scratch marks and abrasions were common in the implants collected. The study has several limitations, which are clearly stated in the manuscript. Further research is needed to investigate these factors in more detail, using a larger number of implants and a wider population of surgeons, and to develop strategies to improve the success of these procedures.