Friction最新文献

This work explores the bi-stable behaviour of a frictional system susceptible to mode-coupling instability. The focus is placed on the variations of the energy flows at the contact, due to external perturbations, and the role of contact nonlinearities on the system dynamic response. A lumped parameter numerical model, incorporating contact nonlinearities, is developed, allowing transitions between sliding, sticking, and detachment contact conditions. While prestressed Complex Eigenvalue Analysis (CEA) allows to identify instabilities in the linearized frictional system, transient simulations were conducted to investigate the nonlinear system dynamics and the possibility to switch between two stable states (mode-coupling or stable sliding) by an external perturbation. The investigation of the bi-stable state has been carried out by performing an energy balance of the system, accounting for the exchanged mechanical energies at the contact, to highlight the key role of contact nonlinearities in driving the power flows at the origin of the different stable states and respective limit cycle. The findings underscore the critical role of contact nonlinearities in shaping the power flows at the contact interface, determining the transition between stable sliding and mode-coupling, providing further insights on the “fugitive” feature of mode-coupling instabilities.

In this work, a friction-induced vibration model for water-lubricated bearings (WLBs) is developed. The model incorporates interfacial mechanical effects, including the stiffness and damping coefficients of the water film, contact stiffness of asperities, elastic deformation of the bush, etc. To evaluate the friction-induced vibration state, i.e., stability of WLBs, the complex eigenvalue analysis is employed. A frictional noise experiment for a WLB is performed to validate the effectiveness of the developed model. Based on this model, the stability diagram of friction-induced vibrations in WLBs under various parameters is obtained, and the effects of key parameters, such as radial clearance, angular groove amplitude, and boundary coefficient of friction, on the stability are investigated. Numerical results indicate that increasing the boundary coefficient of friction, surface roughness, radial clearance, and angular groove amplitude elevates the risk of unstable friction-induced vibration. Furthermore, numerical studies reveal the existence of a critical rotational speed at which friction-induced vibration transitions from being unstable to stable. As the rotational speed approaches the critical value, the risk of unstable friction-induced vibration rapidly decreases. Within the hydrodynamic lubrication regime, the maximum vibration attenuation index tends to remain constant, regardless of any further increases in the rotational speed.

To address the persistent environmental concerns of petroleum-based lubricants, we report on a new class of high-performance, sustainable greases. These were formulated by combining select vegetable oils, including the structurally unique Orychophragmus violaceus (OV) seed oil, with an oleic acid-functionalized organo-modified montmorillonite nanoclay thickener. The sustainable samples consistently outperformed an industrial-grade lithium soap grease in tribological experiments. Remarkably, under harsh conditions of high load (up to 500 N) and temperature (150 °C), both 5 and 10 wt.% nanoclay-thickened OV-based greases exhibited superlubricity, with the coefficient of friction of approximately 0.008. Surface analysis using Raman spectroscopy after the tribological experiments revealed that this exceptional performance was not a result of typical carbon-based tribofilm formation but from a novel lubrication mechanism: the in-situ formation of an adhesive, low-shear solid tribofilm composed of the nanoclay itself. The formulated greases also exhibited excellent low-temperature fluidity and structural stability, with complete recovery after freeze-thaw cycles to -20 °C. These results highlight a new design methodology for creating reliable and biodegradable lubricants suitable for extreme industrial applications.

Lubrication failure of moving parts at extremely cryogenic temperatures poses a major challenge for advancements in space exploration, superconductivity, and other technologies. This study systematically investigates the tribological behavior of hydrogenated amorphous carbon (a-C:H) films in vacuum from -200 to 25 ℃. Notably, as temperature decreases, the friction coefficient and the wear life of the a-C:H films exhibit an abnormal increase. At -200 ℃, wear life exhibits a remarkable enhancement of at least two orders of magnitude. Introducing in situ mass spectrometry and cryogenic micro/nano indentation, the dynamic monitoring of interface damage, hydrogen passivation, and hardness evolution was conducted during the friction process. The work indicated that cryogenic temperatures significantly reduce damage of a-C:H films, leading to changes in the synergistic lubrication involving hydrogen passivation, graphitization, and transfer films, resulting in high friction and low wear. It is fundamentally attributed to cryogenic temperatures altering the interfacial activity, which is the key factor in activating the synergistic lubrication of the above mechanisms.

Crucially，with a suitable interfacial activity at -75 ℃, a-C:H films can achieve an ultralow friction coefficient of ~0.015 and a wear rate of ~ 10⁻⁸ mm³/N·m. The work provides critical insights and establishes a foundation for deploying a-C:H films for cryogenic applications.

Wear debris particles play a crucial role in frictional interfaces. Conventional understanding holds that the debris accumulation causes severe wear. Interestingly, the debris from metal friction pairs includes anti-wear metal oxides generated by tribochemical reactions, which can form a protective oxidation film to resist wear. However, minimizing the abrasive damage caused by accumulated debris and using the anti-wear property of the metal oxides can be mutually exclusive. Here, a rational design of a coupling surface that manipulates nanoscale wear debris to resist further wear is reported. It consists of surface textures used to capture and temporarily store excess nanoscale wear debris, a deposited self-cleaning coating that subsequently helps transfer part of the captured debris into the sliding-contact interface, where it converts into a protective oxidation film. The coexistence of the two elements with contrasting properties in manipulating nanoscale wear debris considerably reduces wear under conditions of water lubrication, oil lubrication, and macroscale superlubricity. Our strategy achieves the manipulation and utilization of wear debris for anti-wear purposes. This work holds the potential to promote further investigation into the role of nanoscale wear debris and its utilization approaches.

This study uses rollers with curved profiles to explore the phase stability of austenitic matrix AISI 304 exposed to the rolling contact wear under the variable slip ratios in the range from -0.23 up to 8.81%. The wear track is formed under the cyclic surface delamination and the asymmetric extrusion of matrix towards the track side. The size of wear track and the fraction of extruded materials grow along with the slip ratio. Severe plastic deformation makes the parental matrix susceptible to phase transition when the ferromagnetic martensite replaces the paramagnetic austenite. The extent of this transition and the preferential alignment of neighbouring phases were found to be more complex under the higher slip ratios. The matrix is preferentially strained along the wear track width when the slip ratio is close to zero, whereas the presence of friction components at the higher slip ratios preferentially strains the matrix closer to the rolling direction. The sandwich-like microstructure is formed when the extremely refined layer containing a mixture of strain-induced martensite and austenite on the free surface is replaced by the thermally softened one due to friction heating, containing mostly strain-induced martensite, followed by the plastically strained region with prevailing austenite fraction. The heat release due to plastic deformation of austenite, phase transformation, and friction contribute to elevated temperatures, especially at the higher slip ratios. The enhanced thermal softening at the higher slip ratios reduces the compressive residual stresses, but the dislocation density in austenite as well as martensite is increasing. The study also demonstrates that this phase transformation can be reliably monitored by magnetic Barkhausen noise, which increases with the slip ratio due to the growing preferential straining of both phases into the direction of rolling. The high sensitivity of this non-destructive magnetic method is based on the medium-increasing fraction of the strain-induced martensite and its distinct preferential straining along the rolling direction at higher strain rates. The influence of residual stress state on magnetic emission is only minor.

Rubber products used in shoe soles and tires need high friction, especially under lubrication, to ensure safety in daily life. The frictional behavior of rubber under lubrication is influenced by various parameters, including the properties of the rubber tread, lubricant, mating surface, and sliding conditions. The effects of these parameters on friction have been investigated, but the mechanisms for enhancing friction under lubrication have not been studied systematically. This review is a summary of the methods used to enhance rubber friction under lubrication, including optimizing the tread groove geometry to improve drainage, texturing to increase the contact area, preventing liquid from flowing into the contact interface, increasing hysteresis friction, and controlling the surface free energy of rubber to promote dewetting. Additionally, suction-based attachment is emphasized as an effective mechanism for liquid-covered surfaces. Integrating these approaches may significantly advance the design of high-friction rubber tread, enabling the development of materials that maintain high friction regardless of lubrication conditions.

During cardiovascular interventional surgeries, catheters come into mechanical contact with vascular tissues, resulting in friction, collisions, and compression that can damage the tissue. To address this, surface engineering is essential for modifying the catheter surface. Effective catheter coatings require high adhesion strength to prevent peeling or delamination from the inner surface, while the outer surface must provide excellent lubricity and biocompatibility. In this study, we use the layer-by-layer (LbL) technique to introduce catechol-modified chitosan (CC) and dopamine-modified oxidized hyaluronic acid (DOHA), forming a nanoscale, superhydrophilic, strongly adhesive, and biocompatible coating on cardiovascular catheters. Tight binding of CC and DOHA results from electrostatic interactions, chemical reactions, and catechol group enrichment, yielding an adhesion strength up to 1 MPa. These CC/DOHA multilayers greatly enhance the lubrication performance of the TPU substrate, reducing the coefficient of friction (COF) by up to 95% compared to the uncoated state. After a 30-minute friction test, the COF of the CC/DOHA₁₆ coating only slightly increased from 0.032 to 0.044, demonstrating excellent stability. Evaluations showed a reduction in vascular intima damage from grade 5 without coating to grade 3, confirming the coating's effectiveness in minimizing friction-induced damage.

Pitch deviation, a type of longwave deviation, has been demonstrated to be a significant contributor to noise generation in the drive trains of motor vehicles operating at relatively high speeds. To date, numerous studies have investigated the effects of pitch deviation on the dynamic characteristics of gear systems. However, these models often exhibit inconsistencies with real-world observations due to the simplification of the contact between teeth pairs. This simplification ignores clearance compensation and damping of the oil film. To fill this gap, a new numerical model of spur gear systems with pitch deviation considering lubrication is proposed. An analytical model of the time-varying mesh stiffness (TVMS) and meshing impact force for spur gear pairs is proposed, in which the geometric relationship and iterative solution process are investigated. An improved tribodynamic model is established to investigate the excitation behavior of gear pairs with pitch deviation under lubrication. The accuracy of the proposed model is verified through comparisons with references and experimental results. The results show that the random impact phenomenon of the TVMS is significantly suppressed by the oil film, and the meshing impact force is also reduced. The transmission stability of spur gear pairs with tooth pitch deviation is improved under lubrication. The research results provide theoretical support for accurately predicting the dynamic responses of spur gear pairs with different precision levels while considering lubrication.

Under special operation conditions such as the starting, stopping, and turning of marine engines, it is challenging to establish a stable water lubricating film for water-lubricated bearings. Lubrication failure leads to severe friction-induced vibration behaviours, which threatens the reliability of bearings and the stealthiness of ship. In this study, a novel vibration dissipation mode via the mechanical-electrical-thermal energy conversion pathway was applied for the design of water-lubricated bearing materials. The piezoelectric damping composites (PDCs) with different deformation responses were fabricated. Under water-lubricated conditions, M3055 had excellent tribological properties, including a low average friction coefficient (COF) of about 0.22 and a low wear rate of 0.0066 mm³/(N·h). Furthermore, M3055 demonstrated excellent vibration-noise attenuation with maximum vibration and noise amplitudes of 4.2 m/s² and 0.88 Pa, respectively. These results were attributed to the fact that M3055 provided suitable deformation resistance and optimal piezoelectric damping effect, and the mechanical energy (vibration) was successfully converted into Joule heat. The knowledge gained could not only contribute to a better understanding of PDCs but also provide a theoretical reference for the development of novel anti-wear and vibration-attenuated water-lubricated bearing polymers.