Friction最新文献

Friction properties of rock are closely connected with the anisotropy. The anisotropy of rock friction can provide a valuable assessment for geotechnical and geological engineering. In this study, the rotary friction tests were conducted to analyze the water effect on the friction property and the friction anisotropy of the four types of rock. The drilling response model (DD-model) was employed to characterize the rotary friction behavior of the rocks. The parameters of this model include the three types of friction parameters: 1/ς, μ, and f, where 1/ς and μ are constant, and f is a variable. A quantitative method is proposed for assessing the anisotropy of rock friction. The results of the rotary friction tests indicate that the relation between torque force and thrust force conforms to the DD-model. The changes of two friction constants 1/ς and μ from dry state to water-saturated state suggest that the water effect on the friction strength of the rocks exhibits significant anisotropy. The friction strength determined by the friction variable f increases first, then decreases, and finally stabilizes with the increasing of depth. AI_f is an anisotropy index calculated by the proposed method. The percentage difference of the average value of AI_f between water-saturated and dry states shows the degree of the water effect on the friction anisotropy of the rocks, mudstone (MU) > granite (GR) > fine sandstone (FS) > argillaceous siltstone (AS). The quantitative model is hopefully constructed for characterizing the relation between the anisotropic friction strength of rock and the moisture state in future.

Three triangular friction block configurations are commonly employed in high-speed train brake systems, namely, unperforated, perforated configuration with one circular hole, and perforated with three circular holes. In this study, we adopted these friction block types to investigate the effect of perforated friction block configurations on the brake performance of high-speed trains based on a self-developed brake test rig. The results indicate the significant impact of the number of the holes on the wear behavior, temperature distribution, and vibration characteristics of the brake interface. The friction surface of the unperforated block is covered by wear debris, while the perforated blocks produce less wear debris. Furthermore, the one-hole block exhibits a more uniform temperature distribution and better vibration behavior than that with three holes. The friction brake is a dynamic process, during which separation and attachment between the pad and disc alternatively occur, and the perforated structure on the friction block can both trap and expel the wear debris.

This study reports on the tribological performance of aromatic thermosetting co-polyester (ATSP) and polyether ether ketone (PEEK)-based polymer composite coatings mixed with PTFE filler. The coatings were tested across a wide temperature range from −180 to 110 °C to simulate the environmental temperatures on Titan, Moon, and Mars, which are of particular interest for NASA’s future exploratory missions. An experimental setup was developed to conduct the pin-on-disk experiments under dry sliding conditions and extreme temperature and contact pressure. Transfer film formation and its characteristics were found to play significant roles in the tribological performance, and the characteristics of the film were temperature-dependent. The XPS and SEM analysis indicated the increase of the PTFE content in the transfer film as the temperature decreased to cryogenic conditions. The coefficient of friction did not follow a linear trend with temperature and was minimum at 110 °C and maximum at −180 °C. ATSP coating showed superior performance with lower friction and unmeasurable wear at all temperatures, whereas PEEK coating exhibited maximum wear at 25 °C followed by −180, and 110 °C.

Recently, various slippery liquid-infused porous surfaces (SLIPS) have been fabricated for the protection of various materials. However, these SLIPSs are limited by their underlying storage structure and superficial lubricant layer, showing poor durability. Herein, inspired by the high-strength structure of Shell nacre’s “brick-mud” layer, we fabricated an all-inorganic composite coating by using wet chemically etched MXene as a brick and an aluminum phosphate binder (AP) as mud. Then, a series of microwell-array structures were designed and prepared on the coating via nanosecond ultrafast laser writing ablation technology. Subsequently, the textured surface was modified by a silane coupling agent. Vinyl-terminated polydimethylsiloxane (PDMS) was tightly grafted onto the porous surface through a thiol-ene click reaction to obtain lubricant grafted texture surface (LGTS). The prepared LGTS showed good lubrication properties for multiple phases, including various liquids, ice crystals, and solids. It exhibits excellent chemical stability and mechanical durability under deionized water impact, centrifugal test, strong acid solutions, anti/de-icing cycles, and high-intensity friction. Thus, the proposed strategy for constructing robust LGTS will greatly promote theoretical research on super wetting interfacial materials and their applications in the fields of antifouling, anti/de-icing, and lubricating protection.

The paper shows that work in a quantum system is quantized with energy; the quantum work is equivalent to the highest eigenenergy (the Debye energy) of the system and the superlubricity of solids is derived from the quantum work. The prerequisite for the superlubricity is that the lateral force at contact surfaces in sliding is less than the Debye force so that the phonon of the solids is not excited.

In this study, a new comprehensive fully coupled elastic-hydrodynamic model is developed for a multi-layer gas foil thrust bearing (GFTB). The interaction effects among the top foil, back board, middle foil, and bottom foil, as well as the Coulomb friction effect, are considered. The stiffness and static characteristics obtained by the experimental and theoretical approaches are in good agreement, which verifies the accuracy of the model. The contribution of each foil layer to the overall stiffness and the load-carrying mechanism are analyzed. Interaction effects of the load, preload, and rotational speed on the static performance are investigated comprehensively. Furthermore, start-stop tests are performed to achieve the lift-off speed, start-up torque, and shut-down torque under various operating conditions.

The contact of an elastic quarter- or eighth-space is studied under the condition that the movement of the side surface of the quarter-space is constrained: It can slide freely along the plane of the side surface but its normal movement is blocked (for example, by a rigid wall). The solution of this contact problem can be easily achieved by additionally applying a mirrored load to an elastic half-space. Non-adhesive contact and the Johnson-Kendall-Roberts (JKR)-type adhesive contact between a rigid sphere and an elastic quarter-space under such a boundary condition is numerically simulated using the fast Fourier transform (FFT)-assisted boundary element method (BEM). Contacts of an elastic eighth-space are investigated using the same idea. Depending on the position of the sphere relative to the side edge, different contact behavior is observed. In the case of adhesive contact, the force of adhesion first increases with increasing the distance from the edge of the quarter-space, achieves a maximum, and decreases further to the JKR-value in large distance from the edge. The enhancement of the force of adhesion compared to the half-space-contact is associated with the pinning of the contact area at the edge. We provide the maps of the force of adhesion and their analytical approximations, as well as pressure distributions in the contact plane and inside the quarter-/eighth-space.

Oil pollution and the energy crisis make oil-water separation an urgent for human need. The widespread use of materials with a single emulsion separation capability is limited. Multifunctional on-demand separation materials can adapt to a wide range of application scenarios, thus having a wider range of applications. The underoil superhydrophilic surface is of great significance for realizing the on-demand separation of oil/water emulsions through the removal of water in the oil and oil in the water. A 3D porous emulsion separation material based on the superhydrophilic principle of sphagnum moss was designed. The material was prepared in a simple step by taking advantage of the adhesion of polydopamine and the introduction of the as-prepared superhydrophilic BaSO₄ nanoparticles to achieve superhydrophilicity with a water contact angle (WCA) of 0° and an oil contact angle (OCA) of 157.3°, resulting in excellent separation performance for both water-in-oil and oil-in-water emulsions. Underoil superhydrophilic porous composite (OSPC) can complete two kinds of emulsion separations by filtration or adsorption. It adsorbs water from water-in-oil emulsion to achieve separation, with a good adsorption capacity of 74.38 g/g and efficiency up to 99%. It can also filter oil-in-water emulsions with an efficiency of 99.92%. The separation efficiencies are all almost unchanged after ten separation cycles. Furthermore, the material has excellent flame retardancy, which reduces the possibility of secondary disasters. The three-dimensional porous sponge has excellent on-demand separation performance for multiple emulsions. It provides a new preparation strategy for underoil superhydrophilic materials and a new idea for the design direction of special wetting materials for the on-demand separation of oil/water emulsions.

This manuscript presents an innovative methodology for the assessment of the friction torque of ball slewing bearings. The methodology aims to overcome the limitations of state-of-the-art approaches, especially when the friction torque is conditioned by the preload of the balls. To this end, the authors propose to simulate the preload scatter when solving the load distribution problem, prior to the friction torque calculation. This preload scatter allows to simulate a progressive transition of the balls from a four-point contact state to a two-point contact one. By implementing this capability into an analytical model, the authors achieve a successful correlation with experimental results. Nonetheless, depending on the stiffness of the structures to which the bearing is assembled, it is demonstrated that the rigid ring assumption can lead to inaccurate friction torque results when a tilting moment is applied. The methodology described in this research work is meant to have a practical application. Therefore, the manuscript provides guidelines about how to use and tune the analytical model to get a reliable friction torque prediction tool.

Inorganic nanoparticles have been proved as powerful lubricant additives at elevated temperature. However, the tribological properties are inevitably impaired due to poor dispersion and insufficient high temperature resistance of organic matter modified nanoparticles. Here, we prepare a self-dispersed molybdenum disulfide quantum dot/graphene crumpled ball (MGCB) comprising molybdenum disulfide quantum dot uniformly interspersed on the wrinkled graphene ball. The crumpled ball composite possesses excellent dispersity in polyalkylene glycol base oil without depending on surface modifiers. Compared with the conventional phosphate esters lubricant, our results indicate MGCB could vastly improve the lubrication performance of polyalkylene glycol with an extremely low concentration (0.05 wt%) at elevated temperature (150 °C), showing a friction reduction of 47% and a wear reduction of 30% compared with the conventional phosphate esters lubricant (tricresyl phosphate, TCP). This is because crumpled ball potentiates synergistic lubrication effect within the boundary lubrication. Overall, we envision our designed self-dispersed MGCB has significant potential in tribological application at elevated temperature.