Wet Friction Research Articles

Comparative Study of the Mechanical and Tribological Characteristics of Fe–Cu–Ni–Sn Composites with Different CrB2 Content under Dry and Wet Friction

Comparative Study of the Mechanical and Tribological Characteristics of Fe–Cu–Ni–Sn Composites with Different CrB2 Content under Dry and Wet Friction

Preparation and properties of resin-based wet friction materials reinforced by SiCp/basalt fiber cloth

A new type of wet friction material was prepared by solution impregnation with phenolic resin as binder, SiC particles and basalt fiber cloth as reinforcement.The hardness, density, porosity, microstructure, tribological properties and surface roughness were analyzed. The results show that the coefficient of friction (COF) and COF stability of the prepared material is more than 0.11 and 90%, respectively, while the wear rate is 0.26 × 10−7 cm3/J. The main wear pattern of this wet friction material is resin shedding. There is no large-sized “third body” abrasive debris on the friction surface, resulting in low wear rate of friction material.

Specific Features of Reactive Pulsed Laser Deposition of Solid Lubricating Nanocomposite Mo-S-C-H Thin-Film Coatings.

This work investigates the structure and chemical states of thin-film coatings obtained by pulsed laser codeposition of Mo and C in a reactive gas (H2S). The coatings were analysed for their prospective use as solid lubricating coatings for friction units operating in extreme conditions. Pulsed laser ablation of molybdenum and graphite targets was accompanied by the effective interaction of the deposited Mo and C layers with the reactive gas and the chemical states of Mo- and C-containing nanophases were interdependent. This had a negative effect on the tribological properties of Mo–S–C–H nanocomposite coatings obtained at H2S pressures of 9 and 18 Pa, which were optimal for obtaining MoS2 and MoS3 coatings, respectively. The best tribological properties were found for the Mo–S–C–H_5.5 coating formed at an H2S pressure of 5.5 Pa. At this pressure, the x = S/Mo ratio in the MoSx nanophase was slightly less than 2, and the a-C(S,H) nanophase contained ~8 at.% S and ~16 at.% H. The a-C(S,H) nanophase with this composition provided a low coefficient of friction (~0.03) at low ambient humidity and 22 °C. The nanophase composition in Mo–S–C–H_5.5 coating demonstrated fairly good antifriction properties and increased wear resistance even at −100 °C. For wet friction conditions, Mo–S–C–H nanocomposite coatings did not have significant advantages in reducing friction compared to the MoS2 and MoS3 coatings formed by reactive pulsed laser deposition.

Condition Monitoring Method for Automatic Transmission Clutches

This paper presents the development of a condition monitoring method for wet friction clutches which might be useful for automatic transmission applications. The method is developedbased on quantifying the change of the relative rotational velocity signal measured between the input and output shaft of a clutch. Prior to quantifying the change, the raw velocity signal is preprocessed to capture the relative velocity signal of interest. Three dimensionless parameters, namely the normalized engagement duration, the normalized Euclidean distance and the spectral angle mapper distance, that can be easily extracted from the signal of interest are proposed in this paper to quantify the change. In order to experimentally evaluate and verify the potential of the proposed method, clutches’ life data obtained by conducting accelerated life tests on some commercial clutches with different lining friction materials using a fully instrumented SAE#2 test setup, are utilized for this purpose. The aforementioned parameters extracted from the experimental data clearly exhibit progressive changes during the clutch service life and are well correlated with the evolution of the mean coefficient of friction (COF), which can be seen as a reference feature. Hence, the quantities proposed in this paper can therefore be seen as principle features that may enable us to monitor and assess the condition of wet friction clutches.

Thermal-hydraulic experimental study of louvered fin-and-flat-tube heat exchanger under wet conditions with variation of inlet humidity ratio

An experimental study for air-side thermal-hydraulic performance of louvered fin-and-flat-tube heat exchanger under dehumidifying conditions has been performed. The louvered fin studied, used for an automotive evaporator, is geometrically defined by a flow depth of 47 mm, a high louver angle of 50° and a louver pitch of 1.1 mm. The test was conducted for frontal air velocity ranging from 1.1 to 4.5 m/s. The heat exchanger was tested first in dry condition, working as a heater. In wet conditions, the cooling coil was tested for three air inlet temperatures, Ta, and for various inlet relative humidity, RH, organized as follows: Ta = 30 °C (RH = 40 to 70%), Ta = 35 °C (RH = 30 to 60%), and Ta = 40 °C (RH = 20 to 60%); which correspond to a variation of the inlet humidity ratio from 9.2 to 27.7 gwater/kgdry-air. It was found that, for all inlet conditions, the wet friction factor is higher than that for dry surface (up to 43%). Concerning the heat transfer performance, at low Reynolds number, the sensible Colburn ‘j’ factor is insensitive to the surface conditions. However, as Re number increases, the sensible wet j factor decreases (up to 23%) compared to the dry one. This degradation is more and more pronounced as the inlet humidity ratio increases.

Modular modeling and dynamic response analysis of a driveline system during start-up process

This study is aimed at exploring the effect of the structural parameters of a driveline system on its dynamic response, during the start-up process. For this purpose, a modular modeling method was proposed, based on the power flow and mechanical connections in the driveline. Moreover, a dynamic model of the driveline for vehicle start-up was established, taking into consideration the time-varying mesh stiffness, shaft stiffness, damping, and the variation of the wet clutch friction coefficient with the relative slipping speed. Modal analysis of the driveline system was performed, and the influence of time-varying mesh stiffness of the gear set on the natural frequency was analyzed. Furthermore, an engine constant start-up control strategy was adopted, and the start-up process of the vehicle was simulated. The simulation model was verified with the experiment data. A new evaluation indicator was proposed taking into consideration the phase difference and amplitude change, induced by the time-varying stiffness and the self-excited vibration in the system. Lastly, the influence of the structural parameters on the dynamic response of the driveline system, during start-up, was investigated.

Wet Surface Attachment: Micro–Nano Hierarchical Structure Enhanced Strong Wet Friction Surface Inspired by Tree Frogs (Adv. Sci. 20/2020)

Wet Surface Attachment: Micro–Nano Hierarchical Structure Enhanced Strong Wet Friction Surface Inspired by Tree Frogs (Adv. Sci. 20/2020)

Toward Bioinspired Wet Adhesives: Lessons from Assessing Surface Structures of the Suction Disc of Intertidal Clingfish

The clingfish attaches to rough surfaces with considerable strength using an intricate suction disc, which displays complex surface geometries from structures called papillae. However, the exact role of these structures in adhesion is poorly understood. To investigate the relationship between papillae geometry and adhesive performance, we developed an image processing tool that analyzed the surface and structural complexity of papillae, which we then used to model hydrodynamic adhesion. Our tool allowed for the automated analysis of thousands of papillae in specimens across a range of body sizes. The results led us to identify spatial trends in papillae across the complex geometry of the suction disc and to establish fundamental structure-function relationships used in hydrodynamic adhesion. We found that the surface area of papillae changed within a suction disc and with fish size, but that the aspect ratios and channel width between papillae did not. Using a mathematical model, we found that the surface structures can adhere considerably when subjected to disturbances of moderate to high velocities. We concluded that a predominant role of the papillae is to leverage hydrodynamic adhesion and wet friction to reinforce the seal of the suction disc. Overall, the trends in papillae characteristics provided insights into bioinspired designs of surface microstructures for future applications in which adhesion is necessary to attach to diverse surfaces (in terrestrial or aquatic environments), even when subjected to disturbance forces of randomized directionality.

Investigation of Tribological Properties of Rail and Wheel Steels

The wear resistance characteristics of AISI 51B60H and 30MnB5 steels with different carbon and boron contents were investigated. The analysis of the microstructure and hardness of steels was carried out. The specific wear rates in the conditions of dry and wet friction were calculated, and the friction coefficients were determined. The worn surfaces of rail samples were studied by electron microscopy. The possibility of using boron steels instead of R260 steel, which is used for manufacturing high-speed railways, was considered.

The Effect of Roughness on the Wet Skid Resistance of Tire Tread Compounds

The wet skid resistance is the main performance of automobile tires, and the excellent wet skid resistance can ensure the safety of the car. In this paper, 60 mesh sandpaper, 400 mesh sandpaper, 1000 mesh sandpaper and 2000 mesh sandpaper were used to pre-grind the sample, which changed the surface roughness of the sample; measured the dry and wet friction coefficient of the sample, and the measured friction pair was smooth Glass surface, rough glass surface and 180 grit sandpaper surface. The experimental results show that the rougher the surface of the sample and the surface of the friction pair, the greater the wet friction coefficient and the better the wet skid resistance.

The effect of humidity on the tribological behavior of the carbon-carbon composite in brakes

The article is devoted to the study of the humidity effect on the antifriction properties of a carbon-carbon composite material under the brake-applied mode. The effect of friction coefficient reducing due to the hygroscopic properties of this carbon composite is justified. The change in the friction coefficient and the composite structure during braking is investigated. In a macro scale it is shown that the structure of the samples working surface is isotropic both in the initial condition and after friction, which is due to the specific location of the fibrous carbon filler (reinforcing frame) in the volume of this carbon-carbon composite material. Based on the theory of errors, metrological assurance for evaluating the reproduction of experimental data was performed. Based on the obtained quantitative data on the reduction of the friction coefficient and the “drying” time of wet friction surfaces, practical recommendations are defined to ensure the effectiveness of vehicle braking arrangements.

Dynamic Friction Performance of Hierarchical Biomimetic Surface Pattern Inspired by Frog Toe‐Pad

AbstractIn this study, the dynamic friction performance of frog toe‐pad inspired surface patterns is investigated in three folds. First, frog toe‐pad morphology is mimicked, designed, and fabricated using 3D printing technology. Friction coefficients of the models are measured experimentally over a wet medium, with varying velocity, load, and sliding direction. Furthermore, time is recorded to reach a 5 mm height by the water flow through a steady model in another experimental set‐up. Second, numerical simulation is employed to study the contact area, sliding displacement, and frictional stress of the model tread patterns. Surfaces with different low frictional coefficients are considered to simulate the presence of wet medium and surface roughness. Third, an analytical model is utilized to calculate the water squeeze‐out time, as well as the height difference of drained water during wet surface conditions. Among three different bioinspired models, built to compare with a sample tire design, the double‐layered studded hexagonal pattern shows the best wet traction performance. This study demonstrates that the bioinspired hierarchical studded hexagonal model can provide design solutions for future tire treads with enhanced wet friction performances, as well as applications in products requiring surface wet traction enhancement, including boot soles, roller surfaces, and surgical grippers.

Micro-Nano Hierarchical Structure Enhanced Strong Wet Friction Surface Inspired by Tree Frogs.

Superior wet attachment and friction performance without the need of special external or preloaded normal force, similar to the tree frog's toe pad, is highly essential for biomedical engineering, wearable flexible electronics, etc. Although various pillar surfaces are proposed to enhance wet adhesion or friction, their mechanisms remain on micropillar arrays to extrude interfacial liquid via an external force. Here, two‐level micropillar arrays with nanocavities on top are discovered on the toe pads of a tree frog, and they exhibit strong boundary friction ≈20 times higher than dry and wet friction without the need of a special external or preloaded normal force. Microscale in situ observations show that the specific micro–nano hierarchical pillars in turn trigger three‐level liquid adjusting phenomena, including two‐level liquid self‐splitting and liquid self‐sucking effects. Under these effects, uniform nanometer‐thick liquid bridges form spontaneously on all pillars to generate strong boundary friction, which can be ≈2 times higher than for single‐level pillar surfaces and ≈3.5 times higher than for smooth surfaces. Finally, theoretical models of boundary friction in terms of self‐splitting and self‐sucking are built to reveal the importance of liquid behavior induced by micro–nano hierarchical structure.

Research of the influence of a hydrocarbon coating on the operational stability of membrane fabric

The effect of the hydrocarbon coating on the operational stability of membrane tissues is investigated. Coating on membrane tissues was obtained by plasma-chemical vapor deposition of the products of polymerization and decomposition of acetylene. As a result, the resulting coating allows to: increase resistance to multi-cycle abrasive impact by 9,9-30,1%, while the resistance of the textile base to light, washing, dry and wet friction does not deteriorate; increase tensile strength by 5,7-13,4%; increase elongation to 11,0-18,4%. The stability of fixation of organosilicon impregnation in the structure of the textile base after modification in a nonequilibrium low-temperature plasma increases to 45,5%.

Maleic Anhydride Modified Dicyclopentadiene Resin for Improving Wet Skid Resistance of Silica Filled SSBR/BR Composites

As commercial rubber in tires, silica-filled solution-polymerized styrene-butadiene rubber/butadiene rubber (SSBR/BR) compounds exhibited preferable wet skid resistance (WSR) properties, which could be further enhanced by the incorporation of some oligomeric resins. However, the untreated dicyclopentadiene (DCPD) resin shows a slight improvement in wet friction even if the good compatibility between DCPD and SBR owing to their common cyclic structures. For this problem to be addressed, we aimed to enhance its resin-silica interaction by reaction with maleic anhydride (MAH). In detail, the effect of MAH content on WSR, curing characteristics, physical-mechanical properties of the silica-filled SSBR/BR composites was investigated. When the MAH content is 4 wt% in the modified DCPD resin, the maximum enhancement of about 15% in tan δ values at 0 °C, as well as that of 17% in British pendulum skidding tester (BPST) index is obtained, indicating a desirable improvement in WSR. In addition of these two commonly used methods, water contact angles of the vulcanizates increase gradually with increasing MAH content, further confirming the remarkable performance of modified DCPD resin in WSR.

Bionic Design to Reduce Jacking Force for Trenchless Installations in Clay Soil

The application of trenchless technology is the trend of underground public facilities’ installation, replacement and repairing. As the soil-engaging component during penetrating bore, the working resistance of penetration head has remarkable effect on energy consumption of the whole working process. Some typical soil-digging animals, like pangolin and earthworm, they own special micro structures on their surface. It has been widely proved that some micro geometrical structures can effectively reduce adhesion resistance. Four kinds of bionic penetration heads were designed by imitating micro geometrical structures inspired by the soil animals. In this work, the real time jacking forces of the bionic penetration heads were measured and compared with a smooth penetration head (control group) without micro geometrical structures. The result indicated that the jacking forces of the bionic penetration heads were smaller than that of the smooth penetration head. This proved that the bionic penetration heads have the ability of reducing adhesion resistance. The vertical concave penetration head got the smallest jacking force, of which the average jacking force was 18.7% lower than that of the smooth penetration head. The interaction between soil and bionic surface of penetration head was discussed on the condition of wet friction. The bionic surface reduced the adhesion resistance by disturbing the soil and braking the continuous water film between soil and the surface of the penetration head.

Laboratory and field performance comparison of dense graded and stone mastic asphalt as a runway surface

ABSTRACT Ungrooved stone mastic asphalt (SMA) provides an alternate to grooved dense graded asphalt (DGA) as a runway surface. To be verified as suitable in this application, SMA must provide similar or better asphalt surface performance, compared to DGA, and must achieve regulated aircraft skid resistance requirements. A pilot runway resurfacing project provided the basis for this laboratory and field comparison of ungrooved SMA to grooved DGA as a runway surface. Laboratory performance testing demonstrated adequate and comparable deformation, fatigue and moisture resistance of SMA, while the quality assurance records indicated acceptable asphalt production. The construction records indicated that SMA had a lower air void content in the compacted layer, which can only provide a more durable surface. Most importantly, the aircraft skid resistance of SMA exceeded that of grooved DGA, based on surface macro-texture and wet friction measurements. It was concluded that SMA performed equal to or better than DGA, particularly with regard to surface texture and wet friction and it is recommended that airports consider ungrooved SMA as a runway surface in the future. The demonstration project should also be monitored in the future, to verify the long-term performance of SMA, compared to the DGA.

High‐speed wire electrical discharge machining to create superhydrophobic surfaces for magnesium alloys with high corrosion and wear resistance

AbstractHerein, a superhydrophobic surface of AZ31B magnesium alloy prepared by high‐speed wire electrical discharge machining and modification with stearic acid is reported. The surface morphology and wettability of the superhydrophobic surface were investigated by scanning electron microscopy and optical contact angle measurement, respectively. A uniform micro‐/nanopetal‐like structure was shown within the superhydrophobic surface, resulting in a contact angle of 151 ± 0.5° and a sliding angle of 4 ± 0.5°. Notably, the superhydrophobic surface had better corrosion resistance than the bare magnesium alloy, and its corrosion current density was reduced by nearly one order of magnitude. Under both dry and wet friction conditions, the friction coefficient of the superhydrophobic surface was lower than that of the bare magnesium alloy surface, with a much lower wear loss. In addition, the friction coefficient of the superhydrophobic sample was lower than that of the bare magnesium alloy sample under both the dry and wet friction conditions. Thus, the superhydrophobic sample experienced reduced wear and had a low wear rate.

Nondimensional Characterization of the Operational Envelope of a Wet Friction Clutch

In recent years, multidisc wet friction clutches are used in demanding powertrains of automatic and dual clutch transmissions targeting high efficiency and smoothness during gearshift. However, the developed flow pattern between the clutch discs is significantly complex and the Computational Fluid Dynamics (CFD) methods employed are quite demanding in terms of computational cost. To deal with this issue semi-analytical solutions were derived, which are limited, however, to specific problems, in order to obtain handy expressions, while also providing insight to the wet clutch physics. Nevertheless, this lack of global validity is counterbalanced by the fact that the governing equations become analytically solvable at specific operational conditions with satisfactory accuracy, provided that the simplifications rendering the truncated terms inactive hold true. In this work, a quantitative way of determining the relative weight of each term of the Navier-Stokes (NS) equations set is presented, based on the post-processing of CFD results using the Buckingham “π-theorem”. The sets of nondimensional numbers created were used to describe and model the physics of the wet clutch.

Wet and dry internal friction can be measured with the Jarzynski equality

The existence of two types of internal friction wet and dry is revisited, and a simple protocol is proposed for distinguishing between the two types and extracting the appropriate internal friction coefficient. The scheme requires repeatedly stretching a polymer molecule, and measuring the average work dissipated in the process by applying the Jarzynski equality. The internal friction coefficient is then estimated from the average dissipated work in the extrapolated limit of zero solvent viscosity. The validity of the protocol is established through analytical calculations on a one-dimensional free-draining Hookean spring-dashpot model for a polymer, and Brownian dynamics simulations of: (a) a single-mode nonlinear spring-dashpot model for a polymer, and (b) a finitely extensible bead-spring chain with cohesive intra-chain interactions, both of which incorporate fluctuating hydrodynamic interactions. Well-established single-molecule manipulation techniques, such as optical tweezer-based pulling, can be used to implement the suggested protocol experimentally.