Actual Contact Research Articles

Sliding bearings with lead-bismuth liquid metal lubrication

The article presents the results of tribological tests of a thrust bearing made of steel-bronze materials when lubricated with a lead-bismuth metal melt under conditions of heating up to 500°C. The influence of steel-bronze on the coefficient of friction in the ranges of load 0.17–1.0 MPa, velocity 0.05–0.23 m/s, and temperature 250–500°C was studied. It has been established that the tribological properties of friction pairs under experimental conditions mainly depend on temperature, which determines the wetting angle and the viscosity of the metal melt. With an increase in the temperature of the melt above 300°C, the viscosity of the melt decreases, and the liquid metal lubricant is squeezed out of the actual contact areas.

A Reduction Technique for the Calculation of the Forced Response of Bladed Disks in the Presence of Contact Mistuning at Blade Root Joints

Abstract The effect of differences in nominally equal sectors of a bladed disk (mistuning) is a well-known problem for designers since the forced response may show localized amplification of the blade response with respect to a cyclically symmetric (tuned) configuration. In order to perform a large number of simulations in a reasonable amount of time to characterize the highest blade response, corresponding to the worst mistuning pattern, reduction techniques have been developed, where the mistuning is introduced directly in a compact, reduced order model (ROM) obtained from very large finite element (FE) models. Typically, mistuning is introduced in the ROM in terms of natural frequency perturbations of the blade; nevertheless, a better insight is specifically required in order to correlate mistuning to a specific source (geometrical, material, contact mistuning). In this paper, a reduction technique is presented to take into account mistuning due to the contact uncertainties at the blade root joint, which can be caused by design tolerances, manufacturing process, assembly procedures, wear, etc. The technique takes its basis from the Craig–Bampton component mode synthesis (CB-CMS) applied to the uncoupled blade and disk sector, which is typically included in most of the FE software for easy implementation in standard industrial practice. The full set of master degrees-of-freedom (DOFs) at the random contacts are purposely reduced using an optimal local modal basis based on the Gram–Schmidt interface (GSI) technique developed by the authors. Experimental evidence of actual uncertain contact obtained during joint preloading is used to find an appropriate base to represent typical contact patterns.

Wettability, Self-Cleaning Property, and Mechanical Durability of A390 Aluminum Alloy with the Major Effect of Si Phases.

Si phases with specific shapes, including primary and eutectic Si phases, were etched on the surface of A390 Al alloy by chemical etching. Meanwhile, their effects on the wettability, self-cleaning properties, and mechanical durability of etched A390 aluminum alloy surfaces were investigated. The results showed that the surface etched for 5 min and modified by 1H,1H,2H,2H-perfluorodecyltrimethoxysilane (FAS-17) demonstrated a contact angle of 153.2 ± 1.4° and a sliding angle of 11.2 ± 1.6°. The theoretical contact angle calculated by constructing a wetting model of Si phases approximates the actual contact angle. Further experiments indicated that the etched surface acquired excellent self-cleaning properties. In addition, the exposed Si phases formed a strong support skeleton, which greatly improved the wear resistance of the hydrophobic surface.

Migrants' willingness to contact local residents in China

Although the integration of migrants has been increasingly pursued by the policy makers in many countries, there remains a paucity of knowledge regarding what migrants themselves think about integration. Based on the 2014 China Migrants Dynamic Survey, this study draws upon new assimilation theory to investigate migrants' willingness to contact locals in China. Our results suggest that most migrants in China are willing to contact natives, but their willingness may not be strong. Different from the actual intergroup contact, migrants' willingness to develop intergroup contact is not affected by their socioeconomic status. Instead, such willingness might be developed based on the cultural commonalities between migrants and natives. Residential segregation can weaken migrants' willingness to contact locals even after the endogeneity bias of residential choice is controlled by using instrumental variable method and focusing on migrants who live in the housing provided by employers. This draws a darker picture of migrant integration. Previous literature explains that migrants faced with residential segregation have fewer opportunities to form intergroup contact which in turn can hamper their overall integration. Our analysis goes a step further by revealing that they may also lose their motivation to make intergroup contact – a preference that is critical to changing their disadvantaged situation.

What does a sliding triboelectrical sensor really measure?

For a decade, Triboelectric Nanogenerators (TENG) have been massively developed and optimized either as (i) triboelectric energy harvesters or, (ii) triboelectric self-powered sensors. Unfortunately, it is obvious that the classical tribologists’ concerns – like the optimization of both real contact area, friction laws and wear processes – have never really been integrated so far neither in their development processes nor in their triboelectrical responses. Thus, a question as basic as “What kind of tribological information can we expect from multi-asperity triboelectric sensor rubbing on a plane in dry friction?” does currently not lead to a trivial answer.This paper tries to get out this paradoxical situation by simulating triboelectrical behavior of a multi-asperity real contact area in dry friction. Results reveal that a sliding triboelectric sensor assesses the instantaneous and averaged velocities of every interacting asperities within the contact area, finally probing, in real time, the tribological actual contact area behavior during sliding.

Investigation of Changes in the Structural-Phase State and the Efficiency of Hardening of 30CrMnSiA Steel by the Method of Electrolytic Plasma Thermocyclic Surface Treatment

Electrolytic plasma thermocyclic surface hardening is an attractive solution for both chemical and heat treatment used to improve the properties of the steel surface by structural and phase transformation. Structural and phase transformations occurring during the process of electrolytic plasma thermocyclic hardening are performed repeatedly at varying heating–cooling temperatures, which radically improve the quality of the part and give them properties unattainable by means of one-time processing. The impact of electrolytic plasma thermocyclic hardening modes on the structure and mechanical and tribological properties of 30CrMnSiA steel is investigated. The structural and phase components were examined using optical and scanning electron microscopy, as well as X-ray phase analysis. It is established that the structure of the cross-section is characterized by the following zonality: zone 1—a near-surface hardened zone, which is composed of hardened martensite; zone 2—thermal influence; and zone 3—a matrix consisting of pearlite and ferrite. The microhardness and wear resistance of the hardened surface were evaluated by nanoindentation and “ball on disk” methods, respectively. Nanoindentation analysis demonstrated that the indentation hardening process provides a maximum increase in hardness by three times and an increase in stiffness with a decrease in the elastic modulus by 38% compared to the original steel. The results of tribological studies show that electrolytic plasma thermocyclic hardening increases the resistance of steel to friction by increasing the surface hardness and reduces the area of actual contact during friction. It is established that the microhardness of the cross-section decreases proportionally from the surface to the depth of the layer, which is associated with a decrease in the volume content of martensite.

Use of temporal contact graphs to understand the evolution of COVID-19 through contact tracing data

Digital contact tracing has been recently advocated by China and many countries as part of digital prevention measures on COVID-19. Controversies have been raised about their effectiveness in practice as it remains open how they can be fully utilized to control COVID-19. In this article, we show that an abundance of information can be extracted from digital contact tracing for COVID-19 prevention and control. Specifically, we construct a temporal contact graph that quantifies the daily contacts between infectious and susceptible individuals by exploiting a large volume of location-related data contributed by 10,527,737 smartphone users in Wuhan, China. The temporal contact graph reveals five time-varying indicators can accurately capture actual contact trends at population level, demonstrating that travel restrictions (e.g., city lockdown) in Wuhan played an important role in containing COVID-19. We reveal a strong correlation between the contacts level and the epidemic size, and estimate several significant epidemiological parameters (e.g., serial interval). We also show that user participation rate exerts higher influence on situation evaluation than user upload rate does, indicating a sub-sampled dataset would be as good at prediction. At individual level, however, the temporal contact graph plays a limited role, since the behavior distinction between the infected and uninfected individuals are not substantial. The revealed results can tell the effectiveness of digital contact tracing against COVID-19, providing guidelines for governments to implement interventions using information technology.

Normal and tangential contact models for mixed lubrication of mechanical interface

Lubricating oil is usually injected in the gap between mechanical interfaces to reduce the friction and wear and improve the normal bearing capacity of the interface. The state of the mixed lubrication is induced from the inadequate lubrication. However, in the investigation of this state, the irregular oil pits of rough surfaces are usually simplified to be the cylindrical or conical in traditional contact models, which is disagreed with the actual contact situations and makes errors in the contact analysis when it is used to reveal the contact performance of the mixed lubrication. To investigate the normal and tangential contact performances for the real mixed lubrication, the normal and tangential contact models reproducing the effects of both the surface roughness and the lubrication viscosity on the normal contact force as well as the tangential fraction force are proposed in this work. Based on the statistical theory, the total area of oil pits is presented to describe irregular oil pits of rough surfaces. Then, the contact performance between the solid and liquid parts involved in the mixed lubrication is analyzed referring to the elastic–plastic theory and the hydrodynamics theory. Finally, several key influencing factors (including the surface roughness, the film thickness, and the lubrication oil viscosity) on the normal and tangential contact performances of mechanical interface are revealed. The main contribution of this work is providing some guidance on the improvement of the normal and tangential contact performances of the mechanical interface by adjusting the normal pressure, the surface roughness, the relative movement speed, and the viscosity of the lubricating medium.

Effect of textures fabricated by ultrasonic surface rolling on dry friction and wear properties of GCr15 steel

Linear textures and cross textures with different processing stepovers were fabricated on the surface of GCr15 steel by ultrasonic surface rolling processing (USRP). The effects of texture shape and processing stepover on the dry friction and wear performance of the samples were investigated using a ball-and-disk friction tribometer. The results showed that all the USRP texturing samples had lower friction coefficients and wear rates than those of the substrate sample under dry friction conditions. The best tribological performance was achieved at the stepover of 150 μm for linear textures and 250 μm for cross textures. For two shapes of textures with the same stepover, the linear texture had better wear resistance and the cross texture had better friction reduction. The texture shape and processing stepover influenced the texture height and the strengthening effect, which in turn affected the tribological properties of the samples. The improvement in friction and wear performance by ultrasonic rolling texturing was attributed to the synergistic effect of the strengthened substrate, reduced actual contact area, and entrapment of wear debris.

Performance analysis and optimization of the rough-contact Bi2Te3-based thermoelectric cooler via metallized layers

We use the numerical simulation to investigate an optimized metallized barrier layer for enhancing the performance of the Bi2Te3-based thermoelectric cooler (TEC). The maximal cooling capacity of 15.627 W and 5.646 W has been obtained under perfect and rough contact TEC. For the actual rough contact TEC, the Ag layer has the best cooling performance among conventional materials such as Ag, Ni, Ti, Co and Pd. The maximal cooling capacity (Qc max) for the rough contact TEC with Ag layer is 84.5% of the perfect contact TEC, which is 2.33 times of the rough contact TEC without any barrier layers. Subsequently, the roughness slope of the TEC and the pressure applied on the TEC have been researched. The Qc max can achieve 90% of the perfect contact TEC when the slope of the roughness is higher than 0.8. Under pressure over 3000 kPa, the Qc max can reach 91% of the perfect contact TEC. For practical production of Bi2Te3-based TEC can introduce Ag as the metallized layer to reduce the contact resistance and improve the cooling performance.

Thermodynamic analysis on wetting state transitions of rough surfaces with 3D irregular microstructure

Exploring the conditions and mechanisms of wetting state transition from the Cassie state to the Wenzel state and determining the final equilibrium wetting state of the system has always been one of most actively studied topics in the field of superhydrophobic behavior of rough surfaces. A three-dimensional (3D) thermodynamic analysis model of an irregular micro-texture surface (IMS) is established for the first time in this research. The IMS is composed of six different protrusions (cone, cylinder, positive truncated cone, inverted truncated cone, paraboloid, and spherical crown) randomly arranged on a hexagonal honeycomb structure. Based on the Gibbs free energy principle and the actual three-phase contact line (TCL) with droplet motion, generalized theoretical expressions for determining the free energy (FE) and free energy barrier (FEB) for equilibrium wetting states have been derived. Evaluating the critical geometric parameter for the Cassie-Wenzel transition has also been inferred. To a considerable extent, the transition mechanism is closely related to the height and maximum diameter of micro protrusions. This study provides a breakthrough theoretical support for the industrial preparation of robust solid superhydrophobic surfaces.

Complex Heat Exchange in Friction Steam of Brakes

In this article the structural features of friction pairs of brakes are analyzed. Heat transfer processes with new boundary conditions are described analytically with the addition of flow conditions and the appearance of a boundary thermal layer to convective heat transfer. The joint action of heat conduction and convection fields is presented. The release of heat during friction is due to the destruction of adhesive bonds in the actual contact zones, and the stress–strain state of micro-roughnesses. It should be said that due to the presence of accompanying transfer processes, complex heat transfer is much more complex compared to purely conductive, convective, and radiative heat transfer, which significantly complicates its analytical and experimental study. In this regard, the processes of complex heat transfer are currently studied little. From the point of view of non-equilibrium thermodynamics, the main task of describing the transfer process is to establish a relationship between the magnitude of the specific flux and the surface-volume temperatures that it causes in the metallic friction elements of the brakes. Additionally, as a result, an assessment of conductive and convective heat transfer in friction pairs of brake devices was made.

Influence of temperature on the dynamics of formation of granic sleeps and connected elevation dynamics in sliding conditions

The running-in process is accompanied by a change in microgeometry, as a result of which some constant roughness is established, which is characteristic of given friction conditions, and the physical and mechanical properties of the surface layers also change, since plastic deformations usually predominate in the contact.
 The thickness of the surface layers that have undergone changes during external friction depends on the stress state in the zones of their actual contact and heating during friction. The stress state in the zone of actual contact of the bodies is characterized by indentation or crushing of surface microroughnesses, as well as by elastic or plastic states of the latter. Surface heating during friction depends on the thermophysical properties of the contacting bodies and the friction mode. An increase in the temperature of the surface layers causes not only their softening, but also greatly increases the rate of physical and chemical processes in them. This leads to saturation of the surface layers with environmental gas molecules, oxidizing slicks, and also to an increase in the concentration of defects in these layers

Development of a compliant dashpot model with nonlinear and linear behaviors for the contact of multibody systems

A comprehensive dashpot model with hysteresis damping factors that can provide a convenient calculation approach for the elastoplastic impact behavior in multibody systems is studied in this paper. At the beginning of contact, the nonlinear hysteresis damping factor in the elastic phase is derived by approximately solving a nonlinear vibration system. When impact happens at a relatively high speed with a large load, elastoplastic deformation is inevitable, and Hertz contact stiffness cannot represent the actual contact stiffness. In order to describe the contact stiffness in the elastoplastic or plastic phase correctly, a static elastoplastic contact model is adopted to calculate the contact stiffness by approximately linearizing the relationship between load and deformation. When the contact comes into the elastoplastic phase, the impact behavior can be treated as a linear vibration system, and the linear hysteresis damping factor can be obtained from this linear system. The energy dissipation in different contact phases can be described by a nonlinear and a linear hysteresis damping factors. Such a nonlinear hysteresis damping factor can make up for the deficiency of the static elastoplastic contact model when describing the energy dissipation in the elastic contact phase. Simulation results show that the proposed dashpot model is more harmonious with the static elastoplastic contact model compared to the existing dashpot models. A slider-crank mechanism with a clearance joint and a Hopkinson incident bar are exemplified in the present work by using experimental data to validate the effectiveness of the proposed dashpot model.

Theoretical Study of the Friction Coefficient in the M-B Model

In order to study the influencing factors of friction coefficient in an M-B model, based on the basic model of fractal theory, the distribution function and probability distribution density of the micro-convex body truncation area are derived by using mathematical and statistical means, and a new model of critical truncation area and friction coefficient in fractal surface contact process are proposed. Considering the differences between the actual contact area and the truncated area during plastic deformation of the micro-convex body, a correction factor is introduced. Focusing on the mechanism of the elastic-plastic transition phase, and finally a friction coefficient model based on the fractal dimension, the normal force and correction factor is derived. Finally, the friction coefficient of fractal surface is simulated and verified by taking nickel as an example, and it is proved that the new model is correct in predicting the change trend of friction coefficient in the M-B model.

Solid–gas interface thermal conductance for the thermal barrier coating with surface roughness: The confinement effect

The yttria-stabilized zirconia (YSZ) is a famous thermal barrier coating material to protect hot-end components of an engine. As a characteristic feature of the YSZ, the surface roughness shall play an important role in the interface thermal conductance between the YSZ and gas, considering that the gas is typically at an extremely high temperature. We investigate the effect of the surface roughness on the thermal conductance of the YSZ–gas interface with surface roughness described by nanoscale pores on the surface of the YSZ. We reveal two competitive mechanisms related to the microstructure of the pore, i.e., the actual contact area effect and the confinement effect. The increase of the pore depth will enlarge the actual contact area between the YSZ and gas, leading to enhancement of the solid–gas interface thermal conductance. In contrast to the positive actual contact area effect, the geometry-induced confinement effect greatly reduces the interface thermal conductance. These findings shall offer some fundamental understandings for the microscopic mechanisms of the YSZ–gas interface thermal conductance.

Molecular dynamics analysis of friction-triggering process with spherical probe

Molecular dynamics simulation with embedded atom method/Finnis–Sinclair potential function was utilised to study the friction properties of the friction-triggering process at the mesoscopic scale. The sliding friction process of a spherical probe with different diameters on the surface of a single-crystal aluminum matrix was simulated and the influences of sliding friction on the atomic behaviour of the surface under different contact conditions were analysed. The relationship between contact force and friction coefficient with spherical diameter was studied. The research shows that the plastic groove, normal force and friction force increase with the increase in probe diameter in the sliding friction process. The contribution of increasing spherical diameter to normal force is greater than that of friction force. The coefficient of friction varies with the ratio of the actual contact area to the normal force. After the running-in process is stable, the friction coefficient fluctuates around a certain value as theoretically verified by the two-term friction law. The increase in the diameter of the spherical probe leads to the increase in the number of dislocation atoms in the workpiece and the formation of dislocation rings, which result in the difference in the sliding friction process under indenters with different diameters. The results provide valuable reference for friction-triggering measurement based on 3D nanometre probes such as friction force microscopy.

Mendel and Darwin

Evolution by natural selection is an explicitly genetic theory. Darwin recognized that a working theory of inheritance was central to his theory and spent much of his scientific life seeking one. The seeds of his attempt to fill this gap, his “provisional hypothesis” of pangenesis, appear in his notebooks when he was first formulating his evolutionary ideas. Darwin, in short, desperately needed Mendel. In this paper, we set Mendel’s work in the context of experimental biology and animal/plant breeding of the period and review both the well-known story of possible contact between Mendel and Darwin and the actual contact between their ideas after their deaths. Mendel’s contributions to evolutionary biology were fortuitous. Regardless, it is Mendel’s work that completed Darwin’s theory. The modern theory based on the marriage between Mendel’s and Darwin’s ideas as forged most comprehensively by R. A. Fisher is both Darwin’s achievement and Mendel’s.

Elucidation of frictional properties between rubber and flooring using a laser microscope with wide field of view

We investigate the measurement of real contact area, denoted by Ar, between various floor materials and rubber. Observation of Ar plays an important role for better understanding of frictional force. The Ar between them, however, has yet to be observed over regions wider than what an optical microscope can cover. Current optical microscopy techniques are limited by the measurement time required to observe the whole apparent contact area. Further, it is difficult to reproduce actual contact situations between the rubber and the floor materials because at least one of the contacting specimens must be transparent. Usually, glass plates much harder than rubber are used as counter specimens. In this study, we observed the Ar between a transparent silicone rubber and the various floor materials by using a laser microscope with a wide field of view, allowing us to observe the whole apparent contact area of 9 mm × 7 mm for a very short time. By using transparent rubber, we were able to reproduce the actual contact situations of the rubber-floor specimens, which is impossible with glass plates. The measurement of Ar when the apparent contact area was too small suffered from large fluctuations in measurement values due to waviness of the floor specimen. We investigated the apparent contact area required to accurately understand the relationship between frictional force and Ar and clarified the influence of waviness of the floor materials on the distribution of Ar.

АСПЕКТЫ ОБЕСПЕЧЕНИЯ ГЕРМЕТИЧНОСТИ ПОСАДОК С НАТЯГОМ В СЛОЖНЫХ УСЛОВИЯХ ЭКСПЛУАТАЦИИ ИЗДЕЛИЙ

The study objective is to discover optimal technical solutions in the field of ensuring tightness of interference fits in difficult operating conditions of products. 
 As a result of analyzing the problem of technological ensuring tightness and reliability of interference fits at the stage of their design, it is found that the use of regular microrelief and special types of coatings of joint faces can provide a solution to the problem. It is found that the characteristics of the microrelief and the physico-mechanical properties of the joint faces, as well as the assembly method, have a significant effect on the contact density and hermetic connection of the parts. It is shown that by controlling the microgeometry of the of joint faces, the use of soft electroplating coatings and anaerobic materials in the assembly, it is possible to increase the actual contact area and tightness of the joints significantly. The novelty of the work is in a broad consideration and generalization of the constructive and technological provision of tightness of interference fits, the aspects of increasing the tightness and reliability of interference fits are also considered. Analysis of scientific sources has shown that the tightness of the joint under dynamic load significantly depends on the parameters of microgeometry and the physical and mechanical characteristics of the materials of the joint faces of the parts that determine their actual contact area. The directions of ensuring tightness of interference fits, especially those operating in critical conditions, are observed.