Showing papers in "Friction in 2022"

Nano-enhanced biolubricant in sustainable manufacturing: From processability to mechanisms

Abstract: Abstract To eliminate the negative effect of traditional metal-working fluids and achieve sustainable manufacturing, the usage of nano-enhanced biolubricant (NEBL) is widely researched in minimum quantify lubrication (MQL) machining. It’s improved tool wear and surface integrity have been preliminarily verified by experimental studies. The previous review papers also concluded the major influencing factors of processability including nano-enhancer and lubricant types, NEBL concentration, micro droplet size, and so on. Nevertheless, the complex action of NEBL, from preparation, atomization, infiltration to heat transfer and anti-friction, is indistinct which limits preparation of process specifications and popularity in factories. Especially in the complex machining process, in-depth understanding is difficult and meaningful. To fill this gap, this paper concentrates on the comprehensive quantitative assessment of processability based on tribological, thermal, and machined surface quality aspects for NEBL application in turning, milling, and grinding. Then it attempts to answer mechanisms systematically considering multi-factor influence of molecular structure, physicochemical properties, concentration, and dispersion. Firstly, this paper reveals advanced lubrication and heat transfer mechanisms of NEBL by quantitative comparison with biolubricant-based MQL machining. Secondly, the distinctive filmformation, atomization, and infiltration mechanisms of NEBL, as distinguished from metal-working fluid, are clarified combining with its unique molecular structure and physical properties. Furtherly, the process optimization strategy is concluded based on the synergistic relationship analysis among process variables, physicochemical properties, machining mechanisms, and performance of NEBL. Finally, the future development directions are put forward aiming at current performance limitations of NEBL, which requires improvement on preparation and jet methods respects. This paper will help scientists deeply understand effective mechanism, formulate process specifications, and find future development trend of this technology.

Metal matrix nanocomposites in tribology: Manufacturing, performance, and mechanisms

Abstract: Abstract Metal matrix nanocomposites (MMNCs) become irreplaceable in tribology industries, due to their supreme mechanical properties and satisfactory tribological behavior. However, due to the dual complexity of MMNC systems and tribological process, the anti-friction and anti-wear mechanisms are unclear, and the subsequent tribological performance prediction and design of MMNCs are not easily possible: A critical up-to-date review is needed for MMNCs in tribology. This review systematically summarized the fabrication, manufacturing, and processing techniques for high-quality MMNC bulk and surface coating materials in tribology. Then, important factors determining the tribological performance (mainly anti-friction evaluation by the coefficient of friction (CoF) and anti-wear assessment with wear rate) in MMNCs have been investigated thoroughly, and the correlations have been analyzed to reveal their potential coupling/synergetic roles of tuning tribological behavior of MMNCs. Most importantly, this review combined the classical metal/alloy friction and wear theories and adapted them to give a (semi-)quantitative description of the detailed mechanisms of improved anti-friction and anti-wear performance in MMNCs. To guarantee the universal applications of these mechanisms, their links with the analyzed influencing factors (e.g., loading forces) and characteristic features like tribo-film have been clarified. This approach forms a solid basis for understanding, predicting, and engineering MMNCs’ tribological behavior, instead of pure phenomenology and experimental observation. Later, the pathway to achieve a broader application for MMNCs in tribo-related fields like smart materials, biomedical devices, energy storage, and electronics has been concisely discussed, with the focus on the potential development of modeling, experimental, and theoretical techniques in MMNCs’ tribological processes. In general, this review tries to elucidate the complex tribo-performances of MMNCs in a fundamentally universal yet straightforward way, and the discussion and summary in this review for the tribological performance in MMNCs could become a useful supplementary to and an insightful guidance for the current MMNC tribology study, research, and engineering innovations.

Unlocking the secrets behind liquid superlubricity: A state-of-the-art review on phenomena and mechanisms

Abstract: Abstract Superlubricity, the state of ultralow friction between two sliding surfaces, has become a frontier subject in tribology. Here, a state-of-the-art review of the phenomena and mechanisms of liquid superlubricity are presented based on our ten-year research, to unlock the secrets behind liquid superlubricity, a major approach to achieve superlubricity. An overview of the discovery of liquid superlubricity materials is presented from five different categories, including water and acid-based solutions, hydrated materials, ionic liquids (ILs), two-dimensional (2D) materials as lubricant additives, and oil-based lubricants, to show the hydrodynamic and hydration contributions to liquid superlubricity. The review also discusses four methods to further expand superlubricity by solving the challenge of lubricants that have a high load-carrying capacity with a low shear resistance, including enhancing the hydration contribution by strengthening the hydration strength of lubricants, designing friction surfaces with higher negative surface charge densities, simultaneously combining hydration and hydrodynamic contribution, and using 2D materials (e.g., graphene and black phosphorus) to separate the contact of asperities. Furthermore, uniform mechanisms of liquid superlubricity have been summarized for different liquid lubricants at the boundary, mixed, and hydrodynamic lubrication regimes. To the best of our knowledge, almost all the immense progresses of the exciting topic, superlubricity, since the first theoretical prediction in the early 1990s, focus on uniform superlubricity mechanisms. This review aims to guide the research direction of liquid superlubricity in the future and to further expand liquid superlubricity, whether in a theoretical research or engineering applications, ultimately enabling a sustainable state of ultra-low friction and ultra-low wear as well as transformative improvements in the efficiency of mechanical systems and human bodies.

Tailored surface textures to increase friction—A review

Abstract: Abstract Surface textures with micro-scale feature dimensions still hold great potential to enhance the frictional performance of tribological systems. Apart from the ability of surface texturing to reduce friction, surface textures can also be used to intentionally increase friction in various applications that rely on friction for their adequate functioning. Therefore, this review aims at presenting the state-of-the-art regarding textured surfaces for high-friction purposes. After a brief general introduction, the recent trends and future paths in laser surface texturing are summarized. Then, the potential of surface textures to increase friction in different applications including adhesion, movement transmission and control, biomimetic applications, and road-tire contacts is critically discussed. Special emphasis in this section is laid on the involved mechanisms responsible for friction increase. Finally, current short-comings and future research directions are pointed out thus emphasizing the great potential of (laser-based) surface texturing methods for innovations in modern surface engineering.

Design and characterization of metallic glass/graphene multilayer with excellent nanowear properties

Abstract: Abstract The excellent properties of metallic glass (MG) films make them perfect candidates for the use in miniature systems and tools. However, their high coefficients of friction (COFs) and poor wear resistance considerably limit their long-term performance in nanoscale contact. We report the fabrication of a MG/graphene multilayer by the repeated deposition of Cu 50 Zr 50 MG with alternating layers of graphene. The microstructure of the multilayer was characterized by the transmission electron microscopy (TEM). Its mechanical and nanotribological properties were studied by nanoindentation and nanoscratch tests, respectively. A molecular dynamics (MD) simulation revealed that the addition of graphene endowed the MG with superelastic recovery, which reduced friction during nanoscratching. In comparison with the monolithic MG film, the multilayer exhibited improved wear resistance and a low COF in repeated nanowear tests owing to the enhanced mechanical properties and lubricating effect caused by the graphene layer. This work is expected to motivate the design of other novel MG films with excellent nanowear properties for engineering applications.

Synthesis of N-doped carbon quantum dots as lubricant additive to enhance the tribological behavior of MoS_2 nanofluid

Abstract: Abstract In this study, a novel lubricant additive nitrogen-doped carbon quantum dot (N-CQD) nanoparticle was prepared by the solvothermal method. The synthesized spherical N-CQD nanoparticles in the diameter of about 10 nm had a graphene oxide (GO)-like structure with various oxygen (O)- and nitrogen (N)-containing functional groups. Then N-CQDs were added to MoS 2 nanofluid, and the tribological properties for steel/steel friction pairs were evaluated using a pin-on-disk tribometer. Non-equilibrium molecular dynamics (NEMD) simulations for the friction system with MoS 2 or MoS2 + N-CQD nanoparticles were also conducted. The results showed that friction processes with MoS 2 + N-CQD nanofluids were under the mixed lubrication regime. And MoS 2 nanofluid containing 0.4 wt% N-CQDs could achieve 30.4% and 31.0% reduction in the friction coefficient and wear rate, respectively, compared to those without N-CQDs. By analyzing the worn surface topography and chemical compositions, the excellent lubrication performance resulted from the formation of tribochemistry-induced tribofilm. The average thickness of tribofilm was about 13.9 nm, and it was composed of amorphous substances, ultrafine crystalline nanoparticles, and self-lubricating FeSO 4 /Fe 2 (SO 4 ) 3 . NEMD simulation results indicated the interaction between S atoms in MoS 2 as well as these O- and N-containing functional groups in N-CQDs with steel surfaces enhanced the stability and strength of tribofilm. Thereby the metal surface was further protected from friction and wear.

Tribo-informatics approaches in tribology research: A review

Abstract: Abstract Tribology research mainly focuses on the friction, wear, and lubrication between interacting surfaces. With the continuous increase in the industrialization of human society, tribology research objects have become increasingly extensive. Tribology research methods have also gone through the stages of empirical science based on phenomena, theoretical science based on models, and computational science based on simulations. Tribology research has a strong engineering background. Owing to the intense coupling characteristics of tribology, tribological information includes subject information related to mathematics, physics, chemistry, materials, machinery, etc. Constantly emerging data and models are the basis for the development of tribology. The development of information technology has provided new and more efficient methods for generating, collecting, processing, and analyzing tribological data. As a result, the concept of “tribo-informatics (triboinformatics)” has been introduced. In this paper, guided by the framework of tribo-informatics, the application of tribo-informatics methods in tribology is reviewed. This article aims to provide helpful guidance for efficient and scientific tribology research using tribo-informatics approaches.

Insights into the tribological behavior of choline chloride—urea and choline chloride—thiourea deep eutectic solvents

Abstract: Abstract Deep eutectic solvents (DESs) have been considered as novel and economic alternatives to traditional lubricants because of their similar physicochemical performance. In this study, choline chloride (ChCl) DESs were successfully synthesized via hydrogen-bonding networks of urea and thiourea as the hydrogen bond donors (HBDs). The as-synthesized ChCl-urea and ChCl-thiourea DESs had excellent thermal stability and displayed good lubrication between steel/steel tribo-pairs. The friction coefficient and wear rate of ChCl-thiourea DES were 50.1% and 80.6%, respectively, lower than those of ChCl-urea DES for GCr15/45 steel tribo-pairs. However, for GCr15/Q45 steel, ChCl-urea DES decreased the wear rate by 85.0% in comparison to ChCl-thiourea DES. Under ChCl-thiourea DES lubrication, the tribo-chemical reaction film composed of FeS formed at the interfaces and contributed to low friction and wear. However, under high von Mises stress, the film could not be stably retained and serious wear was obtained through direct contact of friction pairs. This illustrated that the evolution of the tribo-chemical reaction film was responsible for the anti-friction and anti-wear properties of the DESs.

Predicting EHL film thickness parameters by machine learning approaches

Abstract: Abstract Non-dimensional similarity groups and analytically solvable proximity equations can be used to estimate integral fluid film parameters of elastohydrodynamically lubricated (EHL) contacts. In this contribution, we demonstrate that machine learning (ML) and artificial intelligence (AI) approaches (support vector machines, Gaussian process regressions, and artificial neural networks) can predict relevant film parameters more efficiently and with higher accuracy and flexibility compared to sophisticated EHL simulations and analytically solvable proximity equations, respectively. For this purpose, we use data from EHL simulations based upon the full-system finite element (FE) solution and a Latin hypercube sampling. We verify that the original input data are required to train ML approaches to achieve coefficients of determination above 0.99. It is revealed that the architecture of artificial neural networks (neurons per layer and number of hidden layers) and activation functions influence the prediction accuracy. The impact of the number of training data is exemplified, and recommendations for a minimum database size are given. We ultimately demonstrate that artificial neural networks can predict the locally-resolved film thickness values over the contact domain 25-times faster than FE-based EHL simulations ( R 2 values above 0.999). We assume that this will boost the use of ML approaches to predict EHL parameters and traction losses in multibody system dynamics simulations.

Two-dimensional molybdenum carbide (MXene) as an efficient nanoadditive for achieving superlubricity under ultrahigh pressure

Abstract: Abstract In this study, a robust macroscale liquid superlubricity with a coefficient of friction of 0.004 was achieved by introducing molybdenum carbide (Mo 2 CT x ) MXene nanoparticles as lubricating additives in a lithium hexafluorophosphate-based ionic liquid at Si 3 N 4 —sapphire interfaces. The maximal contact pressure in the superlubricity state could reach 1.42 GPa, which far exceeds the limit of the superlubricity regime in previous studies. The results indicate that a composite tribofilm (mainly containing molybdenum oxide and phosphorus oxide) that formed at the interface by a tribochemical reaction contributed to the excellent antiwear performance. Furthermore, the extremely low shear strength of the tribofilm and the interlayers of Mo 2 CT x MXene contributed to the superlubricity. This work demonstrates the promising potential of Mo 2 CT x MXene in improving superlubricity properties, which could accelerate the application of superlubricity in mechanical systems.

Typical dampers and energy harvesters based on characteristics of ferrofluids

Abstract: Abstract Ferrofluids are a type of nanometer-scale functional material with fluidity and superparamagnetism. They are composed of ferromagnetic particles, surfactants, and base liquids. The main characteristics of ferrofluids include magnetization, the magnetoviscous effect, and levitation characteristics. There are many mature commercial ferrofluid damping applications based on these characteristics that are widely used in numerous fields. Furthermore, some ferrofluid damping studies such as those related to vibration energy harvesters and biomedical devices are still in the laboratory stage. This review paper summarizes typical ferrofluid dampers and energy harvesting systems from the 1960s to the present, including ferrofluid viscous dampers, ferrofluid inertia dampers, tuned magnetic fluid dampers (TMFDs), and vibration energy harvesters. In particular, it focuses on TMFDs and vibration energy harvesters because they have been the hottest research topics in the ferrofluid damping field in recent years. This review also proposes a novel magnetic fluid damper that achieves energy conversion and improves the efficiency of vibration attenuation. Finally, we discuss the potential challenges and development of ferrofluid damping in future research.

Study on the mechanism of rapid formation of ultra-thick tribofilm by CeO_2 nano additive and ZDDP

Abstract: Abstract CeO 2 nanoparticles are potential anti-wear additives because of their outstanding anti-wear and load-bearing capacity. However, the shear-sintering tribo-film formation mechanism of oxide nanoparticles limits the tribo-film formation rate and thickness greatly. In this study, by compounding with zinc dioctyl dithiophosphate (ZDDP), ultra-fine CeO 2 nanoparticles modified with oleylamine (OM) can quickly form 2 µm ultra-thick tribo-film, which is 10–15 times thicker than that of ZDDP and CeO 2 , respectively. The ultra-thick tribo-film presents a nanocomposite structure with amorphous phosphate as binder and nano-CeO 2 as filling phase, which leads to the highest loading capacity of composite additives. The results of adsorption experiments tested by dissipative quartz crystal microbalance (QCM-D) showed that the P B value of additive has nothing to do with its equilibrium adsorption mass, but is directly proportional to its adsorption rate in 10 s. The compound additive of CeO 2 and ZDDP presented the co-deposition mode of ZDDP monolayer rigid adsorption and CeO 2 viscoelastic adsorption on the metal surface, which showed the highest adsorption rate in 10 s. It is found that the tribo-film must have high film forming rate and wear resistance at the same time in order to achieve super thickness. Cerium phosphate was formed from ZDDP and CeO 2 through tribochemistry reaction, which promotes the formation of an ultra-thick tribo-film with nanocomposite structure, which not only maintains the low friction characteristics of CeO 2 , but also realizes high P B and high load-carrying capacity.

A review of advances in tribology in 2020–2021

Abstract: Abstract Around 1,000 peer-reviewed papers were selected from 3,450 articles published during 2020–2021, and reviewed as the representative advances in tribology research worldwide. The survey highlights the development in lubrication, wear and surface engineering, biotribology, high temperature tribology, and computational tribology, providing a show window of the achievements of recent fundamental and application researches in the field of tribology.

An overview of functional biolubricants

Abstract: Abstract At present, more and more diseases are associated with the lubrication dysfunction, which requires a systematic study of the complex lubrication behavior of tissues and organs in human body. Natural biomacromolecular lubricants are essential for maintaining ultra-low coefficients of friction between sliding biological interfaces. However, when the surface lubrication performance of tissues or organs destroys heavily, it will bring friction/shear damage for sliding contact interfaces. Therefore, the application of exogenous biological lubricating materials to improve the lubrication situation of damaged tissue or organ interfaces has attracted extensive attention of researchers. In this review, based on a simple summary of lubrication mechanism at sliding biological interface, we systematically introduce the research progress of several kinds of representatively biolubrication materials, including eye drops, tissue anti-adhesion agents, joint lubricants, and medical device lubricants. Meanwhile, the lubrication mechanism and individual advantage and shortcoming for each of these synthetic exogenous lubricated materials are clarified. Correspondingly, the important lubrication application functionality of these biolubricant materials in typically medical surgery scenes, such as dry eye syndrome, tissue adhesion, arthritis, and interventional medical devices, is discussed. Finally, we look forward to the future development direction of artificial biolubricant materials.

An advanced efficient model for adhesive wear in elastic—plastic spherical contact

Abstract: Abstract A finite element (FE) model combining submodel technique is presented for the adhesive wear in elastic—plastic spherical contact. It consists of a global model, showing the potential location of fracture under combined normal and tangential loading, and a refined mesh submodel covering only the region near the potential fracture. This allows to describe the morphology of wear particle more accurately than that in a previously developed model by the authors. A range of normal loading is studied to show its effect on the shape and volume of wear particles. Two main regimes of mild and severe wear (along with a relatively narrow transition region between them) are found, which show almost linear and power-law dependency of wear rate on normal loading, respectively. Such behavior agrees with published experimental observations. However, the transition region is theoretically predicted here for the first time.

Tribological performance of iron- and nickel-base self-lubricating claddings containing metal sulfides at high temperature

Abstract: Abstract Iron-based coatings with the incorporation of solid lubricants have been prepared by means of laser cladding, in an effort to control friction and decrease tool wear at high temperatures during metal forming applications. The choice of a Fe-based powder has been considered advantageous, as it can lead to decreased costs compared to nickel-based claddings previously studied by the authors, in addition to having a lower environmental impact. In particular, the incorporation of transition metal dichalcogenides such as MoS 2 as precursors leads to the encapsulation of silver in Fe-based self-lubricating claddings, resulting in a uniform distribution of the soft metal across the thickness of the coating. Subsequent tribological evaluation of the claddings at high temperatures shows that the addition of lubricious compounds leads to lower friction at room temperature and significantly decreased wear up to 600 °C compared to the unmodified iron-based reference alloy, although higher than similar self-lubricating Ni-based claddings. In order to cast light into these observed differences, the corresponding microstructures, phase composition, and self-lubricating mechanisms have been studied and compared for Fe- and Ni-based claddings having both of them the addition of silver and MoS 2 . The results suggest a key role of the formation of protective tribolayers on the counter body during high temperature sliding contact. Additional simulation of the phase evolution during solidification reveals that the formation of different chromium- and nickel-based metal sulfides in Fe- and Ni-claddings during laser cladding by the decomposition of MoS 2 plays a key role in determining their tribological behaviour at high temperatures.

Dynamic, thermal, and vibrational analysis of ball bearings with over-skidding behavior

Abstract: Abstract The term “over-skidding” indicates that the cage rotational speed ratio exceeds the theoretical value as ball purely rolls on the raceway. Different from the skidding phenomenon that occurs in low-load and high-speed bearing, over-skidding usually occurs in large-size angular contact bearings, and it is still difficult to suppress under high load conditions. The main forms of damage to the raceway by over-skidding are spinning and gyro slip. To further explore the vibration characteristics and thermal effects of this phenomenon, a set of over-skidding tests of an angular contact bearing with a bore diameter of 220 mm were conducted on an industrial-size test bench. Through the experiment, the influence of axial load, rotational speed, and lubrication conditions on the occurrence of over-skidding were determined. Based on a previous dynamics model, the heat generation and thermal network models were integrated in the present study to predict the over-skidding and its thermal behavior. The model was validated in terms of the measured degree of over-skidding and temperature rise. The results showed that the degree of over-skidding reaches up to 12% of the theoretical value, and the friction power loss of the ball-pocket accounts for 30% of the total power loss. The analysis of the vibration signal showed a strong correlation between the bearing vibration characteristics and over-skidding behavior, thereby providing a way to indirectly measure the degree of over-skidding.

Dry friction damping mechanism of flexible microporous metal rubber based on cell group energy dissipation mechanism

Abstract: Abstract Flexible microporous metal rubber (FMP-MR) is a high-damping material that dissipates energy by dry friction through internal spiral metal wires in contact with each other. However, the FMP-MR energy dissipation mechanism is not fully understood owing to its disordered grid interpenetrating structure. In this work, computer-aided preparation technology is used to accurately reconstruct the complex spiral network structure of FMP-MR multipoint random contact, and a cell group model with an energy dissipation mechanism is proposed to obtain the dynamic energy distribution of the contact friction in both space and time dimensions. By judging the effective contact point, a global displacement ablation phenomenon of hooked staggered porous materials is induced. The macro- and micro-equivalent frictions are introduced to effectively explain the characteristics of the strong energy dissipation in FMP-MR under fretting excitation. A real and effective damping hysteresis constitutive model is then constructed to dynamically capture the mapping relationship between the complex nonlinear topological structure effect of the materials and spatial random contact dry friction in real time. The results indicate that the contact behavior between turns of the FMP-MR wire follows a clear quasi-Gaussian distribution under an external load, forcing the topological results to change. The energy dissipation of the materials revealed peak energy consumption lagging behind the loading limit for a certain distance, which can be determined by the effective contact point and contact dry friction slip. The consistency between the quasi-static compression tests and constitutive curves of the model was quantitatively verified through residual analysis. The data demonstrated the differential behavior of the FMP-MR meso-structure to follow a phased growth law during loading with different action mechanisms in the guiding, main growth, and relaxation stages of the energy consumption displacement curve. In summary, these findings provide an acceptable theoretical basis for the damping energy consumption mechanism and lifetime prediction of FMP-MR.

Mechanism of superlubricity of a DLC/Si3N4 contact in the presence of castor oil and other green lubricants

Abstract: Abstract To meet the surging needs in energy efficiency and eco-friendly lubricants, a novel superlubricious technology using a vegetable oil and ceramic materials is proposed. By coupling different hydrogen-free amorphous carbon coatings with varying fraction of sp 2 and sp 3 hybridized carbon in presence of a commercially available silicon nitride bulk ceramic, castor oil provides superlubricity although the liquid vegetable oil film in the contact is only a few nanometres thick at most. Besides a partial liquid film possibly separating surfaces in contact, local tribochemical reactions between asperities are essential to maintain superlubricity at low speeds. High local pressure activates chemical degradation of castor oil generating graphitic/graphenic-like species on top of asperities, thus helping both the chemical polishing of surface and its chemical passivation by H and OH species. Particularly, the formation of the formation of −(CH 2 −CH 2 ) n − noligomers have been evidenced to have a major role in the friction reduction. Computer simulation unveils that formation of chemical degradation products of castor oil on friction surfaces are favoured by the quantity of sp 2 -hybridized carbon atoms in the amorphous carbon structure. Hence, tuning sp 2 -carbon content in hydrogen-free amorphous carbon, in particular, on the top layers of the coating, provides an alternative way to control superlubricity achieved with castor oil and other selected green lubricants.

A review of current understanding in tribochemical reactions involving lubricant additives

Abstract: Abstract Lubricants have played important roles in friction and wear reduction and increasing efficiency of mechanical systems. To optimize tribological performance, chemical reactions between a lubricant and a substrate must be designed strategically. Tribochemical reactions are chemical reactions enabled or accelerated by mechanical stimuli. Tribochemically activated lubricant additives play important roles in these reactions. In this review, current understanding in mechanisms of chemical reactions under shear has been discussed. Additives such as oil-soluble organics, ionic liquids (ILs), and nanoparticles (NPs) were analyzed in relation to the tribochemical reaction routes with elements in metallic substrates. The results indicated that phosphorus, sulfur, fluorine, and nitrogen are key elements for tribochemical reactions. The resulting tribofilms from zinc dithiophosphates (ZDDP) and molybdenum dithiocarbamate (MoDTC) have been widely reported, yet that from ILs and NPs need to investigate further. This review serves as a reference for researchers to design and optimize new lubricants.

A new mechanism for friction-induced vibration and noise

Abstract: Abstract For years, friction-induced vibration and noise (FIVN) has puzzled many researchers in academia and industry. Several mechanisms have been proposed for explaining its occurrence and quantifying its frequencies, notably for automotive brake squeal, clutch squeal, and even rail corrugation. However, due to the complex and complicated nature of FIVN, there is not yet one fundamental mechanism that can explain all phenomena of FIVN. Based on experimental results obtained on a simple test structure and corresponding numerical validation using both complex eigenvalue analysis (CEA) and transient dynamic analysis (TDA), this study attempts to propose a new fundamental mechanism for FIVN, which is the repeated cycles of partial detachment and then reattachment of the contact surfaces. Since friction is ubiquitous and FIVN is very common, the insight into FIVN reported in this paper is highly significant and will help establish effective means to control FIVN in engineering and daily life.

Is there more than one stickiness criterion?

Abstract: Abstract Adhesion between an elastic body and a smooth, rigid substrate can lead to large tensile stresses between them. However, most macroscopic objects are microscopically rough, which strongly suppresses adhesion. A fierce debate has unfolded recently as to whether local or global parameters determine the crossover between small and large adhesion. Here, we report simulations revealing that the dependence of the pull-off force F n on the surface energy γ does not only have two regimes of high and low adhesion but up to four regimes. They are related to contacts, which at the moment of rupture consist of (i) the last individual Hertzian-shaped contact, in which is linear in γ , (ii) a last meso-scale, individual patches with super-linear scaling, (iii) many isolated contact patches with extremely strong scaling, and (iv) a dominating largest contact patch, for which the pull-off stress is no longer negligible compared to the maximum, microscopic pull-off stress. Regime (iii) can be seen as a transition domain. It is located near the point where the surface energy is half the elastic energy per unit area in conformal contact. A criterion for the transition between regimes (i) and (ii) appears difficult to grasp.

Insight into macroscale superlubricity of polyol aqueous solution induced by protic ionic liquid

Abstract: Abstract Currently, macroscale liquid superlubricity remains limited to low applied loads and typical ceramic friction pairs. In this study, a robust macroscale superlubricity with a coefficient of friction (COF) of approximately 0.006 is realized at the bearing steel interface induced by protic ionic liquids (ILs) in propylene glycol aqueous solution, and the lubrication system exhibits excellent anti-corrosion properties. Results show that superlubricity can be achieved by employing ILs with longer alkyl chains over a wide load (< 350 N) and speed (> 700 r/min) range. By systematically investigating factors affecting superlubricity, including the IL structure, ionization degree, test conditions, polyol, water-to-alcohol ratio, and lubrication state, the superlubricity mechanism is discussed. Notably, a thicker and denser stern layer can be formed using ILs with longer alkyl chains, which participates in the tribochemical reaction with the metal substrate to form a tribofilm during rubbing. The hydrogen bond network layer formed by the hydrogen ion and polycol aqueous solution can withstand high applied loads. Water can be used to reduce the shear stress of polyols, and enable superlubricity to be achieved under high-speed rotations. Moreover, an inevitable running-in period serves as a dispersing contact stress and dynamically forms a lubricating film, where the lubrication state locates mixed lubrication and then transforms into boundary lubrication as the roughness of the contact surface increases. This study is expected to significantly promote the development and application of superlubricity in the engineering field.

Effect of wear debris on fretting fatigue crack initiation

Abstract: Abstract Both wear and fatigue occur in fretting condition, and they interact with one another during the whole process. Fretting fatigue is commonly analysed without considering the effect of wear in partial slip regime, although wear affects the lifetime of crack initiation. This paper investigates, for the first time, the effect of wear debris on fretting fatigue crack initiation. To investigate the effect of debris, first fretting wear characteristics in partial slip regime are analysed for loading conditions. Then, the effect of wear on fretting fatigue crack initiation is investigated using Ruiz parameters and critical plane methods without considering the debris effect. Through the results, we can see that loading conditions affect the wear profiles in different ways. Moreover, wear has a significant effect on the fatigue in partial slip regime without considering debris especially on the crack initiation location. Finally, considering wear debris in the analysis, its effect on critical plane parameters is investigated. It is found that by considering the wear debris effect, the fretting fatigue crack initiation location is shifted towards the trailing edge. The predictions of both crack initiation location and lifetime show a good agreement with the experimental data.

Influence of structural depth of laser-patterned steel surfaces on the solid lubricity of carbon nanoparticle coatings

Abstract: Abstract Carbon nanoparticle coatings on laser-patterned stainless-steel surfaces present a solid lubrication system where the pattern’s recessions act as lubricant-retaining reservoirs. This study investigates the influence of the structural depth of line patterns coated with multi-walled carbon nanotubes (CNTs) and carbon onions (COs) on their respective potential to reduce friction and wear. Direct laser interference patterning (DLIP) with a pulse duration of 12 ps is used to create line patterns with three different structural depths at a periodicity of 3.5 µm on AISI 304 steel platelets. Subsequently, electrophoretic deposition (EPD) is applied to form homogeneous carbon nanoparticle coatings on the patterned platelets. Tribological ball-on-disc experiments are conducted on the as-described surfaces with an alumina counter body at a load of 100 mN. The results show that the shallower the coated structure, the lower its coefficient of friction (COF), regardless of the particle type. Thereby, with a minimum of just below 0.20, CNTs reach lower COF values than COs over most of the testing period. The resulting wear tracks are characterized by scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. During friction testing, the CNTs remain in contact, and the immediate proximity, whereas the CO coating is largely removed. Regardless of structural depth, no oxidation occurs on CNT-coated surfaces, whereas minor oxidation is detected on CO-coated wear tracks.

Investigations for vibration and friction torque behaviors of thrust ball bearing with self-driven textured guiding surface

Abstract: Abstract In order to improve the starved lubrication condition of rolling bearings, three kinds of textures, namely dimple, groove texture, and gradient groove texture, were developed on the guiding surface of thrust ball bearings in this study. The results show that gradient groove texture has the one-way self-driving function of liquid droplets. The root mean square (RMS) value of vibration acceleration of gradient groove textured bearing (GGB) decreased by 49.1% and the kurtosis decreased by 24.6% compared with non-textured bearing (NB) due to the directional spreading effect of gradient groove textures on oil. The frequency domain analysis showed that the textures mainly suppressed the medium and high-frequency energy of bearing vibration, and the GGB was reduced the most with 65.3% and 48%, respectively. In addition, whether the grease is sufficiently sheared has a large impact on the oil guiding effect, and the friction torque of GGB could decrease by 10.5% compared with NB in the sufficiently sheared condition. Therefore, the gradient groove texture with oil self-driven effect on the guiding surface of rolling bearing can effectively improve the lubrication condition of the bearing and thus reduce the bearing vibration and friction torque, which has a promising application prospect.

High temperature induced “glaze” layer formed in HVOF-sprayed NiCrWMoCuCBFe coating and its wear reduction mechanism

Abstract: Abstract The in-situ formation of oxides on alloy surface induced by high temperature can effectively reduce wear and resist oxidation. In consideration of the solid solution strengthening effect and great oxidation resistance of additional elements at elevated temperature, the NiCrWMoCuCBFe coating was prepared by high velocity oxygen flame (HVOF) spraying technology, and its tribological behavior was scrutinized from 25 to 800 °C. By means of high temperature Vickers hardness tester and high temperature X-ray diffractometer, the mechanical properties and microstructures of NiCrWMoCuCBFe coating were measured. And the effect of the mechanical properties and microstructures of the coating on tribological performance was discussed in detail. The results showed both its friction coefficient (0.37) and wear rate (5.067 × 10 −6 mm 3 ·N −1 ·m −1 ) at 800 °C were the lowest, which was mainly related to the formation of “glaze” layer on the coating surface at high temperature. The glaze layer consisted of two parts, which were NiCr 2 O 4 oxide film with the ability of interlaminar slip formed in the outer layer and nano-grains existed in the inner layer. Worth mentioning, these nano-grains provided bearing capability while the oxide film was vital to reduce wear rate and friction coefficient. As the ambient temperature increased, many hard oxides were produced on the wear scars, including NiO, Cr 2 O 3 , MoO 3 , and Mo 2 C. They can improve tribological and mechanical properties of NiCrWMoCuCBFe coating at a wide temperature range.