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 text...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.展开更多
Modeling the real contact area plays a key role in every tribological process,such as friction,adhesion,and wear.Contact between two solids does not necessarily occur everywhere within the apparent contact area.Consid...Modeling the real contact area plays a key role in every tribological process,such as friction,adhesion,and wear.Contact between two solids does not necessarily occur everywhere within the apparent contact area.Considering the multiscale nature of roughness,Persson proposed a theory of contact mechanics for a soft and smooth solid in contact with a rigid rough surface.In this theory,he assumed that the vertical displacement on the soft surface could be approximated by the height profile of the substrate surface.Although this assumption gives an accurate pressure distribution at the interface for complete contact,when no gap exists between two surfaces,it results in an overestimation of elastic energy stored in the material for partial contact,which typically occurs in many practical applications.This issue was later addressed by Persson by including a correction factor obtained from the comparison of the theoretical results with molecular dynamics simulation.This paper proposes a different approach to correct the overestimation of vertical displacement in Persson’s contact theory for rough surfaces with self-affine fractal properties.The results are compared with the correction factor proposed by Persson.The main advantage of the proposed method is that it uses physical parameters such as the surface roughness characteristics,material properties,sliding velocity,and normal load to correct the model.This method is also implemented in the theory of rubber friction.The results of the corrected friction model are compared with experiments.The results confirm that the modified model predicts the friction coefficient as a function of sliding velocity more accurately than the original model.展开更多
In the present work,two types of novel nano additives,titanium sulfonate ligand/black phosphorus(TiLi/BP)and titanium dioxide/black phosphorus(TiCVBP)nanocomposites,were prepared.The tribological behavior of the steel...In the present work,two types of novel nano additives,titanium sulfonate ligand/black phosphorus(TiLi/BP)and titanium dioxide/black phosphorus(TiCVBP)nanocomposites,were prepared.The tribological behavior of the steel/steel friction pairs lubricated by polyalphaolefins type 6(PA06)containing the nanocomposites under boundary lubrication was studied.The worn surfaces were analyzed using modem surface techniques.The experimental results show that the rubbed surfaces became smooth and showed little wear with the addition of the nanocomposites.TiO_(2)/BP nanocomposites can significantly improve the lubricity of BP nanosheets under high contact stress.The synergistic roles of the load-bearing abilities and rolling effect of TiO_(2) nanoparticles,the slip induced by the BP with its layered structure,and the establishment of a tribofilm on the sliding interface are the basis of the tribological mechanisms.展开更多
Biomimetic scaffolds provide a suitable growth environment for tissue engineering and demonstrate good potential for application in biomedical fields.Different-sized copolymerized biomimetic scaffolds degrade differen...Biomimetic scaffolds provide a suitable growth environment for tissue engineering and demonstrate good potential for application in biomedical fields.Different-sized copolymerized biomimetic scaffolds degrade differently,and the degradation rate is affected by the copolymerization ratio.The study of the degradation property is the foundational research necessary for realizing individualized biomimetic scaffold design.The degradation performance of polyesters with different copolymerization ratios has been widely reported;however,the modeling of this performance has been rarely reported.In this research,the degradation of copolymers was studied with multi-scale modeling,in which the copolymers were dispersed in a cellular manner,the chain break time was simulated,and the chain selection was based on the Monte Carlo(MC)algorithm.The probability model of the copolymer's chain break position was established as a//roulette,/model,whose probability values were estimated by the calculation of the potential energy difference at different chain break positions by molecular dynamics that determined the position of chain shear,thereby fully realizing the simulation of the chain micro-break process.The diffusion of the oligomers was then calculated using the macro diffusion equation,and the degradation process of the copolymer was simulated by three-scale coupling calculations.The calculation results were in good agreement with the experimental data,demonstrating the effectiveness of the proposed method.展开更多
Friction is widespread in almost every field in the oil and gas industry,and it is accompanied by huge energy losses and potential safety hazards.To deal with a series of questions in this regard,biomimetic surfaces h...Friction is widespread in almost every field in the oil and gas industry,and it is accompanied by huge energy losses and potential safety hazards.To deal with a series of questions in this regard,biomimetic surfaces have been developed over the past decades to significantly reduce economic losses.Presently,biomimetic surface engineering on different scales has been successfully introduced into related fields of the oil and gas industry,such as drill bits and the inner surfaces of pipes.In this review,we focused on the most recent and promising efforts reported toward the application of a biomimetic surface in oil and gas fields,indicating the necessity and importance of establishing this disciplinary study.Regarding the oil and gas industry,we mainly analyzed and summarized some important research results into the following three aspects:(i)applications in reducing the wear of exploration production equipment and its components,(ii)separation and drag release technologies in oil/gas storage and transportation,and(iii)functional coatings used in oil and gas development in oceans and polar regions.Finally,based on an in‐depth analysis of the development of biomimetic surface engineering in the fields of oil and gas,some conclusions and perspectives are also discussed.It is expected that biomimetic surface engineering can be used in oil and gas fields more widely and systematically,providing important contributions to green development in the near future.展开更多
Two-dimensional materials having a layered structure comprise a monolayer or multilayers of atomic thickness and ultra-low shear strength.Their high specific surface area,in-plane strength,weak layer-layer interaction...Two-dimensional materials having a layered structure comprise a monolayer or multilayers of atomic thickness and ultra-low shear strength.Their high specific surface area,in-plane strength,weak layer-layer interaction,and surface chemical stability result in remarkably low friction and wear-resisting properties.Thus,2D materials have attracted considerable attention.In recent years,great advances have been made in the scientific research and industrial applications of anti-friction,anti-wear,and lubrication of 2D materials.In this article,the basic nanoscale friction mechanisms of 2D materials including interfacial friction and surface friction mechanisms are summarized.This paper also includes a review of reports on lubrication mechanisms based on the film-formation,self-healing,and ball bearing mechanisms and applications based on lubricant additives,nanoscale lubricating films,and space lubrication materials of 2D materials in detail.Finally,the challenges and potential applications of 2D materials in the field of lubrication were also presented.展开更多
In this work, the friction characteristics of single-layer MoS_2 prepared with chemical vapor deposition(CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS_...In this work, the friction characteristics of single-layer MoS_2 prepared with chemical vapor deposition(CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS_2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS_2 specimens were characterized using an optical microscope, atomic force microscope(AFM), and Raman spectroscopy. The surfaces of the MoS2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS2 at 800 °C and 850 °C were relatively similar to those of mechanically exfoliated MoS2 whereas the crystalline quality of the CVD-grown single-layer MoS_2 at 900 °C was lower. The CVD-grown single-layer MoS_2 exhibited higher friction than mechanically exfoliated single-layer MoS_2, which might be related to the crystalline imperfections in the CVD-grown MoS_2. In addition, the friction of CVD-grown single-layer MoS_2 increased as the CVD growth temperature increased. In terms of tribological properties, 800 °C was the optimal temperature for the CVD process used in this work. Furthermore, it was observed that the friction at the grain boundary was significantly larger than that at the grain, potentially due to defects at the grain boundary. This result indicates that the temperature used during CVD should be optimized considering the grain size to achieve low friction characteristics. The outcomes of this work will be useful for understanding the intrinsic friction characteristics of single-layer MoS2 and elucidating the feasibility of single-layer MoS_2 as protective or lubricant layers for micro- and nano-devices.展开更多
A Nomex fabric/phenolic composite was prepared,and its tribological properties were evaluated under dry and water‐bathed sliding conditions by a pin‐on‐disk tribometer.The resulting size of the friction coefficient...A Nomex fabric/phenolic composite was prepared,and its tribological properties were evaluated under dry and water‐bathed sliding conditions by a pin‐on‐disk tribometer.The resulting size of the friction coefficient for the Nomex fabric/phenolic composite in the study occurred in the following order:dry sliding condition>distilled water‐bathed sliding condition>sea water‐bathed sliding condition.The fabric composite’s wear rate from high to low was as follows:distilled water‐bathed sliding condition>sea water‐bathed sliding condition>dry sliding condition.Under water‐bathed sliding conditions,penetration of water into the cracks accelerated the composite’s invalidation process,resulting in a higher wear rate.We also found that the extent of corrosion and transfer film formed on the counterpart pin significantly influenced the wear rate of the Nomex fabric composite.Discussion of the Nomex fabric composite’s wear mechanisms under the sliding conditions investigated is provided on the basis of the characterization results.展开更多
Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion,resulting in enhanced room temperature formability.The exact mechanism of the formability improvement is...Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion,resulting in enhanced room temperature formability.The exact mechanism of the formability improvement is not yet established.The contribution of interrupted motion in the ductility improvement has been studied through stress relaxation phenomena in uniaxial tensile(UT)tests.However,the reason for improved formability observed when employing servo press is complicated due to the additional contribution from frictional effects.In the present work,an attempt is made to decouple the friction effect on formability improvement numerically.The improved formability is studied using a hole expansion test(HET).The limit of forming during hole expansion is modeled using the Hosford–Coulomb(HC)damage criteria,which is implemented as a user subroutine in a commercial explicit finite element(FE)software.Only the contribution of stress relaxation is accounted for in the evolution of the damage variable during interrupted loading.Therefore,the difference between simulation and experimental hole expansion ratio(HER)can be used to decouple the friction effect from the overall formability improvement during hole expansion.The improvement in HER due to stress relaxation and friction effect is different.The study showed that the model effectively captures the hole expansion deformation process in both monotonic and interrupted loading conditions.Compared to stress relaxation,friction effect played a major role during interrupted HET.展开更多
A thermal model of kinetic friction is assigned to a classical loaded particle moving on a fluctuating smooth surface.A sinusoidal wave resembles surface fluctuations with a relaxation time.The Hamiltonian is approxim...A thermal model of kinetic friction is assigned to a classical loaded particle moving on a fluctuating smooth surface.A sinusoidal wave resembles surface fluctuations with a relaxation time.The Hamiltonian is approximated to the mean energy of the wave describing a system of Harmonic oscillators.The quantization of amplitudes yields in terms of annihilation and creation operators multiplied by a quantum phase.Further,we consider acoustic dispersion relation and evaluate the friction coefficient from the force autocorrelation function.While the sliding particle remains classical describing a nano-particle or a tip with negligible quantum effects like tunneling or delocalization in the wave function,the quantized model of the surface fluctuations results in the temperature dependence of the kinetic friction coefficient.It follows an asymptotic value for higher temperatures and supper-slipperiness at low temperatures.展开更多
This study evaluated dry-in-place lubricants used for cold forging.A group of isothermal compression tests with a strain rate(ε)e&range of 0.001–1 s^(−1) and temperature(T)range of 30–400℃ were completed.The f...This study evaluated dry-in-place lubricants used for cold forging.A group of isothermal compression tests with a strain rate(ε)e&range of 0.001–1 s^(−1) and temperature(T)range of 30–400℃ were completed.The flow stress(σ)curves of annealed steel S45C were obtained,and a corresponding Hensel–Spittel model was developed to support finite element(FE)simulation.The sensitivity of the steady combined forward and backward extrusion(SCFBE)test proposed in another study was improved by approximately 20%after it was optimized using the results of the FE simulations.Key parameters were identified,and the calibration curves after optimization were obtained.On the basis of the optimized test,a friction testing setup with a heating system was developed,in which the die temperature could be adjusted from room temperature(RT)to 230℃.Three dry-in-place lubricants and conventional phosphating lubricant were tested,and the friction factors(m),forming loads,and ejection loads were measured.The surface features of the specimens after testing were also investigated.According to the testing results,of the three tested dry-in-place lubricants,the mica type was the best.In addition,the optimized friction testing design was verified as effective.展开更多
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 demonstr...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.展开更多
Relationship between contact size(A)and static friction(f)has been studied for rigid crystalline systems.We built a series of systems with two identical surfaces but different orientations and investigated the effects...Relationship between contact size(A)and static friction(f)has been studied for rigid crystalline systems.We built a series of systems with two identical surfaces but different orientations and investigated the effects of the size and shape of the contact area on static friction.In these systems,there are numerous nontrivial commensurate contacts.Our results confirmed that the relationship between A and f was determined by both commensurability and shape of the contact.For commensurate contacts,f∝A independent of the shape.For incommensurate contacts,generally f∝A^(0) for regular shapes or f∝A^(1/4) for irregular shapes;however,in very few cases of regular shapes,f∝A^(1/2).Moreover,in above systems,commensurability of a contact can be easily changed by a perturbation of the misfit angle.Therefore,if the perturbation caused by the lateral force and the deformation of the surface are considered(as is the case in real systems),further research is necessary.展开更多
Electronic content:friction.tsinghuajournals.com,1.Manuscript submission The following types of manuscripts on all topics within the scope of the journal are welcomed:·Original research articles.
Superlubricity,or structural lubricity,is a state that has two contacting surfaces exhibiting no resistance to sliding.This effect has been theoretically described to be possible between two completely clean single cr...Superlubricity,or structural lubricity,is a state that has two contacting surfaces exhibiting no resistance to sliding.This effect has been theoretically described to be possible between two completely clean single crystalline solid surfaces.However,experimental observations of superlubricity were limited to nanoscale and under high vacuum or inert gas environments even after twenty years since the concept of superlubricity has been suggested in 1990.In the last two years,remarkable advances have been achieved in experimental observations of superlubricity ranging from micro-scale to centimeters and in ambient environment.This study aims to report a comprehensive understanding of the superlubricity phenomenon.展开更多
The mechanisms of energy dissipation are discussed in this paper by reviewing the models and research in atomic-scale friction.The study is undertaken to answer a fundamental question in the study of friction:How is f...The mechanisms of energy dissipation are discussed in this paper by reviewing the models and research in atomic-scale friction.The study is undertaken to answer a fundamental question in the study of friction:How is frictional work dissipated,particularly in cases where material damage and wear are not involved.The initiation of energy dissipation,the role of structural commensurability,and the estimation of the interfacial shear strength have been examined in detail by introducing the Tomlinson model,the Frenkel-Kontorova model,and the cobblestone model,respectively.The discussion is extended to energy dissipation progress described in terms of phononic and electronic damping.The contributions from other mechanisms of dissipation such as viscoelastic relaxation and material wear are also included.As an example,we analyzed a specific process of dissipation in multilayer graphene,on the basis of results of molecular dynamics(MD)simulations,which reveal a reversible part of energy that circulates between the system and the external driver.This leads us to emphasize that it is crucial in future studies to clearly define the coefficient of dissipation.展开更多
Thanks to their outstanding mechanical properties,Bulk Metallic Glasses(BMGs)are new alternatives to traditional crystalline metals for mechanical and micromechanical applications including power transmission.However,...Thanks to their outstanding mechanical properties,Bulk Metallic Glasses(BMGs)are new alternatives to traditional crystalline metals for mechanical and micromechanical applications including power transmission.However,the tribological properties of BMGs are still poorly understood,mostly because their amorphous nature induces counter intuitive responses to friction and wear.In the present study,four different BMGs(Cu_(47)Zr_(46)Al_(7),Zr_(46)Cu_(45)Al_(7)Nb_(2),Zr_(60)Cu_(28)Al_(12),and Zr_(61)Cu_(25)Al_(12)Ti_(2))underwent ball-on-disc friction tests against 100Cr6 steel balls(American Iron and Steel Institute(AISI)52100)at different relative humidities(RHs)ranging from 20%to 80%.Controlling humidity enabled to observe a high repeatability of the friction and wear responses of the BMG.Interestingly,the friction coefficient decreased by a factor of 2 when the humidity was increased,and the wear rate of BMGs was particularly low thanks to a 3rd-body tribolayer that forms on the BMG surface,composed of oxidized wear particles originating from the ball.The morphology of this tribolayer is highly correlated to humidity.The study also identifies how the tribolayer is built up from the initial contact until the steady state is achieved.展开更多
In this study,the direct intercalation of gemini ionic liquids(ILs)with different alkyl chains into the bentonite(BT)interlayer as a high-performance lubricating additive for base oil 500SN was investigated.The purpos...In this study,the direct intercalation of gemini ionic liquids(ILs)with different alkyl chains into the bentonite(BT)interlayer as a high-performance lubricating additive for base oil 500SN was investigated.The purpose of modifying BT with an IL is to improve the dispersion stability and lubricity of BT in lubricating oil.The dispersibility and tribological properties of IL–BT as oil-based additives for 500SN depend on the increase in interlamellar space in BT and improve as the chain length is increased.More importantly,the IL–BT nanomaterial outperforms individual BT in improving wear resistance,owing to its sheet layers were deformed and sprawled in furrows along the metal surface,thereby resulting in low surface adhesion.Because of its excellent lubrication performance,IL-modified BT is a potential candidate for the main component of drilling fluid.It can be used as a lubricating additive in oil drilling and oil well construction to reduce equipment damage and ensure the normal operation of equipments.展开更多
Magnetorheological elastomer (MRE) is a type of smart material of which mechanical and electrical properties can be reversibly controlled by the magnetic field. In this study, the influence of the magnetic field on th...Magnetorheological elastomer (MRE) is a type of smart material of which mechanical and electrical properties can be reversibly controlled by the magnetic field. In this study, the influence of the magnetic field on the surface roughness of MRE was studied by the microscopic modeling method, and the influence of controllable characteristics of the MRE surface on its friction properties was analyzed by the macroscopic experimental method. First, on the basis of existing studies, an improved mesoscopic model based on magneto-mechanical coupling analysis was proposed. The initial surface morphology of MRE was characterized by the W–M fractal function, and the change process of the surface microstructures of MRE, induced by the magnetic interaction between particles, was studied. Then, after analyzing the simulation results, it is found that with the increase in the magnetic field and decrease in the modulus of rubber matrix, the surface of MRE changes more significantly, and the best particle volume fraction is within 7.5%–9%. Furthermore, through experimental observation, it is found that the height of the convex peak on the surface of MRE decreases significantly with the action of the magnetic field, resulting in a reduction in the surface roughness. Consistent with the simulation results, a particle volume fraction of 10% corresponds to a maximum change of 14%. Finally, the macroscopic friction experiment results show that the friction coefficients of MREs with different particle volume fractions all decrease with the decrease in surface roughness under the magnetic field. When the particle volume fraction is 10%, the friction coefficient can decrease by 24.7% under a magnetic field of 400 mT, which is consistent with the trend of surface roughness changes. This shows that the change in surface morphology with the effect of the magnetic field is an important factor in the control of MRE friction properties by magnetic field.展开更多
Although graphene is well known for super-lubricity on its basal plane, friction at its step edge is not well understood and contradictory friction behaviors have been reported. In this study, friction of mono-layer t...Although graphene is well known for super-lubricity on its basal plane, friction at its step edge is not well understood and contradictory friction behaviors have been reported. In this study, friction of mono-layer thick graphene step edges was studied using atomic force microscopy (AFM) with a Si tip in dry nitrogen atmosphere. It is found that, when the tip slides over a'buried' graphene step edge, there is a resistive force during the step-up motion and an assistive force during the step-down motion due to the topographic height change. The magnitude of these two forces is small and the same in both step-up and step-down motions. As for the'exposed' graphene step edge, friction increases in magnitude and exhibits more complicated behaviors. During the step-down motion of the tip over the exposed step edge, both resistive and assistive components can be detected in the lateral force signal of AFM if the scan resolution is sufficiently high. The resistive component is attributed to chemical interactions between the functional groups at the tip and step-edge surfaces, and the assistive component is due to the topographic effect, same as the case of buried step edge. If a blunt tip is used, the distinct effects of these two components become more prominent. In the step-up scan direction, the blunt tip appears to have two separate topographic effects–elastic deformation of the contact region at the bottom of the tip due to the substrate height change at the step edge and tilting of the tip while the vertical position of the cantilever (the end of the tip) ascends from the lower terrace to the upper terrace. The high-resolution measurement of friction behaviors at graphene step edges will further enrich understanding of interfacial friction behaviors on graphene-covered surfaces.展开更多
基金This work was supported by ANID-CONICYT within the project Fondecyt 11180121 and Fondequip EQM190057 as well as the VID of the University of Chile in the framework of“U-Inicia UI013/2018”HLC acknowledges financial support from Fapergs/Brazil(No.19/2551-0001849-5)+1 种基金CNPq/Brazil(No.305453/2017-3)JS thanks the German Federal Ministry of Education and Research(BMBF)for financial support in the project FH-Europa 2020:MACH-XLT(No.13FH009EX0).
文摘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.
文摘Modeling the real contact area plays a key role in every tribological process,such as friction,adhesion,and wear.Contact between two solids does not necessarily occur everywhere within the apparent contact area.Considering the multiscale nature of roughness,Persson proposed a theory of contact mechanics for a soft and smooth solid in contact with a rigid rough surface.In this theory,he assumed that the vertical displacement on the soft surface could be approximated by the height profile of the substrate surface.Although this assumption gives an accurate pressure distribution at the interface for complete contact,when no gap exists between two surfaces,it results in an overestimation of elastic energy stored in the material for partial contact,which typically occurs in many practical applications.This issue was later addressed by Persson by including a correction factor obtained from the comparison of the theoretical results with molecular dynamics simulation.This paper proposes a different approach to correct the overestimation of vertical displacement in Persson’s contact theory for rough surfaces with self-affine fractal properties.The results are compared with the correction factor proposed by Persson.The main advantage of the proposed method is that it uses physical parameters such as the surface roughness characteristics,material properties,sliding velocity,and normal load to correct the model.This method is also implemented in the theory of rubber friction.The results of the corrected friction model are compared with experiments.The results confirm that the modified model predicts the friction coefficient as a function of sliding velocity more accurately than the original model.
基金support of the National Natural Science Foundation of China(Grant No.51875155)the Fundamental Research Funds for the Central Universities(Grant No.PA2019GDQT0017).
文摘In the present work,two types of novel nano additives,titanium sulfonate ligand/black phosphorus(TiLi/BP)and titanium dioxide/black phosphorus(TiCVBP)nanocomposites,were prepared.The tribological behavior of the steel/steel friction pairs lubricated by polyalphaolefins type 6(PA06)containing the nanocomposites under boundary lubrication was studied.The worn surfaces were analyzed using modem surface techniques.The experimental results show that the rubbed surfaces became smooth and showed little wear with the addition of the nanocomposites.TiO_(2)/BP nanocomposites can significantly improve the lubricity of BP nanosheets under high contact stress.The synergistic roles of the load-bearing abilities and rolling effect of TiO_(2) nanoparticles,the slip induced by the BP with its layered structure,and the establishment of a tribofilm on the sliding interface are the basis of the tribological mechanisms.
基金This paper is sponsored by the National Study Abroad Fund of China and supported by The National Key Research and Development Program of China(2017YFB1002304).
文摘Biomimetic scaffolds provide a suitable growth environment for tissue engineering and demonstrate good potential for application in biomedical fields.Different-sized copolymerized biomimetic scaffolds degrade differently,and the degradation rate is affected by the copolymerization ratio.The study of the degradation property is the foundational research necessary for realizing individualized biomimetic scaffold design.The degradation performance of polyesters with different copolymerization ratios has been widely reported;however,the modeling of this performance has been rarely reported.In this research,the degradation of copolymers was studied with multi-scale modeling,in which the copolymers were dispersed in a cellular manner,the chain break time was simulated,and the chain selection was based on the Monte Carlo(MC)algorithm.The probability model of the copolymer's chain break position was established as a//roulette,/model,whose probability values were estimated by the calculation of the potential energy difference at different chain break positions by molecular dynamics that determined the position of chain shear,thereby fully realizing the simulation of the chain micro-break process.The diffusion of the oligomers was then calculated using the macro diffusion equation,and the degradation process of the copolymer was simulated by three-scale coupling calculations.The calculation results were in good agreement with the experimental data,demonstrating the effectiveness of the proposed method.
基金supported by the National Natural Science Foundation of China (No. 51675534)the Science Foundation of China University of Petroleum, Beijing (No. 2462017BJB06)
文摘Friction is widespread in almost every field in the oil and gas industry,and it is accompanied by huge energy losses and potential safety hazards.To deal with a series of questions in this regard,biomimetic surfaces have been developed over the past decades to significantly reduce economic losses.Presently,biomimetic surface engineering on different scales has been successfully introduced into related fields of the oil and gas industry,such as drill bits and the inner surfaces of pipes.In this review,we focused on the most recent and promising efforts reported toward the application of a biomimetic surface in oil and gas fields,indicating the necessity and importance of establishing this disciplinary study.Regarding the oil and gas industry,we mainly analyzed and summarized some important research results into the following three aspects:(i)applications in reducing the wear of exploration production equipment and its components,(ii)separation and drag release technologies in oil/gas storage and transportation,and(iii)functional coatings used in oil and gas development in oceans and polar regions.Finally,based on an in‐depth analysis of the development of biomimetic surface engineering in the fields of oil and gas,some conclusions and perspectives are also discussed.It is expected that biomimetic surface engineering can be used in oil and gas fields more widely and systematically,providing important contributions to green development in the near future.
基金supported by the National Natural Science Foundation of China(Grant No.51505442)Guangxi Natural Science Foundation(Grant No.2018GXNSFAA138174)
文摘Two-dimensional materials having a layered structure comprise a monolayer or multilayers of atomic thickness and ultra-low shear strength.Their high specific surface area,in-plane strength,weak layer-layer interaction,and surface chemical stability result in remarkably low friction and wear-resisting properties.Thus,2D materials have attracted considerable attention.In recent years,great advances have been made in the scientific research and industrial applications of anti-friction,anti-wear,and lubrication of 2D materials.In this article,the basic nanoscale friction mechanisms of 2D materials including interfacial friction and surface friction mechanisms are summarized.This paper also includes a review of reports on lubrication mechanisms based on the film-formation,self-healing,and ball bearing mechanisms and applications based on lubricant additives,nanoscale lubricating films,and space lubrication materials of 2D materials in detail.Finally,the challenges and potential applications of 2D materials in the field of lubrication were also presented.
基金supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF)funded by the Ministry of Science, ICT and Future Planning (NRF-2017R1A2B4009651)
文摘In this work, the friction characteristics of single-layer MoS_2 prepared with chemical vapor deposition(CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS_2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS_2 specimens were characterized using an optical microscope, atomic force microscope(AFM), and Raman spectroscopy. The surfaces of the MoS2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS2 at 800 °C and 850 °C were relatively similar to those of mechanically exfoliated MoS2 whereas the crystalline quality of the CVD-grown single-layer MoS_2 at 900 °C was lower. The CVD-grown single-layer MoS_2 exhibited higher friction than mechanically exfoliated single-layer MoS_2, which might be related to the crystalline imperfections in the CVD-grown MoS_2. In addition, the friction of CVD-grown single-layer MoS_2 increased as the CVD growth temperature increased. In terms of tribological properties, 800 °C was the optimal temperature for the CVD process used in this work. Furthermore, it was observed that the friction at the grain boundary was significantly larger than that at the grain, potentially due to defects at the grain boundary. This result indicates that the temperature used during CVD should be optimized considering the grain size to achieve low friction characteristics. The outcomes of this work will be useful for understanding the intrinsic friction characteristics of single-layer MoS2 and elucidating the feasibility of single-layer MoS_2 as protective or lubricant layers for micro- and nano-devices.
基金The authors acknowledge the financial support of the National Science Foundation of China grant Nos.51375472 and 51305429.
文摘A Nomex fabric/phenolic composite was prepared,and its tribological properties were evaluated under dry and water‐bathed sliding conditions by a pin‐on‐disk tribometer.The resulting size of the friction coefficient for the Nomex fabric/phenolic composite in the study occurred in the following order:dry sliding condition>distilled water‐bathed sliding condition>sea water‐bathed sliding condition.The fabric composite’s wear rate from high to low was as follows:distilled water‐bathed sliding condition>sea water‐bathed sliding condition>dry sliding condition.Under water‐bathed sliding conditions,penetration of water into the cracks accelerated the composite’s invalidation process,resulting in a higher wear rate.We also found that the extent of corrosion and transfer film formed on the counterpart pin significantly influenced the wear rate of the Nomex fabric composite.Discussion of the Nomex fabric composite’s wear mechanisms under the sliding conditions investigated is provided on the basis of the characterization results.
文摘Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion,resulting in enhanced room temperature formability.The exact mechanism of the formability improvement is not yet established.The contribution of interrupted motion in the ductility improvement has been studied through stress relaxation phenomena in uniaxial tensile(UT)tests.However,the reason for improved formability observed when employing servo press is complicated due to the additional contribution from frictional effects.In the present work,an attempt is made to decouple the friction effect on formability improvement numerically.The improved formability is studied using a hole expansion test(HET).The limit of forming during hole expansion is modeled using the Hosford–Coulomb(HC)damage criteria,which is implemented as a user subroutine in a commercial explicit finite element(FE)software.Only the contribution of stress relaxation is accounted for in the evolution of the damage variable during interrupted loading.Therefore,the difference between simulation and experimental hole expansion ratio(HER)can be used to decouple the friction effect from the overall formability improvement during hole expansion.The improvement in HER due to stress relaxation and friction effect is different.The study showed that the model effectively captures the hole expansion deformation process in both monotonic and interrupted loading conditions.Compared to stress relaxation,friction effect played a major role during interrupted HET.
文摘A thermal model of kinetic friction is assigned to a classical loaded particle moving on a fluctuating smooth surface.A sinusoidal wave resembles surface fluctuations with a relaxation time.The Hamiltonian is approximated to the mean energy of the wave describing a system of Harmonic oscillators.The quantization of amplitudes yields in terms of annihilation and creation operators multiplied by a quantum phase.Further,we consider acoustic dispersion relation and evaluate the friction coefficient from the force autocorrelation function.While the sliding particle remains classical describing a nano-particle or a tip with negligible quantum effects like tunneling or delocalization in the wave function,the quantized model of the surface fluctuations results in the temperature dependence of the kinetic friction coefficient.It follows an asymptotic value for higher temperatures and supper-slipperiness at low temperatures.
基金This research was partially supported by the National Natural Science Foundation of China(NSFC,No.51875348).
文摘This study evaluated dry-in-place lubricants used for cold forging.A group of isothermal compression tests with a strain rate(ε)e&range of 0.001–1 s^(−1) and temperature(T)range of 30–400℃ were completed.The flow stress(σ)curves of annealed steel S45C were obtained,and a corresponding Hensel–Spittel model was developed to support finite element(FE)simulation.The sensitivity of the steady combined forward and backward extrusion(SCFBE)test proposed in another study was improved by approximately 20%after it was optimized using the results of the FE simulations.Key parameters were identified,and the calibration curves after optimization were obtained.On the basis of the optimized test,a friction testing setup with a heating system was developed,in which the die temperature could be adjusted from room temperature(RT)to 230℃.Three dry-in-place lubricants and conventional phosphating lubricant were tested,and the friction factors(m),forming loads,and ejection loads were measured.The surface features of the specimens after testing were also investigated.According to the testing results,of the three tested dry-in-place lubricants,the mica type was the best.In addition,the optimized friction testing design was verified as effective.
基金support from Pontificia Universidad Católica de Chile.A.Rosenkranz gratefully acknowledges the financial support given by ANID(Chile)in the framework of the Fondecyt projects(Nos.11180121 and EQM190057)Additionally,A.Rosenkranz acknowledges the financial support given by the VID of the University of Chile within the project U-Moderniza(No.UM-04/19).
文摘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.
基金supported by National Natural Science Foundation of China(No.51675395)Special Fund for Outstanding Young and Middle-aged Scientific and Technological Innovation Team in the University from Hubei Province(No.T201709)Doctoral Startup Fund for Scientific Research at Wuhan Polytechnic University(2014RZ31).
文摘Relationship between contact size(A)and static friction(f)has been studied for rigid crystalline systems.We built a series of systems with two identical surfaces but different orientations and investigated the effects of the size and shape of the contact area on static friction.In these systems,there are numerous nontrivial commensurate contacts.Our results confirmed that the relationship between A and f was determined by both commensurability and shape of the contact.For commensurate contacts,f∝A independent of the shape.For incommensurate contacts,generally f∝A^(0) for regular shapes or f∝A^(1/4) for irregular shapes;however,in very few cases of regular shapes,f∝A^(1/2).Moreover,in above systems,commensurability of a contact can be easily changed by a perturbation of the misfit angle.Therefore,if the perturbation caused by the lateral force and the deformation of the surface are considered(as is the case in real systems),further research is necessary.
文摘Electronic content:friction.tsinghuajournals.com,1.Manuscript submission The following types of manuscripts on all topics within the scope of the journal are welcomed:·Original research articles.
基金supports from the National Natural Science Foundation of China(NSFC)(Grant No.10832005)the the National Key Basic Research(973)Program of China(Grants Nos.2007CB936803 and 2013CB934200)are acknowledged.
文摘Superlubricity,or structural lubricity,is a state that has two contacting surfaces exhibiting no resistance to sliding.This effect has been theoretically described to be possible between two completely clean single crystalline solid surfaces.However,experimental observations of superlubricity were limited to nanoscale and under high vacuum or inert gas environments even after twenty years since the concept of superlubricity has been suggested in 1990.In the last two years,remarkable advances have been achieved in experimental observations of superlubricity ranging from micro-scale to centimeters and in ambient environment.This study aims to report a comprehensive understanding of the superlubricity phenomenon.
基金The authors would like to express their appreciations to the financial support from the National Natural Science Foundation of China under the grant Nos.of 51075526,51005129,and 51021064.
文摘The mechanisms of energy dissipation are discussed in this paper by reviewing the models and research in atomic-scale friction.The study is undertaken to answer a fundamental question in the study of friction:How is frictional work dissipated,particularly in cases where material damage and wear are not involved.The initiation of energy dissipation,the role of structural commensurability,and the estimation of the interfacial shear strength have been examined in detail by introducing the Tomlinson model,the Frenkel-Kontorova model,and the cobblestone model,respectively.The discussion is extended to energy dissipation progress described in terms of phononic and electronic damping.The contributions from other mechanisms of dissipation such as viscoelastic relaxation and material wear are also included.As an example,we analyzed a specific process of dissipation in multilayer graphene,on the basis of results of molecular dynamics(MD)simulations,which reveal a reversible part of energy that circulates between the system and the external driver.This leads us to emphasize that it is crucial in future studies to clearly define the coefficient of dissipation.
基金supported by the EUR EIPHI Graduate School(ANR-17-EURE-0002)The authors are thankful for the financial support provided by the French National Research Agency(ANR)(ANR-19-CE08-0015)。
文摘Thanks to their outstanding mechanical properties,Bulk Metallic Glasses(BMGs)are new alternatives to traditional crystalline metals for mechanical and micromechanical applications including power transmission.However,the tribological properties of BMGs are still poorly understood,mostly because their amorphous nature induces counter intuitive responses to friction and wear.In the present study,four different BMGs(Cu_(47)Zr_(46)Al_(7),Zr_(46)Cu_(45)Al_(7)Nb_(2),Zr_(60)Cu_(28)Al_(12),and Zr_(61)Cu_(25)Al_(12)Ti_(2))underwent ball-on-disc friction tests against 100Cr6 steel balls(American Iron and Steel Institute(AISI)52100)at different relative humidities(RHs)ranging from 20%to 80%.Controlling humidity enabled to observe a high repeatability of the friction and wear responses of the BMG.Interestingly,the friction coefficient decreased by a factor of 2 when the humidity was increased,and the wear rate of BMGs was particularly low thanks to a 3rd-body tribolayer that forms on the BMG surface,composed of oxidized wear particles originating from the ball.The morphology of this tribolayer is highly correlated to humidity.The study also identifies how the tribolayer is built up from the initial contact until the steady state is achieved.
基金the funding support of the National Natural Science Foundation of China(Nos.52075524,21972153,and U21A20280)Youth Innovation Promotion Association,Chinese Academy of Scieneces(Nos.2022429 and 2018454)+2 种基金Gansu Province Science and Technology Plan(Nos.20JR10RA060 and 20JR10RA048)the Lanzhou Institute of Chemical Physics(LICP)Cooperation Foundation for Young Scholars(No.HZJJ21-06)Key Research Projects of Frontier Science of Chinese Academy of Sciences(No.QYZDY-SSW-JSC013).
文摘In this study,the direct intercalation of gemini ionic liquids(ILs)with different alkyl chains into the bentonite(BT)interlayer as a high-performance lubricating additive for base oil 500SN was investigated.The purpose of modifying BT with an IL is to improve the dispersion stability and lubricity of BT in lubricating oil.The dispersibility and tribological properties of IL–BT as oil-based additives for 500SN depend on the increase in interlamellar space in BT and improve as the chain length is increased.More importantly,the IL–BT nanomaterial outperforms individual BT in improving wear resistance,owing to its sheet layers were deformed and sprawled in furrows along the metal surface,thereby resulting in low surface adhesion.Because of its excellent lubrication performance,IL-modified BT is a potential candidate for the main component of drilling fluid.It can be used as a lubricating additive in oil drilling and oil well construction to reduce equipment damage and ensure the normal operation of equipments.
基金This work was supported by the National Natural Science Foundation of China (No. 11572320)Science and Technology Research Project of Chongqing Municipal Education Commission (No. KJQN201800644)+1 种基金Special Key Project of Technological Innovation and Application Development in Chongqing (cstc2019jscx-fxyd0005)The authors thank professor Xiaojie WANG from Institute of Advanced Manufacturing Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences and associate professor Shiwei CHEN from Chongqing Institute of Science and Technology for the support and fruitful discussions.
文摘Magnetorheological elastomer (MRE) is a type of smart material of which mechanical and electrical properties can be reversibly controlled by the magnetic field. In this study, the influence of the magnetic field on the surface roughness of MRE was studied by the microscopic modeling method, and the influence of controllable characteristics of the MRE surface on its friction properties was analyzed by the macroscopic experimental method. First, on the basis of existing studies, an improved mesoscopic model based on magneto-mechanical coupling analysis was proposed. The initial surface morphology of MRE was characterized by the W–M fractal function, and the change process of the surface microstructures of MRE, induced by the magnetic interaction between particles, was studied. Then, after analyzing the simulation results, it is found that with the increase in the magnetic field and decrease in the modulus of rubber matrix, the surface of MRE changes more significantly, and the best particle volume fraction is within 7.5%–9%. Furthermore, through experimental observation, it is found that the height of the convex peak on the surface of MRE decreases significantly with the action of the magnetic field, resulting in a reduction in the surface roughness. Consistent with the simulation results, a particle volume fraction of 10% corresponds to a maximum change of 14%. Finally, the macroscopic friction experiment results show that the friction coefficients of MREs with different particle volume fractions all decrease with the decrease in surface roughness under the magnetic field. When the particle volume fraction is 10%, the friction coefficient can decrease by 24.7% under a magnetic field of 400 mT, which is consistent with the trend of surface roughness changes. This shows that the change in surface morphology with the effect of the magnetic field is an important factor in the control of MRE friction properties by magnetic field.
基金This work was supported by the National Science Foundation(Grant No.CMMI-1727571).
文摘Although graphene is well known for super-lubricity on its basal plane, friction at its step edge is not well understood and contradictory friction behaviors have been reported. In this study, friction of mono-layer thick graphene step edges was studied using atomic force microscopy (AFM) with a Si tip in dry nitrogen atmosphere. It is found that, when the tip slides over a'buried' graphene step edge, there is a resistive force during the step-up motion and an assistive force during the step-down motion due to the topographic height change. The magnitude of these two forces is small and the same in both step-up and step-down motions. As for the'exposed' graphene step edge, friction increases in magnitude and exhibits more complicated behaviors. During the step-down motion of the tip over the exposed step edge, both resistive and assistive components can be detected in the lateral force signal of AFM if the scan resolution is sufficiently high. The resistive component is attributed to chemical interactions between the functional groups at the tip and step-edge surfaces, and the assistive component is due to the topographic effect, same as the case of buried step edge. If a blunt tip is used, the distinct effects of these two components become more prominent. In the step-up scan direction, the blunt tip appears to have two separate topographic effects–elastic deformation of the contact region at the bottom of the tip due to the substrate height change at the step edge and tilting of the tip while the vertical position of the cantilever (the end of the tip) ascends from the lower terrace to the upper terrace. The high-resolution measurement of friction behaviors at graphene step edges will further enrich understanding of interfacial friction behaviors on graphene-covered surfaces.
