Diamond-like carbon(DLC)coatings are known to provide beneficial mechanical and tribological properties in harsh environments.Their combination of high wear resistance and low friction has led to their extensive use i...Diamond-like carbon(DLC)coatings are known to provide beneficial mechanical and tribological properties in harsh environments.Their combination of high wear resistance and low friction has led to their extensive use in any number of industries.The tribological performance of a DLC coating is varied however,and the frictional response is known to be strongly dependent on the surrounding environment,as well as the material composition and bonding structure of the DLC coating.This paper presents an up-to-date review on the friction of DLC coatings in a water environment,with a special focus on transfer layer formation and tribochemistry.展开更多
In this study we present a mechanism for the elastohydrodynamic(EHD)friction reduction in steel/steel contacts,which occurs due to the formation of oleophobic surface boundary layers from common boundary-lubrication a...In this study we present a mechanism for the elastohydrodynamic(EHD)friction reduction in steel/steel contacts,which occurs due to the formation of oleophobic surface boundary layers from common boundary-lubrication additives.Several simple organic additives(amine,alcohol,amide,and fatty acid)with different molecular structures were employed as the model additives.It was found that the stronger chemisorption at 100℃,rather than the physisorption at 25℃,is more effective in friction reduction,which reaches 22%.What is more,EHD friction reduction was obtained in steel/steel contacts without use of the diamond-like carbon(DLC)coatings with their wetting or thermal effect,which was previously suggested as possible EHD friction reduction mechanism;yet about the same friction reduction of about 20%was obtained here—but with much simpler and less expensive technology,namely with the adsorbed oleophobic surface layers.A small variation in the additive’s molecular structure results in significant changes to the friction,indicating good potential in future EHD lubrication technology,where these additives could be designed and well optimised for notable reduction of the friction losses in the EHD regime.展开更多
To study the relationships between the friction factor f and the flow type in a single rough fracture, the formulae of f for both unconfined and confined flows are deduced based on previous studies. The relationships ...To study the relationships between the friction factor f and the flow type in a single rough fracture, the formulae of f for both unconfined and confined flows are deduced based on previous studies. The relationships between f and the Reynolds number （Re） for different relative roughnesses are investigated experimentally. The Moody-type diagram, based on the deduced formula of f, is also plotted and the hydraulic characteristics of the flow in a rough fracture are analyzed. Results show that the Moody-type diagram of the experiment has a similar distribution to that of the conventional Moody diagram. It is found that the value of f in the experiment is much smaller than that of the conventional Moody diagram and turbulent flow appears easier for rough fractures, which can be explained by the separation phenomenon in boundary layers. The critical Re ranging from 650 to 700 in rough fractures is concluded based on the experimental results. It also can be concluded that the friction factor f is related not only with the Re and the relative roughness but also with the absolute roughness.展开更多
Corrugated cold roll bonding(CCRB) produces metal composite plate with improved mechanical properties compared with conventional methods,but the interfacial mechanism is not fully understood.Here,Cu/Al composite plate...Corrugated cold roll bonding(CCRB) produces metal composite plate with improved mechanical properties compared with conventional methods,but the interfacial mechanism is not fully understood.Here,Cu/Al composite plate with good plate shape was produced by CCRB,and the bonding mechanism and strength along the corrugated interface were studied by experiments and finite element simulations.The results showed that the average bonding strength of Cu/Al composite plate produced by CCRB was nearly twice that of conventional composite plate at an average reduction of 40% during rolling.Strong friction shear stresses occurred at the interface of the corrugated composite plate,which promoted the plastic deformation of the metals and accelerated the rupture of the brittle interfacial layer.Electron backscattered diffraction analysis showed that higher degrees of grain elongation and refinement occurred in the matrices at the front waist and trough due to the stronger normal and shear stresses.Energy-dispersive spectroscopy line scans showed that the thickest atomic diffusion layer occurred at the front waist.The present combination of experimental and computational analyses provides insights into the underlying mechanism of mechanically improved metal composites prepared by CCRB.展开更多
To reduce the usage of classical lubricants in deep drawing,a new tribological system based on volatile lubricants was developed.Therefore,a volatile medium is injected under high pressure into the interstice between ...To reduce the usage of classical lubricants in deep drawing,a new tribological system based on volatile lubricants was developed.Therefore,a volatile medium is injected under high pressure into the interstice between drawing tool and sheet metal.Depending on temperature and pressure,the temporary lubricant may exist in its gaseous or liquid phase.In this study,a novel high fluid pressure tribometer was designed to investigate the friction and wear of dry steel contacts under comparable conditions like in dry deep drawing.Therefore,a new ball-on-disc tribometer was designed and integrated into a high-pressure vessel.To specifically investigate the effects of different environments(technical air,liquid and gaseous carbon dioxide,nitrogen,argon)at atmospheric and high pressure(0.1 MPa,6 MPa)on tribology,the specimens and all components were operating unlubricated.During the experiments,the friction was measured continuously.Results show that the highest friction occurs in air and the lowest in carbon dioxide environment.Subsequent to the experiments,the wear of the specimens was assessed along with changes in surface chemistry related to tribochemical reactions.Therefore,the tribology of the dry sliding contacts is correlated to changes of the surface chemistry.Also differences as well as similarities regarding the different fluid environments are shown.As the results show,the differences between the media used are most pronounced at elevated pressure.Concluding,this work gives clear indications on the suitability of volatile lubricants in dry friction or rather gas lubrication,especially for dry deep drawing.展开更多
The Kevlar/polytetrafluroethylene（Kevlar/PTFE） fabric composite can be used as a self-lubricating liner of the self-lubricating bearing.Many types of nano-particles can improve the tribological performance of the po...The Kevlar/polytetrafluroethylene（Kevlar/PTFE） fabric composite can be used as a self-lubricating liner of the self-lubricating bearing.Many types of nano-particles can improve the tribological performance of the polymer-based composite.Unfortunately,up to now,published work on the effect of nano-particles on the tribological performance of the fabric composite which can be used as a self-lubricating liner is quite scarce.Therefore,for the purpose of exploring a way to significantly improve the tribological performance of the fabric composite,the tribological performance of the Kevlar/PTFE fabric composite filled with nano-titania is evaluated by using the block-on-ring wear tester.The scanning electron microscopy is utilized to observe the morphologies of worn surfaces of the fabric composites and the counterparts.The tensile properties of the composites are evaluated on the universal material testing machine.The test results show that the addition of nano-titania at a proper mass fraction of the matrix resin improves the wear resistance and the tensile strength,decreases the friction coefficient,and makes the wear volume of the composite reach a relative steady state more quickly;plastic deformation and microcutting are important for the wear of the fabric composite;a lubricating layer is formed on the worn surface of the composite during sliding,and the lubricating layer is critical for the tribological performance of the composite;the formation and properties of the lubricating layer are influenced by the nano-titania particles.The proposed study on the effect of nano-titania on the tribological performance of the Kevlar/PTFE fabric composite,especially on the evolution of the worn surface of the composite,provides the basis for further understanding of the influence mechanism of the nano-particles on the tribological performance of the composite and explores a method of improving the tribological performance of the composite.展开更多
文摘Diamond-like carbon(DLC)coatings are known to provide beneficial mechanical and tribological properties in harsh environments.Their combination of high wear resistance and low friction has led to their extensive use in any number of industries.The tribological performance of a DLC coating is varied however,and the frictional response is known to be strongly dependent on the surrounding environment,as well as the material composition and bonding structure of the DLC coating.This paper presents an up-to-date review on the friction of DLC coatings in a water environment,with a special focus on transfer layer formation and tribochemistry.
基金The authors acknowledge the financial support from the Slovenian Research Agency(Research Core Funding No.P2-0231).
文摘In this study we present a mechanism for the elastohydrodynamic(EHD)friction reduction in steel/steel contacts,which occurs due to the formation of oleophobic surface boundary layers from common boundary-lubrication additives.Several simple organic additives(amine,alcohol,amide,and fatty acid)with different molecular structures were employed as the model additives.It was found that the stronger chemisorption at 100℃,rather than the physisorption at 25℃,is more effective in friction reduction,which reaches 22%.What is more,EHD friction reduction was obtained in steel/steel contacts without use of the diamond-like carbon(DLC)coatings with their wetting or thermal effect,which was previously suggested as possible EHD friction reduction mechanism;yet about the same friction reduction of about 20%was obtained here—but with much simpler and less expensive technology,namely with the adsorbed oleophobic surface layers.A small variation in the additive’s molecular structure results in significant changes to the friction,indicating good potential in future EHD lubrication technology,where these additives could be designed and well optimised for notable reduction of the friction losses in the EHD regime.
基金supported by the Program for New Century Excellent Talents in Universities (Grant No.NCET-06-0541)the National Natural Science Foundation of China (Grant Nos.40672154,40772153)
文摘To study the relationships between the friction factor f and the flow type in a single rough fracture, the formulae of f for both unconfined and confined flows are deduced based on previous studies. The relationships between f and the Reynolds number （Re） for different relative roughnesses are investigated experimentally. The Moody-type diagram, based on the deduced formula of f, is also plotted and the hydraulic characteristics of the flow in a rough fracture are analyzed. Results show that the Moody-type diagram of the experiment has a similar distribution to that of the conventional Moody diagram. It is found that the value of f in the experiment is much smaller than that of the conventional Moody diagram and turbulent flow appears easier for rough fractures, which can be explained by the separation phenomenon in boundary layers. The critical Re ranging from 650 to 700 in rough fractures is concluded based on the experimental results. It also can be concluded that the friction factor f is related not only with the Re and the relative roughness but also with the absolute roughness.
基金financially supported by the Major Program of National Natural Science Foundation of China (No. U1710254)Shanxi Province Science and Technology Major Projects (No.20181101008)+1 种基金the Scientific and Technological Progress of Shanxi Province Colleges and Universities (No.2017132)the National Natural Science Foundation of China (Nos.51974196, 51975398,and 51905372)。
文摘Corrugated cold roll bonding(CCRB) produces metal composite plate with improved mechanical properties compared with conventional methods,but the interfacial mechanism is not fully understood.Here,Cu/Al composite plate with good plate shape was produced by CCRB,and the bonding mechanism and strength along the corrugated interface were studied by experiments and finite element simulations.The results showed that the average bonding strength of Cu/Al composite plate produced by CCRB was nearly twice that of conventional composite plate at an average reduction of 40% during rolling.Strong friction shear stresses occurred at the interface of the corrugated composite plate,which promoted the plastic deformation of the metals and accelerated the rupture of the brittle interfacial layer.Electron backscattered diffraction analysis showed that higher degrees of grain elongation and refinement occurred in the matrices at the front waist and trough due to the stronger normal and shear stresses.Energy-dispersive spectroscopy line scans showed that the thickest atomic diffusion layer occurred at the front waist.The present combination of experimental and computational analyses provides insights into the underlying mechanism of mechanically improved metal composites prepared by CCRB.
文摘To reduce the usage of classical lubricants in deep drawing,a new tribological system based on volatile lubricants was developed.Therefore,a volatile medium is injected under high pressure into the interstice between drawing tool and sheet metal.Depending on temperature and pressure,the temporary lubricant may exist in its gaseous or liquid phase.In this study,a novel high fluid pressure tribometer was designed to investigate the friction and wear of dry steel contacts under comparable conditions like in dry deep drawing.Therefore,a new ball-on-disc tribometer was designed and integrated into a high-pressure vessel.To specifically investigate the effects of different environments(technical air,liquid and gaseous carbon dioxide,nitrogen,argon)at atmospheric and high pressure(0.1 MPa,6 MPa)on tribology,the specimens and all components were operating unlubricated.During the experiments,the friction was measured continuously.Results show that the highest friction occurs in air and the lowest in carbon dioxide environment.Subsequent to the experiments,the wear of the specimens was assessed along with changes in surface chemistry related to tribochemical reactions.Therefore,the tribology of the dry sliding contacts is correlated to changes of the surface chemistry.Also differences as well as similarities regarding the different fluid environments are shown.As the results show,the differences between the media used are most pronounced at elevated pressure.Concluding,this work gives clear indications on the suitability of volatile lubricants in dry friction or rather gas lubrication,especially for dry deep drawing.
文摘The Kevlar/polytetrafluroethylene（Kevlar/PTFE） fabric composite can be used as a self-lubricating liner of the self-lubricating bearing.Many types of nano-particles can improve the tribological performance of the polymer-based composite.Unfortunately,up to now,published work on the effect of nano-particles on the tribological performance of the fabric composite which can be used as a self-lubricating liner is quite scarce.Therefore,for the purpose of exploring a way to significantly improve the tribological performance of the fabric composite,the tribological performance of the Kevlar/PTFE fabric composite filled with nano-titania is evaluated by using the block-on-ring wear tester.The scanning electron microscopy is utilized to observe the morphologies of worn surfaces of the fabric composites and the counterparts.The tensile properties of the composites are evaluated on the universal material testing machine.The test results show that the addition of nano-titania at a proper mass fraction of the matrix resin improves the wear resistance and the tensile strength,decreases the friction coefficient,and makes the wear volume of the composite reach a relative steady state more quickly;plastic deformation and microcutting are important for the wear of the fabric composite;a lubricating layer is formed on the worn surface of the composite during sliding,and the lubricating layer is critical for the tribological performance of the composite;the formation and properties of the lubricating layer are influenced by the nano-titania particles.The proposed study on the effect of nano-titania on the tribological performance of the Kevlar/PTFE fabric composite,especially on the evolution of the worn surface of the composite,provides the basis for further understanding of the influence mechanism of the nano-particles on the tribological performance of the composite and explores a method of improving the tribological performance of the composite.
