Friction is a complex phenomenon that depends on many parameters.Despite this,we still rely on and describe friction in the vast majority of cases with a single value,namely,the coefficient of friction(µ),as firs...Friction is a complex phenomenon that depends on many parameters.Despite this,we still rely on and describe friction in the vast majority of cases with a single value,namely,the coefficient of friction(µ),as first proposed by Amontons in 1699.Later,Coulomb introduced a two-parameter description by separating the adhesive and load-dependent terms.However,experimental evidence that determines under what conditions either of the two historical models is more appropriate has not been investigated in detail.In particular,to take full advantage and achieve better accuracy with the two-parameter equation,the real contact area must be well characterized to determine the constant adhesive component sufficiently and accurately.In this study,we performed sliding experiments and measured friction,but at the same time,we also measured the real contact area with sub-micron lateral resolution,which allowed us to design a two-parameter(Coulomb-type)friction equation.We compared these results with the historical friction models of Amontons and Coulomb to better understand the actual differences between them and how this corresponds to the linearity between the friction force and the normal force,which is a key assumption in the more common and simpler Amontons relation.A strong scaling effect of roughness was observed,as well as the related nonlinearity between the normal load,friction,and real contact area.Under high loads and roughnesses,the one-or two-parameter friction descriptive models differed from experiments by only a few percent,and the variation among them was small in the same range.However,for very low roughnesses and loads(close to the nanoscale region),the two-parameter Coulomb model was required for any relevant friction prediction due to the strong adhesive contribution,while the one-parameter description was not appropriate.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
In this work,we evaluated the effect of the counter-body material(the same or dissimilar)and contact configuration(moving or stationary body),at similar contact tribological conditions,on the tribochemical and nanotop...In this work,we evaluated the effect of the counter-body material(the same or dissimilar)and contact configuration(moving or stationary body),at similar contact tribological conditions,on the tribochemical and nanotopography characteristics of adsorbed surface films.Zinc dialkyldithiophosphate(ZDDP),the best performing anti-wear additive,was used in self-mated steel/steel and DLC/DLC contacts,which were compared with mixed steel/DLC and DLC/steel contacts in 1-h and 6-h sliding tests.The macroscale(tribometer)and nanoscale(atomic force microscopy)friction,thickness,topography,and chemical(attenuated total reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy)properties of the tribofilms were studied.The results revealed unexpectedly large differences in all the studied tribofilm parameters;this is because all the tribofilms are completely different;this includes the chemical composition,which is known to have a crucial effect on the nano-and macro-scale tribological properties.These results clearly demonstrate that the surface material,additives,and common contact operating parameters,that is,pressure,velocity,and temperature,crucially affect the ZDDP tribofilm as well as the position of the moving or stationary surface within the contact,and the material of the moving/stationary bodies.展开更多
The requirements for green and sustainable manufacturing mean that stamping lubricants must be continuously re-evaluated and re-designed.In this investigation,the tribological performance of four base oils with differ...The requirements for green and sustainable manufacturing mean that stamping lubricants must be continuously re-evaluated and re-designed.In this investigation,the tribological performance of four base oils with different chemical structures(paraffinic and naphthenic)and viscosities(2 and 20 cSt),as well as water,was evaluated for the stamping of steel sheets and compared with a non-lubricated contact.Most lubricants reduce the coefficient of friction and maintain a similar wear coefficient for steel sheets as in dry contacts.Low-viscosity(LV)naphthenic oil performs very like both high-viscosity(HV)oils.A surprising exception is the LV paraffinic oil,with several-times-higher friction and wear compared to dry contact.This is due to the excellent wetting-spreading and very low cohesion forces that enable oil to escape from extremely thin-film contacts because the viscosity is so low,leading to lubricant starvation.In contrast,HV oils provide a sufficiently thick lubricating film,while strong cohesive forces help in the film’s strength,lessening wear,and reducing friction.In thin-film lubrication with LV oils,such as when stamping,it is thus extremely important that the lubricant’s wetting behaviour and viscosity are sufficient to provide enough film in the contact and prevent starvation,thus ensuring lower friction,less wear,and a longer lifetime of the contact.展开更多
文摘Friction is a complex phenomenon that depends on many parameters.Despite this,we still rely on and describe friction in the vast majority of cases with a single value,namely,the coefficient of friction(µ),as first proposed by Amontons in 1699.Later,Coulomb introduced a two-parameter description by separating the adhesive and load-dependent terms.However,experimental evidence that determines under what conditions either of the two historical models is more appropriate has not been investigated in detail.In particular,to take full advantage and achieve better accuracy with the two-parameter equation,the real contact area must be well characterized to determine the constant adhesive component sufficiently and accurately.In this study,we performed sliding experiments and measured friction,but at the same time,we also measured the real contact area with sub-micron lateral resolution,which allowed us to design a two-parameter(Coulomb-type)friction equation.We compared these results with the historical friction models of Amontons and Coulomb to better understand the actual differences between them and how this corresponds to the linearity between the friction force and the normal force,which is a key assumption in the more common and simpler Amontons relation.A strong scaling effect of roughness was observed,as well as the related nonlinearity between the normal load,friction,and real contact area.Under high loads and roughnesses,the one-or two-parameter friction descriptive models differed from experiments by only a few percent,and the variation among them was small in the same range.However,for very low roughnesses and loads(close to the nanoscale region),the two-parameter Coulomb model was required for any relevant friction prediction due to the strong adhesive contribution,while the one-parameter description was not appropriate.
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The authors acknowledge the financial support from the Slovenian Research Agency ARRS(research core funding No.P2-0231 Tribology and research project No.J2-7115 Nanoscale engineering of the contact interfaces for green lubrication technology).The authors also acknowledge F.Meunier from Oerlikon Balzers for providing the DLC coatings used in this work.
文摘In this work,we evaluated the effect of the counter-body material(the same or dissimilar)and contact configuration(moving or stationary body),at similar contact tribological conditions,on the tribochemical and nanotopography characteristics of adsorbed surface films.Zinc dialkyldithiophosphate(ZDDP),the best performing anti-wear additive,was used in self-mated steel/steel and DLC/DLC contacts,which were compared with mixed steel/DLC and DLC/steel contacts in 1-h and 6-h sliding tests.The macroscale(tribometer)and nanoscale(atomic force microscopy)friction,thickness,topography,and chemical(attenuated total reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy)properties of the tribofilms were studied.The results revealed unexpectedly large differences in all the studied tribofilm parameters;this is because all the tribofilms are completely different;this includes the chemical composition,which is known to have a crucial effect on the nano-and macro-scale tribological properties.These results clearly demonstrate that the surface material,additives,and common contact operating parameters,that is,pressure,velocity,and temperature,crucially affect the ZDDP tribofilm as well as the position of the moving or stationary surface within the contact,and the material of the moving/stationary bodies.
基金supported by the company Hidria Rotomatika d.o.o and Slovenian Research Agency as part of research project No.L2-9244 and research core funding No.P2-0231.
文摘The requirements for green and sustainable manufacturing mean that stamping lubricants must be continuously re-evaluated and re-designed.In this investigation,the tribological performance of four base oils with different chemical structures(paraffinic and naphthenic)and viscosities(2 and 20 cSt),as well as water,was evaluated for the stamping of steel sheets and compared with a non-lubricated contact.Most lubricants reduce the coefficient of friction and maintain a similar wear coefficient for steel sheets as in dry contacts.Low-viscosity(LV)naphthenic oil performs very like both high-viscosity(HV)oils.A surprising exception is the LV paraffinic oil,with several-times-higher friction and wear compared to dry contact.This is due to the excellent wetting-spreading and very low cohesion forces that enable oil to escape from extremely thin-film contacts because the viscosity is so low,leading to lubricant starvation.In contrast,HV oils provide a sufficiently thick lubricating film,while strong cohesive forces help in the film’s strength,lessening wear,and reducing friction.In thin-film lubrication with LV oils,such as when stamping,it is thus extremely important that the lubricant’s wetting behaviour and viscosity are sufficient to provide enough film in the contact and prevent starvation,thus ensuring lower friction,less wear,and a longer lifetime of the contact.
