To investigate the effect of diesel soot on the distribution,composition and mechanical properties of ZDDP tribofilm,a HFRR tribometer was applied to study the tribological performance.Worn surfaces lubricated with ZD...To investigate the effect of diesel soot on the distribution,composition and mechanical properties of ZDDP tribofilm,a HFRR tribometer was applied to study the tribological performance.Worn surfaces lubricated with ZDDP and soot were analyzed by laser microscopy,SEM/EDS,Raman spectroscopy,XPS,and a nano-indentation equipment.Results show that soot scrapes off the ZDDP tribofilm and can be embedded into the worn surface,leading to the reduction of film thickness and non-uniform distribution of tribofim.The phosphate structure in ZDDP tribofilm changes from short chain pyrophosphate to long chain metaphosphate due to the increased contact stress caused by the soot abrasive wear,which can promote the cross-linking of ZDDP.The hardness(H)and elastic modulus(E)of the worn surfaces increase,while the ratio of hardness to elastic modulus,H/E,decreases,which indicates that the reduction of wear resistance is caused by the soot.展开更多
Ethanol has emerged as a promising alternative to fossil fuels,but its use can lead to significant dilution in lubricants,particularly during cold start or heavy traffic.This dilution can affect the performance of add...Ethanol has emerged as a promising alternative to fossil fuels,but its use can lead to significant dilution in lubricants,particularly during cold start or heavy traffic.This dilution can affect the performance of additives,including friction modifiers like molybdenum dithiocarbamate(MoDTC),which are designed to reduce friction under extreme contact conditions.Prior research suggests that ethanol may impact the performance of MoDTC,prompting this study’s goal to investigate the effects of ethanol on MoDTC tribofilms and their friction response under boundary lubrication conditions.Therefore,reciprocating tribological tests were performed with fully formulated lubricants containing MoDTC with varying ethanol concentrations.The results indicate that a critical ethanol dilution level inhibits friction reduction by MoDTC activation,resulting in friction coefficients(COFs)similar to the base oil.Surfaces tested with simple mixtures of polyalphaolefin(PAO)+MoDTC showed increased COFs with added ethanol.Analysis of tested surfaces using Raman spectroscopy,X-ray photoelectron spectroscopy(XPS),and X-ray absorption spectroscopy near the edge structure(XANES)revealed the presence of sulfates,MoO_(3),MoS_(2),and MoS_(x)O_(y)compounds in the tribofilms formed on the surfaces,with and without ethanol diluted in the lubricant.However,the addition of ethanol increased the sulfates and MoO_(3)content of the tribofilms at the expense of friction-reducing compounds such as MoS_(2)and MoS_(x)O_(y).These findings suggest that ethanol dilution in lubricants containing MoDTC creates an oxygen-rich interfacial medium that favors the formation of compounds with insufficient friction-reducing capabilities.展开更多
This study is an optimized extension based on the authors’previous research on the tribo-chemical reaction under constant temperature field of two-stroke internal combustion engines(ICEs).It establishes a coupled ana...This study is an optimized extension based on the authors’previous research on the tribo-chemical reaction under constant temperature field of two-stroke internal combustion engines(ICEs).It establishes a coupled analysis model that considers the tribo-chemical reactions,dynamic contact,and interface lubrication of the piston ring-cylinder liner(PRCL)system under transient temperature conditions.In this study,for the first time,the prediction of the tribofilm thickness and its influence on the surface micro-topography(the comprehensive roughness)are coupled in the working temperature field of the PRCL system,forming an effective model framework and providing a model basis and analytical basis for subsequent research.This study findings reveal that by incorporating temperature and tribofilm into the simulation model,the average friction deviation throughout the stroke decreases from 8.92%to 0.93%when compared to experimental results.Moreover,the deviation during the combustion regime reduces from 39.56%to 7.34%.The proposed coupled model provides a valuable tool for the evaluation of lubrication performance of the PRCL system and supports the analysis software forward design in two-stroke ICEs.展开更多
Impregnated graphite has attracted considerable attention and has been widely used as an ideal friction material in many fields.However,the influence of the friction temperature on its tribological properties has not ...Impregnated graphite has attracted considerable attention and has been widely used as an ideal friction material in many fields.However,the influence of the friction temperature on its tribological properties has not been clearly studied;furthermore,the evolution mechanism of transferred tribofilm is unknown.In this study,the tribological properties of impregnated graphite were investigated at different friction temperatures,and the evolution of the carbon-based tribofilm was also determined.The results revealed that the tribological properties significantly improved with an increase in friction temperature.The friction coefficient and wear depth of impregnated graphite reduced by 68%and 75%,respectively,at a high temperature of 160℃ compared with those of non-impregnated graphite.The significant properties of the impregnated graphite can be attributed to a transferred carbon-based tribofilm with an ordered structure induced by the friction temperature,which uniformly and stably adsorbs on friction interfaces.This study provides an important basis for designing graphite-based friction materials with improved properties suited for industrial applications.展开更多
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 trib...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 2μm 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 Ps 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,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,but also realizes high Pg and high load-carrying capacity.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 friction properties of wet clutches are highly dependent on the surface tribofilms formed by automatic transmission fluids (ATFs). Here, four commercial ATFs were evaluated with a disc-on-disc tribometer to study ...The friction properties of wet clutches are highly dependent on the surface tribofilms formed by automatic transmission fluids (ATFs). Here, four commercial ATFs were evaluated with a disc-on-disc tribometer to study tribofilm formation on steel surfaces and the effects of tribofilms on the friction properties. The chemical composition, stoichiometry, structure, and thickness of the tribofilms were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). Calcium phosphate (CaP) tribofilms form on the friction surface with all ATFs, which contributes to their antishudder characteristics. The thickness and surface coverage of CaP tribofilms are positively correlated with their antishudder properties.展开更多
In this study,we mainly focus on the structural morphology and inter-atomic bonding state of tribofilms resulting from a highly-hydrogenated amorphous carbon(a-C:H) film in order to ascertain the underlying mechanisms...In this study,we mainly focus on the structural morphology and inter-atomic bonding state of tribofilms resulting from a highly-hydrogenated amorphous carbon(a-C:H) film in order to ascertain the underlying mechanisms for its superlubric behavior(i.e.,less than 0.01 friction coefficient).Specifically,we achieved superlubricity(i.e.,friction coefficients of down to 0.003) with this film in dry nitrogen and argon atmospheres especially when the tribo-pair is made of an a-C:H coated Si disk sliding against an a-C:H coated steel ball,while the a-C:H coated disk against uncoated ball does not provide superlubricity.We also found that the state of superlubricity is more stable in argon than in nitrogen and the formation of a smooth and uniformly-thick carbonaceous tribofilm appears to be one of the key factors for the realization of such superlubricity.Besides,the interfacial morphology of sliding test pairs and the atomic-scale bond structure of the carbon-based tribofilms also play an important role in the observed superlubric behavior of a-C:H films.Using Raman spectroscopy and high resolution transmission electron microscopy,we have compared the structural differences of the tribofilms produced on bare and a-C:H coated steel balls.For the a-C:H coated ball as mating material which provided superlow friction in argon,structural morphology of the tribofilm was similar or comparable to that of the original a-C:H coating;while for the bare steel ball,the sp^2-bonded C fraction in the tribofilm increased and a fingerprint-like nanocrystalline structure was detected by high resolution transmission electron microscopy(HRTEM).We also calculated the shear stresses for different tribofilms,and established a relationship between the magnitude of the shear stresses and the extent of sp^3-sp^2 phase transformation.展开更多
Friction modifiers(FMs)are surface-active additives added to base fluids to reduce friction between rubbing surfaces.Their effectiveness depends on their interactions with rubbing surfaces and may be mitigated by the ...Friction modifiers(FMs)are surface-active additives added to base fluids to reduce friction between rubbing surfaces.Their effectiveness depends on their interactions with rubbing surfaces and may be mitigated by the choice of the base fluid.In this work,the performance of an imidazolium ionic liquid(ImIL)additive in polyethylene-glycol(PEG)and 1,4-butanediol for lubricating steel/steel and diamond-like-carbon/diamond-like carbon(DLC-DLC)contacts were investigated.ImIL-containing PEG reduces friction more effectively in steel-steel than DLC-DLC contacts.In contrast,adding ImIL in 1,4-butanediol results in an increase in friction in steel-steel contacts.Results from the Raman spectroscopy,X-ray photoelectron spectroscopy(XPS),and focused ion beam-transmission electron microscopy(FIB-TEM)reveal that a surface film is formed on steel during rubbing in ImIL-containing PEG.This film consists of two layers.The top layer is composed of amorphous carbon and are easily removed during rubbing.The bottom layer,which contains iron oxide and nitride compound,adheres strongly on the steel surface.This film maintains its effectiveness in a steel-steel contact even after ImIL additives are depleted.Such film is not observed in 1,4-butanediol where the adsorption of ImIL is hindered,as suggested by the quartz crystal microbalance(QCM)measurements.No benefit is observed when the base fluid on its own is sufficiently lubricious,as in the case of DLC surfaces.This work provides fundamental insights on how compatibilities among base fluid,FM,and rubbing surface affect the performance of IL as surface active additives.It reveals the structure of an ionic liquid(IL)surface film,which is effective and durable.The knowledge is useful for guiding future IL additive development.展开更多
基金This research was supported by the National Natural Science Foundation of China(Grant No.52075141).
文摘To investigate the effect of diesel soot on the distribution,composition and mechanical properties of ZDDP tribofilm,a HFRR tribometer was applied to study the tribological performance.Worn surfaces lubricated with ZDDP and soot were analyzed by laser microscopy,SEM/EDS,Raman spectroscopy,XPS,and a nano-indentation equipment.Results show that soot scrapes off the ZDDP tribofilm and can be embedded into the worn surface,leading to the reduction of film thickness and non-uniform distribution of tribofim.The phosphate structure in ZDDP tribofilm changes from short chain pyrophosphate to long chain metaphosphate due to the increased contact stress caused by the soot abrasive wear,which can promote the cross-linking of ZDDP.The hardness(H)and elastic modulus(E)of the worn surfaces increase,while the ratio of hardness to elastic modulus,H/E,decreases,which indicates that the reduction of wear resistance is caused by the soot.
基金funded by Fundação de AmparoàPesquisa do Estado de São Paulo(FAPESP)through the R&D ConsortiumTribological challenges in flex-fuel engines,ref.2009/54891-8,and Centro Nacional de Pesquisa em Energia e Materiais(CNPEM-LNLS-National Synchrotron Light Laboratory,grant number 20170101)the partial financial support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico(CNPq)grant number 305374/2021-4+1 种基金the partial financial support from Fundação de AmparoàPesquisa do Estado do Rio Grande do Sul(FAPERGS),Process number:172551-000901-0the National Research Institute on Green Tribology for the Energy Transition(CT-Trib)with financial support by CNPq/Brazil via the grant number 406654/2022-0.
文摘Ethanol has emerged as a promising alternative to fossil fuels,but its use can lead to significant dilution in lubricants,particularly during cold start or heavy traffic.This dilution can affect the performance of additives,including friction modifiers like molybdenum dithiocarbamate(MoDTC),which are designed to reduce friction under extreme contact conditions.Prior research suggests that ethanol may impact the performance of MoDTC,prompting this study’s goal to investigate the effects of ethanol on MoDTC tribofilms and their friction response under boundary lubrication conditions.Therefore,reciprocating tribological tests were performed with fully formulated lubricants containing MoDTC with varying ethanol concentrations.The results indicate that a critical ethanol dilution level inhibits friction reduction by MoDTC activation,resulting in friction coefficients(COFs)similar to the base oil.Surfaces tested with simple mixtures of polyalphaolefin(PAO)+MoDTC showed increased COFs with added ethanol.Analysis of tested surfaces using Raman spectroscopy,X-ray photoelectron spectroscopy(XPS),and X-ray absorption spectroscopy near the edge structure(XANES)revealed the presence of sulfates,MoO_(3),MoS_(2),and MoS_(x)O_(y)compounds in the tribofilms formed on the surfaces,with and without ethanol diluted in the lubricant.However,the addition of ethanol increased the sulfates and MoO_(3)content of the tribofilms at the expense of friction-reducing compounds such as MoS_(2)and MoS_(x)O_(y).These findings suggest that ethanol dilution in lubricants containing MoDTC creates an oxygen-rich interfacial medium that favors the formation of compounds with insufficient friction-reducing capabilities.
基金supported by the Shandong Provincial Natural Science Foundation(No.ZR2022QE183).
文摘This study is an optimized extension based on the authors’previous research on the tribo-chemical reaction under constant temperature field of two-stroke internal combustion engines(ICEs).It establishes a coupled analysis model that considers the tribo-chemical reactions,dynamic contact,and interface lubrication of the piston ring-cylinder liner(PRCL)system under transient temperature conditions.In this study,for the first time,the prediction of the tribofilm thickness and its influence on the surface micro-topography(the comprehensive roughness)are coupled in the working temperature field of the PRCL system,forming an effective model framework and providing a model basis and analytical basis for subsequent research.This study findings reveal that by incorporating temperature and tribofilm into the simulation model,the average friction deviation throughout the stroke decreases from 8.92%to 0.93%when compared to experimental results.Moreover,the deviation during the combustion regime reduces from 39.56%to 7.34%.The proposed coupled model provides a valuable tool for the evaluation of lubrication performance of the PRCL system and supports the analysis software forward design in two-stroke ICEs.
基金supported by the National Key R&D Program of China(No.2018YFB2000801)the National Natural Science Foundation of China(No.51905027)+1 种基金the Tribology Science Fund of State Key Laboratory of Tribology(No.SKLTKF18A02)the Fundamental Research Funds for the Central Universities(No.BUCTRC201908).
文摘Impregnated graphite has attracted considerable attention and has been widely used as an ideal friction material in many fields.However,the influence of the friction temperature on its tribological properties has not been clearly studied;furthermore,the evolution mechanism of transferred tribofilm is unknown.In this study,the tribological properties of impregnated graphite were investigated at different friction temperatures,and the evolution of the carbon-based tribofilm was also determined.The results revealed that the tribological properties significantly improved with an increase in friction temperature.The friction coefficient and wear depth of impregnated graphite reduced by 68%and 75%,respectively,at a high temperature of 160℃ compared with those of non-impregnated graphite.The significant properties of the impregnated graphite can be attributed to a transferred carbon-based tribofilm with an ordered structure induced by the friction temperature,which uniformly and stably adsorbs on friction interfaces.This study provides an important basis for designing graphite-based friction materials with improved properties suited for industrial applications.
基金We acknowledge the financial support provided by the National Natural Science Foundation of China(Nos.51875172 and 51775168)Scientific and Technological Innovation Team of Henan Province Universities(No.19IRTSTHN024)Zhongyuan Science and Technology Innovation Leadership Program(No.214200510024).
文摘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 2μm 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 Ps 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,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,but also realizes high Pg and high load-carrying capacity.
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.
文摘The friction properties of wet clutches are highly dependent on the surface tribofilms formed by automatic transmission fluids (ATFs). Here, four commercial ATFs were evaluated with a disc-on-disc tribometer to study tribofilm formation on steel surfaces and the effects of tribofilms on the friction properties. The chemical composition, stoichiometry, structure, and thickness of the tribofilms were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). Calcium phosphate (CaP) tribofilms form on the friction surface with all ATFs, which contributes to their antishudder characteristics. The thickness and surface coverage of CaP tribofilms are positively correlated with their antishudder properties.
基金supported by the National Basic Research Program of China (Grant No.2011CB013404)National Natural Science Foundation of China(Grant Nos.51321092,51527901 and 51375010)
文摘In this study,we mainly focus on the structural morphology and inter-atomic bonding state of tribofilms resulting from a highly-hydrogenated amorphous carbon(a-C:H) film in order to ascertain the underlying mechanisms for its superlubric behavior(i.e.,less than 0.01 friction coefficient).Specifically,we achieved superlubricity(i.e.,friction coefficients of down to 0.003) with this film in dry nitrogen and argon atmospheres especially when the tribo-pair is made of an a-C:H coated Si disk sliding against an a-C:H coated steel ball,while the a-C:H coated disk against uncoated ball does not provide superlubricity.We also found that the state of superlubricity is more stable in argon than in nitrogen and the formation of a smooth and uniformly-thick carbonaceous tribofilm appears to be one of the key factors for the realization of such superlubricity.Besides,the interfacial morphology of sliding test pairs and the atomic-scale bond structure of the carbon-based tribofilms also play an important role in the observed superlubric behavior of a-C:H films.Using Raman spectroscopy and high resolution transmission electron microscopy,we have compared the structural differences of the tribofilms produced on bare and a-C:H coated steel balls.For the a-C:H coated ball as mating material which provided superlow friction in argon,structural morphology of the tribofilm was similar or comparable to that of the original a-C:H coating;while for the bare steel ball,the sp^2-bonded C fraction in the tribofilm increased and a fingerprint-like nanocrystalline structure was detected by high resolution transmission electron microscopy(HRTEM).We also calculated the shear stresses for different tribofilms,and established a relationship between the magnitude of the shear stresses and the extent of sp^3-sp^2 phase transformation.
基金Wei SONG is supported by China Scholarship Council.
文摘Friction modifiers(FMs)are surface-active additives added to base fluids to reduce friction between rubbing surfaces.Their effectiveness depends on their interactions with rubbing surfaces and may be mitigated by the choice of the base fluid.In this work,the performance of an imidazolium ionic liquid(ImIL)additive in polyethylene-glycol(PEG)and 1,4-butanediol for lubricating steel/steel and diamond-like-carbon/diamond-like carbon(DLC-DLC)contacts were investigated.ImIL-containing PEG reduces friction more effectively in steel-steel than DLC-DLC contacts.In contrast,adding ImIL in 1,4-butanediol results in an increase in friction in steel-steel contacts.Results from the Raman spectroscopy,X-ray photoelectron spectroscopy(XPS),and focused ion beam-transmission electron microscopy(FIB-TEM)reveal that a surface film is formed on steel during rubbing in ImIL-containing PEG.This film consists of two layers.The top layer is composed of amorphous carbon and are easily removed during rubbing.The bottom layer,which contains iron oxide and nitride compound,adheres strongly on the steel surface.This film maintains its effectiveness in a steel-steel contact even after ImIL additives are depleted.Such film is not observed in 1,4-butanediol where the adsorption of ImIL is hindered,as suggested by the quartz crystal microbalance(QCM)measurements.No benefit is observed when the base fluid on its own is sufficiently lubricious,as in the case of DLC surfaces.This work provides fundamental insights on how compatibilities among base fluid,FM,and rubbing surface affect the performance of IL as surface active additives.It reveals the structure of an ionic liquid(IL)surface film,which is effective and durable.The knowledge is useful for guiding future IL additive development.
