Surface moisture or humidity impacting the lubrication property is a ubiquitous phenomenon in tribological systems,which is demonstrated by a combination of molecular dynamics(MD)simulation and experiment for the orga...Surface moisture or humidity impacting the lubrication property is a ubiquitous phenomenon in tribological systems,which is demonstrated by a combination of molecular dynamics(MD)simulation and experiment for the organic friction modifier(OFM)-containing lubricant.The stearic acid and poly-α-olefin 4cSt(PAO4)were chosen as the OFM and base oil molecules,respectively.The physical adsorption indicates that on the moist surface water molecules are preferentially adsorbed on friction surface,and even make OFM adsorption film thoroughly leave surface and mix with base oil.In shear process,the adsorption of water film and desorption OFM film are further enhanced,particularly under higher shear rate.The simulated friction coefficient(that is proportional to shear rate)increases firstly and then decreases with thickening water film,in good agreement with experiments,while the slip length shows a contrary change.The wear increases with humidity due to tribochemistry revealing the continuous formation and removal of Si–O–Si network.The tribological discrepancy of OFM-containing lubricant in dry and humid conditions is attributed to the slip plane’s transformation from the interface between OFM adsorption film and lubricant bulk to the interface between adsorbed water films.This work provides a new thought to understand the boundary lubrication and failure of lubricant in humid environments,likely water is not always harmful in oil lubrication systems.展开更多
High-performance organic friction modifiers(OFMs)added to lubricating oils are crucial for reducing energy loss and carbon footprint.To establish a new class of OFMs,we measured the friction and wear properties of N-(...High-performance organic friction modifiers(OFMs)added to lubricating oils are crucial for reducing energy loss and carbon footprint.To establish a new class of OFMs,we measured the friction and wear properties of N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide referred to as C12Amide-TEMPO.The effect of its head group chemistry,which is characterized by a rigid six-membered ring sandwiched by an amide group and a terminal free oxygen radical,was also investigated with both experiments and quantum mechanical(QM)calculations.The measurement results show that C12Amide-TEMPO outperforms the conventional OFMs of glyceryl monooleate(GMO)and stearic acid,particularly for load-carrying capacity,wear reduction,and stability of friction over time.The friction and wear reduction effect of C12Amide-TEMPO is also greatly superior to those of C12Ester-TEMPO and C12Amino-TEMPO,in which ester and amino groups replace the amide group,highlighting the critical role of the amide group.The QM calculation results suggest that,in contrast to C12Ester-TEMPO,C12Amino-TEMPO,and the conventional OFMs of GMO and stearic acid,C12Amide-TEMPO can form effective boundary films on iron oxide surfaces with a unique double-layer structure:a strong surface adsorption layer owing to the chemical interactions of the amide oxygen and free radical with iron oxide surfaces,and an upper layer owing to the interlayer hydrogen-bonding between the amide hydrogen and free radical or between the amide hydrogen and oxygen.Moreover,the intralayer hydrogen-bonding in each of the two layers is also possible.We suggest that in addition to strong surface adsorption,the interlayer and intralayer hydrogen-bonding also increases the strength of the boundary films by enhancing the cohesion strength,thereby resulting in the high tribological performance of C12Amide-TEMPO.The findings in this study are expected to provide new hints for the optimal molecular design of OFMs.展开更多
The influence of structural factors on the lubrication performance of organic friction modifiers(OFMs)formulated in Group V(polyol ester oil)base oil was studied using a ball-on-disk tribometer.The results show that O...The influence of structural factors on the lubrication performance of organic friction modifiers(OFMs)formulated in Group V(polyol ester oil)base oil was studied using a ball-on-disk tribometer.The results show that OFMs can mitigate friction under heavy loads,low sliding speeds,and high temperatures.These conditions are commonly encountered in internal-combustion engines between cylinder liners and piston rings.The reduction in friction is ascribed to the boundary lubrication film containing the OFM.The chemical composition analysis of the metal disk surface using energy dispersive X-ray spectroscopy(EDS)confirmed the presence of a protective film of OFM on the wear track,albeit inconsistently deposited.Although the adsorption of the OFM on the metal surface was observed to be dependent on the chemical reactivity of the functional groups,levels of unsaturation,and hydrocarbon chain length of the OFM,the frictional performance was not always directly correlated with the surface coverage and tribofilm thickness.This implies that the friction reduction mechanism can involve other localized processes at the interface between the metal surface and lubricant oil.The occasional variation in friction observed for these OFMs can be attributed to the stability and durability of the boundary film formed during the rubbing phase.展开更多
基金the financial support from the National Natural Science Foundation of China(52105210)Project funded by China Postdoctoral Science Foundation(2022M712593)+1 种基金Research Fund of the State Key Laboratory of Solidification Processing(NPU)(2021-TS-06)Zhejiang Provincial Natural Science Foundation of China(Key Program,Grant No.LZ21A020001).
文摘Surface moisture or humidity impacting the lubrication property is a ubiquitous phenomenon in tribological systems,which is demonstrated by a combination of molecular dynamics(MD)simulation and experiment for the organic friction modifier(OFM)-containing lubricant.The stearic acid and poly-α-olefin 4cSt(PAO4)were chosen as the OFM and base oil molecules,respectively.The physical adsorption indicates that on the moist surface water molecules are preferentially adsorbed on friction surface,and even make OFM adsorption film thoroughly leave surface and mix with base oil.In shear process,the adsorption of water film and desorption OFM film are further enhanced,particularly under higher shear rate.The simulated friction coefficient(that is proportional to shear rate)increases firstly and then decreases with thickening water film,in good agreement with experiments,while the slip length shows a contrary change.The wear increases with humidity due to tribochemistry revealing the continuous formation and removal of Si–O–Si network.The tribological discrepancy of OFM-containing lubricant in dry and humid conditions is attributed to the slip plane’s transformation from the interface between OFM adsorption film and lubricant bulk to the interface between adsorbed water films.This work provides a new thought to understand the boundary lubrication and failure of lubricant in humid environments,likely water is not always harmful in oil lubrication systems.
基金JSPS KAKENHI Grant(Nos.19K21915 and 21H01238)JST Adaptable and Seamless Technology Transfer Program through Target-driven R&D(No.JPMJTM19FN)NSK Foundation for Mechatronics Technology Advancement.We thank Dr.Kin-ichi OYAMA(Research Center for Materials Science,Nagoya University)for mass spectrometry analysis of the synthesized OFMs and associate professor Takayuki TOKOROYAMA(Graduate School of Engineering,Nagoya University)for the help with wear scar measurements.Jinchi HOU is grateful for the financial support from the China Scholarship Council(No.202006030017).
文摘High-performance organic friction modifiers(OFMs)added to lubricating oils are crucial for reducing energy loss and carbon footprint.To establish a new class of OFMs,we measured the friction and wear properties of N-(2,2,6,6-tetramethyl-1-oxyl-4-piperidinyl)dodecaneamide referred to as C12Amide-TEMPO.The effect of its head group chemistry,which is characterized by a rigid six-membered ring sandwiched by an amide group and a terminal free oxygen radical,was also investigated with both experiments and quantum mechanical(QM)calculations.The measurement results show that C12Amide-TEMPO outperforms the conventional OFMs of glyceryl monooleate(GMO)and stearic acid,particularly for load-carrying capacity,wear reduction,and stability of friction over time.The friction and wear reduction effect of C12Amide-TEMPO is also greatly superior to those of C12Ester-TEMPO and C12Amino-TEMPO,in which ester and amino groups replace the amide group,highlighting the critical role of the amide group.The QM calculation results suggest that,in contrast to C12Ester-TEMPO,C12Amino-TEMPO,and the conventional OFMs of GMO and stearic acid,C12Amide-TEMPO can form effective boundary films on iron oxide surfaces with a unique double-layer structure:a strong surface adsorption layer owing to the chemical interactions of the amide oxygen and free radical with iron oxide surfaces,and an upper layer owing to the interlayer hydrogen-bonding between the amide hydrogen and free radical or between the amide hydrogen and oxygen.Moreover,the intralayer hydrogen-bonding in each of the two layers is also possible.We suggest that in addition to strong surface adsorption,the interlayer and intralayer hydrogen-bonding also increases the strength of the boundary films by enhancing the cohesion strength,thereby resulting in the high tribological performance of C12Amide-TEMPO.The findings in this study are expected to provide new hints for the optimal molecular design of OFMs.
基金This study was funded by the Agency for Science,Technology and Research (A*STAR) under a Specialty Chemicals Advanced Manufacturing and Engineering IAF-PP research grant (Grant No. A1786a0026)。
文摘The influence of structural factors on the lubrication performance of organic friction modifiers(OFMs)formulated in Group V(polyol ester oil)base oil was studied using a ball-on-disk tribometer.The results show that OFMs can mitigate friction under heavy loads,low sliding speeds,and high temperatures.These conditions are commonly encountered in internal-combustion engines between cylinder liners and piston rings.The reduction in friction is ascribed to the boundary lubrication film containing the OFM.The chemical composition analysis of the metal disk surface using energy dispersive X-ray spectroscopy(EDS)confirmed the presence of a protective film of OFM on the wear track,albeit inconsistently deposited.Although the adsorption of the OFM on the metal surface was observed to be dependent on the chemical reactivity of the functional groups,levels of unsaturation,and hydrocarbon chain length of the OFM,the frictional performance was not always directly correlated with the surface coverage and tribofilm thickness.This implies that the friction reduction mechanism can involve other localized processes at the interface between the metal surface and lubricant oil.The occasional variation in friction observed for these OFMs can be attributed to the stability and durability of the boundary film formed during the rubbing phase.