A complete understanding of the mechanism of boundary lubrication is a goal that scientists have been striving to achieve over the past century.Although this complicated process has been far from fully revealed,a gene...A complete understanding of the mechanism of boundary lubrication is a goal that scientists have been striving to achieve over the past century.Although this complicated process has been far from fully revealed,a general picture and its influencing factors have been elucidated,not only at the macroscopic scale but also at the nanoscale,which is sufficiently clear to provide effective instructions for a lubrication design in engineering and even to efficiently control the boundary lubrication properties.Herein,we provide a review on the main advances,especially the breakthroughs in uncovering the mysterious but useful process of boundary lubrication by adsorption film.Despite the existence of an enormous amount of knowledge,albeit unsystematic,acquired in this area,in the present review,an effort was made to clarify the mainline of leading perspectives and methodologies in revealing the fundamental problems inherent to boundary lubrication.The main content of this review includes the formation of boundary film,the effects of boundary film on the adhesion and friction of rough surfaces,the behavior of adsorption film in boundary lubrication,boundary lubrication at the nanoscale,and the active control of boundary lubrication,generally sequenced based on the real history of our understanding of this process over the past century,incorporated by related modern concepts and prospects.展开更多
Friction is a phenomenon observed ubiquitously in daily life,yet its nature is complicated.Friction between rough surfaces is considered to arise primarily because of macroscopic roughness.In contrast,interatomic forc...Friction is a phenomenon observed ubiquitously in daily life,yet its nature is complicated.Friction between rough surfaces is considered to arise primarily because of macroscopic roughness.In contrast,interatomic forces dominate between clean and smooth surfaces.“Superlubricity”,where friction effectively becomes zero,occurs when the ratio of lattice parameters in the pair of surfaces becomes an irrational number.Superlubricity has been found to exist in a limited number of systems,but is a very important phenomenon both in industry and in mechanical engineering.New atomistic research on friction is under way,with the aim of refining theoretical models that consider interactions between atoms beyond mean field theory and experiments using ultrahigh vacuum non-contact atomic force microscopy.Such research is expected to help clarify the nature of microscopic friction,reveal the onset conditions of friction and superlubricity as well as the stability of superlubricity,discover new superlubric systems,and lead to new applications.展开更多
Using an atomic force microscope,the running-in process of a single crystalline silicon wafer coated with native oxide layer(Si-SiO_(x))against a SiO_(2)microsphere was investigated under various normal loads and disp...Using an atomic force microscope,the running-in process of a single crystalline silicon wafer coated with native oxide layer(Si-SiO_(x))against a SiO_(2)microsphere was investigated under various normal loads and displacement amplitudes in ambient air.As the number of sliding cycles increased,both the friction force Ft of the Si-SiO_(x)/SiO_(2)pair and the wear rate of the silicon surface showed sharp drops during the initial 50 cycles and then leveled off in the remaining cycles.The sharp drop in Ft appeared to be induced mainly by the reduction of adhesion-related interfacial force between the Si-SiO_(x)/SiO_(2)pair.During the running-in process,the contact area of the Si-SiO_(x)/SiO_(2)pair might become hydrophobic due to removal of the hydrophilic oxide layer on the silicon surface and the surface change of the SiO_(2)tip,which caused the reduction of friction force and the wear rate of the Si-SiO_(x)/SiO_(2)pair.A phenomenological model is proposed to explain the running-in process of the Si-SiO_(x)/SiO_(2)pair in ambient air.The results may help us understand the mechanism of the running-in process of the Si-SiO_(x)/SiO_(2)pair at nanoscale and reduce wear failure in dynamic microelectromechanical systems(MEMS).展开更多
Thermally grown surface oxide layers dominate the single-asperity tribological behavior of a Zr60Cu30Al10 glass.Increase in oxidation time leads to an increased contribution of shearing and a corresponding decreased c...Thermally grown surface oxide layers dominate the single-asperity tribological behavior of a Zr60Cu30Al10 glass.Increase in oxidation time leads to an increased contribution of shearing and a corresponding decreased contribution of ploughing to friction.This change in the dominating friction and wear mechanism results in an overall minor decrease of the friction coefficient of oxidized surfaces compared to the metallic glass sample with native surface oxide.Our results demonstrate the importance of creating a stable oxide layer for practical applications of metallic glasses in micro-devices involving sliding contact.展开更多
Friction force measurements between smooth surfaces across two layers of linear alkanes over five decades of speeds are presented. A maximum friction dissipation is observed at a characteristic speed. The behaviour is...Friction force measurements between smooth surfaces across two layers of linear alkanes over five decades of speeds are presented. A maximum friction dissipation is observed at a characteristic speed. The behaviour is described by a new approach: the formation and destruction of molecular bridges between confined alkane layers. The bridges interdigitated between the layers exhibit a thermally activated resistance to shear.An analytical model involving activation barriers accounts for the overall behaviour of the forces over four decades of speed. This first simple semi-quantitative description sheds new light on the subtle mechanisms of friction at the nanoscale level and shows how the molecular length influences the tribological properties of the liquid.展开更多
基金supported by the National Natural Science Foundation of China(NSFC)with the grant No.91123033.
文摘A complete understanding of the mechanism of boundary lubrication is a goal that scientists have been striving to achieve over the past century.Although this complicated process has been far from fully revealed,a general picture and its influencing factors have been elucidated,not only at the macroscopic scale but also at the nanoscale,which is sufficiently clear to provide effective instructions for a lubrication design in engineering and even to efficiently control the boundary lubrication properties.Herein,we provide a review on the main advances,especially the breakthroughs in uncovering the mysterious but useful process of boundary lubrication by adsorption film.Despite the existence of an enormous amount of knowledge,albeit unsystematic,acquired in this area,in the present review,an effort was made to clarify the mainline of leading perspectives and methodologies in revealing the fundamental problems inherent to boundary lubrication.The main content of this review includes the formation of boundary film,the effects of boundary film on the adhesion and friction of rough surfaces,the behavior of adsorption film in boundary lubrication,boundary lubrication at the nanoscale,and the active control of boundary lubrication,generally sequenced based on the real history of our understanding of this process over the past century,incorporated by related modern concepts and prospects.
文摘Friction is a phenomenon observed ubiquitously in daily life,yet its nature is complicated.Friction between rough surfaces is considered to arise primarily because of macroscopic roughness.In contrast,interatomic forces dominate between clean and smooth surfaces.“Superlubricity”,where friction effectively becomes zero,occurs when the ratio of lattice parameters in the pair of surfaces becomes an irrational number.Superlubricity has been found to exist in a limited number of systems,but is a very important phenomenon both in industry and in mechanical engineering.New atomistic research on friction is under way,with the aim of refining theoretical models that consider interactions between atoms beyond mean field theory and experiments using ultrahigh vacuum non-contact atomic force microscopy.Such research is expected to help clarify the nature of microscopic friction,reveal the onset conditions of friction and superlubricity as well as the stability of superlubricity,discover new superlubric systems,and lead to new applications.
基金the financial support from National Basic Research Program(No.2011CB707604)Natural Science Foundation of China(51175441,90923017).
文摘Using an atomic force microscope,the running-in process of a single crystalline silicon wafer coated with native oxide layer(Si-SiO_(x))against a SiO_(2)microsphere was investigated under various normal loads and displacement amplitudes in ambient air.As the number of sliding cycles increased,both the friction force Ft of the Si-SiO_(x)/SiO_(2)pair and the wear rate of the silicon surface showed sharp drops during the initial 50 cycles and then leveled off in the remaining cycles.The sharp drop in Ft appeared to be induced mainly by the reduction of adhesion-related interfacial force between the Si-SiO_(x)/SiO_(2)pair.During the running-in process,the contact area of the Si-SiO_(x)/SiO_(2)pair might become hydrophobic due to removal of the hydrophilic oxide layer on the silicon surface and the surface change of the SiO_(2)tip,which caused the reduction of friction force and the wear rate of the Si-SiO_(x)/SiO_(2)pair.A phenomenological model is proposed to explain the running-in process of the Si-SiO_(x)/SiO_(2)pair in ambient air.The results may help us understand the mechanism of the running-in process of the Si-SiO_(x)/SiO_(2)pair at nanoscale and reduce wear failure in dynamic microelectromechanical systems(MEMS).
文摘Thermally grown surface oxide layers dominate the single-asperity tribological behavior of a Zr60Cu30Al10 glass.Increase in oxidation time leads to an increased contribution of shearing and a corresponding decreased contribution of ploughing to friction.This change in the dominating friction and wear mechanism results in an overall minor decrease of the friction coefficient of oxidized surfaces compared to the metallic glass sample with native surface oxide.Our results demonstrate the importance of creating a stable oxide layer for practical applications of metallic glasses in micro-devices involving sliding contact.
基金This work was supported by the National Natural Science Foundation of China (Grant No.50305029).
文摘Friction force measurements between smooth surfaces across two layers of linear alkanes over five decades of speeds are presented. A maximum friction dissipation is observed at a characteristic speed. The behaviour is described by a new approach: the formation and destruction of molecular bridges between confined alkane layers. The bridges interdigitated between the layers exhibit a thermally activated resistance to shear.An analytical model involving activation barriers accounts for the overall behaviour of the forces over four decades of speed. This first simple semi-quantitative description sheds new light on the subtle mechanisms of friction at the nanoscale level and shows how the molecular length influences the tribological properties of the liquid.
