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
Diamond-like carbon(DLC)and graphite-like carbon(GLC)coatings have good prospects for improving the surface properties of engine parts.However,further understanding is needed on the effect of working conditions on tri...Diamond-like carbon(DLC)and graphite-like carbon(GLC)coatings have good prospects for improving the surface properties of engine parts.However,further understanding is needed on the effect of working conditions on tribological behaviors.In this study,GLC and two types of DLC coatings were deposited on GCr15 substrate for investigation.The friction and wear properties of self-mated and steel-mated pairs were evaluated.Two temperatures(25 and 90℃),three lubrication conditions(base oil,molybdenum dithiocarbamate(MoDTC)-containing oil,MoDTC+zinc dialkyldithiophosphate(ZDDP)-containing oil),and high Hertz contact stress(2.41 GPa)were applied in the experiments.The results showed that high temperature promoted the effect of ZDDP on steel-mated pairs,but increased wear under base oil lubrication.The increased wear for steel-mated pairs lubricated by MoDTC-containing oil was due to abrasive wear probably caused by MoO_(3) andβ-FeMoO_(4).It was also found that in most cases,the tribological properties of self-mated pairs were better than those of steel-mated pairs.展开更多
New multilayer coatings were produced by incorporating alternating soft and hard DLC layers enabled by varying the bias voltage during deposition process while maintaining a constant hard-to-soft layer thickness ratio...New multilayer coatings were produced by incorporating alternating soft and hard DLC layers enabled by varying the bias voltage during deposition process while maintaining a constant hard-to-soft layer thickness ratio.These coatings were deposited onto a Cr/Cr Cxgraded layer by closed field unbalanced magnetron sputtering(CFUBMS).The cross-sectional analysis of the coatings showed that the multilayer coatings possess sharp interfaces between the soft and hard layers with the hard to soft layer thickness ratio(1:1.33)constant in all the coatings.Raman analysis uncovered the increasing sp^(3)character of the DLC coatings as a result of decreasing ID/IGratio and increasing full width at half maximum(FWHM)values of the G band peak induced supposedly by an increase in bias voltage during hard layer deposition.Nanoindentation tests showed an increase in hardness of the DLC coatings which can be correlated with the increase in the sp^(3)content of the coatings as well as decreasing sp^(2)-C cluster size,as calculated from the ID/IGratio.Furthermore,the coatings exhibited excellent plastic deformation resistance and adhesion strength upon microindentation and scratch testing,respectively.Although further investigations are required to assess coating durability,the multilayer design could offer the DLC coatings with a rare opportunity to combine the high hardness with damage resistance with a constant bilayer thickness and without the need to introduce complex multilayer system.展开更多
A diamond-like carbon(DLC) film was deposited on YT14 substrate using magnetron sputtering(MS). The surface morphologies, roughness and bonding spectra of obtained film were characterized using scanning electron m...A diamond-like carbon(DLC) film was deposited on YT14 substrate using magnetron sputtering(MS). The surface morphologies, roughness and bonding spectra of obtained film were characterized using scanning electron microscopy(SEM), atomic force microscopy(AFM), and X-ray photoelectron spectroscopy(XPS), respectively, and its mechanical property and bonding strength were measured using a nanoindentation and scratch tester, respectively. The results show that the C-enriched DLC film exhibits a denser microstructure and smoother surface with lower surface roughness of 21.8 nm. The ratio of C sp2 at 284.4 e V that corresponds to the diamond(111) and the C sp3 at 285.3 e V that corresponds to the diamond(220) plane for the as-received film is 0.36: 0.64, showing that the C sp3 has the high content. The hardness and Young's modulus of DLC film by nanoindentation are 8.534 41 and 142.158 1 GPa, respectively, and the corresponding bonding strength is 74.55 N by scratch test.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
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
A one-step method was developed to create a highly biocompatible micropatterned surface on a diamond-like carbon(DLC)through irradiation with a nitrogen ion beam and thus enhance the biocompatibility of osseointegrate...A one-step method was developed to create a highly biocompatible micropatterned surface on a diamond-like carbon(DLC)through irradiation with a nitrogen ion beam and thus enhance the biocompatibility of osseointegrated surfaces and biotribological performance of articular surfaces.The biocompatibility and biotribological mechanisms were analyzed in terms of the structure and morphology of DLC.It was demonstrated that a layer enriched in sp^(3) C-N bonds was formed on the surface of the DLC after nitrogen ion beam irradiation.Moreover,with an increase in the radiation dose,the content of sp^(3) C-N on the DLC surface increased significantly,and the biocompatibility was positively correlated with it.The adhesion of the MC3T3 osteoblasts increased significantly from 32%to 86%under an irradiation dose of 8×10^(15) ions/cm^(2).In contrast,the micropattern had a significant negative effect on the adhesion of the osteoblasts as it physically hindered cell expansion and extension.The micropattern with a depth of 37 nm exhibited good friction properties,and the coefficient of friction was reduced by 21%at relatively high speeds.展开更多
To meet the surging needs in energy efficiency and eco-friendly lubricants,a novel superlubricious technology using a vegetable oil and ceramic materials is proposed.By coupling different hydrogen-free amorphous carbo...To meet the surging needs in energy efficiency and eco-friendly lubricants,a novel superlubricious technology using a vegetable oil and ceramic materials is proposed.By coupling different hydrogen-free amorphous carbon coatings with varying fraction of sp^(2) and sp^(3) hybridized carbon in presence of a commercially available silicon nitride bulk ceramic,castor oil provides superlubricity although the liquid vegetable oil film in the contact is only a few nanometres thick at most.Besides a partial liquid film possibly separating surfaces in contact,local tribochemical reactions between asperities are essential to maintain superlubricity at low speeds.High local pressure activates chemical degradation of castor oil generating graphitic/graphenic-like species on top of asperities,thus helping both the chemical polishing of surface and its chemical passivation by H and OH species.Particularly,the formation of the formation of–(CH_(2)–CH_(2))n–noligomers have been evidenced to have a major role in the friction reduction.Computer simulation unveils that formation of chemical degradation products of castor oil on friction surfaces are favoured by the quantity of sp^(2)-hybridized carbon atoms in the amorphous carbon structure.Hence,tuning sp^(2)-carbon content in hydrogen-free amorphous carbon,in particular,on the top layers of the coating,provides an alternative way to control superlubricity achieved with castor oil and other selected green lubricants.展开更多
Molybdenum dialkyldithiocarbamate(MoDTC)is widely used as a friction modifier in engine lubricating oil.Under MoDTC lubrication,the friction and wear behaviors of tungsten-doped diamond-like carbon(W-DLC)films anneale...Molybdenum dialkyldithiocarbamate(MoDTC)is widely used as a friction modifier in engine lubricating oil.Under MoDTC lubrication,the friction and wear behaviors of tungsten-doped diamond-like carbon(W-DLC)films annealed at 100-400℃were discussed and evaluated using scanning electron microscopy(SEM),atomic force microscopy(AFM),and Raman spectroscopy.Under(polymerized alpha olefin)PAO+MoDTC lubrication,the coefficient of friction of all samples decreased,but the wear rates of the W-DLC films annealed at 300℃increased significantly.By interacting with zinc dialkyldithiophosphate(ZDDP),the wear rates of W-DLC films annealed at different temperatures declined significantly owing to the formation of dense phosphate tribofilms on the worn surfaces.展开更多
The effects of surface adsorption of bovine serum albumin(BSA) and human gamma-globulin(HGG) on the tribological performance of a DLC film were investigated using a quartz crystal microbalance with dissipation(QCM-D),...The effects of surface adsorption of bovine serum albumin(BSA) and human gamma-globulin(HGG) on the tribological performance of a DLC film were investigated using a quartz crystal microbalance with dissipation(QCM-D), a ball-on-disk reciprocating tribometer, and a three-electrode electrochemical cell. The results showed that the wear depth in the BSA solution was higher than that in the HGG solution. In the HGG solution, the HGG-adsorbed layer could act as a lubricating layer and protect the DLC film from wear. The wear volume of DLC film in BSA and HGG mixture solution was higher than that in single HGG solution. This may be because the BSA molecules inhibit the formation of HGG adsorbed layer during sliding.展开更多
High-performance diamond films are highly demanded on tool surfaces for wire-drawing and mechanical sealing applications.Herein,this work aims at enhancing the tribological performance of chemical vapor deposition dia...High-performance diamond films are highly demanded on tool surfaces for wire-drawing and mechanical sealing applications.Herein,this work aims at enhancing the tribological performance of chemical vapor deposition diamond films in water-lubricated conditions by utilizing non-hydrogenated and hydrogenated diamond-like carbon(DLC and DLC-H)top layers.The tribological properties of bilayer micro-crystalline diamond(MCD)/DLC,MCD/DLC-H,nano-crystalline diamond(NCD)/DLC and NCD/DLC-H films are evaluated,in terms of maximal and stable coefficients of friction(COFs),C—C bonds transformation,worn surface morphology and specific wear rates.The results show that DLC or DLC-H coated on diamond layer significantly suppresses the initial maximal COF peak and the wear of counterpart ball.Moreover,severe regular arranged sp^(2) C—C bonds transformation is detected on MCD film,in comparison to NCD;while inversely,the NCD/DLC bilayer exhibits severer C—C bonds transformation effect compared with the MCD/DLC.Furthermore,the DLC-H top layer shows a larger decreasing rate of maximal COFs and wear rates of counterpart balls,compared with the DLC coating,which is due to its superior self-lubricity.Among all the tested films,the NCD/DLC-H bilayer shows an optimized tribological performance.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This work was supported by the Beijing Municipal Natural Science Foundation(3182032)the National Natural Science Foundation of China(41772389)+1 种基金the Pre-Research Program in National 13th Five-Year Plan(61409230603)Joint Fund of Ministry of Education for Pre-research of Equipment for Young Personnel Project(6141A02033120).
文摘Diamond-like carbon(DLC)and graphite-like carbon(GLC)coatings have good prospects for improving the surface properties of engine parts.However,further understanding is needed on the effect of working conditions on tribological behaviors.In this study,GLC and two types of DLC coatings were deposited on GCr15 substrate for investigation.The friction and wear properties of self-mated and steel-mated pairs were evaluated.Two temperatures(25 and 90℃),three lubrication conditions(base oil,molybdenum dithiocarbamate(MoDTC)-containing oil,MoDTC+zinc dialkyldithiophosphate(ZDDP)-containing oil),and high Hertz contact stress(2.41 GPa)were applied in the experiments.The results showed that high temperature promoted the effect of ZDDP on steel-mated pairs,but increased wear under base oil lubrication.The increased wear for steel-mated pairs lubricated by MoDTC-containing oil was due to abrasive wear probably caused by MoO_(3) andβ-FeMoO_(4).It was also found that in most cases,the tribological properties of self-mated pairs were better than those of steel-mated pairs.
基金support from the Australian Government Research Training Program Scholarship。
文摘New multilayer coatings were produced by incorporating alternating soft and hard DLC layers enabled by varying the bias voltage during deposition process while maintaining a constant hard-to-soft layer thickness ratio.These coatings were deposited onto a Cr/Cr Cxgraded layer by closed field unbalanced magnetron sputtering(CFUBMS).The cross-sectional analysis of the coatings showed that the multilayer coatings possess sharp interfaces between the soft and hard layers with the hard to soft layer thickness ratio(1:1.33)constant in all the coatings.Raman analysis uncovered the increasing sp^(3)character of the DLC coatings as a result of decreasing ID/IGratio and increasing full width at half maximum(FWHM)values of the G band peak induced supposedly by an increase in bias voltage during hard layer deposition.Nanoindentation tests showed an increase in hardness of the DLC coatings which can be correlated with the increase in the sp^(3)content of the coatings as well as decreasing sp^(2)-C cluster size,as calculated from the ID/IGratio.Furthermore,the coatings exhibited excellent plastic deformation resistance and adhesion strength upon microindentation and scratch testing,respectively.Although further investigations are required to assess coating durability,the multilayer design could offer the DLC coatings with a rare opportunity to combine the high hardness with damage resistance with a constant bilayer thickness and without the need to introduce complex multilayer system.
基金Funded by the Jiangsu Province Science and Technology Support Program(Industry)(No.BE2014818)
文摘A diamond-like carbon(DLC) film was deposited on YT14 substrate using magnetron sputtering(MS). The surface morphologies, roughness and bonding spectra of obtained film were characterized using scanning electron microscopy(SEM), atomic force microscopy(AFM), and X-ray photoelectron spectroscopy(XPS), respectively, and its mechanical property and bonding strength were measured using a nanoindentation and scratch tester, respectively. The results show that the C-enriched DLC film exhibits a denser microstructure and smoother surface with lower surface roughness of 21.8 nm. The ratio of C sp2 at 284.4 e V that corresponds to the diamond(111) and the C sp3 at 285.3 e V that corresponds to the diamond(220) plane for the as-received film is 0.36: 0.64, showing that the C sp3 has the high content. The hardness and Young's modulus of DLC film by nanoindentation are 8.534 41 and 142.158 1 GPa, respectively, and the corresponding bonding strength is 74.55 N by scratch test.
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 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This study was supported by National Research Foundation of Korea(NRF)grant funded by the Korean government(MSIT)(No.2020R1A2C2004714)funding from the State Key Laboratory of Solidification Processing in NWPU(No.SKLSP202002).
文摘A one-step method was developed to create a highly biocompatible micropatterned surface on a diamond-like carbon(DLC)through irradiation with a nitrogen ion beam and thus enhance the biocompatibility of osseointegrated surfaces and biotribological performance of articular surfaces.The biocompatibility and biotribological mechanisms were analyzed in terms of the structure and morphology of DLC.It was demonstrated that a layer enriched in sp^(3) C-N bonds was formed on the surface of the DLC after nitrogen ion beam irradiation.Moreover,with an increase in the radiation dose,the content of sp^(3) C-N on the DLC surface increased significantly,and the biocompatibility was positively correlated with it.The adhesion of the MC3T3 osteoblasts increased significantly from 32%to 86%under an irradiation dose of 8×10^(15) ions/cm^(2).In contrast,the micropattern had a significant negative effect on the adhesion of the osteoblasts as it physically hindered cell expansion and extension.The micropattern with a depth of 37 nm exhibited good friction properties,and the coefficient of friction was reduced by 21%at relatively high speeds.
基金This research is supported by TOTAL,Solaize Research Center and Federal Ministry of Economic Affairs and Energy Germany(BMWi)within project CHEOPS3(Funding number 03ET1286B).
文摘To meet the surging needs in energy efficiency and eco-friendly lubricants,a novel superlubricious technology using a vegetable oil and ceramic materials is proposed.By coupling different hydrogen-free amorphous carbon coatings with varying fraction of sp^(2) and sp^(3) hybridized carbon in presence of a commercially available silicon nitride bulk ceramic,castor oil provides superlubricity although the liquid vegetable oil film in the contact is only a few nanometres thick at most.Besides a partial liquid film possibly separating surfaces in contact,local tribochemical reactions between asperities are essential to maintain superlubricity at low speeds.High local pressure activates chemical degradation of castor oil generating graphitic/graphenic-like species on top of asperities,thus helping both the chemical polishing of surface and its chemical passivation by H and OH species.Particularly,the formation of the formation of–(CH_(2)–CH_(2))n–noligomers have been evidenced to have a major role in the friction reduction.Computer simulation unveils that formation of chemical degradation products of castor oil on friction surfaces are favoured by the quantity of sp^(2)-hybridized carbon atoms in the amorphous carbon structure.Hence,tuning sp^(2)-carbon content in hydrogen-free amorphous carbon,in particular,on the top layers of the coating,provides an alternative way to control superlubricity achieved with castor oil and other selected green lubricants.
基金This research was financially supported by the National Natural Science Foundation of China(Nos.51875537 and 41872183)the Pre-Research Program in National 14th Five-Year Plan(No.61409230614)the Fundamental Research Funds for the Central University(No.2652018094).
文摘Molybdenum dialkyldithiocarbamate(MoDTC)is widely used as a friction modifier in engine lubricating oil.Under MoDTC lubrication,the friction and wear behaviors of tungsten-doped diamond-like carbon(W-DLC)films annealed at 100-400℃were discussed and evaluated using scanning electron microscopy(SEM),atomic force microscopy(AFM),and Raman spectroscopy.Under(polymerized alpha olefin)PAO+MoDTC lubrication,the coefficient of friction of all samples decreased,but the wear rates of the W-DLC films annealed at 300℃increased significantly.By interacting with zinc dialkyldithiophosphate(ZDDP),the wear rates of W-DLC films annealed at different temperatures declined significantly owing to the formation of dense phosphate tribofilms on the worn surfaces.
基金Funded by the National Natural Science Foundation of China(No.31570958)Science and Technology Support Program of Sichuan Province(No.2016SZ0007)
文摘The effects of surface adsorption of bovine serum albumin(BSA) and human gamma-globulin(HGG) on the tribological performance of a DLC film were investigated using a quartz crystal microbalance with dissipation(QCM-D), a ball-on-disk reciprocating tribometer, and a three-electrode electrochemical cell. The results showed that the wear depth in the BSA solution was higher than that in the HGG solution. In the HGG solution, the HGG-adsorbed layer could act as a lubricating layer and protect the DLC film from wear. The wear volume of DLC film in BSA and HGG mixture solution was higher than that in single HGG solution. This may be because the BSA molecules inhibit the formation of HGG adsorbed layer during sliding.
基金the National Natural Science Foundation of China(No.51705155)the Tribology Science Fund of State Key Laboratory of Tribology(No.SKLTKF17B09)。
文摘High-performance diamond films are highly demanded on tool surfaces for wire-drawing and mechanical sealing applications.Herein,this work aims at enhancing the tribological performance of chemical vapor deposition diamond films in water-lubricated conditions by utilizing non-hydrogenated and hydrogenated diamond-like carbon(DLC and DLC-H)top layers.The tribological properties of bilayer micro-crystalline diamond(MCD)/DLC,MCD/DLC-H,nano-crystalline diamond(NCD)/DLC and NCD/DLC-H films are evaluated,in terms of maximal and stable coefficients of friction(COFs),C—C bonds transformation,worn surface morphology and specific wear rates.The results show that DLC or DLC-H coated on diamond layer significantly suppresses the initial maximal COF peak and the wear of counterpart ball.Moreover,severe regular arranged sp^(2) C—C bonds transformation is detected on MCD film,in comparison to NCD;while inversely,the NCD/DLC bilayer exhibits severer C—C bonds transformation effect compared with the MCD/DLC.Furthermore,the DLC-H top layer shows a larger decreasing rate of maximal COFs and wear rates of counterpart balls,compared with the DLC coating,which is due to its superior self-lubricity.Among all the tested films,the NCD/DLC-H bilayer shows an optimized tribological performance.