Yfi ZHOU ,Zhihao CHEN ,Zhonghui HU ,Li LI ,Qingxiang YANG ,Xiaoli XING,*
a College of Mechanical Engineering,Yanshan University,Qinhuangdao 066004,China
b State Key Laboratory of Metastable Materials Science &Technology,Yanshan University,Qinhuangdao 066004,China
c Aviation Key Laboratory of Science and Technology on Generic Technology of Self-Lubricating Spherical Plain Bearing,Yanshan University,Qinhuangdao 066004,China
d AVIC the First Aircraft Institute,Xi’an 710089,China
e Northwest Institute for Nonferrous Metal Research,Xi’an 710016,China
KEYWORDS 60NiTi;Diamond-Like Carbon(DLC) coating;Graphitization;Transfer film;Tribology
Abstract It is imperative to develop a novel matching of metallic substrate and self-lubricating coating for aircraft spherical plain bearing in a wide range of service conditions.As a new type of superelastic material,60NiTi alloy meets the performance requirements of aerospace bearing materials,but exhibits poor tribological performance,especially under the conditions of dry sliding friction.A Hydrogenated Diamond-Like Carbon (H-DLC) coating was deposited on the 60NiTi alloy to improve its tribological performance.The microstructure and mechanical behavior of the 60NiTi alloy and its H-DLC coating were explored.Results show that improvement of friction and wear performance of the H-DLC coating deposited on the 60NiTi substrate is mainly achieved by graphitization at the friction interface and the transfer film produced on the counterpart ball.The increased friction load leads to intensification of graphitization at the friction interface and formation of continuous and compact transfer film on the surface of the counterpart ball.
In the aerospace field,an effective way to increase energy efficiency and reduce energy consumption is reducing the mass of aerospace equipment.1Therefore,research and development on basic parts in the aerospace field is in constant pursuit of lightweight.Compared with traditional bearing materials,60NiTi alloy,which exhibits lower density (6.7 g/cm3),higher hardness(about 60 HRC),higher compressive strength(about 2.5 GPa) and lower elastic modulus (about 100 GPa),2–4has increasingly revealed as a suitable lightweight candidate material for aerospace spherical plain bearings and similar moving parts,such as shaft bearings,connecting spline and aerohydraulic rods.5,6However,the significantly inferior sliding wear resistance of 60NiTi alloy under oil starvation conditions limits its potential to be employed in some severe working applications.5–8To improve the tribological performance of the alloy,spraying or depositing a solid coating on the alloy is an effective way,which has been widely used in the aerospace industry for light leakage prevention,corrosion protection,environmental barrier and water resistance.9–11
The effectiveness of Polydopamine (PDA)/Polytetrafluoroethylene (PTFE) coatings on 60NiTi alloy has been investigated by Miller et al.,12and their results showed that the Coefficient of Friction (COF) between 60NiTi alloy and Si3N4can be reduced over 85% by 1.5 μm thick coating.Furthermore,Choudhury et al.13added 0.25wt% of graphite particle into the PDA/PTFE coating,and their results showed that the adhesion strength of the coating and 60NiTi alloy can be significantly improved.Although the COF of the 60NiTi alloy can be effectively reduced by the polymer-based coatings,the polymer phase can be easily rubbed off mechanically.Therefore,the abrasive resistance of the PDA/PTFE coating is still not satisfactory.
One effective way for this issue is to deposit 60NiTi alloy with Diamond-Like Carbon (DLC) coating,which has high hardness,excellent chemical inertness,and excellent tribological performances with the structures of three-dimensional networks of sp3hybrid carbon atoms and sp2hybrid carbon atoms.14–17However,the mechanical properties and tribological performance of the DLC coating deposited on 60NiTi alloy are still unclear.
The Hydrogenated Diamond-Like Carbon (H-DLC) coating has received wide attention due to its low COF and good wear resistant in vacuum and atmospheric environment.18,19Sharma et al.20found that the COF value of DLC coating can be maintained at a very low level under both unlubricated and lubricated conditions.Erdemir et al.21conducted friction and wear tests with steel ball steel disc and sapphire ball sapphire disc as counterpart balls in the same test environment,and found that the lower COF is related with the higher proportion of hydrogen in the reaction gas source,regardless of the matrix materials.Lugo et al.22indicated that the COF of H-DLC coating in vacuum decreases with the increase of hydrogen content,which is due to the fact that hydrogen atoms in H-DLC coatings can passivate the unbound σbonds on the contact surface of DLC coatings and avoid the formation of covalent bonds between friction interfaces,thus effectively reducing friction.23–25
The adhesion,mechanical properties and tribological performance of H-DLC coating can also be influenced by the hardness,elasticity and plasticity of the substrate.16,26,27Huang et al.28deposited Ti-DLC/α-C:H multilayer coatings on cemented carbide tools,and obtained that life of the tools can be increased by 1.7 times by Ti-DLC/α-C:H multilayer coatings and the COF of the tools can be reduced to 0.075.Dalibo´n et al.29deposited DLC coating on AISI 316L stainless steel,which significantly improved the friction and wear properties of the steel.Wang and Liu30deposited DLC coating on NiTi alloy,and the COF of the alloy can be reduced to 0.132.Amanov et al.31found that the friction and fretting wear behavior of the Ti-6Al-4V alloy can be improved by DLC coatings.However,the friction and wear properties of the H-DLC coating deposited on the superelastic 60NiTi alloy is seldomly investigated,especially the tribological evolutions under various loads and frequencies histories configurations.
In the present work,the structure,mechanical properties and tribological performance of the 60NiTi alloy were investigated.To improve the tribological performance of the 60NiTi alloy,a H-DLC coating was prepared on the 60NiTi substrate by an ion beam assisted enhanced unbalanced magnetron sputtering equipment,and then the structural evolution of the friction interface between the H-DLC coating and the counterpart balls was systematically studied under different loads and frequencies.The effect of the 60NiTi substrate on the tribological performance of H-DLC coating was furtherly discussed.
2.1.Coating deposition
The H-DLC coating was deposited on the 60NiTi substrate by the ion beam assisted enhanced unbalanced magnetron sputtering equipment of Diamant-VII-340.The 60NiTi substrate was immersed in anhydrous ethanol for ultrasonic cleaning for 30 min to remove oil stains and impurities on the surface.The cleaned 60NiTi substrate was dried with nitrogen and then put into the magnetron sputtering chamber.The chamber temperature and base pressure were maintained at 150 °C and 3×10-3Pa prior to deposition.After that,under the working pressure of (1.0 ± 0.3) Pa,high-purity argon gas with a flow rate of 30 sccm (sccm means standard cubic centimeter per minute) was injected into the magnetron sputtering chamber,and the substrate surface was cleaned to remove contaminants with a bias voltage of–600 V.The Ti/TiC transition layer was primarily deposited with the working temperature and pressure of (240 ± 5) °C and (1.0 ± 0.2) Pa,respectively.During the deposition,a current of 2 A was firstly supplied to the Ti target with the Ar gas flow rate of 30 sccm for 60 min.Then,the Ti target current and the Ar gas flow rate remained unchanged,and 2 A current was supplied to the C target for 60 min.The H-DLC coating was deposited at a working pressure of (1.0 ± 0.2) Pa and a stable gas flow rate (Ar/C2H2=30 sccm/50 sccm)with the C target current of 3 A for 180 min.
2.2.Coating and substrate characterization
The phase composition and microstructure of the 60NiTi substrate were characterized by the D/max-2500/PC X-Ray Diffraction (XRD) and Hitachi S3400N Scanning Electron Microscopy (SEM) equipped with an energy dispersive X-ray spectrometer (EDS).Before analysis,the 60NiTi substrate was ground,mechanically polished and etched (the etched reagent was 1vol% hydrofluoric acid and 10vol% concentrated nitric acid).
The coating thickness was measured by Calotest produced by Anton Paar.A rotating ball with a radius of 12.5 mm(the rotating ball speed is 1200 r/min) was used to grind a crater on the coating surface,the crater morphology was observed with an optical microscope,and the coating thickness was calculated by
whereHCis the coating thickness;Ris the radius of the rotating ball;Dis the substrate diameter;dis the attrition diameter.
The adhesion between the coating and the substrate was tested by the CSM scratch tester.The diamond cone head with a radius of 200 μm and a cone angle of 120° was scratched 3 mm on the coating surface at a loading rate of 0.5 N/s and a maximum loading force of 80 N.After that,the morphology of the scratches was observed through an optical microscope to determine the adhesion between the coating and the substrate.
The mechanical properties of the 60NiTi substrate and H-DLC coating were tested using a nanoindentation tester equipped (NHT2) with a Berkovich indenter.The mechanical properties of 60NiTi substrate and H-DLC coating were determined by matrix mode under a load of 2 mN.The influence of 60NiTi substrate on the mechanical properties of H-DLC coating was tested by controlling the load(2,10,20,30,40,50,70,100,150 mN).The measurements were repeated six times.
Before the tribological test,the impurities on the surface of the 60NiTi substrate,H-DLC coating and counterpart balls were removed by an ultrasonic cleaning machine,and then dried with dry nitrogen.Counterpart balls were commercially available WC-Co balls,which possess a diameter of 3.0 mm and a roughness of about 25 nm.The CSM TRB ball-disk friction and wear tester with the friction load of 5 N and the friction frequency of 5.0 Hz (reciprocating distance is 4 mm) was used to characterize the tribological behaviors of the 60NiTi substrate at a temperature of 25 °C and a relative humidity of (25 ± 3)%.
Additionally,an orthogonal test for H-DLC coating was conducted.The friction loads were 2,5,10 N.The friction frequencies were 2.5,5.0,10.0 Hz.Other conditions remained unchanged in the orthogonal test.After that,the threedimensional morphology of each wear scar was observed by the white light interferometer,and the wear rate of the samples were calculated according to
whereVis the total loss volume of the coating after friction,m3;Fis the positive pressure of friction,N;Sis the total distance of the whole friction process,m.
The microstructure and composition phase of a H-DLC coating with a thickness of 50 nm were detected by the Talos F200X Transmission Electron Microscopy (TEM).The TEM sample was prepared by the following steps.Firstly,a 5 mm×5 mm×0.6 mm silicon wafer was placed in the chamber to deposit the H-DLC coating.Secondly,after deposition,the side of the H-DLC/silicon wafer sample without H-DLC coating was ground with sandpaper until the thickness of H-DLC/silicon wafer sample was less than 20 μm.Thirdly,the side of the H-DLC/silicon wafer sample without H-DLC coating was ion milled by the ion milling instrument to prepare the final sample for TEM observation.Meanwhile,an optical microscope was used to determine the wear spot position on counterpart balls.The Raman spectrometer (XploRA PLUS)with 532 nm optical maser wavelength was used to analyze the surface composition of the wear spot,and the microstructure characteristics of the H-DLC coating surface and the wear scars.
3.1.Composition and microstructure characterizations
Fig.1 shows the phase constitution of the 60NiTi substrate characterized by XRD.As expected,the NiTi matrix phase and Ni4Ti3phase with a rhombohedral structure were found in the 60NiTi substrate.
Fig.1 XRD diffraction pattern of 60NiTi substrate.
The surface microstructural characteristics of the 60NiTi substrate is shown in Fig.2.The elongated needle-like precipitating phase and gray-black precipitating phase were observed by the SEM in the processed 60NiTi substrate.To better identify the microstructure and different phases,EDS element analysis was also performed on the selected area of the 60NiTi substrate.The EDS analysis results are shown in Table 1.According to the EDS results,it can be determined that Area-1 is the NiTi matrix phase,the elongated needle-like precipitating phase of Area-2 is Ni3Ti phase,and the gray-black precipitating phase of Area-3 is the Ni2Ti3phase.According to previous reports,the nucleation time of Ni4Ti3precipitates is on the order of milliseconds.32Therefore,the Ni4Ti3phase in the NiTi matrix phase cannot be avoided even through rapid quenching.Fortunately,the hardness of 60NiTi alloy can be improved by the metastable Ni4Ti3phases.33The arithmetical mean deviation of the profile (Ra) of 60NiTi substrate before magnetron sputtering was 0.38 μm as detected by the white light interferometer.
Fig.2 Microstructural characteristics of 60NiTi substrate.
Table 1 EDS analysis results of 60NiTi substrate.
The Raman spectrum is a commonly used non-destructive method for characterizing the structure of DLC coating.As shown in Fig.3(a),the wide peak in the range of 800–2000 cm-1in the Raman spectrum is fitted by the Gaussian function to get the D peak near 1300–1380 cm-1and the G peak near 1520–1580 cm-1(the G peak is generated by the stretching motion of all sp2atom pairs in the carbon ring or long chain,and the D peak is generated by the breathing vibration mode of the sp2atom in the carbon ring),showing typical characteristics of the H-DLC coating.The integrated intensity ratio of the D peak and G peak (ID/IG),G peak position and full width at half maximum (GFWHM) obtained by analyzing the Raman spectrum can be used to characterize the microstructure of the H-DLC coating.C2H2is introduced during the deposition of the H-DLC coating,and H in the DLC coating mainly passes CHxgroups and freeway exist.34,35
Fig.3 Microstructure of H-DLC coating.
The hydrogen content is an important influence factor on the tribological performance of H-DLC coating.As we all know,the commonly used method to estimate the hydrogen content in the H-DLC coating is to calculate the spectral background ratio and G peak intensity of the Raman spectrum.However,this method is only applicable to the H-DLC coating with a hydrogen content of more than 20%.35,36The Raman spectrum’s background slope of the H-DLC coating was calculated,and the result shows that the value of the slope is negative,which does not meet the positive background slope value requirement in the hydrogen content formula described in Refs.35,36.Therefore,it can be inferred that the hydrogen content of the DLC coating investigated should be less than 20%.Fig.3(b)shows the TEM micrograph and Selected Area Electron Diffraction (SAED)of H-DLC coating.The H-DLC coating mainly presents in the form of typical amorphous carbon structure observed from SAED.Fig.A1 shows the coating after being processed,and the thickness of the coatings can be obtained by Eq.(1).
3.2.Mechanical behavior
The large load scratch tester is widely used to evaluate the adhesion between the coating and the substrate.In the scratch image,Lc1can be defined as the place where fine lines appear on the coating;Lc2can be defined as the point where the coating begins to peel off,and the load at this time is the first chipping load;Lc3can be defined as the point where the coating is completely peeled off,and the load at this time is the fully delamination load.The scratch image of the H-DLC coating deposited on 60NiTi substrate is shown in Fig.4.It can be seen that different degrees of delamination cracks appear on the surface of the coating due to the shearing effect with the increase of the loading force.As the loading force continues to increase,the H-DLC coating can be completely fallen off.In detail,the first chipping load (Lc2) of the H-DLC coating is 21.4 N,and its fully delamination load (Lc3) is 64.6 N.The scratch result indicates that the H-DLC coating deposited on the 60NiTi substrate has a better adhesion property in comparison with the TC4 substrate.37
Fig.4 Scratch morphologies of H-DLC coating.
The typical indentation load–displacement curves of the 60NiTi substrate and the H-DLC coating deposited on the 60NiTi substrate are shown in Fig.5.In Fig.5(a),the typical indentation load–displacement curve,hardness,elastic modulus,H/E(hardness/elastic modulus),andH3/E2of the 60NiTi substrate under an applied load of 2 mN can be obtained.As reported,hardness represents the material’s ability to resist pressure on its surface,elastic modulus represents the material’s ability to resist deformation,H/Eis related to the strain tolerance and fracture toughness of the material,H3/E2is related to the plastic deformation resistance of the material.38
It can be seen that the 60NiTi substrate has a good elastic deformation ability.When subjected to large load,elastic deformation can increase the contact area,thus increasing the bearing capacity,recovering the deformation after unloading,and reducing the plastic deformation.Meanwhile,a higherH/EandH3/E2represent that the 60NiTi substrate has a good fracture toughness and plastic deformation resistance.Fig.5(b) is the typical indentation load–displacement curve of the H-DLC coating under different loading forces.
Fig.5 Typical indentation load–displacement curves of 60NiTi and H-DLC coating.
Fig.6 show the changes of hardness,elastic modulus,H/EandH3/E2of H-DLC coating under different loading forces.When the loading force was 2 mN,the indentation depth of the coating did not exceed 1/10 of the thickness of the coating,and the measured hardness and elastic modulus data can be considered without the influence of the substrate.The H-DLC coating deposited on the 60NiTi substrate effectively improves the hardness,fracture toughness and plastic deformation resistance of the 60NiTi substrate.When the loading force reached 10 mN,the loading depth exceeded 1/10 of the coating thickness,and the result was affected by the substrate.It can be clearly seen that compared with that under the condition of the loading force of 2 mN,the elastic modulus significantly decreased,but the hardness did not change significantly.This trend of change remained unchanged as the loading force continued to increase,but the hardness decreased suddenly when the loading force exceeded 40 mN.This is because the elastically affected region is larger than the plastically affected region,causing significant change of the elastic modulus at a lower indentation depth.In contrast,hardness is affected by the plastic field,which is smaller than that of the elastic field.When the loading force reached 150 mN,the indentation depth exceeded 900 nm,and the measured hardness and elastic modulus of the H-DLC coating are close to the performance of the 60NiTi substrate.
Fig.6 Mechanical behaviors of H-DLC coating with increase loading force.
3.3.Tribological performance
Tribological performance is an important indicator to evaluate the performance of materials.39–42The friction test of the 60NiTi substrate was carried out by 20000 friction cycles with the friction load of 5 N and friction frequency of 5.0 Hz,and the COF curve and wear profile of 60NiTi substrate are shown in Fig.7.During the friction test,the COF curve of the 60NiTi substrate exhibited a large fluctuation range,which is because a stable oxide layer cannot be formed and maintained at the friction interface of the the 60NiTi substrate under the test conditions.Moreover,it can be seen that the COF of the 60NiTi substrate is relatively high,and the COF value was 0.809 ± 0.047 at the later stage of the friction test.
Fig.7 COF curve and wear profile of 60NiTi substrate.
In addition,it can be seen from the wear profile that the wear rate of the 60NiTi substrate,which can be calculated by Eq.(2),is (206.2 ± 15.3) × 10-6mm3· N-1· m-1.The comparatively high wear rate is mainly because of the brittle fracture type damage of the alloy,which in turn leads to the formation,propagation and spalling of fatigue cracks under repeated shear stress.7The friction and wear performance of the H-DLC coating deposited on the 60NiTi substrate was tested,and the effects of friction loads and frequencies on the friction performance were studied.Fig.8 shows the COF curves of H-DLC coating with different loads and frequencies,whereLfandfare friction load and frequecny.It can be seen that the COF curves have the same changing trend under different test conditions.After a short running-in period,the COF of the H-DLC coating under each condition is basically stable and does not fluctuate greatly.
Fig.8 COF of H-DLC coating with different friction loads and frequencies.
The image and 3D morphology of the wear scars under different conditions are shown in Fig.9.As the friction loads and frequencies increase,the width and depth of the wear scars begin to enlarge.It can be seen from the COF curve and the wear profile that the friction and wear performance of the 60NiTi substrate can be effectively improved by the H-DLC coating.Figs.10(a)and(b)show the changes in COF and wear rate under different test conditions,respectively.When the friction load is kept constant,the wear rate of the H-DLC coating increases with the increase of frequency.Actually,the increased friction frequency causes the increased wear rate due to the transportation and reaction rate of the wear debris particles at the wear scars,which was also reported in the previous work.43Meanwhile,the change of COF with friction load has no obvious regularity.When the friction load was 2 N,the COF was consistent with the change in friction frequency;when the friction load was 5 N,the COF firstly decreases and then increases;when the friction load was 10 N,the COF changed unobviously in the frequency range from 2.5 Hz to 5.0 Hz,and decreased significantly when the friction frequency reached 10.0 Hz.When the friction frequency was kept constant,the COF and wear rate simultaneously decreased with the increase of friction load.
Fig.9 Morphologies and 3D profiles of wear scars of H-DLC coating with different friction loads and frequencies.
Fig.10 Variation of COF and wear rate of H-DLC coating with different friction loads and frequencies.
The hydrogen carbon coating has super-low friction COF in vacuum because the friction interface is hydrogen passivated surface.However,in the air the COF will eventually reach a stable value as the pressure increases because gas adsorption destroys the interface.In the air,the adsorption and desorption of gas at the friction interface will also affect the tribological performance of H-DLC coating.According to previous reports,44–46an increase in friction speed leads to a shortening of the contact time between the DLC coating surface and the air,resulting in a decrease in the adsorption time of active gas molecules with the friction surface and a decrease in the amount of adsorption.Therefore,when counterpart balls pass the same point again,the energy required for adsorption and desorption decreases,and therefore the COF of H-DLC coating decreases.In addition,the degree of graphitization at the friction interface and the formation of a transfer film on the surface of counterpart balls also have an impact on the friction wear performance of the H-DLC coating.The above changes cannot be explained simply by graphitization,transfer film,gas adsorption and desorption or the passivation of dangling bonds on the H-DLC coating surface by hydrogen atoms.In order to further clarify the tribological mechanism of the H-DLC coating deposited on the 60NiTi substrate,wear scars and counterpart balls were analyzed by the Raman spectrum.
Fig.11(a)shows the Raman spectra of the H-DLC coating for each wear scar,which shows obvious D peak and G peak in each curve.Through Gaussian fitting,ID/IG,G peak position andGFWHMof the surface and wear scars are obtained.As shown in Table 2,theID/IGof most of the wear scars increased when compared with the surface of H-DLC coating without wear.Besides,the G peak position of all wear scars have different degrees of increase,indicating that graphitization of different degrees appeared at the friction interface.The change ofID/IGis widely used to measure the degree of graphitization of the DLC coating.
Table 2 Raman spectroscopy results of surface and wear scars.
As shown in Fig.11(b),theID/IGof H-DLC coating deposited on the 60NiTi substrate shows an obvious upward trend with the increase of friction load,which indicates that the graphitization degree of the friction interface aggravates with the increase of friction load.At the same time,the friction frequency has no obvious effect on graphitization under low load,but with the increase of friction load,the effect of friction frequency on graphitization grows obvious,that is,with the increase of friction load,the phenomenon of graphitization is more obvious.Graphitization transition is mainly caused by the heating during the friction process,and the increase of friction loads and frequencies can effectively introduce friction heating,which in turn accelerates graphitization transformation during the friction process.
Fig.11 Raman results of wear scars of H-DLC coating.
Figs.12 and 13 are the Raman spectra and optical images of the counterpart balls under different test conditions.In the Raman spectra of the counterpart balls surface under different test conditions,D peaks and G peaks around 1360 cm-1and 1560 cm-1can be observed,which indicates that the carbon transfer film is generated on the surface of the counterpart ball during the friction process,and the generation of the transfer film can reduce the COF of the coating.
Fig.12 Raman spectra of counterpart balls with different friction loads and frequencies.
It can be clearly seen from Fig.13 that with the increase of friction load,the generated area and density of the transfer film increased,but the evolution of the transfer film did not show a trend of monotonous change with the friction frequency.Under the condition of a friction load of 2 N,the area of the transfer film decreased as the frequency increased,and it can be clearly seen that the transfer film was denser at a lower friction frequency.Under the condition of a friction load of 5 N,the area of the transfer film firstly increased and then decreased.When the friction load reached 10 N,the change trend of the area of the transfer film was consistent with the friction frequency.It can be seen that the transfer film produced on the surface of the counterpart balls was relatively loose and easy to fall off under low friction load as the friction frequency increased.While under a relatively high friction load,a more uniform and dense transfer film can be formed on the surface of the counterpart balls and can exist stably under higher friction frequency.
Fig.13 Optical images of counterpart balls with different friction loads and frequencies.
The low COF and low wear rate of the H-DLC coating deposited on the 60NiTi substrate can be ascribed to the combination of gas adsorption and desorption at the friction interface,graphitization of the coating,and transfer film on the counterpart balls.Fig.14 shows the schematic diagram of the friction mechanism of the H-DLC coating deposited on 60NiTi substrate.When the friction frequency is constant,the increased friction load leads to an increase in graphitization at the wear scars surface,and a densely continuous transfer film can be formed on the surface of the counterpart balls.47–50Therefore,the COF of the H-DLC coating decreases with the increase of the friction load.Under low friction load conditions (2 N),graphitization on the wear scars was not obvious,and with the increased friction frequency,the loose and discontinuous transfer film can be formed on the surface of the counterpart balls,which in turn increases the COF of the H-DLC coating.When the friction load was increased to 5 N,the contact time between the friction interface and air during the friction process was shortened with the increased friction frequency,which in turn reduced the adsorption amount of active gas molecules.In this way,the COF of HDLC coating can be decreased due to low amount of gas adsorption together with graphitization and transfer film.However,when the friction frequency reached 10.0 Hz,the transfer film on the counterpart balls became discontinuous and loose,which in turn increased the COF of the H-DLC coating.Under the friction load of 10 N,the transfer film on the counterpart balls was stable at higher friction frequency,and the H-DLC coating exhibited low COF by the combined effect of the transfer film,graphitization and gas adsorption and desorption.The results also reveal that the most excellent tribological performance of the H-DLC coating can be achieved at high frequency and high load conditions due to the combination of higher graphitization and less gas adsorption.
Fig.14 Schematic diagram of friction mechanism of H-DLC coating deposited on 60NiTi substrate.
Meanwhile,it can be found that the degree of graphitization at the friction interface is approximately positively correlated with the area and density of the transfer film on the surface of the counterpart balls,which indicates that the combination of graphitization phenomenon at the friction interface of the H-DLC coating and the transfer formed on the surface of the counterpart balls play a role in mutual promotion.It can be also found that the changes of the above two factors are consistent with the changes of the friction coefficient and wear rate of the H-DLC coating under different experimental conditions.
Besides,it can be seen from Figs.5,6 and 9 that the mechanical properties of the H-DLC coating at the friction interface are affected by the 60NiTi substrate.According to the Hertz elastic contact theory,the tribological performance of the H-DLC coating can be affected by high elasticity of the 60NiTi substrate.The mechanical properties of the contact interface between the H-DLC coating and the counterpart balls was calculated by
whereE1is the combined elastic modulus;E2andv1are the elastic modulus and Poisson’s ratio of the counterpart balls,respectively;E3andv2are the elastic modulus and Poisson’s ratio of the coating,respectively.
Therefore,it can be considered that in comparison with other substrate materials of poor elasticity,the 60NiTi substrate with high elasticity will have a greater impact on the elasticity of the H-DLC coating deposited on the 60NiTi substrate,which can in turn reduce the combined elastic modulusEat the friction interface.Moreover,the contact radius α can be calculated by
where α is the contact radius;R1is the radius of the counterpart ball;Eis integrated elastic modulus;F1is the load.
Hence,the contact radius between the H-DLC coating and the counterpart balls increases as the 60NiTi substrate has lower combined elastic modulusE.Under the same friction load conditions,the actual contact area of the H-DLC coating deposited on the 60NiTi substrate is much larger than that deposited on other substrate materials with less elasticity.The COF of the coating can be calculated by
whereSSis the shear strength of the coating;Ais the actual contact area;Fis positive pressure of friction.
In the friction process,it can be considered that theSSof the H-DLC coating on different substrates is the same,hence the COF is proportional to theA/Fvalue.Therefore,it can be concluded that the high COF of H-DLC coating deposited on the 60NiTi substrate is dependent on the increased actual contact areaAof the 60NiTi substrate with high elasticity.
A H-DLC coating was deposited on the 60NiTi substrate by ion beam assisted enhanced unbalanced magnetron sputtering equipment.The microstructure,mechanical properties and tribological performance of the 60NiTi alloy were firstly explored,and the tribological behaviors of the H-DLC coating deposited on the 60NiTi substrate with different loads and frequencies were further discussed systematically.The conclusions are as follows:
(1) The 60NiTi alloy is mainly composed of NiTi matrix phase and Ni4Ti3phase,and has good elastic deformation ability and fracture toughness.By depositing the H-DLC coating on the 60NiTi substrate,the hardness,fracture toughness and plastic deformation resistance of the 60NiTi substrate can be effectively improved.
(2) The deposited H-DLC coating can effectively reduce the wear rate of the 60NiTi alloy.The increased friction load leads to the intensification of graphitization at the friction interface and the formation of continuous and compact transfer film on the surface of counterpart balls.The friction frequency has different effects on the carbonaceous lubrication layer at the friction interface according to different friction loads.
(3) During the friction process,the elastic modulus of the H-DLC coating is gradually reduced as the increase of the depth of wear scar due to the high elasticity of the 60NiTi substrate.Meanwhile,the increased COF of the H-DLC coating is also attributed to the larger actual contact area,which is caused by the high elasticity of the 60NiTi substrate.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was co-supported by the National Natural Science Foundation of China (No.51905466),the Aeronautical Science Foundation of China(No.201945099002),the Natural Science Foundation of Hebei Province,China (Nos.E2021203191 and E2020203184),and the Youth Top Talent Project of Hebei Province Higher Education,China (No.BJ2019058).
Appendix A.
Fig.A1 Thickness of H-DLC coating.