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Wear269 (2010) 118–124Contents lists available at ScienceDirectWearj o u r n a l h o m e p a g e:w w w.e l s e v i e r.c o m/l o c a t e/w e arFriction and wear characteristics of hydrogenated diamond-like carbonfilms formed on the roughened stainless steel surfaceAkihito Suzuki∗,Yusuke Aiyama,Maiko Tokoro,Hidetoshi Sekiguchi,Masabumi MasukoDepartment of Chemical Engineering,Graduate School of Science and Engineering,Tokyo Institute of Technology,12-1-S1-27,Ookayama2-chome,Meguro-ku,Tokyo152-8552, Japana r t i c l e i n f oArticle history:Received15October2009Received in revised form10March2010 Accepted15March2010Available online 20 March 2010Keywords:Diamond-like carbonSurface roughnessSliding friction and wearPlasma-enhanced chemical vapor depositionWater lubrication a b s t r a c tThe effects of the surface roughness of hydrogenated diamond-like carbon(DLC)films on friction and wear properties have been studied using a ball-on-disk type tribometer in air and water environments.Rough DLCfilms were formed by depositing DLC with the plasma-enhanced chemical vapor deposition method on roughened stainless steel substrate prepared by argon plasma sputter etching.DLCfilms deposited on a smooth stainless steel substrate showed unstable friction behavior in both air and water environments. Thefinal friction coefficient values ranged from0.10to0.15in air and0.15to0.20in water.Line-shaped large-scale delamination of DLCfilm from stainless steel substrate was observed for smooth DLCfilms rubbed in water.In an air environment,large-scale delamination was not observed,but wear of the DLC film could be seen.The wear scar formed on the stainless steel ball under water lubrication was larger than that formed in air environment.In contrast to smooth DLCfilm,DLCfilms having a rough surface showed stable frictional behavior.The friction coefficients in water and air environments were approximately0.1 and0.2,respectively.No large-scale delamination of DLCfilm was observed for rough DLCfilms.Although friction characteristics were improved by roughing the surface of the DLCfilms,the wear scars forming on the ball surface became larger with increases in the surface roughness of the DLCfilm.From the results of the tribotest in n-decane,which has almost the same viscosity as water,it was suggested that not only the hydrodynamic effect but also the interaction between DLCfilms and water affect the friction and wear behavior.DLCfilms changed from hydrophobic to hydrophilic by tribochemical reaction in water environment.The low friction and wear of DLCfilms under water lubrication is considered to be caused by hydrophilication of the DLCfilms.© 2010 Elsevier B.V. All rights reserved.1.IntroductionDiamond-like carbon(DLC)is an amorphous and meta-stable material with some properties similar to those of diamond.DLC has many superior properties such as high mechanical hardness, high wear resistance,a low friction coefficient,chemical inertness,a high gas barrier property,high electrical resistivity,and high optical transparency in the IR spectral region.Since DLCfilms have many excellent features,their application to variousfields is expected. Now DLCfilms are used practically for electromagnetic clutches, fuel injection pumps for diesel engines,piston rings,the frictional parts of hydraulic equipment,molds,combination faucets,cutting tools,and coatings for the inner walls of polyethylene terephthalate (PET)bottles.DLCfilms can be formed on substrates by various methods such as sputtering,ion beam assisted deposition,arc ion plat-∗Corresponding author.Tel.:+81357342628;fax:+81357342628.E-mail address:asuzuki@chemeng.titech.ac.jp(A.Suzuki).ing,pulsed laser deposition,plasma-based ion-implantation,and plasma-enhanced chemical vapor deposition(PECVD).Physical vapor deposition(PVD)is the method to deposit thinfilms by the condensation of a vaporized material onto surfaces.In the PVD methods such as ion beam assisted deposition,arc ion plating,sput-tering and pulsed laser deposition,graphite is used as a starting material so that the resultingfilms contain almost no hydrogen.The property of hydrogen-free carbonfilm depends on the sp3carbon content.Carbonfilms deposited by PVD method sometimes con-tain a significant amount of sp2-hybridized carbon,and are quite soft.However this sp2amorphous carbon(a-C)is not DLC.On the other hand,an a-C that contains some measure of sp3content is a DLC and is obtained by sputtering.The carbonfilm that contains a larger amount of sp3content can be made by ion beam assisted deposition,arc ion plating or pulsed laser deposition.Thisfilm is designated as tetrahedral amorphous carbon(ta-C)and shows very high hardness.In the case of chemical vapor deposition(CVD),hydrogenated amorphous carbon(a-C:H)DLCfilm is obtained because hydrocar-bons such as methane,acetylene,benzene,and toluene are used as0043-1648/$–see front matter© 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2010.03.013A.Suzuki et al./Wear269 (2010) 118–124119starting materials.The hardness of the a-C:H is lower than that of ta-C.It is well known that the tribological characteristics of DLCfilms are strongly dependent on sp2,sp3,hydrogen content and friction environment.ta-Cfilm exhibited a high friction coefficient above 0.6and severe wear under high vacuum(10−6Pa)at room tempera-ture.However,the friction coefficient of ta-Cfilms decreased to0.07 in a humid environment[1].In addition,the steel pin/hydrogen-free DLC disk pair lubricated with glycerol mono-oleate containing poly-alpha olefin(PAO)showed a very low friction coefficient of 0.006[2].In contrast,highly hydrogenated DLCfilm shows excellent wear resistance and a very low friction coefficient below0.01under high vacuum or a dry nitrogen environment[3–7].Furthermore,it has been reported that a self-mated hydrogenated DLC pair lubricated with PAO showed a very low friction coefficient of0.018[8].The hardness and hydrogen content of DLCfilms depend on the methods and conditions offilm deposition,and these properties influence the tribological characteristics of the DLCfilms.In our previous work,the tribological characteristics of two types of DLC films having different hardness have been investigated in additive-free distilled water[9].As a result,it became obvious that transfer of some material from DLCfilms to the mating surfaces affects the friction coefficient.Furthermore,the authors have investigated the influence of the surface polishing of DLCfilms on the tribological properties in both air and water environments[10].The results showed that the friction coefficient became lower and more stable by polishing.Moreover,the friction coefficient was lower in water than in air.It is considered that these results were brought about by the change in surface properties and the hydrodynamic effect of water.In addition to thefilm and surface properties such as hard-ness,hydrogen content,and wettability,tribological characteristics are likely influenced by the surface roughness of DLCfilms.Most of the research regarding the tribological properties of DLCfilms has been conducted for smooth surfaces,and research regarding DLCfilms with rough surfaces is limited.Jiang and Arnell[11] have investigated the effects of surface roughness on the wear of DLCfilms in a dry air environment.They reported that the wear rate of the DLCfilms sliding against tungsten carbide balls increased significantly with increases in the surface roughness, whereas the frictional behavior did not appear to be affected.Ohana et al.[12]have investigated the influence of the surface rough-ness of DLCfilms on their tribological properties against AISI440C, AISI304stainless steel,and brass balls in a water environment. They prepared rough stainless steel substrates by grinding with an abrasive paper and by the treatment of shot-peening with ceram-ics and a steel ball,and deposited DLCfilms on these substrates by the electron-excited DC plasma CVD method.They showed that thefilm having a smooth surface had severe damage,but that with a rough surface had no damage at the same load con-dition.They also reported that the specific wear rate of mating material increased with increases in the surface roughness,but the friction coefficient was not as affected by the surface rough-ness.However,since very little research has been reported,the effects of the surface roughness of DLCfilms on tribological performance remain unclear.In particular,because there has been no report comparing the tribological performance of rough DLCfilms both in air and water environments,it is unknown whether there is a difference between air and water.When applying the DLCfilms to practical friction materials,deposition onto substrate having some measure of roughness cannot be avoided.It is therefore important to understand the tribological characteristics of rough DLCfilms under various environments.In order to investigate the effects of surface roughness on the friction and wear behavior of DLCfilms, stainless steel substrates having different surface roughnesses were Table1Gas compositions and deposition times in DLC coating on substrates.Gas composition[vol.%]Deposition time[min]Ar HMDS CH4Intermediate layer50500380200395503Graded layer70201057010205DLC layer70030120prepared and the hydrogenated DLCfilms were deposited onto substrates by the PECVD method.2.Experimental details2.1.DLC coatingsDLCfilms were deposited on the disk substrates using a stan-dard capacitively coupled parallel plate electrode reaction chamber powered by a13.56MHz RF power source.The diameter of powered electrode was80mm and the distance between the powered and ground electrode was60mm.The substrate material used in this study was martensitic precipitation hardening stainless steel with a hardness of400HV(JIS SUS630).The original substrates were polished to a surface roughness of Ra=0.03±0.01␮m.The sub-strates were cleaned with ethanol in an ultrasonic bath for10min before deposition.The substrates were then placed directly on the powered cathode.Prior to deposition,the substrates were sputter-cleaned in an argon atmosphere(flow rate of argon:10.0sccm)at a self-bias voltage of−700V(RF power120W)and a pressure of 2Pa.Roughened surface substrates were prepared by changing the argon sputtering time.In order to improve the adhesion of DLCfilms to the substrate, argon gas and hexamethyldisiloxane(HMDS)vapor were intro-duced into the chamber and the Si-containing intermediate layer was deposited on the substrate at a self-bias voltage of−500V(RF power60W)and a total pressure of2Pa.The fraction of argon gas and HMDS vapor was changed in steps during deposition of the interlayer,and total three intermediate layers were deposited.Gas compositions of Ar/HMDS were50/50vol.%(flow rate:Ar5.0sccm, HMDS5.0sccm)for thefirst intermediate layer,80/20vol.%(flow rate:Ar8.0sccm,HMDS2.0sccm)for the second intermediate layer,and95/5vol.%(flow rate:Ar9.5sccm,HMDS0.5sccm)for the third intermediate layer.Each intermediate layer was deposited for 3min.After deposition of the interlayer,argon,HMDS,and methane were introduced into the chamber and the stepwise-graded inter-layer was deposited at a self-bias voltage of−500V(RF power 60W)and a total pressure of2Pa.Total two graded layers were deposited.Gas compositions of thefirst and the second graded layer were Ar/HMDS/CH4=70/20/10vol.%(flow rate:Ar7.0sccm,HMDS 2.0sccm,CH41.0sccm)and Ar/HMDS/CH4=70/10/20vol.%(flow rate:Ar7.0sccm,HMDS1.0sccm,CH42.0sccm),respectively.Each intermediate layer was deposited for3min.Finally,argon and methane were introduced,and the hydro-genated DLC top layer was deposited at a self-bias voltage of−500V (RF power60W)and a total pressure of2Pa for120min.Gas composition of Ar/CH4=70/30vol.%(flow rate:Ar7.0sccm,CH4 3.0sccm).The details of the deposition conditions are given in Table1.The changes in chemical composition of stainless steel surface before and after argon plasma sputtering were investigated by elec-120 A.Suzuki et al./Wear269 (2010) 118–124Fig.1.Relationship between the surface roughness of argon plasma sputtered sub-strates and sputtering time.tron probe X-ray microanalysis(EPMA:JEOL Ltd.,JXA-8200)under an acceleration voltage of15kV and beam current of170nA.The DLCfilm thickness was determined using a variable angle spectroscopic ellipsometer(J.A.Woollam Co.Inc.,M-2000UI).The surface roughness of the substrate and the DLC coatings was mea-sured using a contact stylus surface roughness tester(ULVAC Inc., Dektak3).The hardness of the DLC coatings was measured using a nano-indentation instrument(Elionix Co.Ltd.,ENT-1100)with a Berkovich-type diamond indenter.2.2.Ball-on-disk testsFriction and wear tests were conducted using a ball-on-disk tri-bometer consisting of a stationary stainless steel ball(JIS SUS630, 9.525mm in diameter)sliding on a rotating DLC-coated disk in an air environment and distilled water at room temperature (23–25◦C).Some of the tribotests were also conducted under lubri-cation by n-decane at room temperature.Water and n-decane have almost the same viscosity at room temperature.The dynamic vis-cosity of water is0.890mPa s at a temperature of25◦C,which is almost the same as the0.861mPa s of n-decane.The surface rough-ness(Ra)of the balls was0.03␮m.The normal force applied in the tests was4.7N.The normal force created initial maximum Herzian contact pressures of784MPa and a Hertzian contact diameter of 0.107mm.The sliding speed was0.1m s−1,and the total sliding distance was1000m.During testing,the coefficient of friction was monitored continuously.The wear surfaces of the steel balls and the DLC-coated disks were observed using a laser scanning microscope (VK-8510,Keyence Corporation).3.Results and discussion3.1.Change in the substrate surface roughness and temperatureby argon plasma sputteringThe relationships between the surface roughness of argon plasma sputtered substrates and sputtering time are shown in Fig.1.The surface roughness of substrates increases with sputter-ing time.Fig.2(a)and(b)shows the scanning electron microscope (SEM)images before and after the argon plasma sputtering of the substrate,respectively.Table2shows the results of EPMA for the same substrates.Before argon plasma sputtering,polish marks were observed across the whole surface.On the other hand,after argon plasma sputtering,polish marks on the substrate surface disappeared and the metal structure was observed.Since the sput-ter rates vary with regard to crystal structures,the surface shown in Fig.2(b)was obtained by argon plasma sputtering.Except for carbon,there was a slight difference in the surface element com-position of the substrate before and after sputtering.Thecarbon Fig.2.Scanning electron microscope(SEM)images of substrate:(a)before argon plasma sputtering and(b)after argon plasma sputtering.ratio of the substrate surface before sputtering was approximately 10times higher than that of the base material.Mirror polishing and/or precipitation hardening treatment might influence the high carbon content of the substrate surface.The amount of surface car-bon on the substrate after argon sputtering was clearly decreased compared with that before argon plasma sputtering.The carbon existing on the surface was more easily sputtered than other ele-ments.The change in the substrate temperature against sputtering time is shown in Fig.3.The substrate temperature was measured using a temperature sensor label pasted on the substrate surface in the chamber.The substrate temperature rose sharply at the beginning of argon plasma sputtering,but it did not exceed150◦C.It is thought that no change in the quality of the substrate occurred at such a low temperature.In the present study,several substrates having different surface roughnesses were prepared by changing the argon sputtering time.3.2.Surface roughness of DLCfilmsDLCfilms were deposited on the roughened stainless steel sub-strates.Thefilm thickness of the Si-containing intermediate layer ranged from211to296nm,and that of the DLC top layer ranged from720to854nm.The totalfilm thickness deposited on the sub-strate was approximately1␮m.Fig.4shows the relation betweenA.Suzuki et al./Wear 269 (2010) 118–124121Table 2Chemical composition of substrate surface before and after argon plasma sputtering determined by EPMA.[unit:mass%].SubstrateC Si Mn P S Cu Ni CrNb and Ta FeBefore sputtering 0.650.300.420.020.00 2.85 3.9815.900.08Balance After sputtering0.190.330.400.020.00 3.103.9115.690.10Balance SUS630stainless steel<0.07<1.00<1.00<0.04<0.033.00–5.003.00–5.0015.50–17.500.15–0.45Balancethe substrate roughness before DLC deposition and the DLC film roughness.The dotted line shows the line with a slope of 1.If a plotted point exists on this line,it means that surface roughness does not change before and after the deposition of DLC film.All plotted data are near this line.The surface roughness of DLC films is almost the same as that of substrates.3.3.Tribological characteristics of rough DLC filmsFour DLC-coated specimens having a surface roughness Ra of 0.07,0.15,0.19,and 0.48␮m were prepared.A smooth DLC film sample,which was deposited on the smooth original substrate,was also prepared.The surface roughness Ra of this smooth DLC film was 0.03␮m.The hardness of the DLC film measured with a nano-indentation instrument was approximately 15.5GPa.Fig.5shows the change in the friction coefficient against the sliding distance in an air environment and in distilled water at room temperature.Optical microscopic photographs of wear scars formed on the balls after the tribotest are also shown in Fig.5.The initial friction coefficient of the smooth DLC films lubricated in water was approximately 0.1.On the other hand,the initial fric-tion coefficient of the smooth DLC films in air was high and its value exceeded 0.2.Friction coefficients of the smooth DLC films fluctu-ated both in air and water environments,and the reproducibility was not good.The final friction coefficient ranged from 0.10to 0.15in an air environment,and from 0.15to 0.20under water lubri-cation.The wear scar formed on the stainless ball under water lubrication was larger than that formed in an air environment.Unlike the smooth DLC films,the friction behavior of rough DLC films was stable and showed good reproducibility.The final fric-tion coefficient was approximately 0.2in an air environment,and approximately 0.1under water lubrication.The wear scar formed on the ball under water lubrication was larger than that formed in an air environment.The wear scar became larger with increases in the surface roughness.Because ofhigh hardness and low adhe-sive properties of DLC films,abrasive wear of the stainless steel ball is unavoidable.DLC film with rougher surface has larger abrasive effect and larger wear scar was formed on the stainless steel ball.Transfer material was observed on the wear scar,but there was lit-Fig.3.Change in the substrate temperature against sputtering time.tle accumulated material in the entrainment zone.The amount of transfer material generated in air was larger than that generated in water.It is supposed that water inhibited adhesion of transfer material to the stainless steel surface.The difference in adhesion strength causes the difference in the amount of transfer material between two friction environments.Formation of transfer mate-rial is often observed in tribotest of DLC films.In general,formation of transfer layer on the sliding surface reduces the friction coeffi-cient.It is suggested that the low friction of DLC films arise from the graphitization of the hydrogenated DLC film structure.Sliding-induced heat accumulation causes a destabilization of C–H bonds in hydrogenated DLC films and formation of graphitic structure.The difference in the amount of transfer material between two environ-ments may be brought by the cooling effect of water.It is considered that transfer material protects the wear of the ball because wear scar formed on the ball in an air environment was small.However DLC film with rough surface showed lower friction coefficient in water and transfer material did not reduce the friction coefficient effectively.Microscopic photographs and surface profiles of wear tracks on the disk specimens are shown in Fig.6.In the case of a smooth sur-face (Ra 0.03␮m),line-shaped large-scale delamination of DLC film occurred under water lubrication.It is thought that the delamina-tion occurred at the interface between the stainless steel substrate and the interlayer,as the difference in elevation between the DLC surface and the delaminated part was approximately 1␮m.How-ever,wear of the DLC film hardly progressed in the part where the DLC film did not flake off.In an air environment,delamination of DLC film could not observed,but the wear of the DLC film was severe and the depth of wear reached approximately 0.1–0.2␮m.In contrast to smooth DLC film,only a small amount of delamination occurred in rough DLC films (Ra 0.15␮m)in both air and water envi-ronments.Thereason why the delamination occurred only in the smooth DLC film under water lubrication is considered that water penetrates to the interface between substrate and DLC film.Adhe-sion of DLC film to the stainless steel substrate is weakened by the penetration of water,and delamination occurs at the interface.InFig.4.Relationship between the substrate roughness before DLC deposition and the DLC film roughness.122 A.Suzuki et al./Wear269 (2010) 118–124the case of the DLCfilm deposited on the smooth substrate surface, once small delamination occurs,water penetrates into the inter-face further from there,and then delamination crack grows up at a dash and causes a large-scale delamination.On the other hand,in the case of the rough DLCfilm,roughness of the substrate prevents the growth of delamination crack,and no large-scale delamination is developed.It is thought that the cause of poor reproducibility of frictional properties shown in Fig.5is in a large-scale delamination of DLCfilm.Except for the large-scale delamination of smooth DLC film in water lubrication,wear of DLCfilm is higher in air than in water.Because the transfer material formed on the ball in air envi-ronment is hard enough,wear amount of DLCfilm becomes higher in air.In rough DLCfilms,the frictional behavior was different in the test environment.The friction coefficient of rough DLC is lower in water than in air.The hydrodynamic effect of water and/or the interaction between DLCfilms and water can be considered to be the reason for this difference.In order to investigate the effects of water on frictional behavior,a non-polar hydrocarbon,n-decane, was used as a lubricant.The viscosity of n-decane is almost the same as that of water,and if the friction characteristics under n-decane lubrication are similar to those under water lubrication,the differ-ence in frictional behaviors between air and water is caused by the hydrodynamic effect of liquid lubricant.Tribotests under n-decane lubrication were conducted at room temperature for the smooth DLCfilm with a surface roughness of0.03␮m and the rough DLCfilm with a surface roughness of 0.15␮m.Fig.7shows the change in friction coefficient against the sliding distance,the wear scar formed on the ball,and the wear track of the DLC-coated disk.The results of friction coefficient mea-surements obtained in air and in water are also plotted together for comparison.In the case of smooth DLCfilm,the frictionalbehav-Fig.5.Change in the friction coefficient against the sliding distance and optical microscopic photographs of wear scars formed on the balls after the tribotest:(a)Ra0.03␮m, (b)Ra0.07␮m,(c)Ra0.15␮m,(d)Ra0.19␮m,and(e)Ra0.48␮m.A.Suzuki et al./Wear269 (2010) 118–124123Fig.6.Microscopic photographs and surface profiles of wear tracks on the disk specimens:(a)smooth surface in water,(b)smooth surface in air,(c)rough surface in water, and(d)rough surface in air.ior under n-decane lubrication was similar to that under water lubrication rather than that in an air environment.Thefinal fric-tion coefficient with n-decane lubrication was approximately0.15, which was midway between the values in water and in air,respec-tively.The appearance of transfer materials on the ball specimen was similar to that observed under water lubrication.However, accumulated material was not observed in the entrainment zone. Wear scar diameter formed on the ball in n-decane environment was352␮m,and this was larger than that in air of307␮m,and was smaller than that in water of433␮m.As in water,line-shaped delamination of DLCfilm was observed.On the other hand,in the case of rough DLCfilm,the initial fric-tion coefficient under n-decane lubrication was close to that under water lubrication,but it gradually increased and thefinal value was the same as that in air.Transfer materials were observed over the entire wear scar of the ball specimen.Wear scar diameter was 450␮m,and this was larger than the that in air of310␮m,and was about the same as that in water of458␮m.Wear could not be detected on the friction track of the DLCfilm.There was no consis-tent similarity in the friction characteristics of DLCfilms lubricated in n-decane and those in water.These results indicate that not only the hydrodynamic effect of water,but also the interaction between DLCfilms and water affects the tribological behavior of DLCfilms.The contact angle of water on DLCfilms deposited in this work was between70◦and80◦,which shows hydrophobic.However it was observed that the friction track on the DLCfilms after tribotest turned into hydrophilic and did not show water repellency.From this obvious change in the property of124 A.Suzuki et al./Wear269 (2010) 118–124Fig.7.Change in friction coefficient against the sliding distance,the wear scar formed on the ball and the wear track on the DLC-coated disk under n -decane lubrication:(a)smooth surface (Ra 0.03␮m),and (b)rough surface (Ra 0.15␮m).DLC film,tribochemical reaction of DLC films in water environment is expected.Recent researches that used a stable isotopic tracer D 2O and H 218O reported that DLC films tribochemically reacted with water to form hydrophilic hydroxyl and carboxyl groups on surface [13,14].As the mechanism of this tribochemical reaction it is suggested that carbon radicals generate from the C–C bond by mechanical stimulation reacts with water [13,15].The formation of hydrophilic hydroxyl and carboxyl groups by the tribological action in water environment promotes the hydrophilicity of the DLC films,and water molecules adsorb easily on the DLC films.Adsorbed water molecules probably contribute to the low friction coefficient of DLC films under water lubrication.In addition,the low wear of DLC films in water environment is also possibly caused by the friction reduction of the hydrophilic DLC films.4.ConclusionsFriction and wear characteristics between hydrogenated DLC film having a rough surface and stainless steel pairs were investigated in air,water,and n -decane environments at room temperature.The rough-surface DLC film showed quite different tribological characteristics from smooth-surface DLC film.The fric-tion coefficient of rough-surface DLC films was independent of the surface roughness.The friction coefficient of rough-surface DLC was an almost constant value in each lubrication environment.That value was approximately 0.1in water,0.15in n -decane,and 0.2in an air environment.In the case of tribotest in water environ-ment,friction track on DLC films changed from hydrophobic to hydrophilic by tribochemical reaction with water.The low fric-tion coefficient of DLC films under water lubrication is considered to be caused by hydrophilication of the DLC films.The large-scale delamination that occurred on smooth-surface DLC films was not observed on rough surface DLC films.The wear of DLC films was remarkable in an air environment,but was not observed in water environments.This low wear characteristic of DLC films in water environment is considered to be related to the hydrophilication of the DLC films.However,wear of the counter material is consider-able,and the wear scar formed on the ball increased with increases in the surface roughness of DLC film.AcknowledgmentsThe authors thank Center for Advanced Materials Analysis,Tech-nical Department,Tokyo Institute of Technology,for the support of 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