黉 Available online at\^n^n^,.sciencedirect.com ,・ Transactions of ScienceDirect Nonferrous Metals Society of China Press Trans.Nonferrous Met.Soc.China 22(2012)1372—1380 www.tnmsc.cn Influence of Ti target current on microstructure and properties of Ti--doped graphite--like carbon ilfms WANG Yong.xin 一,WANG Li.ping ,XUE Qun-ji 1.State Key Laboratory of Solid Lubrication,Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,Lanzhou 730000,China; 2.Graduate University ofChinese Academy ofSciences,Beijing 100039,China Received 28 June 201 1;accepted 5 September 201 1 Abstract:Ti—doped graphite—like carbon(Ti—GLC)films were synthesized successfully by magnetron sputtering technique.The compositions,microstructures and properties of the Ti—doped GLC films dependent on the parameter of Ti target current were systemically investigated by Raman spectra,X—ray photoelecrton spectroscopy(xPs),x—ray difraction(XRD),scanning electron microscopy(SEM).atomic force microscopy(AFM).nanoindentation and bal1.on.disk tribometer.With the increase of the Ti target current.the ratio of sp bond and the content of Ti as well as the film hardness and compressive internal stress increase.but the high content of the Ti would result in the loose film due to the formation of the squamose structure.Less incorporated Ti reduces the friction of the GLC film in dry—sliding condition,while pure GLC film exhibits the lowest friction coe街cient in water.1ubricated condition.Ti—GLC film deposited with low Ti target current shows hJ gh wear resistance in both dry—sliding and water-lubricated conditions. Key words:Ti--doped graphite--like carbon film;microstructure;tribological performance;target current alloyed in the amorphous carbon matrixes to improve the 1 Introduction Amorphous carbon fa C1 films mainly composed of sp and sp bonds have a great potential to be widely used in modem industry for their attractive properties such as high hardness,high wear resistance and low mechanica1.chemica1 and tribological properties of DLC t5lms.It was known that the addition of F or Si in DLC ilfms improves the friction and wear due to the beneficia1 tribochemical reaction on the contact surfaces[14],and the incorporation of H,Ar or N could adjust the microstructure and mechanical properties of DLC films friction coeficifent『1-4].In order to distinguish the famous a.C films which were named the diamond・like 『151,furthermore,the main emphasis had been placed on the doping of Ti,Cr and W in amorphous carbon matrix carbon(DLC)according to their relatively high content of sp bonds[5—9],the graphite—like carbon(GLC)film was proposed recently due to its high ratio of sp 一 hybridized carbon inside the carbonaceous amorphous since they were strong—carbide—forming(SCF)metals and possess an attractive combination of properties (corrosion resistance,wear resistance,etc.)[16—18]. Although above studies had been undertaken to matirx『10-13].These sudites have demonstrated the high hardness and excellent tribological performance in investigate the deposition and properties of DLC films, the research on the Ti—doped GLC composite films,in particular the relationship between the deposition parameters the mechanica1 and tribological behaviors has not yet systemically reported. both the ambient air and water environment for the GLC ilms.But most of fthe reported GLC films were the pure carbon matrix without any alloying elements.It was rare to know the influence of the element doping on the microstructure and properties of the GLC ilfms. On the side,various additive elements have been In this work.Ti.doped graphite-like carbon (Ti——GLC)films were synthesized successfully by magnetron sputtering technique.A middle frequency Foundation item:Project(50905178)supposed by the National Namral Science Foundation ofChina;Project(2011CB706603)supposed by the National Basic Research Program ofChina Corresponding author:WANG Li—ping;Tel:+86—93 1—4968080;E—mail:lpwang@licp.cas-ca DOI:10.1016/S1003—6326(11)61328—9 WANG Yong-xin,et al/Trans.Nonferrous Met.Soc.China 22(2012)1372—1380 power was applied to the twinbom Ti targets for the sputtering of Ti.The evolution of microstructures and properties of Ti——GLC composite films as the function of Ti target current were systematically studied. 1373 internal stress of the films with different Ti contents was calculated based on the Stoney’s equation『1 91.The tribological behaviors were tested by a ball—on.disk reciprocating 仃ibo—meter both in dry nd awater-lubricated conditions.The Si N4 balls were used as 2 Experimental The Ti—GLC composite films were deposited on Si a counterpart with a diameter of 3 nllT1.AU the tribological tests were performed under a load of 2 N. while the reciprocating ampliude was 5 nlltTl and the requency was 5 Hz.Affter each friction test,the wear wafer(p-type(1 00))and stainless steel by magnetron sputtering deposition process in the argon(Ar) volume lOSS was determined from the wear track profiles atmosphere.The depositing system was configured three magnetron target positions which focused on a subs仃ate seat.High pure graphite target was fixed in the middle of these target positions,and two twinbom Ti targets were fixed on the other two target positions.A DC power was used on the high-pure graphite target to sputter carbon, while a mid.frequency AC power was applied to the twinbom Ti targets to sputter Ti.Prior to deposition,the substrates were cleaned ultrasonically in ethanol and acetone bathes in succession and dried with a blower. The base pressure of the chamber before deposition was pumped to 1.0x l0-j Pa.and the deposition pressure 1.0 Pa was reached with a constant flow of Ar gas.The substrates were DC sputter.cleaned for 1 5 min at a bias voltage of—l 000 V with duty cycle of 50%.Then the interlayers of Ti were pre—deposited to improve the adhesion between the substrates and GLC films.The Ti target current was 2.0 A,while the bias voltage was-500 V with duty cycle of 50%. On the top of the Ti interlayers.GLC films were deposited with incorporation of Ti.Different mid.frequency currents were applied to the Ti targets to adiust the Ti content in the GLC films. The detailed deposition parameters were as follows:an applied DC current of 1 I2 A to graphite target.an applied mid—frequency current increased from 0 to 1.0 A to Ti targets.a deposition bias voltage of一3O0V(duty cycle 50%),Ar gas flow rate of 50 mL/min a deposition pressure of 1 Pa and a deposition time of 1 00 min. The microstructures of the as—deposited Tj—GLC iflms were characterized by Raman spectroscopy (Raman,HR800 Raman spectroscopic measurement), X—ray photoelectron spectroscopy(XPS,Perkin—Elmer PH1-5702 multi.functional X.ray photoelectron spectroscope),X—ray diffraction(XRD,Rigaku Dmax 2400 X—ray difract meter). scanning electron microscopy(SEM JSM-670 1 scanning electron microscope)and atomic force microscopy fAFM. SPM-9500 atomic force microscope).The wom morphology of the contact surfaces of mating balls were investigated by a JSM-5600 SEM. Nanohardness of each Ti-GLC film was measured by a Nanotest600 nanoindenter aDparatus(Micro Materials Ltd.,UK).Atier measuring the bending of the coated Si subs仃ates with a surNce profilometer,the which were obtained using a non.contact 3D surNce profiler(model MicroMAXTM.made by ADE Phase Shitf,Tucson,AZ,USA).Then the speciifc wear rates of all the films were calculated using the equation: K=V/(SF)[20],where Vis the wear volume in m ,S is the total sliding distance in m and F is the normaIlpad in N.The resulted specific wear rate of each film was obtained by averaging four wear tests under the same condition. 3 Results and discussion 3.1 MicrOstructure of Ti—GLC films Raman spectroscopy is a standard tool for the characterization of carbon materials『2 1 1.Typical Raman spectrum of a—C usually shows a D peak and a G peak, which lie at around 1 350 and 1 560 cm- .respectively 『22—241.The G peak is due to the bond stretching of all pairs of sp atoms in both rings and chains.while the D Deak is due to the breathing modes of sp atoms in rings 『241.Because of the different ratios of sp bonds or sp bonds in various a—C films.the location and intensity of the two peaks are different.The enhancement of D peak and the ncrease of spectrum intensity ratio IDfIG can be considered the evidence of the increase in graphite structure(sp bond),which was also named as graphitization for the a.C films『25,261.The Raman spectra of the as.deposited Ti—GLC films are shown in Fig.1.As shown in Fig.1,the intensity Of D peak increases drastically with increasing the mid.frequency current on Ti targets,and the value of IDfIG shown in Fig. 2 greatly increases from 1.7 t0 4.0 as the Ti target current ncreases from 0 to 0.8 A、and then the value InfIR suddenly increases to 1 0.0 as the Ti target current increases to 1.0 A,which indicates that the ratio of s bonds increases in the meantime.The weakened Raman signals of the films with high Ti target current might be related to the decrease of the carbon inside the as—deposited films.The weakened Raman signals of the iflms with high Ti target current might be related to the decrease of hte carbon inside the as—deposited films. The C1s spectra of Ti—GLC films with diferent Ti target currents acquired from the XPS analysis are shown in Fig.3.It can be clearly seen that the bond shifts l374 WANG Yong—xin,et al/TransNonferrous Met.Soc.China 22(20121 1372—1380 .Raman shift/cm一1 Fig.1 Raman spectra of Ti—GLC films at different Ti target currents 20 16 12 8 4 O 0 Ti target current/A Fig.2 I IG and ratio of sp2 bonds in carbon of Ti—GLC films towards lower binding energy with the increase of Ti target current,which indicates that the ratio of sp bond in carbon increases due to the relatively low bindmg energy of sp (284.4 eV)compared with sp bond f285.2 eV)[27,28].In detail,since the intensity of the binding energy is linearly proportional to the fraction of sp and sp bonds.the ratio of the sp bond of the carbon inside the as.deposited Ti-GLC films can be calculated approximately after peak fitting shown in Fig.2.The result shows that the ratio of sp bonds increases from 61%tO 82%with the Ti target current increasing from 0 tO 1.0 A.This result supports the Raman analysis. indicating the graphitization effect of GLC films during the Ti target current increasing.which can be attributed to the following two factors.One is the increase of Binding energy/eV Fig.3 Cls spectra ofTi—GLC films at diferent currents temperature resulti踟 ng rf印 om ∞ the enha加 nced ions bombarding;the other is t/puocINdsJ00甚 he incorporation of Ti in carbon matrix[291.Figure 4 demonstrates that the temperature inside the vacuum chamber and the content of Ti in Ti-GLC film both increase with the increase of Ti target current.As shown in Fig.4,the temperature in the vacuum chamber increases from approximately 58 to 105。C as the Ti target current increases from 0 to 1.0 A. Since the heat transmission in vacuum is very limited. the temperature on the surface should be relatively high. The enhanced temperature during the deposition process 1eads to the transformation from sp bond to sp bond since the sp carbon phase is thermodynamically less satble than the sp carbon phase[30],promoting the increase of sp bonds in carbon matrix.In another view point,higher target current supplies more Ti ions incorporated in a-C films which can cause damage by breaking some of hte sp bonds.1eading to the formation of a stable sp ̄-bonded phase.The contents of Ti in the as—deposited Ti—GLC films calculated from the XPS analysis are also shown in Fig.4.As seen from Fig.4, the content of Ti increases from 0 to 20%as the Ti target current increases from 0 to 1.0 A.thereby to increase the ratio of sp bonds in the carbon marx『3 1]. l20 10O 80 0 60彗 4O Q 20 O Titarget current/A Fig.4 Content of Ti in Ti—films and temperature in vacuum chamber WANG Yong—xin,et al/Trans.Nonferrous Met.Soc.China 22(2012)1372—1380 ≈ 0/s器叩朋I10蠹Z 1377 ∞ 加 m 8 6 4尝 吕 詈 2 Current/A Fig.9 Nanohardnesses and internal stress ofTi—GLC films current increases from 0 to 0.4 A due to the exotic element of Ti embedded in the carbon matrix increases from 0 to 1 0%.The enhanced compressive internal stress increased cohesion of the as.deposited GLC film, resulting in the high resistance under pressure.Thus the hardness of the film was increased with the increase of the compressive internal stress.However,though the content of Ti increases gradually,the compressive internal stress does not increase after Ti target current increases to 0.6 A,as seen in Fig.9,which can be attributed to the well linking between the Ti atoms and the carbon matrix due to the formation of TiC.The changeless compressive internal stress made the nanohardness vary slightly after the Ti target current increases to 0.6 A.The highest hardness of 24 GPa for iflm deposited with Ti target current Of 1.0 A might be related to its high content of hard TiC particles. The friction coefficients of Ti—GLC films with diferent Ti target currents in both dry and water-lubricated conditions are shown in Fig.1 0.In dry—sliding condition,it can be seen clearly that the friction coefifcient of pure GLC film deposited without Ti is 0.063.and the friction coefifcient of GLC doped 5%Ti using Ti target current Of 0.2 A is reduced to 0.043 However,the friction coefifcient of the as.deposited Ti—GLC films increases to 0.1 05 gradually as the Ti target current increases to 1.O A.This variation of friction coemcient might be related to the evolution of surface roughness as well as the microstructure of the as-deposited films.Compared Fig.1 0 with Fig.7,the variations of roughness and friction coefifcient of the Ti—GLC films in ambient air are similar.Obviously,1ow roughness gives the low friction coemcient due to the low friction shear resistance of hte smooth film surface. However,the surface roughness ofwear仃acks will be changed certainly after running-in period.Here,the evolution of microstructure must be the deep reason to the developing of friction coefifcient in ambient air.With the increase of Ti target current,the graphitization effect f0r Ti—GLC flms resulted in the increase of the sp bond in carbon,whereas,the content of C of Ti decreased or increased.The increase of the sp bond enhanced the solid 1ubricating e疏ct of the GLC films due to the weak adhesion of兀bond in sp2-hybrized carbon on the film surface.but the increase of Ti content in the high range will increase the friction shear resistance.The two opposite effects make the lowest friction coefficient of Ti—GLC films perfotin when the Ti is incorporated at a low target current of 0.2 A. 0.14 O.12 a 0.10 甚 8 0.08 0 堇0.06 0.04 0.02 Current/A Fig.10 Friction coefifcients of Ti—GLC films at different Ti target currents However,the friction coe伍cient of Ti-GLC film in water-lubricated wear condition increases from 0.036 to 0.075 gradually as the Ti traget current increases from 0 to 1.0 A.as seen from Fig.10.Additionally,each of the friction coefifcients in water-lubricated wear condition is lower than that in dry.sliding condition for hte same film. A11 can be attributed to the mutual effect between the solid lubricating and hydraulic 1ubricating.According to the present studies『32,33],one of the most effective solid lubricating effects of amorphous carbon was the ofrmation of仃ansferred tribolayer on mating surface. From Fig. 1l(a),the thick lubricating transferred tribolayer on hte contact surface of mating Si3N4 ball can be seen clearly when sliding in dry condition.But only a few of transferred materl’als can be seen on the edge of the wear scar of the mating Si3Nd ball in water.1ubricated wear condition(Fig.1l(b)),which suggests that water prevents the formation of the仃ansferred tribolayer, resulting in the little signiifcance of bonding structure in carbon to the friction shear resistance in water-lubricated wear condition.In addition,the surface roughness cannot affect the friction coefflcient of amorphous films in the short running in period in water.Therefore.the variation of friction coefifcient in water-lubricated condition is only affected by the content of C or Ti,which makes the friction coefifcient increase gradually with the decrease l380 WANG Yong-xin,et al/Trans.Nonferrous Met.Soc.China 22(2012)1372—1380 corrosion and tribological performance[J Jl Nanotechnology,2006 1 7:4614-4623. ZHOU Hui—di.LrU Hui.wen.Tribochemica1 effects on the friction and wear behagiors of a-C:H and a—C films in diierfent environment 『J1.Tribol Int,2007,40:132-138. [21] PIscANEC S, MAURI F, FERRARI A C,LAZZERI M, ROBERTSON J.Ab initio resonant Raman spectra of dimond—laike 【9] RONKAINEN H.VARJ1 S S.HOLMBERG K.1 ibologica1 performance ofdiferent DLC coatings in water-lubricated conditions .carbons[J]l Diamond Relat Mater,2005,14:1078—1083. [22] GILKES K w R,PRAWER S,NUGENT K W,ROBERTSON J, SANDS H S.L1FSHITZ Y.SHI X.Direct quantitative detection of Wear.2001,249:267-271. [1O] LACERDA R G,HAMMER P,LEPIENSKI C M,ALVAREZ R. MARQUES F C.Hard graphitic-like amorphous carbon films with he sp3 bonditng in dimond—alike carbon films using ultraviolet and hi曲stress and local microscopic density[J]l J Vac Sci Technol A, 2001.19:97l-975. FIELD S K,JARRATT M,TEER D G.Tribogical properties of visible Raman spectroscopy[J]l J Appl Phys,2000,87:7283—7289. [23]FERRARI A C,ROBERTSON J.Resonant Raman spectroscopy of disordered,amorphous,and diamondlike carbon[J1.Phys Rev B, 2001,64:075414—075426. graphite—like and diamond—like carbon coatings[J].Tribol Int,2004, 37:949-956. KOVSKY V.METLOV K.KURDYUMOV A.BOHAC P, [12] KULIJASTRABIK L.Study of the structure of hard graphite.1ike amorphous carbon films by electron diffraction .Diamond R 1at Mater.2002.11:1467-1471. [24] CASIRAGHI C, FERR ARI A C, R0BERTsON J. Raman spectroscopy of hydrogenated amorphous carbons[J].Phys Rev B, 2005,72:085401—085414 [25] ROBERTSON J.Diamond-like aorphous carbon[J].Mater Sci Eng R,2002,37:129—281. RE F L Jr.MARQUES F C. [13】 VIANA G A,LACERDA R G.FREIESR investigation of graphite—like amorphous carbon ilfms revealing [26] CHIU SM,LEE S C,WANG CH,TAI F C,CHU CW,GAND. Electrical and mechanical properties of DLC coatings modiied by fitinerant states as the ones responsible for the signal[J】J Non—Cryst Solids,2008,354:2135-2137. plasma immersion ion implntataion[J].J Alloys Compd,2008,449: 379-383. X,OHANA T,TANAKA A,KUB0 T,H NANAO,MINAMI I, [14】 WU MORI S.Tribochemical reaction of Si.DLC coating in water studied [27] LIAO J x,LIU W M,Xu T,XuE Q J.Characteristics of carbon iflms prepared by plasma—based ion implntaation[JJ.Carbon,2004, 42:387-393. by stable isotopic tracer[JJ.Dimond Relaat Mater,2008,17: 147-153. [15] zHANG Rui-jun.MA Hong-tao.Nano—mechanical properties and nano—tribological behaviors of nitrogen—doped diamond一1ike carbon [28] RYBACHUK M,BELL J M.Electronic states oftrns-polayacetylene. poly(p-phenylene vinylene)and sp—hybridised carbon species in amorphous hydrogenated carbon probed by resonant Ralnan (DLC)coatings IJ1.J Mater Sci,2006,41:1705—1709. scattering[J1l Carbon,2009,47:2481—2490 [16] SU Y L.KAO W H.1 ibological behavior and wear mechanisms of Ti-C:H,TiC/TiCN/TiN/Ti coatings when sliding against stee1.bronze [29]ZHANG S,BUI X L,FU Y.Magnetron—sputtered nc-TiC/a-C(A1) tough nanocomposite coatings[J].Thin Solid Films,2004,467: 261-266. nd aaluminum alloy rods『J1.J Mater Sci,2001,36:l89-199. NGH V.JIANG J C.MELETIS E I.Cr—diamond like carbon [17] SInanocomposite films:Synthesis,characterization and properties[J1. Thin Solid Films.2005.489:l50-158. min,WU Wei—dong,LI Sheng—yin,BAl Li,CAO [18】 WANG Xue—[30] MA T B,HU Y Z,WANG H.Moleculra dynamics simulation of shear-induced raphigtization of amorphous carbon films[J].Carbon, 2009,47:1953—1957. [31]Ⅵ NG K W,CHANG C L,wANG D Y.Efrect ofion energy on degradation of dimond-a・like carbon films exposed to high--energy Lin-hong,CHEN Song—lin,TANG Yong-jin Praoperties of w incorporated diamond-like carbon films prepared by pulsed-laser bombardment from an ion implntaer[J】.Diamond Relat Mater,2002, 1 l:1447-1453. deposition[J1.J Alloys Compd,2009,479:741—745. T.Internal stress reduction by incorporation [19] BAN M.HASEGAWA of silicon in diamond—like carbon filmsⅢ.Surf Coat Technol,2002, 162:1-5. [32】TANAKA A,SUZUKI M,OHANA T Friction and wear of various DLC films in water and air environment[J].Tribol Lett,2004,17: 917-924. —ping,ZHANG Jun-yan,ZENG Zhi-xing,LIaN Yi-ming, [20] WANG Li[33】SCHARFTW,SINGERIL.Role ofthetrnsfaerfilm onthefriction nd weaar of meta1 carbide reinforced amorphous carbon coatings Hu Li-tin.xuE Qun ̄ia.Fabrication of a nanocrystalline Ni—Co/CoO functionally graded layer with excellent electrochemical during run-in[J].Tr|b0l Lett,2009,36:43—53. 不同Ti靶电流对Ti掺杂类石墨碳膜的结构和性能的影响 王永欣 一,王立平 ,薛群基 1.中国科学院兰州化学物理研究所,固体润滑国家重点实验室,兰州730000; 2.中国科学院研究生院,北京100039 摘要:利用磁控溅射的方法成功制备Ti掺杂类石墨碳(Ti—GLC)膜。采用拉曼光谱、x射线光电子谱(XPS)、扫 描电子显微镜(SEM)、原子力显微镜(AFM)、纳米压痕仪和球盘式摩擦机分别表征不同Ti靶电流下制备的Ti—GLC 膜的成分、结构和性能。随着Ti靶电流的增加,薄膜中sp 键的比率和Ti含量增加,同时薄膜的硬度和内应力 也增大,但较高的Ti靶电流将会促使薄膜产生鳞片状结构从而使其变疏松。较少的Ti掺入量可以降低GLC膜 的干摩擦因数,纯GLC膜在水润滑条件下的摩擦因数最低。在较低Ti靶电流下制备的Ti—GLC膜在干摩擦及水 润滑条件下均具有较高的抗磨性能。 关键词:Ti掺杂类石墨碳膜;微观结构;摩擦学性能;靶电流 (Edited by LI Xiang-qun)