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Tribo-ecological coatings prepared by ECR sputtering

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Tribo-ecological coatings prepared by ECR sputtering
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  Vacuum 76 (2004) 119–126 Tribo-ecological coatings prepared by ECR sputtering Cristian Petrica Lungu a,b,  , Kunihiko Iwasaki b , Katsuhiro Kishi b ,Masafumi Yamamoto b , Ryohei Tanaka b a National Institute for Lasers, Plasma and Radiation Physics, 111 Atomistilor, P.O. Box MG-36, Magurele-Bucharest, Romania b Japan Ultra-High Temperature Materials Research Institute, 573-3 Okiube, Ube, Yamaguchi 755-0001, Japan Abstract A new process for tribo-ecological coatings was developed by using electron cyclotron resonance-direct current(ECR-DC) hybrid plasma sputtering method. Ag-matrix overlays containing C and Mo for plain bearings to be used inautomobile engines were successfully prepared by this method. The crystallographic state of C included into the Ag-matrix was identified as amorphous or diamond-like carbon (DLC) with high content of sp 2 bonds. Mo was partlytransformed into solid lubricants such as MoO x  ( x ¼ 2 ; 3) during sliding and decreased both the coefficient of frictionand the wear rate. It also made the overlay softer to improve conformability and embeddability. The optimum Mo andC concentrations were determined to be about 1 and 2mass%, respectively. r 2004 Published by Elsevier Ltd. Keywords:  Ag-matrix overlay; C; Mo; DLC; ECR-DC sputtering; Tribology 1. Introduction Three-layer plain bearings currently used inhigh-performance engines of automobiles consistof an electroplated Pb-matrix overlay containingSn, In and Cu, a sintered Cu–Pb bearing alloy anda steel backing [1]. They, however, containenvironmentally toxic element Pb and the overlaysare a little too soft to be used under high workingpressure. In order to improve these shortcomings,Ag-matrix overlays have been developed byelectron cyclotron resonance-direct current(ECR-DC) hybrid plasma sputtering by the pre-sent research group [2–8], where a cylindrical target consisting of   N   graphite rings (hereafterabbreviated as Gr) and (10- N  ) Ag rings was used.The ratio  N  /( N  -10) was roughly equal to the ratioof the effective area of the Gr rings to that of theAg rings and the actual concentration of C in theAg-matrix could be controlled by changing thisratio. The C addition was found to be veryeffective to reduce both the coefficient of frictionand the wear rate in dry and lubricated sliding. Forexample, as the ratio was increased from 0/10 to2/8, the coefficient of friction and the wear rate in ARTICLE IN PRESS www.elsevier.com/locate/vacuum0042-207X/$-see front matter r 2004 Published by Elsevier Ltd.doi:10.1016/j.vacuum.2004.07.001  Corresponding author. Fax: +40-21-4574468. E-mail address:  lungu@alpha2.infim.ro (C.P. Lungu).  the dry sliding condition decreased from 0.8 to0.6 7 0.05 and from 40  10  15 m 2 N 1 to20  10  15 m 2 N  1 , respectively [4]. However, the Vickers hardness of the overlays increased fromabout 100 to about 150 leading to a decrease inconformability and embeddability. Generally amoderate level of hardness, not too soft but nottoo hard, is required to attain high conformabilityand embeddability, though the absolute valuesdepend on the specific application.In order to improve the overall bearing perfor-mances further a third element was tried to beadded to the Ag–C overlays. After several trials tolook for appropriate additional elements, Mo waschosen as one of the most promising thirdelements [7,8]. Mo is expected not only to control the hardness but also to decrease the coefficient of friction and the wear rate by forming solidlubricants. In the present study Ag-based overlayscontaining C and Mo were prepared by ECR-DChybrid plasma sputtering and the synergisticeffects of simultaneous addition of C and Mo ontheir tribological properties were inferred. 2. Experimental procedure Ag-matrix overlays containing C and Mo wereprepared by ECR-DC hybrid plasma sputtering.The normal processing conditions were: depositiontime; 120min, sputtering distance (the nearestdistance between the target and the substrate);30mm, Ar pressure; 0.092Pa, microwave power;400W, the current applied to the magnetic coil;15.5A and the bias potential applied to the target;  800V.The target consists of a stack of Mo, Gr and Agrings as shown in Fig. 1. The thickness and theinternal and external diameters of each ring were5, 80 and 90mm, respectively. This method is veryuseful for the preliminary experiments to deter-mine the optimum compositions and is called‘‘ring method’’ here. The ratios of the number of Mo, Gr and Ag rings were set at 0/2/8; 1/2/7; 2/2/6and 3/2/5. The number of Gr rings was kept at 2for all the cases here because the ratio Gr/Ag=2/8had been found to be the best in the previouswork [6].An electrically grounded flange made of stain-less steel was set at the edges of the target as shownin the lower part of  Fig. 1, to prevent the plasmafrom squeezing into the backside of the target.This gives rise to a shadowing effect on the ringslocated at the edges of the target to decrease theireffective areas. Then the ratio of the effective areasdepends not only on that of the number of therings but also on their stacking position orsequence. By making use of this shadowing effect,however, it is possible to prepare an overlay withlow Mo concentration as will be shown later.It is to be noted here that the deposition ratedepends on the distance between the target and thesubstrate [4]. The rings located closer to the substrate are more effective to provide sputteredmaterial than those located far from it. Furtherdetails of the ECR-DC hybrid plasma have beendescribed elsewhere [3].The substrate (30mm  30mm  2mm) is of atwo-layer structure consisting of a Pb-free bronze ARTICLE IN PRESS Mo ringGraphite ringAg ringFlanges Fig. 1. Arrangement of the sputtering target consisting of Mo,Gr and Ag rings. C.P. Lungu et al. / Vacuum 76 (2004) 119–126  120  plate and a steel backing for strengthening. Thebronze surface onto which an overlay was to beformed was polished with emery paper  ] 600 beforethe deposition.The chemical composition and the distributionof C, Mo and O were investigated with anelectron-probe micro-analyzer (EPMA). Ramanspectroscopy was used to identify the crystal-lographic state of C in the Ag-matrix. Its spectrawere obtained in a back-scattering configurationusing the 514.5nm line of an Ar laser with 5mWpower and 50 m m spot diameter. The signal wasdetected with a photomultiplier using a standardphoton counting system with the acquisition timeof 60s. X-ray diffraction (XRD) was carried outwith Cu-K a  line operated at 30kV and 200mA.The micro-Vickers hardness was measured underthe load of 0.98N and the loading time of 15s.The coefficient of friction and the wear rate weremeasured with a ball-on-disk type tribometer,where a bearing steel (SUJ2 equivalent to AISI52100) ball of 6mm in diameter was used. Load of 5N and sliding radii of 3, 4 or 6mm were adopted.The sliding speed was 0.01 or 0.1ms  1 for themeasurements performed in the dry sliding and0.1ms  1 in the lubricated sliding. The coefficientsof friction were continuously monitored andevaluated after a total sliding time of 1000s. Thedry sliding tests were carried out at roomtemperature (297K) in air with 40–50% relativehumidity and the lubricated ones at room tem-perature in an oil (SAE  ] 30) bath. The coefficientof friction was obtained from the torque exertedon the arm holding the ball and the wear rate wasevaluated from the cross-sectional area of the wearscar. 3. Results and discussion 3.1. Overlay composition and structure Four kinds of overlays (a)–(d) were preparedhere by using the stacking number and sequenceshown in the second column of  Table 1, where 1/2/7, for example, indicates 1 Mo, 2 Gr and 7 Agrings were stacked together from left to right asshown in Fig. 1. The concentrations of Mo, C andAg of the obtained overlays are summarized in thetable. It is clearly seen that Mo concentrationincreased more rapidly than that predicted fromthe increase in the number of Mo rings. Theconcentrations of C and Ag were kept almostconstant except the case of (d), where a suddenchange in the concentrations was observed. Theseare explained by taking the peculiar characteristicsof the ring method into account, that is, thechemical compositions of the overlays depend notonly on the number of the constituent target ringsbut also on their stacking sequence.Fig. 2 shows the Raman spectra of all the kindsof overlays prepared here, where a Gaussian fittingis shown only for the overlay (d) as an example.This kind of fitting was performed for all the otheroverlays to separate D- and G-bands and theresults are summarized in Table 2. The D- and ARTICLE IN PRESS Table 1Chemical compositions of overlays prepared by the ringmethodThe number and Mo C Agstacking sequence (mass%) (mass%) (mass%)(Mo/Gr/Ag) (Balance)(a) 0/2/8 0.06 1.7 98.2(b) 1/2/7 0.8 1.6 97.6(c) 2/2/6 4.2 1.5 94.3(d) 3/2/5 45.0 1.0 54.0 100012001400160018002000 I D  /I G 1.641.801.751.57 D-bandG-band (d)(c)(b)(a)    I  n   t  e  n  s   i   t  y   (  a  r   b .  u  n   i   t  s   ) Raman shift, cm -1 Fig. 2. Raman spectra of the overlays prepared by the Mo/Gr/Ag ring combinations: (a) 0/2/8, (b) 1/2/7, (c) 2/2/6, (d) 3/2/5. C.P. Lungu et al. / Vacuum 76 (2004) 119–126   121  G-bands were found to be located at about 1370and 1575cm  1 on the average, respectively. Theirpositions are very near to those which arecharacteristic to the sp 2 sites of disorderedcarbons, 1350 and 1570cm  1 , respectively [9].The development of the D-band indicates thedisordering of Gr and the ordering of anamorphous or DLC structure simultaneously. Itsintensity is proportional to the number and thesize of sp 2 clusters, while its width is related to thedistribution of clusters with different order anddimensions. The G-band of Gr is related with thein-plane bond-stretching motion of sp 2 C atompairs, which does not require the presence of six-fold rings and occurs at all the sp 2 sites [9]. The risein hardness and brittleness with the increase in theMo concentration arises from the increase in theinternal stress in the overlay. This can also becorrelated with the increase in the G-band half-width (from 117.9 to 128.4cm  1 ), as suggested byTay [10].XRD patterns of the deposited overlays areshown in Fig. 3. The Ag peaks corresponding tothe (111), (200), (220) and (311) planesdecreased in intensity and were broadened withthe increase in the Mo concentration. For exam-ple, the half-width of the (311) Ag peak increasedfrom 0.48 1  for (a) to 0.54 1  and 0.77 1  for (b) and (c),respectively. In (d) only the Mo and the small Cupeaks were observed.The broadening of the Ag peaks indicates thatthe Ag lattice becomes more distorted and/or thatthe crystallographic grain size of the Ag matrixbecomes smaller by the Mo addition. SinceAg–Mo system is not known to form solid solution[11], the latter possibility seems to be larger thanthe former. The Ag–C system also does not formsolid solution [11]. According to the previouspaper [4], the C atoms were found to be distributeduniformly in the Ag matrix in the form of agglomerate particles of about 1 m m in diameter. 3.2. Mechanical and tribological properties The results of the micro-hardness measurementsare summarized in Fig. 4, where the average values of more than 3 measurements are plotted witherror bars. Fig. 5 shows the imprints of theanalyzed overlays after indentations using 0.49N(50g) load and 15s indentation time. It is to benoted here that the overlay (b) with Mo has lowerhardness than the overlay (a) without Mo. This iscontradictory to the well-known fact that Mo hashigher hardness than Ag. This is explained, ARTICLE IN PRESS Table 2Summary of D- and G-band characteristics of Raman spectroscopyPosition of D-band Position of G-band Half-width of D-band Half-width of G-band Intensity ratio( n /cm  1 ) ( n /cm  1 ) ( n /cm  1 ) ( n /cm  1 ) ( I  D / I  G )(a) 1363.65 1573.93 213.5 122.1 1.635(b) 1374.74 1580.09 232.9 117.9 1.803(c) 1374.77 1577.80 228.2 123.8 1.747(d) 1367.20 1572.39 261.9 128.4 1.570    I  n   t  e  n  s   i   t  y   (  a  r   b .  u  n   i   t  s   ) Cu (200) 30 o  40 o  50 o  60 o  70 o  80 o Cu (111)    A  g   (   2   2   0   ) Mo (211)    A  g   (   2   0   0   )   A  g   (   1   1   1   )   A  g   (   3   1   1   ) (d)(b)(c ) (a) 2θ Fig. 3. XRD patterns of the overlays prepared by the Mo/Gr/Ag ring combinations: (a) 0/2/8, (b) 1/2/7, (c) 2/2/6, (d) 3/2/5. C.P. Lungu et al. / Vacuum 76 (2004) 119–126  122  however, by the formation of a significant porousstructure by the addition of Mo as shown in Fig.5(b). This porous structure is the main cause forthe softening. The porosity, however, becomessmaller when the Mo concentration is increasedfurther as in the cases of (c) and (d), and thehardness drastically increases. The brittleness of the overlay (c) and (d) is also high as can bededuced from the cracks formed around theindentation imprints. The softening that wasobserved in the overlay (b) is beneficial for theimprovement of conformability and embeddabilityof the plain bearing overlay.Fig. 6 shows the typical results of the ball-on-disk tests for all the kinds of overlays in the drysliding at 0.01 and 0.1ms  1 . It is clearly seen thatthe results depend strongly on the Mo concentra-tion. The coefficient of friction decreased with theincrease in the Mo concentration at the initialrunning-in stage. The low adhesive force betweenthe hard ball and the top of the relatively softcone-structured overlay surface [4] is considered todominate the friction behavior at this stage. Afterthe initial stage is over, the energy necessary toplow the extremity of the cone-structured surfaceas well as to overcome the abrasive wear leads toan increase in the coefficient of friction [3,12]. Thiscorresponds to the cases of (c) and (d) with highMo contents. Overlay (b) exhibits the most stableand the lowest coefficient of friction both at low(0.01ms  1 ) and at high (0.1ms  1 ) sliding speeds.Fig. 7 shows the typical results of the ball-on-disk tests for all the kinds of overlays in lubricated ARTICLE IN PRESS 050100150200250300350400(a)(c)(b)(d)    V   i  c   k  e  r  s   h  a  r   d  n  e  s  s ,   H   V Fig. 4. Micro-Vickers hardness of the overlays prepared by theMo/Gr/Ag ring combinations: (a) 0/2/8, (b) 1/2/7, (c) 2/2/6, (d)3/2/5.Fig. 5. Indentation imprints of the overlays prepared by the Mo/Gr/Ag ring combinations after Vickers hardness tests using 0.49N(50gf) load and 15s indentation time: (a) 0/2/8, (b) 1/2/7, (c) 2/2/6, (d) 3/2/5. 05001000150020000.10.20.30.40.50.60.7 (d ) (d)(c)(c)(b)(b)(a)(a) Speed: 0.1 ms -1 Radius: 40 mmSpeed: 0.01 ms -1 Radius: 30 mm    C  o  e   f   f   i  c   i  e  n   t  o   f   f  r   i  c   t   i  o  n Sliding time, s Fig. 6. Evolution of the coefficient of friction during the ball-ondisk test of the overlays prepared by the Mo/Gr/Ag ringcombinations: (a) 0/2/8, (b) 1/2/7, (c) 2/2/6, (d) 3/2/5.Temperature; 297K, relative humidity of air; 45%, balldiameter; 6mm, applied load; 5N. C.P. Lungu et al. / Vacuum 76 (2004) 119–126   123
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