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Tribological Properties of Ag/Graphite/Mo Overlays for Plain Bearings Prepared by Ecr-DC Sputtering

In order to improve the electron cyclotron - direct current (ECR-DC) sputtering process, Ag/Graphite/Mo overlays were produced by using composite target made by sintering and compared with those prepared by the "ring method". Wear and
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   NATIONALTRIBOLOGYCONFERENCE24-26 September 2003THE ANNALS OF UNIVERSITY“DUN Ă REA DE JOS“ OF GALA Ţ IFASCICLE VIII, TRIBOLOGY2003 ISSN 1221-4590 93 TRIBOLOGICAL PROPERTIES OF AG/GRAPHITE/MOOVERLAYS FOR PLAIN BEARINGS PREPARED BY ECR-DCSPUTTERING Cristian Lungu 1,2 ,Kunihiko Iwasaki 2 1  National Institute for Lasers, Plasma and Radiation Physics, Bucharest, Romania, 2 Japan Ultra-high Temperature Materials Research Institute, Ube-city, ABSTRACT  In order to improve the electron cyclotron – direct current (ECR-DC) sputtering process, Ag/Graphite/Mo overlays were produced by using compositetarget made by sintering and compared with those prepared by the ”ring method”.Wear and frictional testing was carried out with a CSEM (Switzerland) ball-on-disk tribometer. The counter material was SUJ2 ball bearings with 6 mm in diameter. Measurements were made in air (45-48% relative humidity) at room temperature(25-29 o C) and both in dry and in lubricated (SAE#30 oil) conditions. The propertiesof the prepared overlays were studied and compared with those of the Ag/Graphiteoverlays. Optical microscopic observation of the wear scars of overlays after testing at different sliding speeds in dry and lubricated conditions shows that the smoothwear scar appears at a low sliding speed. At a sliding speed of 0.1 ms -1  a plastic flow can be observed along the sliding direction.  KEYWORDS:   Ag, graphite, ECR-DC sputtering, plain bearing, dry and lubricated sliding. 1.   INTRODUCTION Ag-matrix coatings acting as anti-friction layersfor the engine plain bearings were found to have asuperior combination of mechanical and physical properties such as high toughness, high fatigueresistance, low coefficient of friction in dry andlubricated sliding and excellent resistance againstwear and seizure [1-2]. They are also ecologicallyfriendly, replacing Pb base overlays currently in use.Addition of graphite and metal dopants to Agmatrix overlays was found to reduce the coefficient of friction in dry and lubricated sliding, respectively [2]. 2. EXPERIMENTAL METHODS 2.1 Overlay deposition Ag matrix overlays were prepared using anECR-DC hybrid system presented in detail elsewhere[3]. The Pb-free bronze substrates (30 mm x 30 mm x2 mm) were ground with 600 grit emery paper to havea rough uniform initial surface. The deposition timewas 7.2 ks and the processing conditions were:sputtering distances; 20; 30 and 40 mm, pressure;0,093 Pa, Ar flow rate; 160 mm 3 s -1 , microwave power; 400 W, microwave frequency; 2,45 GHz, potential applied to target; 800 V, DC current; 280 – 500 mA.Fig. 1 The principle of the ring assembling.The target consists of a stack of Mo, Gr and Agrings as shown in figure 1. The thickness and theinternal and external diameters of each ring were 5mm, 80 mm and 90 mm, respectively. This method isvery useful for the preliminary experiments to   NATIONALTRIBOLOGYCONFERENCE24-26 September 2003THE ANNALS OF UNIVERSITY“DUN Ă REA DE JOS“ OF GALA Ţ IFASCICLE VIII, TRIBOLOGY2003 ISSN 1221-4590 94determine 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; 2/2/6 and3/2/5. The number of Gr rings was kept at 2 for all thecases here because the ratio Gr/Ag = 2/8 had beenfound to be the best in the previous work [5]. Anelectrically grounded flange made of stainless steelwas set at the edges of the target as shown in thelower part of figure 1 to prevent the plasma fromsqueezing into the backside of the target. This givesrise to a shadowing effect on the rings located at theedges of the target to decrease their effective areas.Then the ratio of the effective areas depends not onlyon that of the number of the rings but also on theirsstacking positions or sequence. By making use of thisshadowing effect, however, it is possible to prepare anoverlay with low Mo concentration as will be shownlater.A composite target was made by sintering 97mass% Ag + 2 mass% graphite + 1 mass% Mo powders of 10-50 µ m mean diameter. The powderswere milled for 72ks and compacted by pressing themat 60MPa in a special mold of 60 mm in diameter.The compacts were cold-isostatic-pressed (CIP) at294MPa for 1.8ks. Finnaly the compound wassintered in air at 873K for 900s. The sinteredcomposites were shaped in a cylindrical mold with a45 mm radius in order to be fitted in the target holder.The thickness of the overlays prepared by usinga pile of rings and a composite target was in the rangeof 12 – 17 µ m, commonly used thickness for theoverlay of a three-layer engine bearing [4].Wear and frictional testing was carried out witha CSEM (Switzerland) ball-on-disk tribometer. Thecounter material was SUJ2 ball bearings with 6 mm indiameter. Measurements were made in air (45-48%relative humidity) at room temperature (25-29 o C) and both in dry and in lubricated conditions using aSAE#30-oil. Load of 5N and sliding radii between 2and 5 mm were chosen. The sliding speed was 0.01ms -1  and 0.1 ms -1  in dry condition and 0.1 ms -1  inlubricated condition. Wear rate was calculated fromthe cross-sectional area of the wear scar measuredwith a profilometer. The calculated wear volume wasdivided then by load and sliding distance.Micro Vickers hardness was measured using anAkashi microhardness tester. Loads of 9.8 mN, 98 mNand 15 s indentation time were used. After indenta-tion, the indentation imprints were analyzed optically.A thrust-type test was carried out using aspecifically designed specimen. The flat-to-flat slidingwas performed keeping in Hertzian contact a coronalsurface of the substrate and a shaft counter part.During the test, the applied load was increasedstepwise by 3MPa at 300s time interval. The specimentemperature and the coefficient of friction werecontinuously monitored. A sharp raise of thecoefficient of friction or specimen temperature wasnoticed 3.   RESULTS AND DISCUSSION 3.1   Overlays prepared by ring method 3.1.1 Composition of the overlays Chemical composition of the overlays prepared by the ring method was found to depend not only onthe number of the constituent target rings but also ontheirs stacking sequence. The rings located near thecenter of the cylindrical target is more effective thanthose located near the edges because the former areexposed to the plasma more effectively than the latter due to the presence of the grounded flange as wasdescribed before (Fig. 1).Four kinds of overlays (a), (b), (c) and (d) were prepared here by using the stacking number andsequence shown in the second column of Table 1,where 1/2/7, for example, indicates 1 Mo, 2 Gr and 7Ag rings were stacked together from left to right asshown in figure 1 The concentrations of Mo, C andAg are summarized in the table. It is clearly seen thatMo concentration increased more rapidly than that predicted from the increase in the number of Morings. The concentrations of C and Ag were keptalmost constant except the case (d). The suddenchange observed in the concentrations of Mo, C andAg in the case of (d) comes not only from theshadowing effect but also from the staking positionof the Mo rings that was close to the substratecompared to that of the Ag rings. A trace of Modetected in the overlay (a) may be due to thecontamination of the former experiments. Table 1. Composition of the overlays (a) – (d).Mo/Gr/AgMomass%Cmass%Agmass%(a)0/2/80.061.798.2(b)1/2/70.81.697.6(c)2/2/64.21.594.3(d)3/2/545.01.054.0 3.1.2 Mechanical and tribological  propetries The results of the micro-hardness measure-ments are summarized in figure 2, where the averagevalues of more than 3 measurements are plotted witherror bars. It is to be noted here that the overlay (b)with Mo has lower hardness than the overlay (a)without Mo. This is quite unexpected because Mo isknown to have higher hardness than Ag. This isexplained by the formation of a significant porousstructure by the addition of Mo as shown in figure3(b), where the indentation imprints after themicrohardness test of the overlays (a) – (d) are presented, though its formation mechanism has not been known yet. This porous structure is the maincause for the softening. The porosity, however, becomes smaller when the Mo concentration is   NATIONALTRIBOLOGYCONFERENCE24-26 September 2003THE ANNALS OF UNIVERSITY“DUN Ă REA DE JOS“ OF GALA Ţ IFASCICLE VIII, TRIBOLOGY2003 ISSN 1221-4590 95increased further as in (c) and (d), and microhardnessdrastically increases. 0/2/81/2/72/2/63/2/5050100150200250300350400 (a)(c)(b)(d)     V   i  c   k  e  r  s   h  a  r   d  n  e  s  s ,   H   V Mo/Gr/Ag rings Fig. 2 Micro-Vickers hardness of the deposited filmsusing Mo/Gr/Ag ring combinations: (a); 0/2/8, (b);1/2/7, (c); 2/2/6, (d); 3/2/5. 30 m(a) (b) (c) (d) Fig. 3 Indentation imprints of the developed overlaysafter the indentation test (Load: 98mN).The brittleness of the film (c) and (d) alsoincreases, as can be deduced from the cracks formedaround the indentation imprints. The softening thatwas observed in the overlay (b) is considered to bevery beneficial for the improvement of conformabilityand embeddability of the plain bearing overlay. It is to be noted here that the overlay (b) with Mo has lower hardness than the overlay (a) without Mo. This isquite unexpected because Mo is known to have higher hardness than Ag. This is explained by the formationof a significant porous structure by the addition of Moas shown in figure 3b though its formation mechanismhas not been known yet. This porous structure is themain cause for the softening. The porosity, however, becomes smaller when the Mo concentration isincreased further as in (c) and (d), and microhardnessdrastically increases. The brittleness of the film (c)and (d) also increases, as can be deduced from thecracks formed around the indentation imprints. Thesoftening that was observed in the overlay (b) isconsidered to be very beneficial for the improvementof conformability and embeddability of the plain bearing overlay.Figure 4 shows the typical results of the ball-on-disk tests for all kinds of overlays in dry sliding at0.01 ms -1  and 0.1 ms -1 . It is clearly seen that theresults depend strongly on the Mo concentration. Thecoefficient of friction in dry sliding decreased with theincrease in the Mo concentration at the initial running-in stage. The low adhesive force between the hard balland the top of the relatively soft cone-structuredoverlay surface [4] is conside-red to dominate thefriction behavior at this stage. After the initial stage isover, the energy necessary to plow the extremity of the cone-structured surface as well as to overcomethe abrasive wear leads to an increase in thecoefficient of friction [3, 13]. This corresponds to thecases of (c) and (d) with high Mo contents. Theoverlay (b) exhibits the most stable and the lowestcoefficient of friction both at low (0.01ms -1 ) and athigh (0.1ms -1 ) sliding speeds. 05001000150020000,20,40,6 (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. 4 Coefficient of friction behavior during ball-ondisk test of the films prepared using Mo/Gr/Ag ringcombinations: (a); 0/2/8, (b); 1/2/7, (c); 2/2/6, (d);3/2/5. Temperature: 24 o C, relative humidity of air:45%, ball diameter: 6 mm, applied load: 5N. 3.2   Overlays prepared by composite targets3.2.1   Target preparation For the industrial use of the ECR-DC sputteringmethod is necessary to have composite sputteringtargets in order to insure the reproducibility of the process. For this purpose we prepared Mo-Gr-Agcomposite targets, used them for sputtering and madeoverlays characterization.The flow chart of composite target preparationis shown in figure 5. Mo-Gr-Ag powders with 20-50 µ m grain size were milled during 20 h in dry air. Thechosen mixtures were: A: 97mass%Ag, 2mass%Gr,1mass%Mo and B: 96mass%Ag, 2mass%Gr,2mass%Mo.After milling, the mixtures were compactedusing molds having 60 mm in diameter. After 60MPa pressing, 30-g powder was compacted as about 2 mmthickness disks. The disks were pressed using the CIP(cold isostatic pressing) method at 294MPa, 30minutes. After that the compacts were sintered andcylindrical shaped in dry air at about 600 O C for 15minutes. Figure 6 shows such a process when acylindrical load is pressed on the compact disc settledin a cylindrical mold made by Ti.   NATIONALTRIBOLOGYCONFERENCE24-26 September 2003THE ANNALS OF UNIVERSITY“DUN Ă REA DE JOS“ OF GALA Ţ IFASCICLE VIII, TRIBOLOGY2003 ISSN 1221-4590 96 AgGraphiteMoMilling: 20 hPressing: 60 MPa Cold Isostatic Pressing (294 MPa)PowdersTHE COMPOSITE TARGET METHODSintering (600 o C, 15 min) Fig .5 Flow chart of composite target preparationFig. 5 Cylindrical shaping of the compact disc: 90 mmradius of curvatureFig. 6 Composite target appearance after thesputtering process.The prepared disks were cut in cylindricalsectors about 45mm in length and then assembled intothe target holder of the ECR-DC sputtering system.Figure 6 shows the photograph of the developedcomposite target mounted into the ECR-DC water cooled target holder after a sputtering process. 3.2.2    Ball-on-disk tribological tests Ag-matrix including Mo and Gr were depo-sited on rectangular substrates (30 mm x 30 mm x2mm size) made of rough (#600 grit) bronze as wellas on the special designed substrates for the thrust-type seizure resistance tests. Deposition time was 120minutes and the processing conditions were:sputtering distance: 2-4cm, pressure: 0.092Pa (7 x 10 -4  Torr), Ar flow rate: 10 sccm, microwave power: 400W, DC bias: 800 V, DC current: 350 - 400mA.Fig. 7 Wear scar after tribological test of theoverlays prepared using 97mass%Ag, 2mass%Gr and1mass%Mo powder  An optical micrograph of the wear scars of overlays prepared using 97mass%Ag, 2mass%Gr,1mass%Mo powder combination, 30 mm sputteringdistance after testing at different sliding speeds in dryand lubricated conditions are shown in figure 7. 020040060080010000,00,20,40,60,8      C  o  e   f   f   i  c   i  e  n   t  o   f   f  r   i  c   t   i  o  n Sliding time, s  40 mm 30 mm 20 mm BronzeDry slidingSliding speed: 0.1 m/s Fig. 8 Frictional characteristics in dry sliding,high sliding speed, of the coatings prepared at 20, 30and 40 mm sputtering distance, high speed .  At a low sliding speed the wear tracks aresmooth. At a sliding speed of 10 mms -1  a plastic flowalong the sliding direction can be observed. Inlubricated conditions at a speed of 100mms -1  the   NATIONALTRIBOLOGYCONFERENCE24-26 September 2003THE ANNALS OF UNIVERSITY“DUN Ă REA DE JOS“ OF GALA Ţ IFASCICLE VIII, TRIBOLOGY2003 ISSN 1221-4590 97surface was smooth too, with small grooves along thesliding direction.The coefficients of friction of the overlays prepared at 20, 30 and 40mm sputtering distance werein the range of 0.65 ± 0.05, lower than that of the bronze substrate (0.80 ± 0.05) as can be seen in figure8 for the high sliding speed test (0.1ms -1 ) and in figure9 for higher sliding sped test (0.01ms -1 ) performed indry condition. In lubricated conditions, thecoefficients of friction of the coatings were lower andmore stable that that of bronze substrate, after acertain sliding distance (50m), as can be seen in figure10. The coefficient of friction of bronze was low atthe beginning due to the sliding on the top of the“hills” present on the surface. Latter, a close contactwith material increases the coefficient of friction. 0200400600800100012000,00,20,40,60,8      C  o  e   f   f   i  c   i  e  n   t  o   f   f  r   i  c   t   i  o  n Sliding time, s  40 mm 30 mm 20 mm BronzeDry slidingSliding speed: 0.01 m/s Fig. 9 Frictional characteristics in dry sliding of thecoatings. Sliding at low speed. 20040060080010000.0900.0950.1000.1050.110 Bronze (b)(a)(c)      C  o  e   f   f   i  c   i  e  n   t  o   f   f  r   i  c   t   i  o  n Sliding time, s (a): sputtering distance = 20 mm(b): sputtering distance = 30 mm(c): sputtering distance = 40 mmSliding speed: 0.1 ms -1 Load: 5 N, Lubricant: Oil SAE #30 Fig. 10 Frictional characteristics in lubricated sliding,high sliding speed, of the coatings prepared at 20, 30and 40 mm sputtering distance.Figure 11 shows the effect of the sliding speedand sliding conditions on the wear rates. The wear rates and the scar surface appearance reveal a mildwear both in dry and in lubricated conditions of theoverlays prepared using the composite target method. Bronze20 mm30 mm40 mm 0123406080      W  e  a  r  r  a   t  e ,  x   1   0   -   1   5    m    2    N   -   1 Sputtering distance  0.01 ms -1  (dry) 0.1 ms -1  (dry) Lubricated (0.1ms -1 )Load: 5 N, 45% Relative humidity Fig. 11 Wear rates in dry and lubricated sliding, highsliding speed and low sliding speed of the coatings prepared at 20, 30 and 40 mm sputtering distance. Ascomparison, the wear rates relative to the bronzesubstrate are shown, too. 3.2.3 Thrust-type tribological tests In the thrust type test, the sliding contact occursonly on the segmented circular part with 22 and27.2mm in internal and in external diameters,respectively. The schematic diagram of the testequipment is shown in figure 12. The test conditionsare listed in the caption of the same figure. Typicaltemperature and torque monitoring chart during thetest is shown in figure 13, together with those ones of the bronze substrate. The results of the test shown infigure 14 reveal the higher value of seizure resistanceof the composite overlay deposited at 40mmsputtering distance, in connection with the lower roughness of the overlay.Fig. 11 Schematic diagram of the fundamentalseizure test. Test conditions: Shaft roughness: R  max : 1 µ m, Bearing size: ∅ 27.2x ∅  22mm, Sliding velocity:2m/s (1500 rpm), Oil grade: SAE #30, Oil supplyrate: 20 ml/min, Load: increased with 3MPa at every30 minutes.
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