Low friction coatings prepared by high performance type spray gun

Low friction coatings prepared by high performance type spray gun
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  Surface and Coatings Technology 169–170  ( 2003 )  415–4180257-8972/03/$ - see front matter   2003 Elsevier Science B.V. All rights reserved.PII: S0257-8972 Ž 03 . 00142-7 Low friction coatings prepared by high performance type spray gun O. Fukumasa *, K. Osaki , S. Fujimoto , C.P. Lungu , Ana Mihaela Lungu a  ,  a a b b  Department of Electrical and Electronic Engineering, Faculty of Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube, a Yamaguchi 755-8611, Japan National Institute for Lasers, Plasma and Radiation Physics, Atomistilor No. 1, Bucharest, Romania b Abstract To produce low friction coatings to be used as overlays for automobile journal bearings and to perform depositions with highrate, we prepared composite coatings of Ag and graphite by using the new-type spray gun based on the forced constricted typeplasma jet generator  ( i.e. the high performance type spray gun ) . Characterization of the films showed the graphite phase dispersedinto the Ag matrix, analyzed by optical microscopy, electron probe microanalysis and X-ray diffraction. The coefficient of frictionof the Ag y graphite overlay measured using ball-on-disc tribometer was reduced by a factor of five compared with that of bronzesubstrate and by factor of two compared with that of the Ag y graphite overlay prepared by ECR sputtering.   2003 Elsevier Science B.V. All rights reserved. Keywords:  Plasma spraying; Silver; Graphite; X-ray diffraction; Ball-on-disc test; Coefficient of friction 1. Introduction Low friction coating materials are new classes of advanced materials, which exhibit a reduced coefficientof friction in dry sliding and raised wear resistance. Dueto the demand of high power and high-speed automotiveengines, the engine moving parts have to be improvedin tribological characteristics. The plain bearings consistof steel backing, bearing alloy and an electroplatedsliding layer or overlay. The bearing alloy is copper oraluminum base as a rule, the sliding layer consists of aternary lead–tin–copper alloy which principally per-forms the following functions:  ( i )  adapts to the surfaceof the shaft during the running phase;  ( ii )  absorbsforeign particles potentially present in the oil;  ( iii ) provides corrosion protection for the underlying lead–bronze alloy and  ( iv )  provides emergency runningbehavior under conditions of oil shortage  w 1,2 x . Theoverlays that provide seizure and wear resistance andconformability are usually made by electroplating of 10–30  m m thick lead alloy. The lead matrix is soft andhas relatively low fatigue resistance under high-pressureconditions. New ecologically tailored materials and new *Corresponding author. Tel.: q 81-836-85-9445; fax: q 81-836-85-9449.  E-mail address:  fukumasa@plasma.eee.yamaguchi-u.ac.jp ( O. Fukumasa ) . deposition procedures are needed to improve tribologicaland mechanical characteristics of the overlay.We previously reported the formation of Ag y graphiteoverlays by plasma sputtering  w 3 x  and spray coating of alumina mono-layer w 4 x by using thermal plasma reactorbased on the forced constricted type plasma jet generator w 5 x . Ag matrix overlays having low coefficient of frictionin dry sliding were found to increase significantly theseizure resistance of the plain bearings tested in condi-tions close to real operation w 6 x . In this paper, we studyfurther tribological coatings of Ag and Ag y graphite byusing the new-type spray gun  ( i.e. the high performancetype spray gun )  w 7,8 x , in order to obtain overlays withlower coefficient of friction and higher deposition rates. 2. Experimental procedures The high performance type device consists of the rodanode, the insulated constrictor disk and the rod cathode,as can be seen schematically in Fig. 1. They have thenozzle structure with the same diameter. The arc in thissystem is maintained at a constant length, and stronglyconstricted by the insulated disk, the local constrictingring and the working gas  ( Ar ) . As a result, a stable andhigh power plasma jet is produced under various oper-ating conditions. More details were presented elsewhere w 7,8 x .  416  O. Fukumasa et al. / Surface and Coatings Technology 169–170 (2003) 415–418 Fig. 1. Schematic draws of the high performance type spray gun.Fig. 2. OM cross-sectional view of the Ag and Ag y graphite overlays. In this system, powder particles are injected concen-trically into the arc column. So, the whole arc columnand the plasma jet region work as the interaction regionwith the injected powder particles. Injected powderparticles are efficiently heated and melted and areejected from the nozzle symmetrically. The plasma spraygun is fed with Ag powder and a mixture of Ag andgraphite of 20  m m in mean diameter. The mixture  ( Ag80 wt.% and graphite 20 wt.% )  was prepared by millingusing SiO balls during 24 h. 2 Typical processing conditions were: Ar flow rate; 50l y min, plasma arc power; 3 kW, powder feeding rate;0.1–0.2 g y min, deposition time; 1 min and pressure;ambient. Prior to the depositions the substrates wereexposed to the argon plasma in order to increase thesubstrate temperatures and consequently the bondingstrengths of the overlays. Temperatures of the substratesmeasured by a thermocouple were found to be 250, 180and 100  8 C during depositions accomplished at 3, 4 and7 cm distance, respectively.Characterization of prepared films was made as fol-lows:  ( i )  the inclusion of the graphite into the silvermatrix was detected by optical microscopy  ( OM ) ; elec-tron probe microanalysis  ( EPMA )  and X-ray diffraction ( XRD ) ,  ( ii )  Vickers hardness of the coatings wasmeasured by using an Akashi instrument, using 25 gf load and 15 s indentation time and  ( iii )  the coefficientof friction of the deposited film was measured by usinga ball-on-disk tribometer  ( CSEM Instrument-Switzer-land )  at room temperature and 50% relative humidityof air using bearing steel balls of 6 mm in diameter asa counter material. The applied loads were 1 and 5 Nand sliding radii 3 and 4 mm were chosen. The slidingspeed was kept constantly at 0.1 m y s in all themeasurements. 3. Experimental results and discussion For preparation of spray coatings, the substrate wassweeping perpendicularly on the plasma jet axis in ahorizontal plane or fixed at a certain substrate positionrelative to the nozzle exit. When the substrate was fixed,thickness distribution of the Ag and Ag y graphite over-lays was uniform around the center axis of the jetsuggesting symmetrical ejection of the melted particles.Maximum height of the depositions shaped as form of hills prepared at 3, 4 and 7 cm distances from the nozzleexit were 640, 240 and 100  m m, respectively. When asweeping device moved substrates perpendicularly tothe jet axis the prepared overlays were uniform alongthe sweeping direction and thickness was 100, 35 and25  m m correspondingly to the deposition distances of 3,4 and 7 cm. Deposition rates were between 100 and 25 m m y min, larger than in the case of the ECR sputtering ( f 0.1  m m y min ) w 3,6 x .Fig. 2 shows an OM cross-sectional view of the Agand Ag y graphite overlays. The Ag overlay was densewith small number of pores included. The graphiterepresented by black zones is clearly seen in the Ag y graphite overlay. EPMA cross-sectional analysis of theAg y graphite overlay reveals the dispersion of the graph-ite into the Ag matrix as can be seen in Fig. 3. Graphiteconcentrations into the overlays were: 16 " 2, 12 " 2 and8 " 3 wt.% for 3, 4 and 7 cm deposition distances,respectively.The graphite phase  ( Graphite-2H, hexagonal )  wasidentified from the XRD pattern of the Ag y graphiteoverlay as can be seen from the zoom part of  Fig. 4.Mainly, peaks were assigned to silver.Vickers hardness of the coatings was found to be25 " 3, 36 " 4 and 45 " 5 for 3, 4 and 7 cm depositiondistances, respectively. The Vickers hardness of thebronze substrate was 300 " 15, of the pure Ag coatingwas 100 " 10, being in the same range as the Ag coating  417 O. Fukumasa et al. / Surface and Coatings Technology 169–170 (2003) 415–418 Fig. 3. EPMA characteristic X-ray images of C inclusions into the Agmatrix presented as a gray scale.Fig. 4. XRD pattern of Ag y graphite coating deposited at 3-cm distancefrom the nozzle exit. The zoom part of the figure shows the charac-teristic peak   ( 2 u s 26.4 8 )  of graphite  ( Graphite-2H, hexagonal ) .Fig. 5. Frictional characteristics of the Ag and Ag y graphite coatingsprepared at 3-cm distance from the nozzle and of the bronze substrate.In this case substrates were fixed. prepared by sputtering. The fact that the hardness wasreduced with decreasing the deposition distance wasdetermined by the concentration of the graphite into thesilver matrix. At lower distance between the nozzle exitand the substrate, the graphite concentration was foundto be higher. At increased distances, some graphiteparticles were oxidized CO or CO gases. 2 Fig. 5 shows frictional characteristics of bronze, Ag y graphite and pure Ag coatings tested at loads of 1 and5 N, respectively. The reduction of the coefficient of friction can be observed in both cases. For the Ag y graphite coatings tested at high load,  ( i.e. 5 N case )  thereduction of the coefficient of friction appears moreremarkably. This suggests the predominant influence of the graphite solid lubricant inclusion in the silver matrix w 9 x . As shake of comparison, the coefficients of frictionof the Ag overlay and Ag y graphite overlays preparedby ECR sputtering were in the range of 0.6 " 0.1 and0.45 " 0.05, respectively w 3,6 x . The coefficient of the Agoverlay was in the same range for both cases, depositionrate being higher in the plasma spray case. Contrarily,the coefficient of friction of the Ag y graphite overlayprepared by plasma spray was two times lower than theAg y graphite overlay prepared by sputtering. We notethat this behavior is caused by the graphite phase thatwas preserved by using only plasma spray depositionmethod.The low coefficients of friction of the Ag y graphiteoverlays prepared by the method presented in this paperallow them to be used as sliding layers in order toincrease the seizure resistance of the plain bearings aswas demonstrated previously by sputtering  w 10 x . Thehardness of Ag y graphite overlay was about one quarterlower than that of the Ag overlay and about one-tenthlower than that of the bronze substrate, as mentionedabove, leading to increase the embedability and theconformability of the overlays. 4. Conclusions In order to demonstrate the application feasibility of the plasma electrode type spray gun reactor based onthe forced constricted type plasma jet generator forfabricating tribological materials, we have prepared theAg and Ag y graphite overlays for engine bearings. Thepowder particles injected into the arc were efficientlyheated and melted and were ejected from the nozzlesymmetrically determining high deposition rates. Thecoefficients of friction of the Ag and Ag y graphite  418  O. Fukumasa et al. / Surface and Coatings Technology 169–170 (2003) 415–418 overlays was found to be reduced by a factor of twoand five, respectively, compared with that of the bronzeused as substrate measured using a CSEM pin-on-disctribometer in dry sliding. The coefficient of friction of the Ag y graphite overlay was found to be two timeslower than of the Ag y graphite overlay prepared bysputtering. The graphite phase was found to be preservedinto the silver matrix. Acknowledgments One of the authors  ( C.P. Lungu )  would like to expresshis gratitude to New Energy and Industrial DevelopmentOrganization  ( NEDO )  Japan for the financial support.A part of work is financially supported by a Grant-in-aid for University and Society Collaboration from Jap-anese Ministry of Education, Culture, Sports, Scienceand Technology. References w 1 x  H. Ishikawa, K. Nomura, Y. Mizuno, H. Michioka, Y. Fuwa,S. Yasuhara, SAE Technical Paper Series No. 960988,  ( 1996 ) 1. w 2 x  K.H. Grunthaler, H.U. Huhn, K. Muller, K. Statschko, J.R.Toth, SAE Technical Paper Series No. 960984,  ( 1996 )  1. w 3 x  C.P. Lungu, K. Iwasaki, J. IAPS 8  ( 2000 )  65. w 4 x  O. Fukumasa, Oyo Butsuri 67  ( 1998 )  181, in Japanese. w 5 x  O. Fukumasa, Thin Solid Films 390  ( 2001 )  39. w 6 x  C.P. Lungu, K. Iwasaki, Vacuum 66  ( 2002 )  385. w 7 x  K. Osaki, H. Ashida, O. Fukumasa, N. Tada, A. Kobayashi,Proceeding First International Symposium on Applied PlasmaScience,  ( 1997 )  61. w 8 x  K. Osaki, O. Fukumasa, S. Fujimoto, A. Kobayashi, Vacuum65  ( 2002 )  305. w 9 x  A. Savan, E. Pfluger, R. Goller, W. Gissler, Surf. Coat. Technol.126  ( 2000 )  159. w 10 x  C.P. Lungu, K. Iwasaki, T. Kawachi, H. Ishikawa, Formationof seizure resistant surface microstructures by electron cyclo-tron resonance plasma sputtering, in: F. Franek, W.J. Bartz, A.Pauschitz  ( Eds. ) , Tribology 2001, The Austrian TribologySociety, 2001, pp. 291–298.
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