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Low friction silver-DLC coatings prepared by thermionic vacuum arc method

Low friction silver-DLC coatings prepared by thermionic vacuum arc method
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  Vacuum 76 (2004) 127–130 Low friction silver-DLC coatings prepared by thermionicvacuum arc method C.P. Lungu a,  , I. Mustata a , G. Musa a , V. Zaroschi a ,Ana Mihaela Lungu a , K. Iwasaki b a National Institute of Lasers, Plasma and Radiation Physics, Atomisitilor 111, 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 By using a new type of plasma discharge, thermionic vacuum arc (TVA), silver diamond-like carbon (DLC)composite overlays were prepared successfully for tribological applications. Carbon was incorporated into the silvermatrix at concentration between 19 and 42 mass% as a function of the distance to the two TVA guns used to evaporatesimultaneously silver grains and a graphite rod. It was found that the surface of the obtained overlays was smooth(5–10nm peak-to-valley roughness) as measured by atomic force microscope (AFM) in contact mode. Carbon inclusionas DLC phase in the silver matrix reduced the coefficient of friction in dry sliding up to 2.5 times compared to that of the bronze substrate. r 2004 Published by Elsevier Ltd. Keywords:  Thermionic vacuum arc (TVA); Silver-DLC composite; Carbon; AFM; Coefficient of friction 1. Introduction A new class of advanced materials with con-trolled tribological properties and environmentalfriendliness is currently being developed in orderto be applied in the automotive industry as lowfriction coatings for plain bearings by usingelectron cyclotron resonance-direct current(ECR-DC) sputtering [1,2]. Thin film depositionprocess by thermionic vacuum arc (TVA), a newdischarge type in pure metal vapor plasma, mightbecome one of the most suitable technologies tosignificantly improve the tribological properties of the surfaces covered with different materials. TVAcan be ignited only in high vacuum (HV) or ultrahigh vacuum (UHV) conditions between a heatedcathode surrounded by an electron focusingWhenelt cylinder and an anode (crucible) contain-ing the material to be deposited [3,4]. Due to thecontinuous electron bombardment of the anode(positively charged with controllable high-voltagesupply) by the accelerated thermo-electrons com-ing from the grounded cathode, the anode material ARTICLE IN PRESS$-see front matter r 2004 Published by Elsevier Ltd.doi:10.1016/j.vacuum.2004.07.002  Corresponding author. Fax: +40-21-457-4468. E-mail address: (C.P. Lungu).  first melts and afterwards starts to evaporateensuring a steady state concentration of theevaporated atoms in the cathode–anode space.At further increase in the applied high voltage, abright discharge is established inside the vacuumvessel in the vapors of the pure anode material.The energy of ions of the TVA plasma can bedirectly controlled and established at the requiredvalue even during arc running by changing thecathode heating current and anode potential. Asmentioned in a recently published paper, ionbombardment ensures better quality of the depos-ited thin film [5]. 2. Experimental set up and method The experimental set-up is shown in Fig. 1. Both Ag and graphite were evaporated simultaneouslyusing two different TVA guns. The cathode of each of the guns consisted of a heated tungstenfilament surrounded by molybdenum Wheneltcylinder, which had an aperture of 10mm indiameter. The filament for the silver discharge wasmade of a tungsten wire of 0.6mm in diameter,while the filament for the carbon discharge wasmade of thoriated tungsten wire of 3mm indiameter. The filaments were arranged in theapertures of the Whenelt cylinders in the plane of their front surface. A hydrogen free graphite rod20mm in length and 10mm in diameter was usedas anode in the carbon discharge case, and silvergrains of 5mm in diameter filled the anodecrucible in the silver discharge case. The inter-electrode gap was adjusted in the range of 4–8mm.The ion energy was evaluated by using a retardingfield analyser of multiple mesh type. Two bronzesubstrates of 25mm  25mm  3mm were posi-tioned above the TVA guns at 240mm distance tothe carbon anode and 320mm to the silver anode,sample (a) and 340 and 240mm, respectively,sample (b).The film composition was measured by using aX-ray fluorescence (XRF) analyzer in quantitativemode and the crystallographic phase of carbonincluded in the prepared films was characterized bya Laser–Raman spectrometer using 514.4nmwavelength radiation of an Ar ion laser, 5mWpower and 1 m m spot diameter. The film morphol-ogy was analyzed by using an atomic forcemicroscope (AFM) in contact mode. The coeffi-cient of friction of the deposited film was measuredby using a ball-on-disk tribometer at roomtemperature and 50% relative humidity of air. Abearing steel ball of 6mm in diameter was used asa counter material. Load of 5N and sliding radiusof 4mm were chosen. The sliding speed was keptat 0.1m/s in all the measurements. 3. Results and discussion The following parameters were found to controlthe TVA: arc currents;  I  arc ; cathode thermoelec-tronic current (controlled by the heated cathodetemperature)  T  c ; Inter-electrode distance,  d  , theangle between an imaginary perpendicular linefrom the anode and the axis of the heated cathode.Both the cathode of TVA and the vacuum vesselare at earth potential, the plasma has a positivepotential with respect to the vacuum vessel wallwhich is roughly equal to the cathode potentialfall. In these circumstances, the sample (and thegrowing layer too) is subjected to an intensebombardment by both evaporated atoms andenergetic ions during the film deposition.The ionic energies in the vicinity of the substrateholder were found to be in the range of 50–300eVleading to the formation of composite films with asmooth morphology as was shown by the AFM ARTICLE IN PRESS Fig. 1. Experimental set-up. C.P. Lungu et al. / Vacuum 76 (2004) 127–130 128  analysis. Fig. 2 shows the surface of the sample (a)scanned on an area of 1 m m  1 m m size. The peak-to-valley roughness was found in the range of 5–10nm. The surfaces of the deposited films weresmooth and reproduced properly the initial rough-ness of the bronze used as substrates allowing thecoating to keep the embeddability of foreignparticles in the running-in process of the enginebearings.XRF analyses of the prepared films showed aconcentration of about 44 mass% C in the sample(a) fixed close to the carbon anode and about 19mass% C in the sample (b) positioned closer to thesilver anode.Carbon was incorporated into the compositefilms as DLC phase, as defined recently by Ferrariand Robertson [6] with a low ratio of characteristicG-/D-band intensities. Fig. 3 shows a typicalRaman spectrum of the prepared film—sample(a). A Gaussian fit was made in order to separatethe D and G peaks.  I  G / I  D  was found to be 0.34and 0.23 for the samples (a) and (b), respectively.The D and G peaks are characteristic of the sp 2 sites of all disordered carbons at about 1350 and1570cm  1 , respectively. Development of the D-band indicates disordering of graphite but order-ing of an amorphous carbon structure; its intensityis proportional to the number and size of sp 2 clusters, while its width is more related to anarrower distribution of clusters with differentorder and dimensions. The G-band of graphiteinvolves the in-plane bond-stretching motion of pairs of carbon sp 2 atom; this mode does notrequire the presence of six-fold ring and so itoccurs at all sp 2 sites [6]. The influence of carbonincorporation into the film as DLC led to a drasticdecrease in the coefficient of friction tested in drycondition. Fig. 4 shows the frictional behavior of the Ag-DLC films compared to that of the bronzesubstrate.The reduction of the coefficient of friction canbe observed for both the coatings compared tothat of the bronze substrate. A stable and lowcoefficient of friction was exhibited by sample (a),where carbon concentration was higher. Thissuggests the predominant influence of the DLC ARTICLE IN PRESS Fig. 2. AFM image of the sample (a). Scanned area:1 m m  1 m m.Fig. 3. Typical Raman spectrum of the sample (a).Fig. 4. Coefficient of friction versus sliding distance. C.P. Lungu et al. / Vacuum 76 (2004) 127–130  129  acting as a solid lubricant inclusion in the silvermatrix.Due to the ion bombardment, the filmswere compact and very smooth, as shown inFig. 2. Due to the incident energetic ions, theadherence of the thin film to the substrateincreases remarkably. In this case the adherencewas directly related to the value of the energy of ions, increasing with this energy. Taking thepeculiarities of the carbon film deposition intoconsideration, TVA method is considered to beone of the most adequate technologies for this fieldof applications. Indeed, due to the ensured highpurity of the deposition process (in vacuum vesselonly carbon and silver being introduced besidesrefractory metals used as electrodes) completelyhydrogen free carbon films can be obtained. At thesame time, TVA technology ensures high efficiencyin producing high energy electrons to heat carbonwhich needs temperatures higher than 4000K forsublimation.Due to the vacuum conditions and highsublimation temperature of the carbon and re-lative low melting temperature of the silver themain energy losses are practically only by radia-tion. Taking these advantages into account TVA isexpected to be very promising for preparation of Ag-DLC tribological coatings. 4. Conclusions Silver-DLC composite overlays for tribologicalapplications were successfully prepared by TVAmethod. The ion energy near the substrate wasfound to be in the range of 50–300eV. Carbon wasincorporated into the silver matrix at concentrationbetween 19 and 42 mass%, as a function of distanceto the two TVA guns used to evaporate simulta-neously silver and carbon anodes. It was found thatthe surface of the obtained overlays was smooth(5–10nm peak to valley roughness) as measured byAFM in contact mode. Carbon inclusion as DLCphase in the silver matrix reduced the coefficient of friction in dry sliding up to 2.5 times compared tothat of the bronze substrate. References [1] Iwasaki K, Lungu CP, Takayanagi S, Ohkawa K. Int JAppl Mech Eng 2002;7:351.[2] Lungu CP, Iwasaki K. Vacuum 2002;66:385.[3] Musa G, Ehrich H, Schuhmann J. IEEE Trans Plasma Sci1997;25:386.[4] Ehrich H, Schuhmann J, Musa G, Popescu A, Mustata I.Thin Solid Films 1998;333:95.[5] Biloiu C, Erich H, Musa G. J Vac Sci Technol2001;A19:757.[6] Ferrari AC, Robertson J. Phys Rev 2000;B61:14095. ARTICLE IN PRESS C.P. Lungu et al. / Vacuum 76 (2004) 127–130 130
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