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diamond-like carbon multilayer coatings on steel for tribological applications
Surface and Coatings Technology 148 (2001) 277–283
Tungsten carbideydiamond-like carbon multilayer coatings on steel for tribological applications ´ a,*, G. Zambranob, A. Carvajalb, P. Prietob, H. Galindoc, E. Martınez ´ d, A. Lousad, C. Rincon J. Esteved a
´ ´ Departamento de Fısica, Universidad Autonoma de Occidente, Cali, Colombia b ´ Departamento de Fısica, Universidad del Valle, Cali, Colombia c ´ ´ Departamento de Fısica, Universidad de los Andes, Merida, Venezuela d ` ´ Universitat de Barcelona, Departament de Fısica Aplicada i Optica, Av. Diagonal, 647 E-08028 Barcelona, Catalunya, Spain Received 30 April 2001; accepted in revised form 3 July 2001
Abstract Tungsten carbideydiamond like (W–CyDLC) multilayers have been investigated as low friction coatings on high-speed steel substrates. The coatings are composed of a W–C multilayer base and an upper lubricious DLC layer and they are obtained by reactive r.f. magnetron sputtering from a single target comprising of two equal halves; one-half carbon and the other half tungsten. The whole coating structure was obtained in situ, without any interruption of the sputtering process. Transmission electron microscopy (TEM) and SIMS were used to assess the multilayer structure and XPS, XRD, and Raman spectroscopy was used to analyze its composition. The tribological properties of the coatings in sliding wear were investigated by means of scratch test and ball-on-disc test measurements. It was found that the multilayer W–C base improves the adhesion of the upper DLC layer to steel substrates while maintaining its low friction coefficient. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Tungsten carbideydiamond like multilayers; Low friction coating; High-speed steel substrates
1. Introduction Diamond like carbon (DLC) coatings technology is an expanding area of applied materials due to the unique combination of properties they exhibit. For example, their surface smoothness, hardness and chemical inertness in combination with a low friction coefficient against most metals, make DLC films interesting for wear-protective purposes. Nowadays, the use of wearprotective DLC films is common on automotive applications, magnetic recording media w1,2x, optics w3x, biomedical prostheses w4x and hard coatings for cutting tools. However, DLC films present high compressive internal stress and low chemical interaction between films and their metal substrates, this causes poor adhe* Corresponding author. ´ E-mail address: [email protected] (C. Rincon).
sion to metal substrates thus limiting their commercial use. At the present time, the most general approach to solve the adhesion problem is to use metal or silicon interlayers between steel and DLC. It is of special interest the use of interlayers with gradually changing composition incorporating metal, ceramic and DLC w5– 8 x. Magnetron sputtering is a useful technique for multilayer deposition. This technique allows good control of deposition parameters such as temperature, deposition rate and gas composition during the deposition process. In the case of multilayer films, they are generally obtained by using multitarget sputtering systems. In this work, we introduce a novel method for the deposition of W–C and DLC multilayer coatings by using a single sputtering target comprising of two equal halves; onehalf carbon, the other tungsten. The different layers are obtained by the variation of the methaneyargon sputter-
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 6 0 - 3
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´ et al. / Surface and Coatings Technology 148 (2001) 277–283 C. Rincon
Fig. 1. Schematic of the magnetron sputtering system.
ing gas composition without any interruption in the process. Using this deposition method we have prepared multilayer coatings with the aim of improving both adhesion and wear resistance of DLC protective coatings onto steel substrates. The coatings are formed by a multilayer stack of different W–C layers and an upper DLC layer. The objective of the multilayer base is to improve the adhesion and the mechanical resistance of the upper DLC layer while keeping its low friction coefficient w9x. 2. Experimental details 2.1. Film deposition A set of coatings were deposited by means of a r.f. (13.56 MHz) magnetron sputtering system, which is schematically represented in Fig. 1. The W–C sputtering target was made of two halves cut from 2.5-cm diameter W (99.99%) and C (99.99%) targets, which were bonded to a water-cooled backing plate. The r.f. power was 14 Wycm2. Coatings were deposited onto silicon wafers and also onto M2 (AISI) high-speed steel substrates polished with 1-mm diamond paste and ultrasonically cleaned in alcohol. The substrates were fixed to a holder 6 cm below the sputtering target. The discharge chamber was evacuated to a base pressure of 10y3 Pa before the introduction of the sputtering gas. Prior to coating deposition the target was sputter cleaned in argon gas
at a pressure of 4 Pa for 10 min, behind a shutter, in order to remove the possible target contamination. Substrates were electrically grounded. Two different types of WC single films were deposited by two different processes: (i) pure Ar at 4 Pa (process A) and; (ii) 20% CH4 yAr mixture (process B). Substrate temperature was 5008C for both processes. The DLC film was obtained by using a 40% CH4 yAr gas mixture and a substrate temperature of 1508C w10x (process C). The deposition parameters are listed in Table 1. The multilayer coatings were obtained by alternatively changing the deposition conditions from process A to process B until the desired number of bilayers were deposited. Three different multilayer structures were deposited, formed by one, six and seven bilayers, the deposition time being 15 min for each process in the first structure, and 5 min for each process in the other two. An upper DLC layer was grown on the top of the multilayer structure during 30 min. The DLC thickness was approximately 0.3 mm and the total thickness was approximately 1 mm. 2.2. Film characterization The thickness of the coatings was measured with a Dektak 3030 profilometer from the steps produced in the coatings during the deposition with the assistance of a silicon mask. The multilayer composition and chemical bonding were investigated with an ESCA-PHI 5500
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photoelectron spectrometer (XPS) equipped with an Al Ka monochromatic X-ray source. The microstructure of the coatings was analyzed by grazing incidence X-ray diffraction (GIXRD) (18 incidence angle) using a parallel optics diffractometer (Philips-MRD) with a Cu anode and working at power settings of 35 kV=40 mA. ˚ (Cu–Ka douThe radiation wavelength, ls1.5418 A blet), was selected by a graphite monochromator in the diffracted beam. The microstructure of the upper DLC layer was analyzed by micro-Raman spectroscopy (Jobin Ivon T 64000) using an Arq incident laser beam (514.5 nm). The coating’s cross-section morphology was determined by SEM (Cambridge Instruments Stereoscan 360). The multilayer microstructure was analyzed by TEM using a Philips CM 30 microscope operating at 300 kV. The multilayer elemental analysis was obtained by SIMS (Atomika 488). The depth profiling was carried out with an oxygen ion beam. The adhesion of the films to the steel substrate was evaluated using a microscratch technique (NanoTest 550 Micro Materials Ltd., UK) with a spherical diamond indenter of 50-mm radius. Scratches 2 mm long were produced with a scan velocity of 20 000 nmys, a loading rate of 32.43 mNys and a maximum load of 3000 mN. The failure mechanisms and the critical load values have been established from SEM images of the scratches. The friction and wear behavior was studied using a ballon-disk micro-tribometer. This is a home-built tribometer capable of testing the wear and measuring the friction at room conditions (temperature of 248C and humidity of 40%). It is of reduced size as compared to standard pin-on-disk systems. It uses 6-mm-diameter test balls of alumina and steel and the track radius described on the sample is 1.5 mm. The tests were conducted with a load of 350 mN, under a sliding velocity of 0.060 msy1 and were stopped at sample failure. 3. Results and discussion 3.1. Composition and microstructure In order to understand the individual layer composition, XPS analysis was made for single films deposited
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Fig. 2. Deconvoluted tungsten (a) and carbon (b) XPS spectra of the layer deposited from 20% of CH4 in the CH4yAr gas mixture.
by process A and B. Fig. 2 shows the XPS spectra of W and C measured in the B type single layer. Fig. 2a shows the W 4f7y2 doublet. The deconvolution of this doublet shows two contributions: W–C and W–O. The corresponding areas indicate that the W is predominantly present as carbide, with some minor amount of oxide. Fig. 2b shows the C 1s peak and its deconvolution indicates the presence of a majority of C–W bonds and
Table 1 Process parameters for the deposition of the W–CyDLC multilayers and the upper DLC layer Process
WC multilayer A
Reactive sputtering gas Substrate temperature Sputtering gas pressure r.f. power Target Substrates Target-substrate distance
Upper DLC B
C
20% CH4yAr mixture 5008C 6 Pa
40% CH4yAr mixture 1508C 8 Pa
Pure Ar 5008C 4 Pa 14 Wycm2 W–C binary M2 (AISI) high-speed steel, silicon 6 cm
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Micro-Raman spectroscopy analysis of the single layer deposited by process C is shown in Fig. 3a. The intensities of the two characteristic G and D bands (centered at 1350 cmy1 and 1550 cmy1) are typical of DLC films. Fig. 3b shows the micro-Raman spectrum of the 20% CH4 yAr W–C sample (B type). In this last figure, a weak G and D signal can be seen. This reflects the C–C bonded carbon phase observed in XPS analysis (the spectrum also contains a peak centered at 1290 cmy1 due to an instrumental fluorescence). No Raman signal was detected for the sample deposited by process A as could be expected from the XPS results. Grazing incidence XRD (GIXRD) spectra provided additional information on the structure of the W–C single layers. Fig. 4a,b shows the diffraction pattern of layers deposited by the two processes A and B. Broad diffraction peaks can be observed, indicating a poorly crystalline structure. The peak positions were indexed to the diffraction lines of different tungsten carbide phases in powder diffraction diagrams. The layer deposited with pure Ar gas (process A) showed predominant formation of the hexagonal W2C phase with a (002) texture. On the other hand, the layer deposited by process B, showed predominant formation of the cubic substoichiometric WC1yx phase with a (111) texture. This interpretation of the XRD patterns agrees well with previously published results of W–C coatings deposited by magnetron sputtering and related techniques w12,14,15x. From all XPS, Raman and GIXRD results we can conclude that the material deposited by processes A, B, C consist of W2C phase, substoichiometric WC1yx phase with the presence of segregated carbon and DLC, respectively. Fig. 3. Raman spectra of the single DLC layer (a) and of the W–C layer deposited from 20% CH4yAr.
also some C–C bonding fraction. The Gaussian contribution in the C 1s peak associated to the C–W bonding appears centered at 283.5 eV. This corresponds well to values reported for the non-stoichiometric cubic phase WC1yx w11–13x. From the W–C contributions in the W4f7y2 doublet (Fig. 2a) and in the C 1s peak (Fig. 2b), the stoichiometry of the W–C bonded fraction has been deduced as WC0.63 in the sample deposited from 20% CH4 yAr. From the C–C contribution in the C 1s peak (Fig. 2b), it can be deduced that this sample also contains 12% of carbon bonded as C–C. On the other hand, the A type sample deposited from pure argon showed similar XPS spectra, but different area relationship, from which a WC0.40 stoichiometry (a W rich layer) and a non-significant fraction of C–C bonds was deduced.
Fig. 4. GIXRD patterns of tungsten carbide films deposited on steel from pure argon (a) and from 20% CH4yAr gas mixture (b).
´ et al. / Surface and Coatings Technology 148 (2001) 277–283 C. Rincon
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Fig. 5. Cross-sectional SEM micrograph of a W–CyDLC multilayer coating.
3.2. Multilayer structure
Fig. 6. Cross-sectional TEM image of the multilayer stack.
3.3. Mechanical properties Fig. 5 shows the SEM cross-section morphology of a W–CyDLC multilayer. We can distinguish clearly the DLC upper layer and the multilayer stack in which a columnar dense structure is observed. The multilayer structure cannot be resolved with this technique. Fig. 6 shows the TEM cross-section of the multilayer stack with enough resolution to clearly differentiate the multilayer structure. The inset in Fig. 6 shows the corresponding electron diffraction pattern. The rings correspond to (111) and (222) diffraction planes of a WC1yx phase w12x. A similar result has also been obtained from the GIXRD diffraction pattern of the multilayer stack, where exclusively the WC1yx phase appears. In Fig. 6 it can also be observed that the expected W2CyWC1yx bilayer structure cannot be distinguished. Instead of this, dark homogenous layers limited by narrow bright stripes are observed. These dark homogenous layers probably correspond to the WC1yx phase and the bright stripes correspond to a carbon rich interface region. Therefore, because of the multilayer deposition process, the hexagonal W2C phase does not form, and carbon rich interlayers appear in the multilayer interfaces. SIMS depth profile analysis of multilayer coatings shows a pure carbon layer at the beginning of the ion etching corresponding to the upper DLC layer and, afterwards, it shows the periodic modulation of the multilayer (Fig. 7). These profiles also show the narrow carbon rich interlayers. In the SIMS profile these interlayers can be markedly seen for the first layers but they fade out for the deeper layers due to the mixing effect of the ion milling.
Scratch test measurements were performed on single DLC layers and on W–CyDLC multilayers deposited on hardened steel substrates. The critical load value for the single DLC coating onto steel was 530 mN. This is a low value when comparing with typical critical load values obtained with a 200-mm sphere, but it is in the
Fig. 7. SIMS depth profile of a W–CyDLC multilayer coating on silicon substrate.
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the steady state, the friction coefficients are similar for single DLC and multilayer coatings, with a value of 0.12 when sliding against steel balls and 0.08 when sliding against alumina balls. The same ball-on-disc test allows us to determine the lifetime of the coatings under sliding wear. Fig. 8a plots the evolution of the friction coefficient during the test for a multilayer coating. The test against alumina balls shows that the failure of the coating happens very early as compared to the test against steel balls. The wear scar was observed in the optical microscope after stopping the test at different stages. The images indicated that, when sliding against alumina balls, the coatings are progressively worn, but when sliding against steel balls the coatings suffer no visible wear until failure. In both cases, failures appear to be adhesion failures. When comparing single DLC coatings and multilayer coatings the lifetime increases noticeably as it can be seen in Fig. 8b. All the multilayers showed a similar behavior in this test. The ensemble of mechanical tests demonstrates the advantage of the multilayer structures to improve wear resistance of DLC coatings on steel substrates without disturbing their lubricious friction properties. 4. Conclusions
Fig. 8. (a) Ball-on-disc results as observed on a multilayer coating of seven bilayers with an upper DLC layer. The solid line corresponds to alumina ball and the dotted line corresponds to steel ball. (b) Lifetime of three different coatings on hardened steel under the ball-ondisc tests: a DLC single layer and two different multilayers.
range of previously reported values measured with a 50mm radius sphere w16x. A general improvement of the adhesion has been obtained for all the multilayer coatings as compared to single DLC coatings. The critical load values obtained for all the multilayers were approximately 1300 mN, without any particular difference among them. Moreover, SEM images of the scratches showed a clear difference in the failure mechanisms at the critical load. The single DLC coatings suffer a catastrophic failure with the complete delamination of the coatings in a flaking mode w17x. On the contrary, the multilayer coatings showed a cohesive failure of the coatings with internal transverse cracking due to the material fragility. Ball-on-disc tests have allowed us to determine the friction coefficient of the coatings when sliding against steel and alumina balls. The measured friction coefficient stabilizes after a short initial transition period. In
Magnetron sputtering from a single tungsten–carbon binary target proves to be an alternative method for the deposition of W–C multilayers and DLC layers in a single process. The multilayer stack is obtained by alternatively changing the methaneyargon sputtering gas composition. The structure of this multilayer stack consists of cubic WC1yx polycrystalline layers separated by carbon rich narrow interlayers. The upper DLC layer has been obtained in the same process by increasing the methane concentration in the gas mixture. The experiments have demonstrated that the multilayer coatings improve the adhesion and the wear resistance of the DLC coatings on steel substrates while keeping their low friction properties. Acknowledgements The authors acknowledge the technical staff of the ´ ` Serveis Cientifıco-Tecnics of the Universitat de Barcelona for their collaboration in TEM, XPS and XRD measurements. This work has been financially supported by the CYTED (project VIII.7A). References w1x H.C. Tsai, D.B. Bogy, J. Vac. Sci. Technol. A5 (1987) 3287. w2x N.A. Morrison, S.E. Rodil, A.C. Ferrari, J. Robertson, W.I. Milne, Thin Solid Films 333 (1999) 71. w3x N. Savvides, B. Window, J. Vac. Sci. Technol. A3 (1985) 2386.
´ et al. / Surface and Coatings Technology 148 (2001) 277–283 C. Rincon w4x S.J. Harris, A.W. Weiner, S.C. Tung, S.J. Simko, M.C. Militello, Surf. Coat. Technol. 62 (1993) 550. w5x H. Sjostrom, ¨ ¨ L Hultman, J.E. Sundgren, L.R. Wallenberg, Thin Solid Films 232 (1993) 169. w6x S. Veprek, J.Vac. Sci. Technol., A 17 (1999) 2401. w7x K. Vandierendonck, M. Nesladek, S. Kadlec, C. Quaeyhaegens, M. Van Stapeen, L.M.M. Stals, Surf. Coat. Technol. 98 (1998) 1060. w8x A.A. Voevodin, M.A. Capano, S.J.P. Laube, M.S. Donley, J.S. Zabinski, Thin Solid Films 298 (1997) 107. w9x A.A. Voevodin, J.P. O’Neill, J.S. Zabinski, Thin Solid Films 342 (1999) 194. w10x N.A. Sanchez, C. Rincon, G. Zambrano, H. Galindo, P. Prieto., Thin Solid Films 373 (2000) 247.
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w11x O.R. Monteiro, M.P. Delplancke-Ogletree, R.Y. Lo, R. Winand, I. Brown, Surf. Coat. Technol. 94-95 (1997) 220. w12x A.A. Voevodin, J.P. O9Neill, S.V. Prasad, J.S. Zabinski., J. Vac. Sci. Technol. A 17 (1999) 9863. w13x J. Esteve, G. Zambrano, C. Rincon, ´ E. Martınez, ´ H. Galindo, P. Prieto, Thin Solid Films 373 (2000) 282. w14x P.K. Srivastva, V.D. Vankar, K.L. Chopra, J. Vac. Sci. Technol. A4 (1986) 2819. w15x N. Radic, B. Grzeta, O. Milat, J. Ivkov, M. Stubicar, Thin Solid Films 320 (1998) 192. w16x E. Martınez, ´ M.C. Polo, E. Pascual, J. Esteve, Diamond Relat. Mater. 8 (1999) 563–566. w17x P. Hendenqvist, M. Olsson, S. Jacobson, S. Soderberg, ¨ Surf. Coat. Technol. 41 (1990) 31.
Tungsten carbideydiamond-like carbon multilayer coatings on steel for tribological applications ´ a,*, G. Zambranob, A. Carvajalb, P. Prietob, H. Galindoc, E. Martınez ´ d, A. Lousad, C. Rincon J. Esteved a
´ ´ Departamento de Fısica, Universidad Autonoma de Occidente, Cali, Colombia b ´ Departamento de Fısica, Universidad del Valle, Cali, Colombia c ´ ´ Departamento de Fısica, Universidad de los Andes, Merida, Venezuela d ` ´ Universitat de Barcelona, Departament de Fısica Aplicada i Optica, Av. Diagonal, 647 E-08028 Barcelona, Catalunya, Spain Received 30 April 2001; accepted in revised form 3 July 2001
Abstract Tungsten carbideydiamond like (W–CyDLC) multilayers have been investigated as low friction coatings on high-speed steel substrates. The coatings are composed of a W–C multilayer base and an upper lubricious DLC layer and they are obtained by reactive r.f. magnetron sputtering from a single target comprising of two equal halves; one-half carbon and the other half tungsten. The whole coating structure was obtained in situ, without any interruption of the sputtering process. Transmission electron microscopy (TEM) and SIMS were used to assess the multilayer structure and XPS, XRD, and Raman spectroscopy was used to analyze its composition. The tribological properties of the coatings in sliding wear were investigated by means of scratch test and ball-on-disc test measurements. It was found that the multilayer W–C base improves the adhesion of the upper DLC layer to steel substrates while maintaining its low friction coefficient. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Tungsten carbideydiamond like multilayers; Low friction coating; High-speed steel substrates
1. Introduction Diamond like carbon (DLC) coatings technology is an expanding area of applied materials due to the unique combination of properties they exhibit. For example, their surface smoothness, hardness and chemical inertness in combination with a low friction coefficient against most metals, make DLC films interesting for wear-protective purposes. Nowadays, the use of wearprotective DLC films is common on automotive applications, magnetic recording media w1,2x, optics w3x, biomedical prostheses w4x and hard coatings for cutting tools. However, DLC films present high compressive internal stress and low chemical interaction between films and their metal substrates, this causes poor adhe* Corresponding author. ´ E-mail address: [email protected] (C. Rincon).
sion to metal substrates thus limiting their commercial use. At the present time, the most general approach to solve the adhesion problem is to use metal or silicon interlayers between steel and DLC. It is of special interest the use of interlayers with gradually changing composition incorporating metal, ceramic and DLC w5– 8 x. Magnetron sputtering is a useful technique for multilayer deposition. This technique allows good control of deposition parameters such as temperature, deposition rate and gas composition during the deposition process. In the case of multilayer films, they are generally obtained by using multitarget sputtering systems. In this work, we introduce a novel method for the deposition of W–C and DLC multilayer coatings by using a single sputtering target comprising of two equal halves; onehalf carbon, the other tungsten. The different layers are obtained by the variation of the methaneyargon sputter-
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 6 0 - 3
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Fig. 1. Schematic of the magnetron sputtering system.
ing gas composition without any interruption in the process. Using this deposition method we have prepared multilayer coatings with the aim of improving both adhesion and wear resistance of DLC protective coatings onto steel substrates. The coatings are formed by a multilayer stack of different W–C layers and an upper DLC layer. The objective of the multilayer base is to improve the adhesion and the mechanical resistance of the upper DLC layer while keeping its low friction coefficient w9x. 2. Experimental details 2.1. Film deposition A set of coatings were deposited by means of a r.f. (13.56 MHz) magnetron sputtering system, which is schematically represented in Fig. 1. The W–C sputtering target was made of two halves cut from 2.5-cm diameter W (99.99%) and C (99.99%) targets, which were bonded to a water-cooled backing plate. The r.f. power was 14 Wycm2. Coatings were deposited onto silicon wafers and also onto M2 (AISI) high-speed steel substrates polished with 1-mm diamond paste and ultrasonically cleaned in alcohol. The substrates were fixed to a holder 6 cm below the sputtering target. The discharge chamber was evacuated to a base pressure of 10y3 Pa before the introduction of the sputtering gas. Prior to coating deposition the target was sputter cleaned in argon gas
at a pressure of 4 Pa for 10 min, behind a shutter, in order to remove the possible target contamination. Substrates were electrically grounded. Two different types of WC single films were deposited by two different processes: (i) pure Ar at 4 Pa (process A) and; (ii) 20% CH4 yAr mixture (process B). Substrate temperature was 5008C for both processes. The DLC film was obtained by using a 40% CH4 yAr gas mixture and a substrate temperature of 1508C w10x (process C). The deposition parameters are listed in Table 1. The multilayer coatings were obtained by alternatively changing the deposition conditions from process A to process B until the desired number of bilayers were deposited. Three different multilayer structures were deposited, formed by one, six and seven bilayers, the deposition time being 15 min for each process in the first structure, and 5 min for each process in the other two. An upper DLC layer was grown on the top of the multilayer structure during 30 min. The DLC thickness was approximately 0.3 mm and the total thickness was approximately 1 mm. 2.2. Film characterization The thickness of the coatings was measured with a Dektak 3030 profilometer from the steps produced in the coatings during the deposition with the assistance of a silicon mask. The multilayer composition and chemical bonding were investigated with an ESCA-PHI 5500
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photoelectron spectrometer (XPS) equipped with an Al Ka monochromatic X-ray source. The microstructure of the coatings was analyzed by grazing incidence X-ray diffraction (GIXRD) (18 incidence angle) using a parallel optics diffractometer (Philips-MRD) with a Cu anode and working at power settings of 35 kV=40 mA. ˚ (Cu–Ka douThe radiation wavelength, ls1.5418 A blet), was selected by a graphite monochromator in the diffracted beam. The microstructure of the upper DLC layer was analyzed by micro-Raman spectroscopy (Jobin Ivon T 64000) using an Arq incident laser beam (514.5 nm). The coating’s cross-section morphology was determined by SEM (Cambridge Instruments Stereoscan 360). The multilayer microstructure was analyzed by TEM using a Philips CM 30 microscope operating at 300 kV. The multilayer elemental analysis was obtained by SIMS (Atomika 488). The depth profiling was carried out with an oxygen ion beam. The adhesion of the films to the steel substrate was evaluated using a microscratch technique (NanoTest 550 Micro Materials Ltd., UK) with a spherical diamond indenter of 50-mm radius. Scratches 2 mm long were produced with a scan velocity of 20 000 nmys, a loading rate of 32.43 mNys and a maximum load of 3000 mN. The failure mechanisms and the critical load values have been established from SEM images of the scratches. The friction and wear behavior was studied using a ballon-disk micro-tribometer. This is a home-built tribometer capable of testing the wear and measuring the friction at room conditions (temperature of 248C and humidity of 40%). It is of reduced size as compared to standard pin-on-disk systems. It uses 6-mm-diameter test balls of alumina and steel and the track radius described on the sample is 1.5 mm. The tests were conducted with a load of 350 mN, under a sliding velocity of 0.060 msy1 and were stopped at sample failure. 3. Results and discussion 3.1. Composition and microstructure In order to understand the individual layer composition, XPS analysis was made for single films deposited
279
Fig. 2. Deconvoluted tungsten (a) and carbon (b) XPS spectra of the layer deposited from 20% of CH4 in the CH4yAr gas mixture.
by process A and B. Fig. 2 shows the XPS spectra of W and C measured in the B type single layer. Fig. 2a shows the W 4f7y2 doublet. The deconvolution of this doublet shows two contributions: W–C and W–O. The corresponding areas indicate that the W is predominantly present as carbide, with some minor amount of oxide. Fig. 2b shows the C 1s peak and its deconvolution indicates the presence of a majority of C–W bonds and
Table 1 Process parameters for the deposition of the W–CyDLC multilayers and the upper DLC layer Process
WC multilayer A
Reactive sputtering gas Substrate temperature Sputtering gas pressure r.f. power Target Substrates Target-substrate distance
Upper DLC B
C
20% CH4yAr mixture 5008C 6 Pa
40% CH4yAr mixture 1508C 8 Pa
Pure Ar 5008C 4 Pa 14 Wycm2 W–C binary M2 (AISI) high-speed steel, silicon 6 cm
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Micro-Raman spectroscopy analysis of the single layer deposited by process C is shown in Fig. 3a. The intensities of the two characteristic G and D bands (centered at 1350 cmy1 and 1550 cmy1) are typical of DLC films. Fig. 3b shows the micro-Raman spectrum of the 20% CH4 yAr W–C sample (B type). In this last figure, a weak G and D signal can be seen. This reflects the C–C bonded carbon phase observed in XPS analysis (the spectrum also contains a peak centered at 1290 cmy1 due to an instrumental fluorescence). No Raman signal was detected for the sample deposited by process A as could be expected from the XPS results. Grazing incidence XRD (GIXRD) spectra provided additional information on the structure of the W–C single layers. Fig. 4a,b shows the diffraction pattern of layers deposited by the two processes A and B. Broad diffraction peaks can be observed, indicating a poorly crystalline structure. The peak positions were indexed to the diffraction lines of different tungsten carbide phases in powder diffraction diagrams. The layer deposited with pure Ar gas (process A) showed predominant formation of the hexagonal W2C phase with a (002) texture. On the other hand, the layer deposited by process B, showed predominant formation of the cubic substoichiometric WC1yx phase with a (111) texture. This interpretation of the XRD patterns agrees well with previously published results of W–C coatings deposited by magnetron sputtering and related techniques w12,14,15x. From all XPS, Raman and GIXRD results we can conclude that the material deposited by processes A, B, C consist of W2C phase, substoichiometric WC1yx phase with the presence of segregated carbon and DLC, respectively. Fig. 3. Raman spectra of the single DLC layer (a) and of the W–C layer deposited from 20% CH4yAr.
also some C–C bonding fraction. The Gaussian contribution in the C 1s peak associated to the C–W bonding appears centered at 283.5 eV. This corresponds well to values reported for the non-stoichiometric cubic phase WC1yx w11–13x. From the W–C contributions in the W4f7y2 doublet (Fig. 2a) and in the C 1s peak (Fig. 2b), the stoichiometry of the W–C bonded fraction has been deduced as WC0.63 in the sample deposited from 20% CH4 yAr. From the C–C contribution in the C 1s peak (Fig. 2b), it can be deduced that this sample also contains 12% of carbon bonded as C–C. On the other hand, the A type sample deposited from pure argon showed similar XPS spectra, but different area relationship, from which a WC0.40 stoichiometry (a W rich layer) and a non-significant fraction of C–C bonds was deduced.
Fig. 4. GIXRD patterns of tungsten carbide films deposited on steel from pure argon (a) and from 20% CH4yAr gas mixture (b).
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Fig. 5. Cross-sectional SEM micrograph of a W–CyDLC multilayer coating.
3.2. Multilayer structure
Fig. 6. Cross-sectional TEM image of the multilayer stack.
3.3. Mechanical properties Fig. 5 shows the SEM cross-section morphology of a W–CyDLC multilayer. We can distinguish clearly the DLC upper layer and the multilayer stack in which a columnar dense structure is observed. The multilayer structure cannot be resolved with this technique. Fig. 6 shows the TEM cross-section of the multilayer stack with enough resolution to clearly differentiate the multilayer structure. The inset in Fig. 6 shows the corresponding electron diffraction pattern. The rings correspond to (111) and (222) diffraction planes of a WC1yx phase w12x. A similar result has also been obtained from the GIXRD diffraction pattern of the multilayer stack, where exclusively the WC1yx phase appears. In Fig. 6 it can also be observed that the expected W2CyWC1yx bilayer structure cannot be distinguished. Instead of this, dark homogenous layers limited by narrow bright stripes are observed. These dark homogenous layers probably correspond to the WC1yx phase and the bright stripes correspond to a carbon rich interface region. Therefore, because of the multilayer deposition process, the hexagonal W2C phase does not form, and carbon rich interlayers appear in the multilayer interfaces. SIMS depth profile analysis of multilayer coatings shows a pure carbon layer at the beginning of the ion etching corresponding to the upper DLC layer and, afterwards, it shows the periodic modulation of the multilayer (Fig. 7). These profiles also show the narrow carbon rich interlayers. In the SIMS profile these interlayers can be markedly seen for the first layers but they fade out for the deeper layers due to the mixing effect of the ion milling.
Scratch test measurements were performed on single DLC layers and on W–CyDLC multilayers deposited on hardened steel substrates. The critical load value for the single DLC coating onto steel was 530 mN. This is a low value when comparing with typical critical load values obtained with a 200-mm sphere, but it is in the
Fig. 7. SIMS depth profile of a W–CyDLC multilayer coating on silicon substrate.
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the steady state, the friction coefficients are similar for single DLC and multilayer coatings, with a value of 0.12 when sliding against steel balls and 0.08 when sliding against alumina balls. The same ball-on-disc test allows us to determine the lifetime of the coatings under sliding wear. Fig. 8a plots the evolution of the friction coefficient during the test for a multilayer coating. The test against alumina balls shows that the failure of the coating happens very early as compared to the test against steel balls. The wear scar was observed in the optical microscope after stopping the test at different stages. The images indicated that, when sliding against alumina balls, the coatings are progressively worn, but when sliding against steel balls the coatings suffer no visible wear until failure. In both cases, failures appear to be adhesion failures. When comparing single DLC coatings and multilayer coatings the lifetime increases noticeably as it can be seen in Fig. 8b. All the multilayers showed a similar behavior in this test. The ensemble of mechanical tests demonstrates the advantage of the multilayer structures to improve wear resistance of DLC coatings on steel substrates without disturbing their lubricious friction properties. 4. Conclusions
Fig. 8. (a) Ball-on-disc results as observed on a multilayer coating of seven bilayers with an upper DLC layer. The solid line corresponds to alumina ball and the dotted line corresponds to steel ball. (b) Lifetime of three different coatings on hardened steel under the ball-ondisc tests: a DLC single layer and two different multilayers.
range of previously reported values measured with a 50mm radius sphere w16x. A general improvement of the adhesion has been obtained for all the multilayer coatings as compared to single DLC coatings. The critical load values obtained for all the multilayers were approximately 1300 mN, without any particular difference among them. Moreover, SEM images of the scratches showed a clear difference in the failure mechanisms at the critical load. The single DLC coatings suffer a catastrophic failure with the complete delamination of the coatings in a flaking mode w17x. On the contrary, the multilayer coatings showed a cohesive failure of the coatings with internal transverse cracking due to the material fragility. Ball-on-disc tests have allowed us to determine the friction coefficient of the coatings when sliding against steel and alumina balls. The measured friction coefficient stabilizes after a short initial transition period. In
Magnetron sputtering from a single tungsten–carbon binary target proves to be an alternative method for the deposition of W–C multilayers and DLC layers in a single process. The multilayer stack is obtained by alternatively changing the methaneyargon sputtering gas composition. The structure of this multilayer stack consists of cubic WC1yx polycrystalline layers separated by carbon rich narrow interlayers. The upper DLC layer has been obtained in the same process by increasing the methane concentration in the gas mixture. The experiments have demonstrated that the multilayer coatings improve the adhesion and the wear resistance of the DLC coatings on steel substrates while keeping their low friction properties. Acknowledgements The authors acknowledge the technical staff of the ´ ` Serveis Cientifıco-Tecnics of the Universitat de Barcelona for their collaboration in TEM, XPS and XRD measurements. This work has been financially supported by the CYTED (project VIII.7A). References w1x H.C. Tsai, D.B. Bogy, J. Vac. Sci. Technol. A5 (1987) 3287. w2x N.A. Morrison, S.E. Rodil, A.C. Ferrari, J. Robertson, W.I. Milne, Thin Solid Films 333 (1999) 71. w3x N. Savvides, B. Window, J. Vac. Sci. Technol. A3 (1985) 2386.
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