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Raman study of carbon clusters in W–C thin films
Materials Science and Engineering A 396 (2005) 290–295
Raman study of carbon clusters in W–C thin films N. Radi´c a , B. Pivac a, ∗ , F. Meinardi b , Th. Koch c b
a Ru der - Boˇskovi´c Institute, P.O. Box 180, HR-10000 Zagreb, Croatia Dipartimento di Scienza dei Materiali, Universita degli Studi di Milano-Bicocca, I-20125 Milano, Italy c Institut f¨ ur Werkstoffkunde und Materialpr¨ufung, Technische Universit¨at Wien, A-1040 Wien, Austria
Received 15 November 2004; received in revised form 17 January 2005; accepted 18 January 2005
Abstract A dispersion of nanocrystalline tungsten carbides in an amorphous carbon (a-C) matrix blends the hardness and thermal stability of carbides with the low-friction coefficient of a-C to form a superior coating. A dc-reactive (Ar + C6 H6 ) magnetron sputtering of tungsten was used to produce tungsten–carbon films with variable fraction of incorporated unreacted carbon. Raman spectroscopy was used to study the carbon phase in these films, and the nanoindentation measurements were employed for their mechanical characterization. All Raman spectra indicate occurrence of the graphitic carbon, characterized by two peaks, one related to graphite mode (G-line), and another which is due to disorder (D-line). It is concluded that the enhancement of graphitic carbon fraction phase in the films is effected through the increase in concentration of the presumably uniformly distributed carbon clusters less than 1 nm in size. © 2005 Elsevier B.V. All rights reserved. Keywords: Thin films; Tungsten–carbon; Raman spectroscopy; Nanohardness
1. Introduction Tungsten carbide is a widely used coating material for protection against wear, erosion, and high-temperature oxidation due to its high-hardness and excellent thermal stability [1]. Plasma spray [1] and CVD [2] processes, both produce WC with thermodynamically stable simple hexagonal structure. Magnetron sputtering of tungsten in hydrocarbon environment was used to produce WC crystalline stable and metastable phases at lower deposition temperatures in comparison with those of CVD process [3–7]. The main limitation of the sputtering process is high-residual stress and brittleness of deposited films, which may be corrected to a certain extent by optimization of the deposition process. Control of stress and hardness has been achieved by varying the film composition and by substrate biasing [6,8]. Some studies have shown that it is possible to form hard W–C solid solutions in addition to crystalline carbides and carbon phases [9,10]. Nanocomposite films containing both tungsten carbides and
∗
Corresponding author. Tel.: +385 1 4561 068; fax: +385 1 4680 114. E-mail address: [email protected] (B. Pivac).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.01.007
amorphous carbon phases (which was often erroneously referred to as diamond-like carbon, DLC) exhibit a blend of good properties of both materials [11–15]. Different types of amorphous carbon (hydrogenated or not) are classified on the basis of their composition. For unhydrogenated films, composition ranges from sp2 -rich, glassy or amorphous carbon to sp3 -rich, usually known as tetrahedral a-C or amorphic diamond. If hydrogen is incorporated into the films, then the physical properties will vary over a wider range, and are classified in at least four types of hydrocarbons, ranging from hard to soft material [16]. Therefore, recently we applied a novel approach in tungsten carbide film preparation by dc magnetron reactive sputtering of W with benzene as a carbon precursor [7]. It is expected that the use of carbon-rich molecules, and therefore reduced hydrocarbon partial pressure will affect the sputtering process itself, and hence the films produced. Moreover, at higher benzene partial pressure the process might not completely break all benzene rings, and smaller or larger parts of complete rings might be incorporated into the growing layer, affecting therefore its tribological properties. In this work, the Raman spectroscopy was employed to characterize the carbon phase in the sputter-deposited
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tungsten–carbon films in a function of the film deposition parameters. The aim was to correlate the parameters of the film deposition with the unreacted carbon (carbon not bound in the WC phase) incorporation, and to estimate the carbon clusters size. The observed mechanical properties of the prepared films were interpreted as a consequence of the film structure.
2. Experimental Tungsten–carbon thin films were deposited onto monocrystalline silicon substrates by reactive sputtering in a twosource device described in more details previously [7]. The magnetron discharges operated in argon + benzene gas mixture at 2 Pa total pressure. The current density was 6 mA/cm2 in both magnetrons, while discharge voltage increased with the C6 H6 partial pressure and varied in the 330–450 V range. Deposition rate was 0.1–0.2 nm/s and the final film thickness were about 0.5 m. Benzene partial pressures were 1.25, 2.5 or 5% of total pressure, respectively. The substrate temperature was varied in steps of 200 ◦ C, while substrate potential was floating or biased −70 V with respect to discharge plasma. Raman scattering measurements were carried out with a Labram (Dilor) confocal spectrometer in backscattering configuration equipped with an argon-ion laser (λ = 488 nm). Due to very low-Raman scattering intensity, every Raman spectrum was accumulated during 30 repeated scans with 4 cm−1 spectral resolution over the spectral range of interest. Duration of data acquisition and the apparent absence of photoluminescence signal increase at about 2000 cm−1 determined the upper limit of the investigated spectral range at 1850 cm−1 . The hardness and elastic modulus were measured by nanoindentation on a Nano Indenter XP apparatus. Five indents were made on each sample, and the indentation depths were about 40 nm.
3. Results 3.1. Raman spectroscopy As already noted, all Raman spectra in this work were recorded in the 80–1850 cm−1 range. In the following, we present the variation of shape and amplitude of Raman spectra in the 1200–1850 cm−1 range, where Raman “fingerprints” of carbon phases are observed. Fig. 1 shows (as an example) Raman spectra in the 200–1850 cm−1 spectral region of three WC samples deposited with increasing content of benzene in reactive atmosphere. The spectral variation above 1180 cm−1 clearly indicates that increase in the benzene partial pressure leads to enhanced portion of incorporated unreacted carbon in the growing sample. Since hardness, one of fundamental proper-
Fig. 1. Raman spectra of tungsten–carbon films deposited with increasing benzene concentration in the reactive atmosphere. A case of substrates held at 200 ◦ C and at floating potential is shown.
ties of WC material, is significantly affected by the embedded carbon, it is of major importance to understand how carbon is incorporated in the growing film. The analysis of the broad humps seen in Fig. 1 takes into account several features observed in the recorded spectra: a) The broad lines observed below 1180 cm−1 are less sensitive to the variation of benzene partial pressure than the lines above that wavenumber. This allows to consider them not important for the analysis of the unreacted carbon fraction in the examined films. b) Well-separated D- and G-carbon lines were observed with films prepared at high-benzene admixture concentration. We assume a persistence of these two lines in a lessdiscerned spectra of films prepared at lower benzene partial pressure. The spectrum of single-crystal graphite exhibits a single high-frequency peak at about 1580 cm−1 , called G (graphite) peak [17,18]. For microcrystalline graphite, two peaks centered at 1575 and 1355 cm−1 were found [18]. The additional peak at 1355 cm−1 was assigned to first-order scattering from a zone boundary phonon activated by disorder due to the finite crystalline size [18], and is often referred as D (disordered) peak. c) Since benzene was a carbon-supplying gas, the benzene or condensed benzene rings vibrations at about 1470 and 1740 cm−1 were included into analysis. The peak at 1470 cm−1 can be identified with semicircle stretching vibration of carbon atoms in benzene or benzene clusters [19]. Moreover, the feature around 1740 cm−1 may arise from benzene-related stretching [20]. Both peaks around 1470 and 1740 cm−1 was also observed in the Raman scattering from MeV ion implanted diamond [21] and pulsed laser deposited amorphous carbon films. Thus, only 1200–1850 cm−1 part of the spectra, which exhibits a clear dependence upon benzene partial pressure, is analyzed in what follows. The deconvolution procedure included four Gaussian peaks centered at fixed positions (only
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N. Radi´c et al. / Materials Science and Engineering A 396 (2005) 290–295 Table 1 Nanohardness values for the W–C films deposited under indicated conditions
Fig. 2. An example of the deconvolution procedure. The benzene-related peaks at 1470 and 1740 cm−1 and graphitic-phase related peaks at 1380 and 1580 cm−1 are considered.
small adjustments were allowed) with adjustable line widths. Such approach was necessary in order to obtain a meaningful results from deconvolution of the broad and smooth complex peaks. The baseline for the fit was taken to be linear with end points defined by fitting range. As seen in Fig. 1, the overall background seems to be due in a greater part to the far wings of Rayleigh scattering. No photoluminescence enhancement at the upper end of the examined spectral range is observed. Moreover, the C/W atomic ratio in the films prepared at 1.25% benzene partial pressure suggests that almost all carbon atoms should be bounded to tungsten in the observed WC1−x structure. In such case, the unreacted carbon signal should be very low and the actually observed signal should be considered as a true background. The above justifies the linear background subtraction in the analysis of the Raman signal in the 1200–1850 cm−1 range. As an example, a numerical deconvolution of this complex peak into peaks centered at about 1380, 1470, 1580 and 1740 cm−1 is shown in Fig. 2. As expected, the overall Raman spectrum is dominated by the G peak at 1580 cm−1 and the D peak around 1380 cm−1 . The effects of benzene partial pressure in the working gas upon the intensity of respective Raman lines are shown in Fig. 3. As expected (see Fig. 1), a total Raman signal in the
Standard Deviation (S.D.) is calculated from five measurements.
1200–1850 cm−1 increases with benzene fraction. However, various carbon fractions behave differently. The benzenerelated signals (1470 and 1740 cm−1 ) taken together remain small throughout the composition range, as expected since only a minor part of benzene from the gas would escape decomposition and end incorporated into the growing film. On the other hand, carbon-related G- and D-lines steadily grow as more benzene is supplied. As the substrate temperature is higher, the enhancement of D- and G-line is more pronounced. A better insight into the benzene partial pressure effects upon the unreacted carbon structure in the examined films is obtained by the analysis of the variation of D- and G-lines intensity and line width with deposition parameters. 3.2. Nanohardness
Fig. 3. Raman peaks intensity (peak area) as a function of benzene partial pressure in magnetron working gas. Peaks-related to benzene (: at 1470 and : at 1740 cm−1 ) and graphitic phase (: D-line at 1380 and : G-line at 1580 cm−1 ) are shown. Open and full symbols correspond to deposition at room temperature and 400 ◦ C, respectively.
The results of nanoindentation measurements are given in Table 1. The values for the monocrystalline silicon substrate are shown for reference. Two trends are discernible: (a) nanohardness of the prepred films generally decreases with the benzene content in the working gas and (b) increase in substrate temperature produces harder films. The observed range of nanohardness, from 38.5 to about 20 GPa is comparable with the recent results obtained on similar films prepared with acetylene as a carbon-supplying gas [15], with the corresponding carbon content increasing from about 45 to 60 at.%.
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4. Discussion The nanocomposite materials allow to blend the properties of different phases in order to achieve the required properties. A generic design principle was recently proposed [22] to produce superhard coatings (Hv ≥ 40 GPa) by embedding the nanocrystals of transition metal nitrides into the amorphous matrix of non-metallic nitrides. The analogue approach with the transition metal carbides in amorphous carbon matrix yielded somewhat different result—no superhardness was achieved [14], but the coatings exhibited a significant increase in toughness [13]. It is expected that the volume ratio of nanocrystalline carbide to the amorphous hydrocarbon matrix predominantly determines the mechanical and tribological properties of the carbon-based nanocomposites. However, the spatial distribution of both phases is important as well. It is well-known that the excessive amount of carbonsupplying admixture (CH4 , C2 H2 , C6 H6 ) in the magnetron working gas results in a formation of amorphous tungsten carbide phases [5–7,23–25]. The occurrence of amorphous phase is detrimental to the film hardness [6,7,13,26], and therefore quite undesirable in its role of hard coating. The amorphisation is presumably due to the ample incorporation of unreacted carbon, hydrogen or hydrocarbon fragments into the growing film. The microscopic phenomena active in the W–C film formation by plasma process are insufficiently known. The unreacted carbon might be incorporated by covering of the hydrocarbon fragments adsorbed at the film surface by the incoming tungsten atoms, or indirectly subplanted into the growing film [27]. Its subsequent agglomeration into a carbon clusters is probably effected through the short range atomic rearrangements, and driven by the low-solid solubility of carbon in all tungsten carbides. The results of Raman spectroscopy allow to correlate the deposition parameters, microscopic configuration of such carbon clusters, and film physical properties, in order to exert a control of the latter by the first. Since the optical properties of different samples did not vary (within the experimental error), as found by reflectivity measurements, it is concluded that from the analysis of Raman data, a reliable information on structural properties can be obtained. In actual W–C films Raman analysis revealed following: On average, about 20% of total peak area (below: line intensity, I) between 1200 and 1850 cm−1 is due to the 1470 and 1740 cm−1 vibrations of benzene rings/molecules. In a tentative model of surface-reaction growth of tungsten–carbon films by the Ar + C6 H6 reactive sputtering [7], the adsorbed benzene molecules and/or fragments could be buried in the growing film by the incoming tungsten atoms, if the number of incoming W-atoms is insufficient to effect chemical reaction with all available carbon atoms. The carbon contribution through the D- and G-lines of graphitic clusters dominates the examined frequency range. As already noted, the carbon lines are readily observed even when the carbon to tungsten atomic ratio approaches the stoichiometric W2 C subcarbide.
Fig. 4. The line intensity ID /IG ratio as a function of total (ID + IG ) graphitic fraction in the prepared films. Benzene partial pressure: () 1.25%; () 2.5%; () 5%; open and full symbols mark deposition at room temperature and 400 ◦ C, respectively.
It indicates that in the actual deposition process a fraction of deposited tungsten also remained unbound to carbon, although it cannot be observed by the XRD method (probably due to a very fine dispersion of tungsten particles across the film). The analysis of Raman spectra of various carbon phases is rather complex [19,28]. Here, we adopt a simplified model of carbon clusters embedded in a matrix of surrounding material. In this model, the G-line corresponds to the carbon atoms in a sp2 configuration, organized in a graphite-like fused sixfold rings, while D-line corresponds to the disordered sp2 bonds of carbon atoms in contact with the surrounding material at cluster boundary. The line intensity (peak area) is a relative measure of respective carbon species abundance in the film, and the line width indicates a cluster size and ordering within it. The results of Raman analysis of the carbon-phase in the examined WC films can be summarized as follows: (i) The ID /IG ratio increases as the total (ID + IG ) carbon fraction (predominantly controlled by the benzene partial pressure) in the film increases (Fig. 4). This indicates enhanced disordering of the carbon-phase effected through the increase of boundary fraction relative to the graphitic cluster core. This model, developed by Ferrari et al. [28,29] implies increase in a number of nanocrystalline graphitic inclusions, i.e. excludes a model of graphitic inclusions growing bigger as the carbon supply increases. (ii) Variation in the full width at half maximum (FWHM) with line intensity is opposite in two lines: in the case of G-line it decreases (150 → 120 cm−1 ), while in the case of D-line it increases (175 → 230 cm−1 ), as seen in Fig. 5. Line broadening due to the stress in the films is expected to follow the same trend in both G- and D-Raman lines [19]. In pure tungsten films deposited in the same device at 2 Pa argon pressure, the macroscopic compressive pressure of only about 1 GPa was measured. Therefore, it seems that the line widths are predominantly related to the cluster size and distribu-
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Fig. 5. Variation of the FWHM with corresponding Raman line intensity: upper panel D-line and lower panel G-line. Benzene partial pressure: () 2.5%; () 5%; open and full symbols mark deposition at room temperature and 400 ◦ C, respectively. The points corresponding to the lowest benzene partial pressure (1.25%) are rather scattered due to the least accuracy of deconvolution procedure, and are therefore omitted.
tion. The observed behaviour may be interpreted as the enhanced ordering in the shrinking graphitic core, with increased distortion in the outer layers. (iii) As shown in Fig. 6, the FWHM of G-line steadily decreases with the increase in benzene content, while the corresponding ratio of D- and G-line intensities increases. Putting aside the points corresponding to the lowest benzene partial pressure (1.25%), due to the least accuracy of the deconvolution procedure, the correlation is rather clear, and allows one to estimate the size of the carbon clusters in the film [19]. Thus, according to the measured ID /IG ratio and G-line FWHM, the graphiticcluster (presumably dominated by small aromatic rings) size is estimated to be less than 1 nm.
The above findings suggest that the graphitic phase in the examined W–C films consists of a uniform dispersion of small clusters. With increase in carbon content, the size of the clusters remains essentially the same, while their number grows. The effect of such increase in graphitic cluster concentration reduced translational periodicity of WC1−x crystals, allowing growth of only nanocrystalline dimensions. A similar change in W–C films structure with the increase of carbon content was already observed [5,13]. If the average size of the WC1−x crystallites is less than about 3 nm, the discrimination between the grain boundaries and grains is blurred to the extent that the material W–C:H is practically amorphous. The consequence in physical properties is a significant decrease in nanohardness, as confirmed by the results in Table 1. The parameter, which governs the grain-size structure in the asdeposited W–C films is in the greatest extent the unreacted carbon content. The effects of substrate temperature and potential bias during deposition are inconclusive, and the work on their clarification is in progress.
5. Conclusions Tungsten–carbon thin films were prepared by reactive magnetron sputtering, with benzene as a carbon-supplying admixture. The structure of the unreacted carbon incorporated into the film was examined by Raman spectroscopy. It was found that, to a larger extent, the carbon is incorporated as a graphitic phase. The enhancement of the graphitic fraction in the film is effected through the growing number of uniformly distributed small size (<1 nm) graphitic nanoclusters. Increased concentration of graphitic inclusions prevents formation of tungsten carbide microcrystals, leading to the eventual amorphization of the WC film structure, and consequent decrease in film hardness. Thus, a controlled incorporation of unreacted carbon allows an optimization between film hardness and other properties.
Acknowledgements The authors acknowledge the technical assistance of A. Pavleˇsin and I. Kovaˇcevi´c.
References
Fig. 6. The correlation between G-line width and the D-line and G-line intensity ratio. Benzene partial pressure: () 1.25%; () 2.5%; () 5%; open and full symbols mark deposition at room temperature and 400 ◦ C, respectively. A dashed circle and a full line are only guidance for the eye, separating the least accurate, and therefore scattered results for 1.25% benzene partial pressure from the results for higher benzene fraction.
[1] R.F. Bunshah (Ed.), Handbook of Deposition Technologies for Films and Coatings, Noyes, Park Ridge, NJ, 1994, p. 591. [2] C.M. Kelly, D. Garg, P.N. Dyer, Thin Solid Films 219 (1992) 103. [3] P.K. Srivastava, V.D. Vankar, K.L. Chopra, J. Vac. Sci. Technol. A 4 (1986) 2819. [4] K. Fuchs, P. Rodhammer, E. Bertel, F.P. Netzer, E. Gornik, Thin Solid Films 151 (1987) 383. [5] Y. Pauleau, Ph. Gouy-Pailler, J. Mater. Res. 7 (1992) 2070. [6] Ph. Gouy-Pailler, Y. Pauleau, J. Vac. Sci. Technol. A 11 (1993) 96.
N. Radi´c et al. / Materials Science and Engineering A 396 (2005) 290–295 [7] N. Radi´c, B. Grˇzeta, O. Milat, J. Ivkov, M. Stubiˇcar, Thin Solid Films 320 (1998) 192. [8] A. Cavaleiro, T.M. Vieira, G. Lemp´eriere, Thin Solid Films 185 (1990) 199. [9] Y. Pauleau, Ph. Gouy-Pailler, Mater. Lett. 13 (1992) 157. [10] C. Rebholz, J.M. Schneider, H. Ziegele, B. Rahle, A. Leyland, A. Matthews, Vacuum 49 (1998) 265. [11] O.R. Monteiro, M.-P. Delplancke-Ogletree, I.P. Brown, Thin Solid Films 342 (1999) 100–107. [12] A.A. Voevodin, J.P. O’Neill, J.S. Zabinski, Thin Solid Films 342 (1999) 194–200. [13] A.A. Voevodin, J.P. O’Neill, S.V. Prasad, J.S. Zabinski, J. Vac. Sci. Technol. A 1A7 (1999) 986. [14] W. Precht, A. Czyzniewski, Surf. Coat. Technol. 174–175 (2003) 979. [15] A. Czyzniewski, Thin Solid Films 433 (2003) 180. [16] G. Fanchini, S.C. Ray, A. Tagliaferro, Surf. Sci. Technol. 151–152 (2002) 233. [17] M.H. Grimdisditch, A.K. Ramdas, Phys. Rev. B 11 (1975) 3139.
295
[18] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [19] J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, S.R.P. Silva, J. Appl. Phys. 80 (1996) 440. [20] Z.Y. Chen, J.P. Zhao, T. Yano, T. Ooie, M. Yoneda, J. Sakakibara, J. Appl. Phys. 88 (2000) 2305. [21] S. Prawer, K.W. Nugent, D.N. Jamieson, Diamond Relat. Mater. 7 (1998) 106. (22( S. Veprek, S. Reiprich, Thin Solid Films 268 (1995) 64. [23] K. Fuchs, P. R¨odhammer, E. Bertel, F.P. Netzer, E. Gornik, Thin Solid Films 151 (1987) 383–397. [24] W. van Duyn, B. van Lochem, Thin Solid Films 181 (1989) 497–503. [25] K. Machida, M. Euyo, I. Toyoshima, Thin Solid Films 161 (1988) L91–L95. [26] P. Juliet, A. Rouzaud, K. Aabadi, P. Monge-Cadet, Y. Pauleau, Thin Solid Films 290/291 (1996) 232–237. [27] J. Schwan, S. Ulrich, H. Roth, H. Ehrhardt, S.R.P. Silva, J. Robertson, R. Samlenski, R. Brenn, J. Appl. Phys. 79 (1996) 1416–1422. [28] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [29] A.C. Ferrari, Mat. Res. Symp. Proc. 675 (2001) 11.5.1.
Raman study of carbon clusters in W–C thin films N. Radi´c a , B. Pivac a, ∗ , F. Meinardi b , Th. Koch c b
a Ru der - Boˇskovi´c Institute, P.O. Box 180, HR-10000 Zagreb, Croatia Dipartimento di Scienza dei Materiali, Universita degli Studi di Milano-Bicocca, I-20125 Milano, Italy c Institut f¨ ur Werkstoffkunde und Materialpr¨ufung, Technische Universit¨at Wien, A-1040 Wien, Austria
Received 15 November 2004; received in revised form 17 January 2005; accepted 18 January 2005
Abstract A dispersion of nanocrystalline tungsten carbides in an amorphous carbon (a-C) matrix blends the hardness and thermal stability of carbides with the low-friction coefficient of a-C to form a superior coating. A dc-reactive (Ar + C6 H6 ) magnetron sputtering of tungsten was used to produce tungsten–carbon films with variable fraction of incorporated unreacted carbon. Raman spectroscopy was used to study the carbon phase in these films, and the nanoindentation measurements were employed for their mechanical characterization. All Raman spectra indicate occurrence of the graphitic carbon, characterized by two peaks, one related to graphite mode (G-line), and another which is due to disorder (D-line). It is concluded that the enhancement of graphitic carbon fraction phase in the films is effected through the increase in concentration of the presumably uniformly distributed carbon clusters less than 1 nm in size. © 2005 Elsevier B.V. All rights reserved. Keywords: Thin films; Tungsten–carbon; Raman spectroscopy; Nanohardness
1. Introduction Tungsten carbide is a widely used coating material for protection against wear, erosion, and high-temperature oxidation due to its high-hardness and excellent thermal stability [1]. Plasma spray [1] and CVD [2] processes, both produce WC with thermodynamically stable simple hexagonal structure. Magnetron sputtering of tungsten in hydrocarbon environment was used to produce WC crystalline stable and metastable phases at lower deposition temperatures in comparison with those of CVD process [3–7]. The main limitation of the sputtering process is high-residual stress and brittleness of deposited films, which may be corrected to a certain extent by optimization of the deposition process. Control of stress and hardness has been achieved by varying the film composition and by substrate biasing [6,8]. Some studies have shown that it is possible to form hard W–C solid solutions in addition to crystalline carbides and carbon phases [9,10]. Nanocomposite films containing both tungsten carbides and
∗
Corresponding author. Tel.: +385 1 4561 068; fax: +385 1 4680 114. E-mail address: [email protected] (B. Pivac).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.01.007
amorphous carbon phases (which was often erroneously referred to as diamond-like carbon, DLC) exhibit a blend of good properties of both materials [11–15]. Different types of amorphous carbon (hydrogenated or not) are classified on the basis of their composition. For unhydrogenated films, composition ranges from sp2 -rich, glassy or amorphous carbon to sp3 -rich, usually known as tetrahedral a-C or amorphic diamond. If hydrogen is incorporated into the films, then the physical properties will vary over a wider range, and are classified in at least four types of hydrocarbons, ranging from hard to soft material [16]. Therefore, recently we applied a novel approach in tungsten carbide film preparation by dc magnetron reactive sputtering of W with benzene as a carbon precursor [7]. It is expected that the use of carbon-rich molecules, and therefore reduced hydrocarbon partial pressure will affect the sputtering process itself, and hence the films produced. Moreover, at higher benzene partial pressure the process might not completely break all benzene rings, and smaller or larger parts of complete rings might be incorporated into the growing layer, affecting therefore its tribological properties. In this work, the Raman spectroscopy was employed to characterize the carbon phase in the sputter-deposited
N. Radi´c et al. / Materials Science and Engineering A 396 (2005) 290–295
291
tungsten–carbon films in a function of the film deposition parameters. The aim was to correlate the parameters of the film deposition with the unreacted carbon (carbon not bound in the WC phase) incorporation, and to estimate the carbon clusters size. The observed mechanical properties of the prepared films were interpreted as a consequence of the film structure.
2. Experimental Tungsten–carbon thin films were deposited onto monocrystalline silicon substrates by reactive sputtering in a twosource device described in more details previously [7]. The magnetron discharges operated in argon + benzene gas mixture at 2 Pa total pressure. The current density was 6 mA/cm2 in both magnetrons, while discharge voltage increased with the C6 H6 partial pressure and varied in the 330–450 V range. Deposition rate was 0.1–0.2 nm/s and the final film thickness were about 0.5 m. Benzene partial pressures were 1.25, 2.5 or 5% of total pressure, respectively. The substrate temperature was varied in steps of 200 ◦ C, while substrate potential was floating or biased −70 V with respect to discharge plasma. Raman scattering measurements were carried out with a Labram (Dilor) confocal spectrometer in backscattering configuration equipped with an argon-ion laser (λ = 488 nm). Due to very low-Raman scattering intensity, every Raman spectrum was accumulated during 30 repeated scans with 4 cm−1 spectral resolution over the spectral range of interest. Duration of data acquisition and the apparent absence of photoluminescence signal increase at about 2000 cm−1 determined the upper limit of the investigated spectral range at 1850 cm−1 . The hardness and elastic modulus were measured by nanoindentation on a Nano Indenter XP apparatus. Five indents were made on each sample, and the indentation depths were about 40 nm.
3. Results 3.1. Raman spectroscopy As already noted, all Raman spectra in this work were recorded in the 80–1850 cm−1 range. In the following, we present the variation of shape and amplitude of Raman spectra in the 1200–1850 cm−1 range, where Raman “fingerprints” of carbon phases are observed. Fig. 1 shows (as an example) Raman spectra in the 200–1850 cm−1 spectral region of three WC samples deposited with increasing content of benzene in reactive atmosphere. The spectral variation above 1180 cm−1 clearly indicates that increase in the benzene partial pressure leads to enhanced portion of incorporated unreacted carbon in the growing sample. Since hardness, one of fundamental proper-
Fig. 1. Raman spectra of tungsten–carbon films deposited with increasing benzene concentration in the reactive atmosphere. A case of substrates held at 200 ◦ C and at floating potential is shown.
ties of WC material, is significantly affected by the embedded carbon, it is of major importance to understand how carbon is incorporated in the growing film. The analysis of the broad humps seen in Fig. 1 takes into account several features observed in the recorded spectra: a) The broad lines observed below 1180 cm−1 are less sensitive to the variation of benzene partial pressure than the lines above that wavenumber. This allows to consider them not important for the analysis of the unreacted carbon fraction in the examined films. b) Well-separated D- and G-carbon lines were observed with films prepared at high-benzene admixture concentration. We assume a persistence of these two lines in a lessdiscerned spectra of films prepared at lower benzene partial pressure. The spectrum of single-crystal graphite exhibits a single high-frequency peak at about 1580 cm−1 , called G (graphite) peak [17,18]. For microcrystalline graphite, two peaks centered at 1575 and 1355 cm−1 were found [18]. The additional peak at 1355 cm−1 was assigned to first-order scattering from a zone boundary phonon activated by disorder due to the finite crystalline size [18], and is often referred as D (disordered) peak. c) Since benzene was a carbon-supplying gas, the benzene or condensed benzene rings vibrations at about 1470 and 1740 cm−1 were included into analysis. The peak at 1470 cm−1 can be identified with semicircle stretching vibration of carbon atoms in benzene or benzene clusters [19]. Moreover, the feature around 1740 cm−1 may arise from benzene-related stretching [20]. Both peaks around 1470 and 1740 cm−1 was also observed in the Raman scattering from MeV ion implanted diamond [21] and pulsed laser deposited amorphous carbon films. Thus, only 1200–1850 cm−1 part of the spectra, which exhibits a clear dependence upon benzene partial pressure, is analyzed in what follows. The deconvolution procedure included four Gaussian peaks centered at fixed positions (only
292
N. Radi´c et al. / Materials Science and Engineering A 396 (2005) 290–295 Table 1 Nanohardness values for the W–C films deposited under indicated conditions
Fig. 2. An example of the deconvolution procedure. The benzene-related peaks at 1470 and 1740 cm−1 and graphitic-phase related peaks at 1380 and 1580 cm−1 are considered.
small adjustments were allowed) with adjustable line widths. Such approach was necessary in order to obtain a meaningful results from deconvolution of the broad and smooth complex peaks. The baseline for the fit was taken to be linear with end points defined by fitting range. As seen in Fig. 1, the overall background seems to be due in a greater part to the far wings of Rayleigh scattering. No photoluminescence enhancement at the upper end of the examined spectral range is observed. Moreover, the C/W atomic ratio in the films prepared at 1.25% benzene partial pressure suggests that almost all carbon atoms should be bounded to tungsten in the observed WC1−x structure. In such case, the unreacted carbon signal should be very low and the actually observed signal should be considered as a true background. The above justifies the linear background subtraction in the analysis of the Raman signal in the 1200–1850 cm−1 range. As an example, a numerical deconvolution of this complex peak into peaks centered at about 1380, 1470, 1580 and 1740 cm−1 is shown in Fig. 2. As expected, the overall Raman spectrum is dominated by the G peak at 1580 cm−1 and the D peak around 1380 cm−1 . The effects of benzene partial pressure in the working gas upon the intensity of respective Raman lines are shown in Fig. 3. As expected (see Fig. 1), a total Raman signal in the
Standard Deviation (S.D.) is calculated from five measurements.
1200–1850 cm−1 increases with benzene fraction. However, various carbon fractions behave differently. The benzenerelated signals (1470 and 1740 cm−1 ) taken together remain small throughout the composition range, as expected since only a minor part of benzene from the gas would escape decomposition and end incorporated into the growing film. On the other hand, carbon-related G- and D-lines steadily grow as more benzene is supplied. As the substrate temperature is higher, the enhancement of D- and G-line is more pronounced. A better insight into the benzene partial pressure effects upon the unreacted carbon structure in the examined films is obtained by the analysis of the variation of D- and G-lines intensity and line width with deposition parameters. 3.2. Nanohardness
Fig. 3. Raman peaks intensity (peak area) as a function of benzene partial pressure in magnetron working gas. Peaks-related to benzene (: at 1470 and : at 1740 cm−1 ) and graphitic phase (: D-line at 1380 and : G-line at 1580 cm−1 ) are shown. Open and full symbols correspond to deposition at room temperature and 400 ◦ C, respectively.
The results of nanoindentation measurements are given in Table 1. The values for the monocrystalline silicon substrate are shown for reference. Two trends are discernible: (a) nanohardness of the prepred films generally decreases with the benzene content in the working gas and (b) increase in substrate temperature produces harder films. The observed range of nanohardness, from 38.5 to about 20 GPa is comparable with the recent results obtained on similar films prepared with acetylene as a carbon-supplying gas [15], with the corresponding carbon content increasing from about 45 to 60 at.%.
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4. Discussion The nanocomposite materials allow to blend the properties of different phases in order to achieve the required properties. A generic design principle was recently proposed [22] to produce superhard coatings (Hv ≥ 40 GPa) by embedding the nanocrystals of transition metal nitrides into the amorphous matrix of non-metallic nitrides. The analogue approach with the transition metal carbides in amorphous carbon matrix yielded somewhat different result—no superhardness was achieved [14], but the coatings exhibited a significant increase in toughness [13]. It is expected that the volume ratio of nanocrystalline carbide to the amorphous hydrocarbon matrix predominantly determines the mechanical and tribological properties of the carbon-based nanocomposites. However, the spatial distribution of both phases is important as well. It is well-known that the excessive amount of carbonsupplying admixture (CH4 , C2 H2 , C6 H6 ) in the magnetron working gas results in a formation of amorphous tungsten carbide phases [5–7,23–25]. The occurrence of amorphous phase is detrimental to the film hardness [6,7,13,26], and therefore quite undesirable in its role of hard coating. The amorphisation is presumably due to the ample incorporation of unreacted carbon, hydrogen or hydrocarbon fragments into the growing film. The microscopic phenomena active in the W–C film formation by plasma process are insufficiently known. The unreacted carbon might be incorporated by covering of the hydrocarbon fragments adsorbed at the film surface by the incoming tungsten atoms, or indirectly subplanted into the growing film [27]. Its subsequent agglomeration into a carbon clusters is probably effected through the short range atomic rearrangements, and driven by the low-solid solubility of carbon in all tungsten carbides. The results of Raman spectroscopy allow to correlate the deposition parameters, microscopic configuration of such carbon clusters, and film physical properties, in order to exert a control of the latter by the first. Since the optical properties of different samples did not vary (within the experimental error), as found by reflectivity measurements, it is concluded that from the analysis of Raman data, a reliable information on structural properties can be obtained. In actual W–C films Raman analysis revealed following: On average, about 20% of total peak area (below: line intensity, I) between 1200 and 1850 cm−1 is due to the 1470 and 1740 cm−1 vibrations of benzene rings/molecules. In a tentative model of surface-reaction growth of tungsten–carbon films by the Ar + C6 H6 reactive sputtering [7], the adsorbed benzene molecules and/or fragments could be buried in the growing film by the incoming tungsten atoms, if the number of incoming W-atoms is insufficient to effect chemical reaction with all available carbon atoms. The carbon contribution through the D- and G-lines of graphitic clusters dominates the examined frequency range. As already noted, the carbon lines are readily observed even when the carbon to tungsten atomic ratio approaches the stoichiometric W2 C subcarbide.
Fig. 4. The line intensity ID /IG ratio as a function of total (ID + IG ) graphitic fraction in the prepared films. Benzene partial pressure: () 1.25%; () 2.5%; () 5%; open and full symbols mark deposition at room temperature and 400 ◦ C, respectively.
It indicates that in the actual deposition process a fraction of deposited tungsten also remained unbound to carbon, although it cannot be observed by the XRD method (probably due to a very fine dispersion of tungsten particles across the film). The analysis of Raman spectra of various carbon phases is rather complex [19,28]. Here, we adopt a simplified model of carbon clusters embedded in a matrix of surrounding material. In this model, the G-line corresponds to the carbon atoms in a sp2 configuration, organized in a graphite-like fused sixfold rings, while D-line corresponds to the disordered sp2 bonds of carbon atoms in contact with the surrounding material at cluster boundary. The line intensity (peak area) is a relative measure of respective carbon species abundance in the film, and the line width indicates a cluster size and ordering within it. The results of Raman analysis of the carbon-phase in the examined WC films can be summarized as follows: (i) The ID /IG ratio increases as the total (ID + IG ) carbon fraction (predominantly controlled by the benzene partial pressure) in the film increases (Fig. 4). This indicates enhanced disordering of the carbon-phase effected through the increase of boundary fraction relative to the graphitic cluster core. This model, developed by Ferrari et al. [28,29] implies increase in a number of nanocrystalline graphitic inclusions, i.e. excludes a model of graphitic inclusions growing bigger as the carbon supply increases. (ii) Variation in the full width at half maximum (FWHM) with line intensity is opposite in two lines: in the case of G-line it decreases (150 → 120 cm−1 ), while in the case of D-line it increases (175 → 230 cm−1 ), as seen in Fig. 5. Line broadening due to the stress in the films is expected to follow the same trend in both G- and D-Raman lines [19]. In pure tungsten films deposited in the same device at 2 Pa argon pressure, the macroscopic compressive pressure of only about 1 GPa was measured. Therefore, it seems that the line widths are predominantly related to the cluster size and distribu-
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Fig. 5. Variation of the FWHM with corresponding Raman line intensity: upper panel D-line and lower panel G-line. Benzene partial pressure: () 2.5%; () 5%; open and full symbols mark deposition at room temperature and 400 ◦ C, respectively. The points corresponding to the lowest benzene partial pressure (1.25%) are rather scattered due to the least accuracy of deconvolution procedure, and are therefore omitted.
tion. The observed behaviour may be interpreted as the enhanced ordering in the shrinking graphitic core, with increased distortion in the outer layers. (iii) As shown in Fig. 6, the FWHM of G-line steadily decreases with the increase in benzene content, while the corresponding ratio of D- and G-line intensities increases. Putting aside the points corresponding to the lowest benzene partial pressure (1.25%), due to the least accuracy of the deconvolution procedure, the correlation is rather clear, and allows one to estimate the size of the carbon clusters in the film [19]. Thus, according to the measured ID /IG ratio and G-line FWHM, the graphiticcluster (presumably dominated by small aromatic rings) size is estimated to be less than 1 nm.
The above findings suggest that the graphitic phase in the examined W–C films consists of a uniform dispersion of small clusters. With increase in carbon content, the size of the clusters remains essentially the same, while their number grows. The effect of such increase in graphitic cluster concentration reduced translational periodicity of WC1−x crystals, allowing growth of only nanocrystalline dimensions. A similar change in W–C films structure with the increase of carbon content was already observed [5,13]. If the average size of the WC1−x crystallites is less than about 3 nm, the discrimination between the grain boundaries and grains is blurred to the extent that the material W–C:H is practically amorphous. The consequence in physical properties is a significant decrease in nanohardness, as confirmed by the results in Table 1. The parameter, which governs the grain-size structure in the asdeposited W–C films is in the greatest extent the unreacted carbon content. The effects of substrate temperature and potential bias during deposition are inconclusive, and the work on their clarification is in progress.
5. Conclusions Tungsten–carbon thin films were prepared by reactive magnetron sputtering, with benzene as a carbon-supplying admixture. The structure of the unreacted carbon incorporated into the film was examined by Raman spectroscopy. It was found that, to a larger extent, the carbon is incorporated as a graphitic phase. The enhancement of the graphitic fraction in the film is effected through the growing number of uniformly distributed small size (<1 nm) graphitic nanoclusters. Increased concentration of graphitic inclusions prevents formation of tungsten carbide microcrystals, leading to the eventual amorphization of the WC film structure, and consequent decrease in film hardness. Thus, a controlled incorporation of unreacted carbon allows an optimization between film hardness and other properties.
Acknowledgements The authors acknowledge the technical assistance of A. Pavleˇsin and I. Kovaˇcevi´c.
References
Fig. 6. The correlation between G-line width and the D-line and G-line intensity ratio. Benzene partial pressure: () 1.25%; () 2.5%; () 5%; open and full symbols mark deposition at room temperature and 400 ◦ C, respectively. A dashed circle and a full line are only guidance for the eye, separating the least accurate, and therefore scattered results for 1.25% benzene partial pressure from the results for higher benzene fraction.
[1] R.F. Bunshah (Ed.), Handbook of Deposition Technologies for Films and Coatings, Noyes, Park Ridge, NJ, 1994, p. 591. [2] C.M. Kelly, D. Garg, P.N. Dyer, Thin Solid Films 219 (1992) 103. [3] P.K. Srivastava, V.D. Vankar, K.L. Chopra, J. Vac. Sci. Technol. A 4 (1986) 2819. [4] K. Fuchs, P. Rodhammer, E. Bertel, F.P. Netzer, E. Gornik, Thin Solid Films 151 (1987) 383. [5] Y. Pauleau, Ph. Gouy-Pailler, J. Mater. Res. 7 (1992) 2070. [6] Ph. Gouy-Pailler, Y. Pauleau, J. Vac. Sci. Technol. A 11 (1993) 96.
N. Radi´c et al. / Materials Science and Engineering A 396 (2005) 290–295 [7] N. Radi´c, B. Grˇzeta, O. Milat, J. Ivkov, M. Stubiˇcar, Thin Solid Films 320 (1998) 192. [8] A. Cavaleiro, T.M. Vieira, G. Lemp´eriere, Thin Solid Films 185 (1990) 199. [9] Y. Pauleau, Ph. Gouy-Pailler, Mater. Lett. 13 (1992) 157. [10] C. Rebholz, J.M. Schneider, H. Ziegele, B. Rahle, A. Leyland, A. Matthews, Vacuum 49 (1998) 265. [11] O.R. Monteiro, M.-P. Delplancke-Ogletree, I.P. Brown, Thin Solid Films 342 (1999) 100–107. [12] A.A. Voevodin, J.P. O’Neill, J.S. Zabinski, Thin Solid Films 342 (1999) 194–200. [13] A.A. Voevodin, J.P. O’Neill, S.V. Prasad, J.S. Zabinski, J. Vac. Sci. Technol. A 1A7 (1999) 986. [14] W. Precht, A. Czyzniewski, Surf. Coat. Technol. 174–175 (2003) 979. [15] A. Czyzniewski, Thin Solid Films 433 (2003) 180. [16] G. Fanchini, S.C. Ray, A. Tagliaferro, Surf. Sci. Technol. 151–152 (2002) 233. [17] M.H. Grimdisditch, A.K. Ramdas, Phys. Rev. B 11 (1975) 3139.
295
[18] F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970) 1126. [19] J. Schwan, S. Ulrich, V. Batori, H. Ehrhardt, S.R.P. Silva, J. Appl. Phys. 80 (1996) 440. [20] Z.Y. Chen, J.P. Zhao, T. Yano, T. Ooie, M. Yoneda, J. Sakakibara, J. Appl. Phys. 88 (2000) 2305. [21] S. Prawer, K.W. Nugent, D.N. Jamieson, Diamond Relat. Mater. 7 (1998) 106. (22( S. Veprek, S. Reiprich, Thin Solid Films 268 (1995) 64. [23] K. Fuchs, P. R¨odhammer, E. Bertel, F.P. Netzer, E. Gornik, Thin Solid Films 151 (1987) 383–397. [24] W. van Duyn, B. van Lochem, Thin Solid Films 181 (1989) 497–503. [25] K. Machida, M. Euyo, I. Toyoshima, Thin Solid Films 161 (1988) L91–L95. [26] P. Juliet, A. Rouzaud, K. Aabadi, P. Monge-Cadet, Y. Pauleau, Thin Solid Films 290/291 (1996) 232–237. [27] J. Schwan, S. Ulrich, H. Roth, H. Ehrhardt, S.R.P. Silva, J. Robertson, R. Samlenski, R. Brenn, J. Appl. Phys. 79 (1996) 1416–1422. [28] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [29] A.C. Ferrari, Mat. Res. Symp. Proc. 675 (2001) 11.5.1.