A study of hard diamond-like carbon films in mid-frequency dual-magnetron sputtering

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1 Diamond & Related Materials 15 (2006) A study of hard diamond-like carbon films in mid-frequency dual-magnetron sputtering Yu Xiang a, *, Wang Cheng-biao a, Liu Yang b, Yu De-yang b a School of Engineering and Technology, China University of Geosciences, Beijing , China b Beijing Powertech Co,. Ltd., Beijing , China Received 14 December 2004; received in revised form 7 August 2005; accepted 22 September 2005 Available online 22 November 2005 Abstract A series of hydrogen-free diamond-like carbon (DLC) films were deposited by a mid-frequency dual-magnetron sputtering under basic conditions of Cr and C target power density between 6 and 18 W/cm 2, bias voltage in a range of 100 V to 200 V, and a pure argon atmosphere. Microstructure, microhardness, adhesion, friction and wear properties were investigated for the DLC films to be used as protective films on cutting tools and forming dies, etc. The DLC films exhibited some combined superior properties: high hardness of GPa, good adhesion of critical load of N, and friction coefficient about 0.1 in air condition. Properties of the magnetron-sputtered carbon films showed a strong dependence on flux and energy of ion bombardment during growth of the films. D 2005 Published by Elsevier B.V. Keywords: Cr-doped DLC film; Mid-frequency dual-magnetron sputtering; Multilayered and graded structure; Properties 1. Introduction Due to having some prominent tribological properties such as high hardness, low friction coefficient and high wear resistance, diamond-like carbon (DLC) films are of great interest for usages of wear-resistant films on cutting tool, forming die, etc. Accordingly, numerous studies on quality improvement of such film have been made in various techniques, e.g. arc evaporation, HF-plasma or microwaveplasma CVD and unbalance magnetron sputtering [1]. Hydrogenated DLC film has limited microhardness due to inclusion of H atoms. A main limitation in hydrogen-free DLC film applications is its poor adhesion to steel substrates due to high residual stress in the DLC films and/or the large discrepancy of mechanical properties between the DLC films and steel substrates [2]. Some studies have attempted to improve the adhesion between DLC film and its substrates by applying adherent layer such as B and Ti to the film interface [3]. Methods enhancing the mechanical properties were also reported, such as doping TiN, TiCN and TiC * Corresponding author. Tel.: ; fax: address: yuyuxiang7546@sina.com (Y. Xiang). layers in DLC matrix [4]. A graded composition and microstructure may be used to improve adhesion and microhardness of the graded film. In this study, several multilayered and graded DLC films with a single-phase homogeneous carbon layer were synthesized by selectively using C and Cr targets in a mid-frequency dual-magnetron sputtering. The pulsed plasma technique was applied to both magnetron drives and substrate bias in this experiment. Resent studies demonstrated that mid-frequency ( khz) magnetron sputtering offered some advantages over traditional DC magnetron sputtering for deposition of metastable films like DLC and CN x [5]. Pulsing the discharge can significantly modify the characteristics of magnetron plasma, pulsing the substrate bias voltage can increase the ion current drawn at the substrate, which have some important effects on properties of the me tastable films [6]. This paper focuses on improving properties of the hydrogen-free DLC film by controlling process parameters like target current, substrate bias voltage, etc., during the midfrequency dual-magnetron sputtering. Structure, deposition process and resulting properties of the DLC film are investigated /$ - see front matter D 2005 Published by Elsevier B.V. doi: /j.diamond

2 1224 Y. Xiang et al. / Diamond & Related Materials 15 (2006) Experimental details 2.1. Coating equipment Cr-doped hydrogen-free DLC films were deposited using unbalanced dual-magnetron sputtering in a coating rig SP8050 (Beijing Powertech Co., Ltd.), whose cross-sectional view is schematically shown in Fig. 1. As can be seen, four pairs of dual-magnetron targets including two chromium (Cr) and six graphite (C) ones were set in the cylindrical coating rig. Two planar magnetrons of each pair of dual-magnetron targets were placed in vacuum chamber in face-to-face configuration approximately 70 cm apart, respectively. The substrate holder was placed in center area. A unipolar-pulse DC power supply on the substrates and four pulsed AC power supplies of 40 khz, on each pair of dual-magnetrons, were used to control the substrate bias voltage, substrate bias current and target power. The duty ratio of the pulsed substrate bias voltage was 80% in Ar ion cleaning and was 60% in the following deposition processes. The mid-frequency power supplies used a currentconstant mode to control the magnetron targets at constant values of 2, 4 and 6 A, respectively, corresponding to target power increased from 2 to 8 kw and target voltage 3 from 200 to 800 V. The C and Cr targets had a uniform size of mm 3 and a purity of 99.9%. Argon gas was introduced through each target to enhance sputtering of the targets. Process parameters in each procedure could be automatically controlled by PLC (Programmable Logical Controller) through presetting Sample preparation and Ar ion cleaning Sheets of M2 high-speed steel (HSS) in a uniform size of mm 3 were used as coupons for mechanical and tribological tests, and some coupons of Si(100) wafers in a uniform size of mm 3 were used for cross-sectional SEM and Raman tests. Surfaces of the steel sheets were mirror polished to surface roughness of R a 0.05 Am. All coupons were ultrasonically cleaned in acetone bath for 20 min, and were then blown dry with nitrogen. After pumping down by a turbo-molecular pump to a base pressure of Pa and before depositing, surfaces of the coupons were further cleaned by Ar ion bombardment under the bias voltage of 800 V at pressure of 3.0 Pa for 15 min, in order to remove some adhering impurities on substrates Deposition procedures After the substrate was heated to 100 -C, deposition process was started in condition that the depositing pressure maintained at 0.3 Pa in argon atmosphere and the bias voltage was in a rang of 100 V to 200 V. The deposition procedures included three optional steps: firstly, Cr interlayer about 0.2 Am on the substrate was sputtered from two Cr targets, where Cr target current kept at 4 A; secondly, transitional layer of Cr x C y about 1 Am on Cr interlayer was co-sputtered from two Cr and six C targets, where Cr target current decreased from 10 A to 1 A and C target current increased from 2 A to 6 A, continuously; thirdly, top DLC layer about 1 Am was sputtered from six C targets, where C target currents were at constant values of 2, 4 and 6 A, respectively Surface analysis methods Scanning electron microscope (SEM) combined with energy-dispersive X-ray (EDX) analyzer, Quant 200, was used to investigate the morphologies and elemental compositions. Raman spectroscope, PHI-6100/SAM, was used to investigate the microstructure. Nanoindenter (MML 1 Nanotester 600) and scratch tester (LICP-CAS SW-97) were used to investigate the microhardness and adhesion, respectively. Ball-on-disk tester (LICP-CAS DD-92) was used to investigate the friction and wear property under conditions: DLC film was deposited on the disc, and counterpart ball used was a A6 mmal 2 O 3 ball; loads of 1 N, 2 N and 5 N were applied in air; rotated speed was constant at 1000 r/min, equivalent to 0.6 m/s of linear speed; tested time was about 15 min. Surface profilometer was used to measure the cross-sectional area of the wear track so as to calculate the wear volume of the DLC film. Optical microscope attached with a camera was used for analysis of wear traces. 3. Results and discussions Properties of the graded DLC film in unbalanced magnetron sputtering show a strong dependence on flux and energy of ion bombardment during growth of the film [7]. According to our experimental results of mid-frequency dual-magnetron sputtering, the substrate bias current, an index of ion flux, had some close correlations with process parameters, such as: target Fig. 1. Schematic diagram of cross-sectional coating rig SP Micro Materials Limited. LICP-CAS: Lanzhou Institute of Chemical Physics, Chinese Academy of Science.

Y. Xiang et al. / Diamond & Related Materials 15 (2006) 1223 1227 1225 power, substrate bias voltage, substrate temperature and gas pressure.

3 Y. Xiang et al. / Diamond & Related Materials 15 (2006) power, substrate bias voltage, substrate temperature and gas pressure. For example, in condition that target power was 6 kw or substrate bias voltage was 250 V at substrate temperature less than 150 -C and argon pressure of 0.3 Pa, the substrate bias current had a high value of 5 A for the carbon deposition. This value could be even higher with increasing argon pressure from 0.3 Pa to 1 Pa and substrate temperature from 150 -C to 450 -C. The energy of ion bombardment was increased with the increasing bias voltage. For a convenient operation, the properties of DLC films were controlled by adjusting the target current and substrate bias voltage under condition of the constant substrate temperature and sputtering pressure Microstructure and deposition character Surface and cross-section SEM images of DLC films at 100 V of the pulsed bias voltage are shown in Fig. 2. As shown in Fig. 2a, the film showed a very smooth morphology with no visible loose particles from the targets, whose average surface roughness, R a, is about 4 nm. During unbalance magnetron sputtering on precise forming die, substrate arcing is somewhat inevitable [8]. If the localized surface arcing persists, some droplets and holes from the running arcs may make the die unusable. The mid-frequency magnetron sputtering may be an effective method to reduce the defects induced by substrate arcing. In Fig. 2b, the multilayered and graded structure, containing layers of Cr, C x Cr y and DLC, can be observed. Apart from the imperfect smoothness of crosssection surface due to sampling difficulty, R a å0.2 Am inthe wide areas; interfaces between each layer of the graded DLC film were well bonded without any visible flaw, of which C x Cr y layer were embedded in DLC matrix. Such graded structure may be used to cope with the residual stress in DLC Fig. 3. A typical Raman spectrum of DLC film deposited at 100 V of pulsed bias voltage. film and the large discrepancy of physical properties between DLC film and the substrate. The integral structure of graded DLC film is dense and compact, which is vital in achieving high hardness for wear resistance. A typical Raman spectrum of the DLC film is shown in Fig. 3. The spectrum can be fitted to two Gaussian peaks, one at approximately 1565 cm 1 ( G peak), and another at approximately 1338 cm 1 ( D peak). The D-to-G bond ratio of I D /I G is Similar to report by Yang and Teer [9], the metal Cr and Cr containing layers bolstered the stress management in hard a-c films. In this way, the stress state of me tal-doped DLC film could be stepwise adjusted in graded structure from outer boundary through multilayer to inner boundary and substrate by specified process control. Deposition rate of the DLC film was high, more than 0.8 Am/h, which was about twice of reported one using traditional sputtering [10]. The deposition rate was closely related with target power in this current-constant mode and thus could be controlled. When the biasing voltages was at 100, 150, and 200 V, meanwhile carbon target current kept at 2 A, the deposition rates were measured at 0.96, 0.94 and 0.87 Am/h, respectively. When the C target currents was at 2, 4 and 6 A, meanwhile the bias voltage kept at 100 V, the deposition rates were measured at 0.96, 1.06, 1.14 Am/h, respectively. Above results were obtained in such condition: compared with chromium, carbon deposition rate was relatively low owing to the low sputtering yield of graphite. However, if the biasing voltage exceeded 200 V or the target current exceeded 6 A in above condition, the deposition rate was not increased and may be decreased little due to too intense ion bombardment, similar results can be seen in literature [11] Mechanical properties Fig. 2. Surface and cross-section SEM images showing structure of Cr-doped DLC film on Si wafer Microhardness To accurately measure microhardness of DLC film in micron-scale thickness, a two-step penetration method in

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3. Friction and wear property Friction and wear property of DLC films of hard one with 34 GPa and superhard one with 45 GPa was investigated in the ball-on-disc tester, whose frictional results are

4 1226 Y. Xiang et al. / Diamond & Related Materials 15 (2006) improvement in the adhesion, and the best one was 65 N of L c from the graded DLC film at substrate bias voltage of 160 V Friction and wear property Friction and wear property of DLC films of hard one with 34 GPa and superhard one with 45 GPa was investigated in the ball-on-disc tester, whose frictional results are shown in Fig. 6. At a 7 load of 5 N, the average friction coefficient of the superhard one is about 0.08, and that of the hard one is about 0.1. At the low loads of 1 N, 2 N and 5 N, the DLC films show some low and constant friction coefficients between 0.15 and 0.08, Fig. 4. Influence of target current and substrate bias voltages on mirohardness of DLC films. nanoindenter tests was used, i.e. after obtaining the test results of large-load, a penetration load range could be determined to ensure that range of deformation under the indenter was within DLC layer and did not extend into the transitional layer or substrate. As shown in Fig. 4, microhardness of the hydrogenfree DLC films is between 34 GPa and 46 GPa. The microhardness value is higher than that of hydrogen carbon films (a-c:h) and a-c films, as reported in literature [12]. The compactness of DLC film may contribute to such improved microhardness. The microhardness value increases with the increasing bias voltages, a typical result of increased residual stress by ion bombardment [13,14]. Target current also shows a clear effect on microhardness of the film Adhesion In case of using the graded structure, influences of interlayer design and substrate bias on film adhesion were evaluated by scratch test, whose results are shown in Fig. 5. Being as index of adhesion force between film and substrate, the critical load L c, was determined by onset of abrupt variations in friction forces, as a diamond stylus scratched with continuous loading on DLC films. Three types of DLC films were tested for evaluating their adhesions. In Fig. 5A, a pure DLC film showed a poor adhesion, peeled off the substrate surface at 16 NofL c, due to the poor interface compatibility. In Fig. 5B, a DLC film with Cr interlayer was used to improve the adhesion. For a convenient analysis, L c is divided into L c1 (critical load for cohesion) and L c2 (critical load for adhesion), determined by the first and secondary onsets of abrupt variations of the friction forces. The DLC film was peeled off the substrate surface at 21 N of L c1, but L c2 was lasted to 60 N. Cr interlayer seemed to play an important role on adhesion improvement. In Fig. 5C,D, the graded DLC films with Cr interlayer and C x Cr y transitional layer at substrate bias voltages of 100 V and 160 V were tested. Scratch results demonstrated a significant Fig. 5. Influence of transitional layer and substrate bias voltages on adhesion of DLC films (target current 6 A).

Y. Xiang et al. / Diamond & Related Materials 15 (2006) 1223 1227 1227 Fig. 6. Two comparative curves of friction coefficients of DLC films with different hardness of 34 GPa and 45 GPa.

5 Y. Xiang et al. / Diamond & Related Materials 15 (2006) Fig. 6. Two comparative curves of friction coefficients of DLC films with different hardness of 34 GPa and 45 GPa. Optical micrographs of wear tracks were from the discs at the end of 500 m of sliding distance. and these fiction coefficients do not show an obvious change with the different loads. As shown from the optical micrographs in Fig. 6, in the test range of 500 m, no obvious failure of DLC films was observed. The sliding wear rates of the DLC films are in a range of mm 3 /Nm at a load of 2 N and at a sliding distance of 500 m. The wear rate of the super-hard DLC film is mm 3 /Nm, and that of the hard one is mm 3 /Nm. After collecting the wear debris from the ball and disc and filtering the debris in acetone bath, it was found in the EDX analysis that the main element content of the debris was carbon, so abrasion was supposed as the wear mechanism. Under the same test condition, the superhard DLC film, microhardness more than 40 GPa, showed the lower volumetric wear rate and friction coefficient than that of the hard one, microhardness between 30 GPa and 40 GPa. In this case, the wear resistance of DLC film might be correlated with the hardness of DLC film [15,16]. 4. Conclusion A series of hydrogen-free DLC films were successfully synthesized using the mid-frequency dual-magnetron sputtering. The process parameters, structure and properties of Crdoped graded DLC film have been investigated for the films to be used as the protective films. The properties of these DLC films showed a clear dependence on the flux and energy of ion bombardment during growth of the film. The DLC film possessed a graded and multilayered structure. The Cr interlayer played a role of adhesion improvement. The structural integrity was beneficial for both the high microhardness and good adhesion of the DLC films. The effect of target current and bias voltage on the properties of the DLC film was also examined. It was found that the DLC film possessed some combined advisable properties: a very smooth morphology, microhardness in range of GPa, adhesion of L c up to 65 N and a reliable friction coefficient about 0.1. The super-hard DLC film had the behavior of both good adhesion and lower friction coefficient, and thus it is predicted that it may be used the one as a high-quality wearresistant film in the industrial applications. Acknowledgements This project has been supported by the National Natural Science Foundation of China with Grant No and by The Tribology Science Fund of National Tribology Laboratory of Tsinghua University in Beijing. References [1] J. Robertson, Mater. Sci. Eng., R Rep. 37 (2002) 129. [2] M. Chhowalla, Diam. Relat. Mater. 10 (2001) [3] S. Zhang, H. Xie, Surf. Coat. Technol. 113 (1999) 120. [4] Y. Lifshitz, Diam. Relat. Mater. 8 (1999) [5] J. Qi, K.H. Lai, C.S. Lee, I. Bello, S.T. Lee, J.B. Luo, S.Z. Wen, Diam. Relat. Mater. 10 (2001) [6] P.J. Kelly, R.D. Arnell, Vacuum 56 (2000) 159. [7] S. Zhang, X.L. Bui, Y.Q. Fu, Surf. Coat. Technol. 167 (2003) 137. [8] J. Stallard, D. Mercs, M. Jarratt, D.G. Teer, P.H. Shipway, Surf. Coat. Technol. 177/178 (2004) 545. [9] S. Yang, D.G. Teer, Surf. Coat. Technol. 131 (2000) 412. [10] M. Jarratt, J. Stallard, N.M. Renevier, D.G. Teer, Diam. Relat. Mater. 12 (2003) [11] M. Stuber, S. Ulrich, H. Leiste, A. Kratzsch, H. Holleck, Surf. Coat Technol (1999) [12] K.R. Lee, K.Y. Eun, I. Kim, J. Kim, Thin Solid Films 377/378 (2000) 261. [13] R.D. Arnell, P.J. Kelly, J.W. Bradley, Surf. Coat. Technol. 188/189 (2004) 158. [14] Y. Kazuhiro, W. Toshiya, W. Koichiro, et al., Diam. Relat. Mater. 10 (2001) 895. [15] X. Yu, X. Zhang, Ch.B. Wang, et al., Vacuum 75 (2004) 231. [16] W. Zhang, A. Tanaka, K. Wazumi, Y. Koga, Diam. Relat. Mater. 11 (2002) 1837.

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