PREPARATIONS AND TRIBOLOGICAL PROPERTIES OF SOFT- METAL / DLC COMPOSITE COATINGS BY RF MAGNETRON SPUTTER USING COMPOSITE TARGETS

Transcription

1 Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal July 216 Editors J.F. Silva Gomes and S.A. Meguid Publ. INEGI/FEUP (216) PAPER REF: 6297 PREPARATIONS AND TRIBOLOGICAL PROPERTIES OF SOFT- METAL / DLC COMPOSITE COATINGS BY RF MAGNETRON SPUTTER USING COMPOSITE TARGETS Minoru Goto (*) Department of Mechanical Engineering, National Institute of Technology, Ube College, Ube, Japan (*) mi-goto@ube-k.ac.jp ABSTRACT This work reports that the characteristics and tribological properties of both Ag/DLC nanocomposite coatings (RF-Ag-DLC) and Cu/DLC nanocomposite coatings (RF-Cu-DLC) with hydrogen-free DLC matrix deposited by RF magnetron sputtering using concentric composite targets. These coatings show granular structure of Ag or Cu cluster. The transition of friction coefficient became stable when metal-rich tribofilms formed on the counterfaces. Keywords: Soft metal, DLC, nanocomposite, RF magnetron sputter, tribology. INTRODUCTION Diamond-like carbon (DLC) is a carbon material exhibiting various kinds of attractive characteristics, and has been attracting a lot of researchers on the field of material science, biology and so on as well as tribology. The characteristics of DLC coatings vary not only by changing the structure of carbon network but also by adding the other elements as dopants or additives (Donnet, 28). Adding metals to DLC coatings (Me-DLC) is thus considered as a powerful method to improve electrical and/or biological properties as well as tribological properties. Copper and silver are good candidates as additives because of their low electrical resistivity, good tribological properties and unique biological properties. In recent years, we have reported that the structures and tribological properties of Cu and Ag doped DLC (H-Cu- DLC and H-Ag-DLC) with hydrogenated DLC matrix deposited by hybrid deposition process of PVD and PECVD (Goto, 215). By changing deposition method from the hybrid process to RF magnetron sputter, the structure of DLC matrix can be changed too as well as that of the coatings itself (RF-Me-DLC). However, it is difficult for the co-sputtering process by both metal target and carbon target to obtain low concentration of metal in RF-Me-DLC, because the sputter yield of metal is significantly higher than that of carbon in general. In this study, the characteristics and tribological properties of both Ag/DLC nanocomposite coatings (RF- Ag-DLC) and Cu/DLC nanocomposite coatings (RF-Cu-DLC) with hydrogen-free DLC matrix deposited by RF magnetron sputtering using concentric composite targets have been reported. EXPERIMENTAL DETAILS Concentric composite target (CCT) RF-Me-DLC coatings were deposited on the substrate by RF magnetron sputtering of a concentric composite target (CCT). The CCT consisted of C base target with a diameter of

2 Symposium_19: Advances in Tribology: Theory and Applications mm and metal tablet with diameters ranging from 14 to 7 mm, and the tablet was located on the center of the base target concentrically. The concentration of metal in RF-Me-DLC was varied by the diameter of tablet and RF power during the deposition. Figure 1 explains the concept of concentric composite target. Figure 1 (a) shows conventional composite target that several sector plates of additive element are located on the base target (Singh, 25, Sediri, 216). The concentration of additive element in the coating varies with the aria ratio between the sector plates and the exposed regions of base target. When we need to get the Me-DLC with low metal concentration, the area ratio of Me/C should be very small, since the sputter yields of Ag and Cu are significantly larger than that of C (Kato, 23 and Eckstein, 22). In this case, we are afraid about ununiformity of the coating composition, because the ununiformity of the metal concentration is amplified by the error of the plate size. However, a ring shaped high plasma density region is formed on the target during RF magnetron sputtering process, and the etching rate of the target in this region is relatively larger than that of the other region, i.e., the etching rate of the center region on the target is considerably lower than that of the high plasma density region. If the metal tablet with high sputter yield, such as Ag or Cu, is placed in the center of the C base target where the etching rate is low as shown in Fig. 1 (b), the Me-DLC with low and uniform metal concentration can be obtained. The target configuration as shown in Fig. 1 (b) is termed concentric composite target (CCT) in this paper. The advantage of CCT configuration is the increase of area ratio between metal/carbon which would be favourable to reduce fluctuation of metal content in the coatings. In addition, the reduction of metal size as a tablet is achievable. (a) conventional composite target Fig. 1 - Schematic images of target layout (b) concentric composite target Experimental procedure A Si (1) wafer with dimensions of mm 3 was used as a substrate for RF-Me- DLC coatings deposition. The Si (1) substrate was ultrasonically cleaned by organic alkaline solution, and then rinsed in super-deionized water several times. Waterdrops on the substrate was removed by clean nitrogen blow, and then the substrate was introduced in the coating chamber after fixing to the substrate holder made of stainless steel. The RF-Me-DLC coatings were deposited on the substrate by RF magnetron sputter using CCT. An Argon gas was introduced in the coating chamber, and the pressure was maintained at Pa during the deposition process. The metal concentration in the coating was varied by changing the tablet diameter and/or RF power, and was estimated by the composition ratio between metal and C in the coating, as measured by Energy Dispersive X-ray Spectroscopy (EDS). The accuracy of quantitative analysis by EDS is lower than that by Wavelength Dispersive X

3 Proceedings of the 5th International Conference on Integrity-Reliability-Failure ray Spectroscopy (WDS), especially for light element such as C. The accuracy of the composition rate of RF-Me-DLC might be lower than a few at.%. Despite this relatively low accuracy, the metal concentration measured by EDS allows to compare the metal/carbon content ratio in the different coatings (Goto, 215). In this paper, RF-Cu-DLC with Cu concentration ranging from 16 to 55 at.% and RF-Ag-DLC with Ag concentration ranging from 19 to 65 at.% were prepared. The deposition parameters are summarized in Table1. The thickness of the coating was defined by the step height between substrate and coating surface, which was measured by a contact-probe-type surface profiler. The nano-structure of the coatings was observed by transmission electron microscopy (TEM). A Raman spectroscopy was used for the characterization of DLC matrix. Table1 - Deposition parameters Tribological experiments on RF-Me-DLC were performed using linear reciprocating tribometer to compare the tribological properties, such as the friction transition and the tribofilm formation on the counterface, with those of H-Me-DLC previously reported. The RF-Me-DLCs which were supplied to the experiment were prepared at the RF power of 125 W in 1 hour using Ag or Cu tablets with diameters of 14 and 1 mm respectively. A mirrorpolished JIS SUJ2 bearing steel ball with a diameter of 6 mm was used as counter materials, after ultrasonically cleaned in acetone. The sliding speed was 2 mm/s in average, and the normal load was 1 N which yields maximum Hertz contact pressure of.6 GPa. The worn surfaces on the RF-ME-DLC and the SUJ2 slider were observed by optical microscope (OM) and scanning electron microscope (SEM) after tribological experiments. The chemical composition on the worn surfaces were also analysed by EDS. RESULTS AND DISCUSSION Characterization of RF-Me-DLC by RF magnetron sputtering using CCT Figure 2 shows the relationship between RF power and coating thickness of RF-Me-DLC at the deposition time of 1 hr. under the tablet diameter of 7 mm, 1 mm and 14 mm, respectively. The result of base target is also shown as a reference. The coating thickness at the deposition time of 1 hr. corresponds to the deposition rate. The deposition rate increases as increase of RF power and tablet size. The difference in deposition rate of RF-Me-DLC becomes small when the tablet diameters were 1 and 7 mm, whereas the deposition rate was increased obviously when the diameter of metal tablet was larger than 1 mm. In addition, the difference in the deposition rate of RF-Ag-DLC in the tablet diameter between 14 and 1 mm was much larger than that of RF-Cu-DLC. On the other hand, the gradient of deposition rate in Fig. 2 was hardly affected by RF power. This means that the evaporation process of metal

4 Symposium_19: Advances in Tribology: Theory and Applications (Ag and Cu) of CCT is not sensitive to the RF power during RF magnetron sputter process in this study. Figure 3 shows the relationship between RF power and metal (Ag, Cu) concentration in the coating for the three kinds of tablet diameters. The concentration varied from 19 to 65 at.% for RF-Ag-DLC and 16 to 55 at.% for RF-Cu-DLC. The metal concentration tends to decrease when the diameter of metal tablet becomes small. The trend of metal concentration against RF power and tablet diameter corresponds to the tendency of the deposition rate of RF-Ag-DLC and RF-Cu-DLC. In case of 14 mm tablet use, Ag concentration decreased from 65 to 4 at.%, as increase of RF power. The variation of Cu concentration against RF power is narrow compared with Ag-DLC, and decreased from 54 to 39 at.% in case of 14 mm tablet use. On the other hand, the variation of metal content in RF- Ag-DLC and RF-Cu-DLC against RF power became small in case of 1 and 7 mm tablet use. Thus, the difference in the thickness between RF-Me-DLC and metal-free DLC is corresponding to the contribution of metal part to the total thickness. 1 Target/Substrate: 7 mm, Ar pressure: 1.1 x 1 1 Pa 1 Target/Substrate: 7 mm, Ar pressure: 1.1 x 1 1 Pa Thickness, nm/hr : Ag tablet 14 mm : Ag tablet 1 mm : Ag tablet 7 mm : metal free (Ref.) Thickness, nm/hr : Cu tablet 14 mm : Cu tablet 1 mm : Cu tablet 7 mm : metal free (Ref.) Ag content, at.% RF Power, W (a) RF-Ag-DLC (b) RF-Cu-DLC Fig. 2 - relationship between RF power and coating thickness of RF-Me-DLC at the deposition time of 1 hr. under the tablet diameter of 7 mm, 1 mm and 14 mm Target / Substrate distance: 7 mm Ar pressure: 1.1 x 1 1 Pa 65 at.% RF Power, W (a) RF-Ag-DLC : Ag tablet 14 mm : Ag tablet 1 mm : Ag tablet 7mm 19 at.% RF Power, W (b) RF-Cu-DLC Fig. 3 - Relationship between RF power and metal (Ag, Cu) concentration in the coating for the three kinds of tablet diameters A surface area ratio of CCT between metal region and carbon region is one of the important parameter to determine the metal content in the coatings. The advantage of CCT is that the composite coatings with low metal concentration can be obtained by one sputter process under relatively large surface area ratio of Me/C, though the sputter yield of metal is Cu content, at.% Target / Substrate distance: 7 mm Ar pressure: 1.1 x 1 1 Pa 55 at.% RF Power, W : Cu tablet 14 mm : Cu tablet 1 mm : Cu tablet 7mm 16 at.%

5 Proceedings of the 5th International Conference on Integrity-Reliability-Failure extremely higher than that of carbon. If the composite target using metal sector plates is achieved to the RF-Me-DLC deposition, the surface area ration should become very small. The estimations of metal concentration in the RF-Me-DLC against surface area ratio using a composite target with sector plate are shown in Fig. 4, together with the experimental data by CCT. In this calculation, a self-bias voltage was assumed at 1 V from literature (ISHIDA, 21) though actual self-bias cannot be measured in our system. A sputter yields of C, Ag and Cu are estimated to atoms/ion, atoms/ion, and atoms/ion, respectively (Kato, 23 and Eckstein, 22). The metal concentration in the coatings decreased drastically by CCT used. Therefore, the configuration of CCT was suitable for this study. 1 1 Ag content, at.% Calcurated Ag content by composit target using sector plates Y Ag =.437 atms/ion, Y C = 8.27x1 4 atoms/ion Process gas: Ar, Self bias: 1 V 75 W 125 W 175 W Cu content, at.% Calcurated Cu content by composit target using sector plates Y Cu =.265 atms/ion, Y C = 8.27x1 4 atoms/ion Process gas: Ar, Self bias: 1 V 75 W 125 W 175 W Surface area ratio of Ag/C (a) RF-Ag-DLC Surface area ratio of Cu/C (b) RF-Cu-DLC Fig. 4 - Metal concentration in the RF-Me-DLC against surface area ratio Differential thickness of RF Me DLC : Ag tablet 14mm : Cu tablet 14mm : Ag tablet 1mm : Cu tablet 1mm RF power, W (a) Differential thickness of RF-Me-DLC RF power, W (b) Ratio of differential thickness Fig. 5 - Relationship between differential thickness and RF power Evaporation process of carbon can be explained mainly by the sputter process of carbon base target, because deposition rate of metal-free DLC is almost proportional to the RF power during the deposition process, as shown in Fig. 2. Indeed the plasma density at the center of carbon target is considerably lower than the ring-shaped dense plasma region and the etching rate at the center part might be insensitive to RF power significantly, but the evaporation process of metal in CCT might be subject not only to sputter process but also to another process. Figure 5 shows a differential thickness versus RF power during RF-Me-DLC deposition. A differential thickness is defined as the difference in deposition rate between RF- Ratio of differential thickness RF Ag DLC RF Cu DLC Ratio of surface area 14 mm tablet /1 mm tables

6 Symposium_19: Advances in Tribology: Theory and Applications Me-DLC and metal-free DLC at same RF power. The differential thickness corresponds to the equivalent thickness which is composed of the metal part in RF-Me-DLC. In the case of RF- Cu-DLC, the differential thickness was hardly affected by RF power at the same tablet diameter. The differential thickness at 14 mm tablet use was approximately times larger than those at 1 mm tablet use, which is comparable to the area ratio of tablet surface (see Fig. 4 (b)). On the other hand, the differential thickness of RF-Ag-DLC increased monotonically as the increase of RF power at the same tablet diameter. The differential thickness at 14 mm tablet use was approximately times larger than those at 1 mm tablet use, which was approximately twice of the area ratio of tablet surface (see Fig. 4 (b)). The temperature of metal tablet must increase as the RF power increases during deposition process, because the CCT surface is heated by hot plasma though backside of the target is cooled by water. A thermal effect must take into account to understand the difference in deposition rate between RF-Ag-DLC and RF-Cu-DLC, because saturate vapour pressure of Ag is approximately two order of magnitude higher than that of Cu. The saturate vapour pressure of Ag at 173 K are estimated at 1-3 Pa, which was two order of magnitude higher than that of Cu (1-5 Pa) (Koma, et al., 1987). Whereas, the sputter yield of Ag at Ar + 1 ev was estimated as atoms/ion, which was comparable to that of Cu ( atoms/ion) (Kato, 23 and Eckstein, 22). Figure 6 shows photographs of CCT (a) with Ag tablet and (b) with Cu tablet after coating preparations. In case of Ag tablet having relatively high partial vapour pressure, the contact surface of C base target (center part) against Ag tablet became white by Ag vapour emitted from the backside of the tablet. On the other hand, the change in contact surface of the base target with Cu tablet having relatively lower vapour pressure was not particularly noticeable. This result clearly showed that Ag vapour was emitted from Ag tablet during deposition process, and that the high deposition rate of RF-Ag- DLC is attributed to the contribution of thermal effect. Therefore, the thermal effect must take into account to the compositions of the coatings as well as the deposition rate when composite target composed of different kinds of element having different magnitude of saturate vapour pressure is utilized in the RF magnetron sputter process. (a) C base target and Ag tablet (dia.: 1 mm) (b) C base target and Cu tablet (dia.: 1 mm) Fig. 6 - Photographs of CCT (a) with Ag tablet and (b) with Cu tablet after coating preparations Center part of the base target of (a) became white by Ag vapour from the bottom of Ag tablet. Figure 7 shows the cross section image and electron diffraction pattern of (a) RF-Ag-DLC and (b) RF-Cu-DLC by transmission electron microscopy (TEM) observation. These coatings were prepared using Ag or Cu tablet with a diameter of 1 mm which the area ratio of Me / C is.42. The structure of RF-Me-DLC is granular structure in which the fine crystals of Ag or

7 Proceedings of the 5th International Conference on Integrity-Reliability-Failure Cu clusters are dispersed homogeneously in an amorphous DLC matrix. The nano-structure of RF-Me-DLC is the same as that of both H-Cu-DLC and H-Ag-DLC having hydrogenated DLC matrix (Takeno, 213). Normalized Raman spectra of metal-free DLC prepared by three kinds of RF power (75, 125 and 175 W) are summarized in Fig. 8. The typical DLC spectra with D peak and G peak were obtained from metal-free DLC, which forms DLC matrix of RF-Me-DLC. The shape of the Raman spectra didn t change under the different RF power during the deposition process. Nanoindentation hardness of RF-Me-DLC deposited by RF power of 125 W during 1 hour is shown in Fig. 9, as a function of metal content. By adding metals to the DLC, the hardness decreases as increase of metal concentration, and the hardness of RF-Ag-DLC tends to lower than that of RF-Cu-DLC at similar metal concentration. 2 nm 2 nm (a) RF-Ag-DLC (b) RF-Cu-DLC 2 2. Target: KCL Co. Ar pressur:.1 Pa.34 mw equiv W.5 : Cu DLC : Ag DLC λ = 532 nm 125 W Hardness, GPa Normalized intensity, a. u. Fig. 7 - Cross section and diffraction pattern of (a) RF-Ag-DLC and (b) RF-Cu-DLC by TEM Max. Load: 1 mn Ind. depth: 5 nm 75 W. 5 Si (1) Wave number, cm Fig. 8- Raman spectra of metal-free DLC Metal content, at. % 5 Fig. 9 - Nanoindentation result of RF-Me-DLC Transition of friction coefficient and metal-rich tribofilm formation Figure 1 shows the transition of friction coefficient of RF-Ag-DLC with metal concentrations of 28 and 46 at.%, respectively. The result of the tribo-test of metal-free DLC is also shown in the figure. The friction coefficient varied from.12 to.26 as the test

8 Symposium_19: Advances in Tribology: Theory and Applications progressed. As increase of Ag concentration, the gradient of friction coefficient against test duration decreased. Figure 11 shows SEM image and chemical maps of the counterface slid against RF-Ag-DLC with 28 at.% of Ag. The whitish transfer layer to the sliding surface is visible in the SEM image, which indicated metal-rich tribofilm formation on the counterface. Friction coefficient µ Load: 1N Average Speed: 2mm/s Ag at.% Ag 28 at.% Ag 46 at.% Duration, min. Fig. 1 - Transition of friction coefficient of RF-Ag-DLC Fig SEM image and chemical maps of counterface of RF-Ag-DLC (Ag: 28 at.% ) The whitish transfer layer emitted Ag Lα, but neither C Kα nor O Kα is visible in the chemical maps. This result indicates that non-oxidized Ag-rich tribofilm is formed on the sliding surface during sliding. This phenomenon is the similar to that on the counterface of the H-Ag-DLC deposited by hybrid process previously reported (GOTO, 215). Figure 12 shows friction transition of RF-Cu-DLC with metal concentrations of 3 and 4 at.%, respectively. By adding Cu to the metal-free DLC, the change of friction coefficient became stable against test duration. The friction coefficient decreased with maintaining the stability as the increase of Cu content, but absolute value of the friction coefficient of RF-Cu-DLC was higher than that of RF-Ag-DLC. The result of SEM and EDS analysis revealed that the Cu-rich tribofilms

9 Proceedings of the 5th International Conference on Integrity-Reliability-Failure formed on steel counterface of RF-Cu-DLC after tribo-test in air, as shown in Fig. 13. Indeed Cu is oxidizable metal, but the Cu-rich tribofilm is neither carbonized nor oxidized even though the tribo-test was performed in oxidizable environment. The fundamental tribological properties, such as friction coefficient and metal-rich tribofilm formation, of RF-Me-DLC with hydrogen-free DLC matrix are similar tendency as H-Me-DLC with hydrogenated DLC matrix (GOTO, 215). However, more detailed research is necessary to understand either effect of metal concentration or mechanical properties on the tribological properties of RF- Me-DLC under the wild-range composition and structure. Friction coefficient µ Load: 1N Average Speed: 2mm/s Cu at.% Cu 4 at.% Cu 3 at.% Duration, min. Fig Friction transition of RF-Cu-DLC Fig SEM image and chemical maps of counterface of RF-Cu-DLC (Cu: 3 at.% ) CONCLUSION This study demonstrated that the configuration of concentric-composite target was suitable for preparing metal/diamondlike carbon composite coatings with a wide range of metal concentration by RF magnetron sputter process. As a result, RF-Ag-DLC with 19~65 at.% of

10 Symposium_19: Advances in Tribology: Theory and Applications Ag and RF-Cu-DLC with 16~55 at.% of Cu have been prepared by RF magnetron sputtering method using concentric composite targets composed of C base target and Ag or Cu tablet. The thermal effect must take into account to the compositions of metal/diamondlike carbon coatings as well as the deposition rate when composite target composed of different kinds of element having different magnitude of saturate vapor pressure is utilized in the RF magnetron sputter process. The structures of RF- Ag-DLC and RF-Cu-DLC are granular structure which is the same structure as the films prepared by the hybrid deposition process of PECVD and PVD. Non-oxidized metal-rich tribofilms were formed on the counterface of RF sputtered Ag-DLC and Cu-DLC, and the transition of friction coefficient became stable relatively than that of RF sputtered metal-free DLC. ACKNOWLEDGMENTS This work was partly supported by Grant-in-Aid for Scientific Research (S) (252292), (C) (264293) of Japan Society for the Promotion of Science (JSPS) and Collaborative Research Project (J1438 and J1518) of the Institute of Fluid Science of Tohoku University. The author expresses his gratitude to Dr. J. CHOI for of Tokyo University for his support of Raman analysis. The author acknowledged Mr. M. Oda and Mr. T. Nawata of students of National Institute of Technology, Ube College for their support of coating preparation. REFERENCES [1]-Donnet, C. and Erdemir, A. Tribology of diamond-like carbon films fundamentals and application. 28, Springer. [2]-Goto, M. Ito, K. Fontaine, J. Takeno, T. Miki, H. and Takagi, T. Formation processes of metal-rich tribofilm on the counterface during sliding against metal/diamondlike-carbon nanocomposite coatings. Tribology Online, Vol. 1, No. 5, 215, pp [3]-Singh, K., Limaye, P.K., Soni, N.L., Grover, A.K., Agrawal, R.G., Suri, A.K., Wear studies of (Ti Al)N coatings deposited by reactive magnetron sputtering, Wear 258 (25) [4]-Sediri, A., Zaghrioui, M., Baricharda,, A., Autret, C., Negulescua, B., Del Campob, L., Echegut, P., Laffez, P., Growth of polycrystalline Pr2NiO4+δ coating on alumina substrate by RF, Thin Solid Films 6 (216) [5]-ISHIDA, T., Preparing Conductive Transparent ITO Films by RF Magnetron Sputtering with Low Self Bias Voltage and Study on its Self Bias, J. Vac. Soc. Jpn. Vol. 44, No. 8 (21) [6]-Kato, D. et al., NIFS Database on Sputtering, Reflection and Range Values, URL= 23. [7]-Eckstein, W., Max-Plank-Institute fur Plasmaphysik, IPP Report, IPP9/132 (22) [8]-Koma, A., Yagi, K., Tsukada, M., Aono M., Handbook of Surface Science & Technology (Japanese), MARUZEN co. Ltd. (1987). [9]-Takeno, T., Saito, H., Goto, M., Fontaine, J., Miki, H., Belin, M., Takagi, T., Adachi, K., Deposition, structure and tribological behaviour of silver carbon nanocomposite coatings, Diamond & Related Materials 39 (213)