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Thermal stability of Cr-doped diamond-like carbon films synthesized by cathodic arc evaporation
Thin Solid Films 476 (2005) 258 – 263 www.elsevier.com/locate/tsf
Thermal stability of Cr-doped diamond-like carbon films synthesized by cathodic arc evaporation Ming-Chieh Chiua,*, Wen-Pin Hsieha, Wei-Yu Hob, Da-Yung Wangb, Fuh-Sheng Shieua a
Department of Materials Engineering, National Chung-Hsing University, Taichung, Taiwan b Center for Applied Science and Technology, Ming-Dao University, Changhua, Taiwan
Received 15 December 2003; received in revised form 8 July 2004; accepted 16 September 2004 Available online 10 November 2004
Abstract Cr-doped diamond-like carbon (DLC) films were synthesized using a cathodic arc evaporation (CAE) process. The thermal oxidation behavior of Cr-doped DLC films was investigated using thermal gravimetric analysis (TGA) and differential thermal analysis (DTA). The phase identification and microstructural examinations were conducted by X-ray diffraction, scanning electron spectroscopy (SEM), transmission electron spectroscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) in order to understand the characteristics of Cr-doped DLC films. The as-deposited Cr-doped DLC film exhibits a lamellar structure observed by TEM. A significant weight loss of film results from the thermal oxidation of carbon occurred at 290 to 342 8C. At the temperature higher than 342 8C, slight weight gain of specimen was observed due to the thermal oxidation of the underlying CrCx Ny and CrN interlayer. By heat-treated specimens from 200 to 400 8C, Raman spectra reveal the increase of I D/I G value conforming to the graphitiation process of the Cr-doped DLC films. Finally, surface reactions of the annealed films using XPS analysis were discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Cathodic arc evaporation; Oxidation; Microstructure
1. Introduction Diamond-like carbon (DLC) films have superior physical and chemical properties in terms of hardness, chemical inertness, high wear resistance, and thermal conductivity. Consequently, DLC coatings are potentially useful for different industrial applications ranging from tribological protective coatings to microelectronics. In general, the nature and properties of the DLC could be modified by controlling the incorporation of dopants, such as silicon, fluorine, nitrogen, metal and metal carbides. However, DLC coatings are known to be extremely sensitive to the presence of oxidizing species including oxygen and water during friction. They have a significant disadvantage of low
* Corresponding author. Tel.: +886 422840500407; fax: +886 422857017. E-mail address: [email protected] (M.-C. Chiu). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.09.029
thermal stability at higher working temperatures. DLC coatings cannot retain their superior characteristics at higher temperature because of the conversion of sp3-bonded carbon to sp2-bonded carbon. It is known that a wide variety of methods can be used to produce DLC coatings, such as plasma deposition [1], ion sputtering [2], magnetron sputtering [3], ion plating [4], and laser plasma deposition [5]. Previous work showed that metal-doped DLC films on top of a CrN/CrCx Ny interlayer were successfully deposited on M2 high-speed steels by cathodic arc evaporation (CAE) process [6]. Significant improvement of adhesion between DLC films and substrate can be obtained by graded design in both composition and microstructure. Further study showed that Ti, Cr, and Zr metals could act as a catalyst to play a key role of DLC deposition using CAE [7]. Some works on the thermal stability of DLC films have been reported. Oxidation studies of the DLC films have been characterized by using thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) techniques. Wang et
M.-C. Chiu et al. / Thin Solid Films 476 (2005) 258–263 Table 1 Deposition parameters of CAE synthesized Cr-DLC films Parameters
Values
Base pressure (Pa) Reactive gas pressure (Pa) Total deposition time (min) Distance of cathode to substrate (mm) CAE target
5103 2.5 65 180 99.99% Cr (100 mm in diameter) 60 1000 100 (100 kHz pulsed) 150
Cathode current (A) Bias voltage at ion cleaning stage (V) Bias voltage at coating stage (V) Substrate temperature (8C)
al. [8] investigated the oxidation kinetics and reaction mechanism of Ti-doped DLC films by TGA/DTA analysis. The results revealed two distinctive oxidation behaviors of DLC films oxidation at 350 8C and TiN/TiCx Ny interlayer oxidation at 450 8C [8]. In this study, the thermal stability of Cr-doped DLC films deposited by CAE process was investigated using TGA/DTA analysis. The effect of annealing treatment on the thermal stability of Cr-doped DLC films in ambient air is confirmed by examining their structural changes.
2. Experiment details Cr-doped DLC films were prepared by a CAE process using 99.99% Cr targets, and acetylene (C2H2) and nitrogen (N2) reactive gases. Cr-doped DLC coatings were deposited on SS304 stainless steel foils of dimensions 30100.025
259
mm that is designed to fit into the TGA/DTA crucibles in spiral forms. During the deposition process, Cr ion bombardment was first executed from Cr targets on the substrates to form a Cr-rich buffer layer of about 60-nm thick, and CrN was deposited as a follow-up interlayer when nitrogen was introduced. The C2H2 was then introduced into the chamber to form a CrCx Ny layer in which the C and N composition graded by decreasing the N2 flow rate and increasing the C2H2 flow rate at the same time. Cr-doped DLC film was finally deposited on the top and the N2 flow rate was turned off at this final stage. The substrate temperature was controlled at 150 C and the deposition time for all the samples was about 65 min. The working pressure of the plasma chamber was 2.5 Pa during DLC deposition stage. Details of the process parameters are listed in Table 1. The as-deposited Cr-doped DLC coatings were then subjected to thermal analysis using a TGA/TDA system, which is equipped with both a Cahn-2000 thermogravimetric unit and an Ulvac TGD-7000HR unit. To understand the oxidation behavior of the coatings, these films were also annealed at varying temperatures of 200, 300, 400 and 500 8C in ambient air for 30 min. The analytical techniques used in this study to characterize the degradation of DLC films include transmission electron microscopy (TEM), scanning electron microscopy (SEM), glancing incidence X-ray diffraction (GIXD). Raman spectroscopy using a 50 mW beam of wavelength 514.5 nm from an argon laser in the wavenumber range 1000–2000 cm1 and X-ray photoelectron spectroscopy (XPS, PHI 1600) using Mg Ka radiation were used to identify the sp3/sp2 bond conversion and chemical bonding at the near-surface region. The
Fig. 1. (a) TEM cross-sectional micrograph, (b) SAD pattern of the as-deposited Cr-DLC coating.
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samples for XPS study were cut into 33 mm square blocks. Samples were load locked to the analysis chamber, surface analyzed prior to ion bombardment, and subsequently sputtered in the analysis position. To reduce the influence of surface contamination, sample surface was presputtered under 3.0 keV Ar+ bombardment for 30 s before the analysis was conducted. Surface charge-up does occur causing the peak binding energy to be shifted from its expected position. The binding energy of the elements was calibrated by using amorphous SiO2 and single crystalline Si as standards. Microstructure of the DLC coatings was investigated by a JOEL JEM-4000EX TEM operated at 400 kV. The crosssection specimen for TEM study was prepared by gluing two 13 mm rectangle blocks face-to-face, and then grinding and polishing from both sides down to ~50 Am. To reduce the milling time, a precision grinding machine (VCR 500i) was used to dimple the specimens. Ion milling was done by a BAL-TEC RES010 ion milling operated at 5 kV, using a single-side sample holder. Surface morphology of the coatings was examined by a JEOL JSM-5400 SEM operated at 20 kV. Crystallinity of the coatings was studied by a SCINTAGXGEN-400 diffractometer using Cu Ka radiation at an incident angle of 28.
Fig. 3. XRD spectrum of the Cr-DLC coating annealed in ambient air.
believed that the metal interlayer can reduce the residual stress and improve the adhesion between coating and substrate. The multilayered and graded film structures
3. Results and discussion 3.1. Microstructure analysis of the as-deposited DLC coatings Details of the microstructure of the as-deposited DLC coatings were revealed by cross-sectional TEM micrograph, as shown in Fig. 1(a), in which the deposition sequence of Cr, CrN, CrCx Ny and DLC layers can be readily identified. The film thickness was controlled at 1.65 Am. The CrCx Ny exhibits a dense and compact microstructure with wellattached CrN interface. As demonstrated by Wang et al. [6– 8] for the design of metal-doped DLC on stainless steel, it is
Fig. 2. TGA/DTA analyses of the Cr-DLC films.
Fig. 4. Scanning electron micrographs of (a) as-deposited, (b) annealed at 200 8C, and (c) annealed at 300 8C Cr-DLC films.
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improved the severe mismatch in lattice and physical properties between DLC and substrate. The interfaces between each layer are free from any noticeable voids, gaps or other defects. The Cr and CrN adhesive layer were found to have a columnar structure. CrCx Ny and DLC films exhibit a lamellar appearance, as shown in Fig. 1(a). Differed from conventional DLC deposition processes, this study used cathodic arc sources as the source energy of C2H2 decomposition for DLC synthesis. During the DLC synthesized process, a joint physical vapor deposition and plasma enhanced chemical vapor deposition process occurred, where the activated C2H2 decomposition and amorphous carbon deposition were augmented by the intense metal plasma generated by cathodic arc sources. When the C2H2 was introduced, the columnar growth stopped and the structure has a lamellar appearance. During DLC deposition stage, it is observed that Cr target surface was partially poisoned by carbon layer. Therefore, the lamellar structure was resulted from the superposition of Crbase carbides and carbon layer. The dimension of each multilayer is about several nanometers, which is much smaller than that in the columnar layer. The selected area diffraction (SAD) pattern covering DLC and CrCx Ny showed diffuse ring patterns indicating an amorphous structure, as shown in Fig. 1(b). GIXD investigation of the as-deposited sample shows diffraction peaks corresponding to CrN and stainless steel. There were no characteristic peaks of crystalline carbon phases indicating again the amorphous characteristic of the film. 3.2. Oxidation performance DLC films were analyzed by TGA/DTA, as shown in Fig. 2. DTA/TGA tests of the DLC films were conducted at a heating rate of 10 8C/min between 25 and 800 8C. From
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Fig. 2, it is clear to see that thermal oxidation of carbon occurred at 290 to 342 8C. The DTA curve reveals a typical exothermic reaction in this temperature range. Below 290 8C, crystallization and graphitization of the amorphous DLC films occurred, as depicted in the DTA curve. Between 290 and 342 8C, a significant weight loss results from the carbon oxidation, according to the (1) C þ O2 YCO2
ð1Þ
The reaction product CO2 escapes with the vent gas out of the crucible. According to the TGA curve, weight gain of specimen changed only slightly as the temperature increased from 400 to 800 8C due to the thermal oxidation of underlying CrN and CrCx Ny interlayers. CrN coatings are known to have good oxidation resistant properties with practical application limits at 700 8C investigated by Hsieh et al. [9,10]. It is known that CrN will easily form a dense and stable Cr2O3 thin film on top when exposed to air at higher temperature [9,10]. The result indicates that the oxidation reaction of CrN and CrCx Ny below the temperature 700 8C is low. Therefore, slightly weight gain may come from oxide layer. Further oxidation of the embedded CrN and CrCx Ny is anticipated to occur above 700 8C. GIXD analysis of specimens heat-treated at 200, 300, and 400 8C, respectively, in ambient air for 30 min is shown in Fig. 3. The as-deposited specimen is also depicted for comparison. The result reveals DLC films with an amorphous structure. The CrN phase in the as-deposited specimen is detected from the underlayer. GIXD analysis of the annealed DLC films showed similar structure with the as deposited specimen. The results indicate that CrN interlayer still remain stable up to 400 8C. Fig. 4 shows surface morphologies of the as-deposited and isothermally oxidized DLC films under exposure of 200 and 300 8C in ambient air at 30 min. At 200 8C exposure, crystallization and
Fig. 5. (a) TEM cross-sectional micrograph, (b) SAD pattern of the Cr-DLC coating annealed at 300 8C.
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graphitization of the amorphous DLC films occurred compared to as-deposited one, as shown in Fig. 4(b). At 300 8C exposure, grooves was formed on DLC films, as shown in Fig. 4(c). It is as a result of stress relief of the highly stress DLC films. Fig. 5(a) shows the TEM crosssectional micrograph of the DLC films annealed at 300 8C. Comparing to the as-deposited specimen, a completely amorphous structure of both CrCx Ny and carbon layers with dispersed nanocrystallites was observed after annealed at 300 8C. Fig. 5(b) depicts the SAD pattern covering DLC and CrCx Ny, in which the nanocrystallites were observed to be Cr2O3, graphite and h-Cr2N signals. These results indicate that structures of Cr-doped DLC films are deteriorated up to 300 8C in ambient air. 3.3. Raman analysis In order to investigate the film properties, we have studied the Raman spectra of Cr-doped DLC coatings to distinguish different bonding types of carbon. For quantitative analysis, the Raman spectra were also fitted to two Gaussian peaks denoted G and D, as shown in Fig. 6. Gband means graphite structure located at approximately 1530–1570 cm1, which is assigned to the E2g symmetric vibrational mode of sp2 bond. D-band represents disordered graphite-like structure centered at 1350 cm1, which is attributed to the bond-angle disorder in sp3 bond [11]. By comparing Raman spectra of Cr-doped DLC films annealed at different temperatures, the thermal stability of these Crdoped DLC films can be distinguished. Characteristics of Cr-doped DLC films of as-deposited specimen and specimens annealed from 200 to 400 8C analyzed by Raman spectra was shown in Fig. 6. Parameters of the Gaussian curves used to fit the Dand G-bands of Raman spectra obtained from annealed Cr-doped DLC films as shown in Table 2. With increase of annealing temperature, the G-band moves to higher wavenumber. For the as-deposited specimen, the G band position centered at 1557.5 cm1 and D-band position at
Fig. 6. The dependence of Raman shift on the annealing temperature of (a) as-deposited, (b) annealed at 200 8C, (c) annealed at 300 8C, (d) annealed at 400 8C DLC films.
Table 2 Parameters of the Gaussian curves used to fit the D- and G-bands of Raman spectra obtained from annealed Cr-DLC films Temperature (8C)
-D (cm1)
CD (cm1)
-G (cm1)
CG (cm1)
As-deposited 200 300
1395.8 1404.2 1397.3
259.8 238.3 169.9
1557.5 1567.6 1600
114.2 107.5 89.1
- D: position of D band; C D: width of D band. - G: position of G band; C G: width of G band.
1395.8 cm1. With increase of annealing temperature, the G-band position of the films shifts to a higher wavenumber from 1570 to 1620 cm1. In a theoretical study of Raman spectroscopy of amorphous carbon, similar results [11,12] concluded that shifting of G-band to higher wavenumber had been associated with increased graphitic composition in the films. Observable separation of the D- and G-band appeared at 300 8C and conversion of sp3-bond carbon to sp2-bond carbon occurred. The DLC films were then completely destroyed over 400 8C indicating that DLC films disappeared from the substrate. The relative intensity of the D-band to the G-band becomes noticeable with the increase of annealing temperature (Fig. 7). The as-deposited Cr-doped DLC film possessed the lowest I D/I G ratio (1.18), corresponding to the highest sp3 bond content. The increase of I D/I G has been conformed by the progress of graphitiation process in the film. Increase of graphite microdomains with increase of I D/I G values was anticipated. This observation is similar to the work of Tallant et al. [13], in which they used Auger electron spectrometer to confirm the conversion of sp3-bonded carbon to sp2-bonded carbon above 300 8C. For Cr-doped DLC films annealed at temperature higher than 300 8C, the carbon structure is converted to nanocrystalline graphite and carbides. The evolution to nanocrystalline graphite must have been accompanied by conversion of sp3 bonds to sp2 bonds.
Fig. 7. The dependence of I D/I G on annealing temperature.
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occurs. For higher annealing temperatures, oxidation of Cr in the film is more pronounced. Oxidation of carbon increases dramatically forming carbon dioxide, which then effuses out of the surface, and with remaining carbon, which converts to a graphite phase.
4. Conclusions Characterization of the Cr-doped DLC coatings oxidized at elevated temperature reveals that distinctive thermal decomposition occurred at 290 to 342 8C. The integrated intensity ratio I D/I G starts to increase at 200 8C, while an observable D-band appears at 300 8C. With increasing annealing temperature, the proportion of conversion of sp3bonded carbon to sp2-bonded carbon increases. Oxidation of the Cr-doped DLC coatings occurs at 300 8C. Oxidation of carbon increases dramatically and the remaining converts to a graphite phase at 300 8C.
Acknowledgements Fig. 8. XPS spectra of the (a) C1s, and (b) O1s core level for annealed CrDLC films.
3.4. XPS analysis Fig. 8a shows the XPS C1s peaks of Cr-doped DLC films annealed at different temperatures, as well as the asdeposited specimen. The spectral line shape of the DLC C1s core level shifts from 284.6 eV of as-deposited specimen toward 284.2 eV of specimen annealed at 300 8C. The shift of binding energy to lower values have significantly been affected by oxidation which suggests the conversion of sp3 site to sp2 site of DLC films. The fullwidth half-maximum of the C1s spectra of the DLC films is about 1.6 eV and is therefore larger than that of specimens annealed at 200 and 300 8C. After annealing at 300 8C, the increase of relative intensity near 288.6 eV indicates formation of CUO and CMO components. The oxidation of the film is further confirmed from XPS O1s spectra, as shown in Fig. 8b. The binding energy shifts from 532.0 eV of as-deposited to a lower value of 530.2 eV of annealed specimens [14]. For the Cr-doped DLC films, oxidation of Cr is more marked at a higher annealing temperature. Most carbon dioxide evolves at 300 8C from the surface and the remaining carbon is mainly a graphite phase. From the results, a reaction mechanism of annealed DLC films is proposed. When the annealing temperature is lower than 200 8C, oxygen is adsorbed on the surface and an insignificant oxidation occurs. When heating at 300 8C, the conversion of sp3-bonded carbon to sp2-bonded carbon
The authors wish to thank Surftech for their valuable assistance and operation.
References [1] D.L. Pappas, J. Hopwood, J. Vac. Sci. Technol., A, 12 (1994) 1576. [2] J.J. Cuomo, J.P. Doyle, J. Brully, J.C. Liu, Appl. Phys. Lett. 58 (1991) 446. [3] S.C. Seo, D.C. Ingram, H.H. Richardson, J. Vac. Sci. Technol. 13 (1995) 2856. [4] R.F. Huang, C.Y. Chan, C.H. Lee, J. Gong, K.H. Lai, C.S. Lee, K.Y. Li, L.S. Wen, C. Sun, Diamond Relat. Mater. 10 (2001) 1850. [5] C.B. Collins, F. Davanloo, D.R. Jander, T.J. Lee, H. Park, J.H. You, J. Appl. Phys. 69 (1991) 862. [6] D.Y. Wang, K.W. Weng, C.L. Chang, X.J. Guo, Diamond Relat. Mater. 9 (2000) 831. [7] Y.Y. Chang, D.Y. Wang, W.T. Wu, Thin Solid Films 420/421 (2002) 241. [8] D.Y. Wang, C.L. Chang, W.Y. Ho, Surf. Coat. Technol. 120/121 (1999) 138. [9] W.P. Hsieh, W.C. Wang, C.C. Wang, F.S. Shieu, J. Vac. Sci. Technol., A, Vac. Surf. Films 19 (2001) 2235. [10] W.P. Hsieh, C.C. Wang, C.H. Lin, F.S. Shieu, J. Electrochem. Soc. 149 (2002) B234. [11] A. Richter, H.J. Scheibe, W. Pompe, K.W. Brezeinka, I. Muhling, J. Non-Cryst. Solids 88 (1986) 131. [12] D. Beeman, J. Silverman, R. Lynds, M.R. Anderson, Phys. Rev., B 30 (1984) 870. [13] D.R. Tallant, J.E. Parmeter, M.P. Siegal, R.L. Simpson, Diamond Relat. Mater. 4 (1995) 191. [14] J.M. Seo, K.H. Lee, Surf. Sci. 369 (1996) 108.
Thermal stability of Cr-doped diamond-like carbon films synthesized by cathodic arc evaporation Ming-Chieh Chiua,*, Wen-Pin Hsieha, Wei-Yu Hob, Da-Yung Wangb, Fuh-Sheng Shieua a
Department of Materials Engineering, National Chung-Hsing University, Taichung, Taiwan b Center for Applied Science and Technology, Ming-Dao University, Changhua, Taiwan
Received 15 December 2003; received in revised form 8 July 2004; accepted 16 September 2004 Available online 10 November 2004
Abstract Cr-doped diamond-like carbon (DLC) films were synthesized using a cathodic arc evaporation (CAE) process. The thermal oxidation behavior of Cr-doped DLC films was investigated using thermal gravimetric analysis (TGA) and differential thermal analysis (DTA). The phase identification and microstructural examinations were conducted by X-ray diffraction, scanning electron spectroscopy (SEM), transmission electron spectroscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) in order to understand the characteristics of Cr-doped DLC films. The as-deposited Cr-doped DLC film exhibits a lamellar structure observed by TEM. A significant weight loss of film results from the thermal oxidation of carbon occurred at 290 to 342 8C. At the temperature higher than 342 8C, slight weight gain of specimen was observed due to the thermal oxidation of the underlying CrCx Ny and CrN interlayer. By heat-treated specimens from 200 to 400 8C, Raman spectra reveal the increase of I D/I G value conforming to the graphitiation process of the Cr-doped DLC films. Finally, surface reactions of the annealed films using XPS analysis were discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Diamond-like carbon; Cathodic arc evaporation; Oxidation; Microstructure
1. Introduction Diamond-like carbon (DLC) films have superior physical and chemical properties in terms of hardness, chemical inertness, high wear resistance, and thermal conductivity. Consequently, DLC coatings are potentially useful for different industrial applications ranging from tribological protective coatings to microelectronics. In general, the nature and properties of the DLC could be modified by controlling the incorporation of dopants, such as silicon, fluorine, nitrogen, metal and metal carbides. However, DLC coatings are known to be extremely sensitive to the presence of oxidizing species including oxygen and water during friction. They have a significant disadvantage of low
* Corresponding author. Tel.: +886 422840500407; fax: +886 422857017. E-mail address: [email protected] (M.-C. Chiu). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.09.029
thermal stability at higher working temperatures. DLC coatings cannot retain their superior characteristics at higher temperature because of the conversion of sp3-bonded carbon to sp2-bonded carbon. It is known that a wide variety of methods can be used to produce DLC coatings, such as plasma deposition [1], ion sputtering [2], magnetron sputtering [3], ion plating [4], and laser plasma deposition [5]. Previous work showed that metal-doped DLC films on top of a CrN/CrCx Ny interlayer were successfully deposited on M2 high-speed steels by cathodic arc evaporation (CAE) process [6]. Significant improvement of adhesion between DLC films and substrate can be obtained by graded design in both composition and microstructure. Further study showed that Ti, Cr, and Zr metals could act as a catalyst to play a key role of DLC deposition using CAE [7]. Some works on the thermal stability of DLC films have been reported. Oxidation studies of the DLC films have been characterized by using thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) techniques. Wang et
M.-C. Chiu et al. / Thin Solid Films 476 (2005) 258–263 Table 1 Deposition parameters of CAE synthesized Cr-DLC films Parameters
Values
Base pressure (Pa) Reactive gas pressure (Pa) Total deposition time (min) Distance of cathode to substrate (mm) CAE target
5103 2.5 65 180 99.99% Cr (100 mm in diameter) 60 1000 100 (100 kHz pulsed) 150
Cathode current (A) Bias voltage at ion cleaning stage (V) Bias voltage at coating stage (V) Substrate temperature (8C)
al. [8] investigated the oxidation kinetics and reaction mechanism of Ti-doped DLC films by TGA/DTA analysis. The results revealed two distinctive oxidation behaviors of DLC films oxidation at 350 8C and TiN/TiCx Ny interlayer oxidation at 450 8C [8]. In this study, the thermal stability of Cr-doped DLC films deposited by CAE process was investigated using TGA/DTA analysis. The effect of annealing treatment on the thermal stability of Cr-doped DLC films in ambient air is confirmed by examining their structural changes.
2. Experiment details Cr-doped DLC films were prepared by a CAE process using 99.99% Cr targets, and acetylene (C2H2) and nitrogen (N2) reactive gases. Cr-doped DLC coatings were deposited on SS304 stainless steel foils of dimensions 30100.025
259
mm that is designed to fit into the TGA/DTA crucibles in spiral forms. During the deposition process, Cr ion bombardment was first executed from Cr targets on the substrates to form a Cr-rich buffer layer of about 60-nm thick, and CrN was deposited as a follow-up interlayer when nitrogen was introduced. The C2H2 was then introduced into the chamber to form a CrCx Ny layer in which the C and N composition graded by decreasing the N2 flow rate and increasing the C2H2 flow rate at the same time. Cr-doped DLC film was finally deposited on the top and the N2 flow rate was turned off at this final stage. The substrate temperature was controlled at 150 C and the deposition time for all the samples was about 65 min. The working pressure of the plasma chamber was 2.5 Pa during DLC deposition stage. Details of the process parameters are listed in Table 1. The as-deposited Cr-doped DLC coatings were then subjected to thermal analysis using a TGA/TDA system, which is equipped with both a Cahn-2000 thermogravimetric unit and an Ulvac TGD-7000HR unit. To understand the oxidation behavior of the coatings, these films were also annealed at varying temperatures of 200, 300, 400 and 500 8C in ambient air for 30 min. The analytical techniques used in this study to characterize the degradation of DLC films include transmission electron microscopy (TEM), scanning electron microscopy (SEM), glancing incidence X-ray diffraction (GIXD). Raman spectroscopy using a 50 mW beam of wavelength 514.5 nm from an argon laser in the wavenumber range 1000–2000 cm1 and X-ray photoelectron spectroscopy (XPS, PHI 1600) using Mg Ka radiation were used to identify the sp3/sp2 bond conversion and chemical bonding at the near-surface region. The
Fig. 1. (a) TEM cross-sectional micrograph, (b) SAD pattern of the as-deposited Cr-DLC coating.
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samples for XPS study were cut into 33 mm square blocks. Samples were load locked to the analysis chamber, surface analyzed prior to ion bombardment, and subsequently sputtered in the analysis position. To reduce the influence of surface contamination, sample surface was presputtered under 3.0 keV Ar+ bombardment for 30 s before the analysis was conducted. Surface charge-up does occur causing the peak binding energy to be shifted from its expected position. The binding energy of the elements was calibrated by using amorphous SiO2 and single crystalline Si as standards. Microstructure of the DLC coatings was investigated by a JOEL JEM-4000EX TEM operated at 400 kV. The crosssection specimen for TEM study was prepared by gluing two 13 mm rectangle blocks face-to-face, and then grinding and polishing from both sides down to ~50 Am. To reduce the milling time, a precision grinding machine (VCR 500i) was used to dimple the specimens. Ion milling was done by a BAL-TEC RES010 ion milling operated at 5 kV, using a single-side sample holder. Surface morphology of the coatings was examined by a JEOL JSM-5400 SEM operated at 20 kV. Crystallinity of the coatings was studied by a SCINTAGXGEN-400 diffractometer using Cu Ka radiation at an incident angle of 28.
Fig. 3. XRD spectrum of the Cr-DLC coating annealed in ambient air.
believed that the metal interlayer can reduce the residual stress and improve the adhesion between coating and substrate. The multilayered and graded film structures
3. Results and discussion 3.1. Microstructure analysis of the as-deposited DLC coatings Details of the microstructure of the as-deposited DLC coatings were revealed by cross-sectional TEM micrograph, as shown in Fig. 1(a), in which the deposition sequence of Cr, CrN, CrCx Ny and DLC layers can be readily identified. The film thickness was controlled at 1.65 Am. The CrCx Ny exhibits a dense and compact microstructure with wellattached CrN interface. As demonstrated by Wang et al. [6– 8] for the design of metal-doped DLC on stainless steel, it is
Fig. 2. TGA/DTA analyses of the Cr-DLC films.
Fig. 4. Scanning electron micrographs of (a) as-deposited, (b) annealed at 200 8C, and (c) annealed at 300 8C Cr-DLC films.
M.-C. Chiu et al. / Thin Solid Films 476 (2005) 258–263
improved the severe mismatch in lattice and physical properties between DLC and substrate. The interfaces between each layer are free from any noticeable voids, gaps or other defects. The Cr and CrN adhesive layer were found to have a columnar structure. CrCx Ny and DLC films exhibit a lamellar appearance, as shown in Fig. 1(a). Differed from conventional DLC deposition processes, this study used cathodic arc sources as the source energy of C2H2 decomposition for DLC synthesis. During the DLC synthesized process, a joint physical vapor deposition and plasma enhanced chemical vapor deposition process occurred, where the activated C2H2 decomposition and amorphous carbon deposition were augmented by the intense metal plasma generated by cathodic arc sources. When the C2H2 was introduced, the columnar growth stopped and the structure has a lamellar appearance. During DLC deposition stage, it is observed that Cr target surface was partially poisoned by carbon layer. Therefore, the lamellar structure was resulted from the superposition of Crbase carbides and carbon layer. The dimension of each multilayer is about several nanometers, which is much smaller than that in the columnar layer. The selected area diffraction (SAD) pattern covering DLC and CrCx Ny showed diffuse ring patterns indicating an amorphous structure, as shown in Fig. 1(b). GIXD investigation of the as-deposited sample shows diffraction peaks corresponding to CrN and stainless steel. There were no characteristic peaks of crystalline carbon phases indicating again the amorphous characteristic of the film. 3.2. Oxidation performance DLC films were analyzed by TGA/DTA, as shown in Fig. 2. DTA/TGA tests of the DLC films were conducted at a heating rate of 10 8C/min between 25 and 800 8C. From
261
Fig. 2, it is clear to see that thermal oxidation of carbon occurred at 290 to 342 8C. The DTA curve reveals a typical exothermic reaction in this temperature range. Below 290 8C, crystallization and graphitization of the amorphous DLC films occurred, as depicted in the DTA curve. Between 290 and 342 8C, a significant weight loss results from the carbon oxidation, according to the (1) C þ O2 YCO2
ð1Þ
The reaction product CO2 escapes with the vent gas out of the crucible. According to the TGA curve, weight gain of specimen changed only slightly as the temperature increased from 400 to 800 8C due to the thermal oxidation of underlying CrN and CrCx Ny interlayers. CrN coatings are known to have good oxidation resistant properties with practical application limits at 700 8C investigated by Hsieh et al. [9,10]. It is known that CrN will easily form a dense and stable Cr2O3 thin film on top when exposed to air at higher temperature [9,10]. The result indicates that the oxidation reaction of CrN and CrCx Ny below the temperature 700 8C is low. Therefore, slightly weight gain may come from oxide layer. Further oxidation of the embedded CrN and CrCx Ny is anticipated to occur above 700 8C. GIXD analysis of specimens heat-treated at 200, 300, and 400 8C, respectively, in ambient air for 30 min is shown in Fig. 3. The as-deposited specimen is also depicted for comparison. The result reveals DLC films with an amorphous structure. The CrN phase in the as-deposited specimen is detected from the underlayer. GIXD analysis of the annealed DLC films showed similar structure with the as deposited specimen. The results indicate that CrN interlayer still remain stable up to 400 8C. Fig. 4 shows surface morphologies of the as-deposited and isothermally oxidized DLC films under exposure of 200 and 300 8C in ambient air at 30 min. At 200 8C exposure, crystallization and
Fig. 5. (a) TEM cross-sectional micrograph, (b) SAD pattern of the Cr-DLC coating annealed at 300 8C.
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graphitization of the amorphous DLC films occurred compared to as-deposited one, as shown in Fig. 4(b). At 300 8C exposure, grooves was formed on DLC films, as shown in Fig. 4(c). It is as a result of stress relief of the highly stress DLC films. Fig. 5(a) shows the TEM crosssectional micrograph of the DLC films annealed at 300 8C. Comparing to the as-deposited specimen, a completely amorphous structure of both CrCx Ny and carbon layers with dispersed nanocrystallites was observed after annealed at 300 8C. Fig. 5(b) depicts the SAD pattern covering DLC and CrCx Ny, in which the nanocrystallites were observed to be Cr2O3, graphite and h-Cr2N signals. These results indicate that structures of Cr-doped DLC films are deteriorated up to 300 8C in ambient air. 3.3. Raman analysis In order to investigate the film properties, we have studied the Raman spectra of Cr-doped DLC coatings to distinguish different bonding types of carbon. For quantitative analysis, the Raman spectra were also fitted to two Gaussian peaks denoted G and D, as shown in Fig. 6. Gband means graphite structure located at approximately 1530–1570 cm1, which is assigned to the E2g symmetric vibrational mode of sp2 bond. D-band represents disordered graphite-like structure centered at 1350 cm1, which is attributed to the bond-angle disorder in sp3 bond [11]. By comparing Raman spectra of Cr-doped DLC films annealed at different temperatures, the thermal stability of these Crdoped DLC films can be distinguished. Characteristics of Cr-doped DLC films of as-deposited specimen and specimens annealed from 200 to 400 8C analyzed by Raman spectra was shown in Fig. 6. Parameters of the Gaussian curves used to fit the Dand G-bands of Raman spectra obtained from annealed Cr-doped DLC films as shown in Table 2. With increase of annealing temperature, the G-band moves to higher wavenumber. For the as-deposited specimen, the G band position centered at 1557.5 cm1 and D-band position at
Fig. 6. The dependence of Raman shift on the annealing temperature of (a) as-deposited, (b) annealed at 200 8C, (c) annealed at 300 8C, (d) annealed at 400 8C DLC films.
Table 2 Parameters of the Gaussian curves used to fit the D- and G-bands of Raman spectra obtained from annealed Cr-DLC films Temperature (8C)
-D (cm1)
CD (cm1)
-G (cm1)
CG (cm1)
As-deposited 200 300
1395.8 1404.2 1397.3
259.8 238.3 169.9
1557.5 1567.6 1600
114.2 107.5 89.1
- D: position of D band; C D: width of D band. - G: position of G band; C G: width of G band.
1395.8 cm1. With increase of annealing temperature, the G-band position of the films shifts to a higher wavenumber from 1570 to 1620 cm1. In a theoretical study of Raman spectroscopy of amorphous carbon, similar results [11,12] concluded that shifting of G-band to higher wavenumber had been associated with increased graphitic composition in the films. Observable separation of the D- and G-band appeared at 300 8C and conversion of sp3-bond carbon to sp2-bond carbon occurred. The DLC films were then completely destroyed over 400 8C indicating that DLC films disappeared from the substrate. The relative intensity of the D-band to the G-band becomes noticeable with the increase of annealing temperature (Fig. 7). The as-deposited Cr-doped DLC film possessed the lowest I D/I G ratio (1.18), corresponding to the highest sp3 bond content. The increase of I D/I G has been conformed by the progress of graphitiation process in the film. Increase of graphite microdomains with increase of I D/I G values was anticipated. This observation is similar to the work of Tallant et al. [13], in which they used Auger electron spectrometer to confirm the conversion of sp3-bonded carbon to sp2-bonded carbon above 300 8C. For Cr-doped DLC films annealed at temperature higher than 300 8C, the carbon structure is converted to nanocrystalline graphite and carbides. The evolution to nanocrystalline graphite must have been accompanied by conversion of sp3 bonds to sp2 bonds.
Fig. 7. The dependence of I D/I G on annealing temperature.
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occurs. For higher annealing temperatures, oxidation of Cr in the film is more pronounced. Oxidation of carbon increases dramatically forming carbon dioxide, which then effuses out of the surface, and with remaining carbon, which converts to a graphite phase.
4. Conclusions Characterization of the Cr-doped DLC coatings oxidized at elevated temperature reveals that distinctive thermal decomposition occurred at 290 to 342 8C. The integrated intensity ratio I D/I G starts to increase at 200 8C, while an observable D-band appears at 300 8C. With increasing annealing temperature, the proportion of conversion of sp3bonded carbon to sp2-bonded carbon increases. Oxidation of the Cr-doped DLC coatings occurs at 300 8C. Oxidation of carbon increases dramatically and the remaining converts to a graphite phase at 300 8C.
Acknowledgements Fig. 8. XPS spectra of the (a) C1s, and (b) O1s core level for annealed CrDLC films.
3.4. XPS analysis Fig. 8a shows the XPS C1s peaks of Cr-doped DLC films annealed at different temperatures, as well as the asdeposited specimen. The spectral line shape of the DLC C1s core level shifts from 284.6 eV of as-deposited specimen toward 284.2 eV of specimen annealed at 300 8C. The shift of binding energy to lower values have significantly been affected by oxidation which suggests the conversion of sp3 site to sp2 site of DLC films. The fullwidth half-maximum of the C1s spectra of the DLC films is about 1.6 eV and is therefore larger than that of specimens annealed at 200 and 300 8C. After annealing at 300 8C, the increase of relative intensity near 288.6 eV indicates formation of CUO and CMO components. The oxidation of the film is further confirmed from XPS O1s spectra, as shown in Fig. 8b. The binding energy shifts from 532.0 eV of as-deposited to a lower value of 530.2 eV of annealed specimens [14]. For the Cr-doped DLC films, oxidation of Cr is more marked at a higher annealing temperature. Most carbon dioxide evolves at 300 8C from the surface and the remaining carbon is mainly a graphite phase. From the results, a reaction mechanism of annealed DLC films is proposed. When the annealing temperature is lower than 200 8C, oxygen is adsorbed on the surface and an insignificant oxidation occurs. When heating at 300 8C, the conversion of sp3-bonded carbon to sp2-bonded carbon
The authors wish to thank Surftech for their valuable assistance and operation.
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