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Improvement of the adhesion of diamond-like carbon coatings induced by ion treatments
Materials Chemistry and Physics 72 (2001) 158–162
Improvement of the adhesion of diamond-like carbon coatings induced by ion treatments Der-Jun Jan∗ , Chi-Fong Ai Physics Division, Institute of Nuclear Energy Research, P.O. Box 3-4, Lungtan, Taoyuan 325, Taiwan
Abstract Diamond-like carbon (DLC) films, with and without nitrogen, were deposited using ion beam technique. Their film structural characterizations were analyzed by Raman spectroscopy and measurement of refractive index. The analysis results indicate that nitrogen-containing diamond-like film shows a higher I(D) to I(G) ratio and a lower refractive index than the film without nitrogen. The nitrogen incorporated into the film combined with ions’ bombarding the surface of the substrate prior to coating promotes film adhesion strength, nanohardness and wear resistance. N2 ion treatment also results in the better adhesion than both Ar and O2 ion treatments. Our results further demonstrate that nitrogen-containing DLC films were firmly deposited on the stainless steel without an intermediate layer. Such a layer is commonly used for other coating processes as a buffer layer to improve adhesion strength. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Adhesion; Hardness; Optical properties
1. Introduction Amorphous hydrogenated carbon (a-C:H) films, referred to as diamond-like carbon (DLC) have excellent hardness, wear resistance and optical properties for a variety of applications such as optical coatings and protective coatings. Until now, using DLC films for practical application has been mainly limited by poor adhesion to substrates due to the large compressive stress developed during their depositions [1]. Recently, the incorporation of nitrogen into DLC as amorphous hydrogenated carbon nitride (a-C:H:N) films has been extensively studied. Related investigations have demonstrated the feasibility of incorporating nitrogen to reduce the internal stress of the films without significant change in the film hardness [2–5]. In particular, Franceschini et al. [6] indicated that reducing the internal stress can effectively improve the film adhesion. In addition, the interfacial bonding is a principal factor affecting the film adhesion. Therefore, bond strength between the substrate and the film must be increased to improve coating durability by modifying the substrate surface properties [7]. For instance, Bertrand et al. [8] studied the adhesion of silica thin films on stainless steel substrate by plasma pretreatment of the substrate surface before deposition. Such pretreatment improved film adhesion considerably. Bertrand ∗
Corresponding author. Tel.: +886-3-4711400, ext: 7440; fax: +886-3-4711408. E-mail address: [email protected] (D.-J. Jan).
concluded that creation of metal–N–Si linkages by N2 and NH3 pretreatments clearly contributed to improve the adhesion. This paper studies the feasibility of incorporating nitrogen into DLC films as a-C:H:N and ion beam treatment with different gases (Ar, O2 , N2 ) on the surface of stainless steel before a-C:H:N deposition on the adhesion of a-C:H:N.
2. Experimental 2.1. Specimen preparation Substrates of AISI 304 sheets of 1 mm thickness were bombarded with about 140 eV Ar, O2 or N2 ions from a gridless ion source (Commonwealth Scientific’s Mark II), and coated subsequently with a-C:H:N by introducing N2 gas into a discharge chamber of gridless ion source and injecting C2 H2 gas against the substrate holder through a gas distribution ring with a diameter of 10 cm. Substrates were held at the ambient temperature during deposition. The deposition conditions were: anode voltage, 130 V; anode current, 4 A; N2 gas flow, 8 sccm; C2 H2 gas flow, 35 sccm. The deposition time was 15 min. 2.2. Evaluation of film properties and adhesion testing Renishaw Raman spectroscopy with He–Ne laser excitation (6328 Å) was carried out over the spectral range 1000– 2000 cm−1 to study the nature of the films. An SCI thin film
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 4 2 7 - 8
D.-J. Jan, C.-F. Ai / Materials Chemistry and Physics 72 (2001) 158–162
159
Fig. 1. Raman spectra of (a) undoped and (b) N-doped DLC films.
metrology system was used to measure the refractive index and extinction coefficients of the films. Several tests were used to evaluate the adhesion of the a-C:H:N coatings. The tests were Scotch tape, thermal shock, saltwater immersion, chemical resistance and nanoindentor hardness tests. In the thermal shock test, the substrate was immersed in a bath of boiling water for 2 min, then
removed and plunged into a bath of ice water for 2 min. This cycle was repeated five times. The wear resistance of the films was assessed using a Falex#6 Abrasion Tester. The total number of rotations was 900 and the wheel load was 45 N. After the completion of each test, the surface of the test samples was examined under an optical microscope at 200×.
Table 1 Fitted Raman parameters of a-C:H and a-C:H:N films Film
a-C:H a-C:H:N
Type of fitting
Gaussian Gaussian
D band
G band
I(D)/I(G)
Position (ω0 )
Width (ω)
Position (ω0 )
Width (ω)
1293 1383
239 291
1506 1561
187 133
0.69 1.48
160
D.-J. Jan, C.-F. Ai / Materials Chemistry and Physics 72 (2001) 158–162
Fig. 2. Plots of the refractive indices and extinction coefficients of (a) undoped and (b) N-doped DLC films.
D.-J. Jan, C.-F. Ai / Materials Chemistry and Physics 72 (2001) 158–162
161
Table 2 Adhesion tests of N-doped DLC coatings with different ion treatments Sample
Defect inspection
Tape test
Thermal shock
Saltwater immersion
Chemical resistance
Untreated N2 ion O2 ion Ar ion
Many Rare Few Few
Partial lifting Pass Pass Pass
Some cracks Pass Pass Pass
Some cracks Pass Pass Pass
Some cracks Pass Few cracks at the edge Rare cracks at the edge
3. Results and discussion 3.1. Raman spectroscopy and optical properties The Raman spectra of undoped and N-doped DLC films are shown in Fig. 1. The spectra were deconvoluted with Gaussian line shapes. Table 1 gives the results of fitted parameters. The I(D)/I(G) ratio of the a-C:H:N film is twice as
high as that of the a-C:H film. The a-C:H:N exhibits D band and G band positions at 1383 and 1561 cm−1 , respectively, rather than at 1293 and 1506 cm−1 as for the a-C:H film. The full width at half maximum (FWHM) of the G band was 133 cm−1 for a-C:H:N rather than 187 cm−1 as for a-C:H. These results agree with those of Freire and Franceschini [9] and indicate that nitrogen incorporation caused an increase in the size and/or the number of graphitic domains in the
Fig. 3. 200× optical micrographs of (a) uncoated substrate and (b) N-doped DLC coating, after abrasion test.
162
D.-J. Jan, C.-F. Ai / Materials Chemistry and Physics 72 (2001) 158–162
films. Lenardi et al. [5] demonstrated that an increase in sp2 bonding in a-C:H:N film resulted in a G band shift to a higher wave number. Fig. 2 shows refractive index and extinction coefficient versus wavelengths for undoped and N-doped DLC films. The figure shows that doping leads to a drop of nearly 0.25 in refractive index. Wood et al. [2] stated that the reduction in refractive index appears to be the result of compositional changes which also significantly affects the atom number density of the films.
to those results, the film hardness decreased gradually under different ion treatments in the order, N2 > Ar > O2 . The N2 ion treated sample showed a hardness of 8 GPa, about 4 times higher than that of uncoated one. The photomicrographs in Fig. 3 display the results of the abrasion tests. The uncoated substrate was severely abraded, while the N-doped DLC coating showed just a little abrasion, indicating that N-doped DLC coating better protects the surface of the metal from scratching and chemical attack than does undoped DLC coating.
3.2. Adhesion tests 4. Conclusions The undoped DLC coating peeled away from the substrates after exposure to the atmosphere for a few days due to the high compressive stress. The N-doped DLC coatings did not show the same problem. Table 2 summarizes the results of adhesion tests for N-doped DLC coatings with three kinds of ion treatments. It is clear that many local spots were present in the untreated DLC since the residual contaminants could not be removed effectively without ion bombardments [10]. Adhesion tests reveal the influence of different ion treatments on the interface bond durability. The N-doped films passed exhibited the best adhesion performance under N2 ion treatment. The literature [8] has established that improvement of the film/metal interface quality can be explained by that N2 ion treatment leads to the formation of metal-nitride groups which in turn induce a larger cohesion of the substrate surface, as compared to the native oxide layer. The creation of metal–N–C linkages after N2 ion treatment clearly contributes to the improved adhesion. 3.3. Measurement of hardness and the abrasion testing The film hardness was measured by using a nanoindenter. The penetration depth was 60 nm. The substrate effect would be significant for a high penetration depth, reducing the observed hardness value [11] due to the fact that the thickness of the films was in the order of only 120 nm. Measurement of the hardness of the uncoated substrate was taken as a baseline to compare with that of the coated substrates. According
This work has successfully developed a novel process that combines nitrogen-containing DLC films from a mixture of C2 H2 and N2 using the ion beam technique with N2 ions bombarding the substrate surface previous to coating the DLC films. The nitrogen incorporated into the films reduced the film stress, and the N2 ion pretreatment induces the formation of metal–N–C linkages. Both changes significantly enhance the film’s adhesion strength. References [1] J.H. Kim, D.A. Ahn, Y.H. Kim, H.K. Baik, J. Appl. Phys. 82 (1997) 658. [2] P. Wood, T. Wydeven, O. Tsuji, Thin Solid Films 258 (1995) 151. [3] Y.H. Cheng, Y.P. Wu, J.G. Chen, X.L. Qiao, C.S. Xie, Diamond Relat. Mater. 8 (1999) 1214. [4] F.L. Freire Jr., Jpn. J. Appl. Phys. 36 (1997) 4886. [5] C. Lenardi, M.A. Baker, V. Briois, L. Nobili, P. Piseri, W. Gissler, Diamond Relat. Mater. 8 (1999) 595. [6] D.F. Franceschini, C.A. Achete, F.L. Freire Jr., Appl. Phys. Lett. 60 (1992) 3229. [7] J. Narayan, R.D. Vispute, K. Jagannadham, J. Adhesion Sci. Technol. 9 (1995) 753. [8] N. Bertrand, B. Drevillon, A. Gheorghiu, C. Senemaud, L. Martinu, J.E. Klemberg-Sapieha, J. Vac. Sci. Technol. A 16 (1998) 6. [9] F.L. Freire Jr., D.F. Franceschini, Thin Solid Films 293 (1997) 236. [10] F. Mild, K. Goedicke, M. Fahland, Thin Solid Films 279 (1996) 169. [11] H.C. Ong, R.P.H. Chang, N. Baker, W.C. Oliver, Surf. Coat. Technol. 89 (1997) 38.
Improvement of the adhesion of diamond-like carbon coatings induced by ion treatments Der-Jun Jan∗ , Chi-Fong Ai Physics Division, Institute of Nuclear Energy Research, P.O. Box 3-4, Lungtan, Taoyuan 325, Taiwan
Abstract Diamond-like carbon (DLC) films, with and without nitrogen, were deposited using ion beam technique. Their film structural characterizations were analyzed by Raman spectroscopy and measurement of refractive index. The analysis results indicate that nitrogen-containing diamond-like film shows a higher I(D) to I(G) ratio and a lower refractive index than the film without nitrogen. The nitrogen incorporated into the film combined with ions’ bombarding the surface of the substrate prior to coating promotes film adhesion strength, nanohardness and wear resistance. N2 ion treatment also results in the better adhesion than both Ar and O2 ion treatments. Our results further demonstrate that nitrogen-containing DLC films were firmly deposited on the stainless steel without an intermediate layer. Such a layer is commonly used for other coating processes as a buffer layer to improve adhesion strength. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Adhesion; Hardness; Optical properties
1. Introduction Amorphous hydrogenated carbon (a-C:H) films, referred to as diamond-like carbon (DLC) have excellent hardness, wear resistance and optical properties for a variety of applications such as optical coatings and protective coatings. Until now, using DLC films for practical application has been mainly limited by poor adhesion to substrates due to the large compressive stress developed during their depositions [1]. Recently, the incorporation of nitrogen into DLC as amorphous hydrogenated carbon nitride (a-C:H:N) films has been extensively studied. Related investigations have demonstrated the feasibility of incorporating nitrogen to reduce the internal stress of the films without significant change in the film hardness [2–5]. In particular, Franceschini et al. [6] indicated that reducing the internal stress can effectively improve the film adhesion. In addition, the interfacial bonding is a principal factor affecting the film adhesion. Therefore, bond strength between the substrate and the film must be increased to improve coating durability by modifying the substrate surface properties [7]. For instance, Bertrand et al. [8] studied the adhesion of silica thin films on stainless steel substrate by plasma pretreatment of the substrate surface before deposition. Such pretreatment improved film adhesion considerably. Bertrand ∗
Corresponding author. Tel.: +886-3-4711400, ext: 7440; fax: +886-3-4711408. E-mail address: [email protected] (D.-J. Jan).
concluded that creation of metal–N–Si linkages by N2 and NH3 pretreatments clearly contributed to improve the adhesion. This paper studies the feasibility of incorporating nitrogen into DLC films as a-C:H:N and ion beam treatment with different gases (Ar, O2 , N2 ) on the surface of stainless steel before a-C:H:N deposition on the adhesion of a-C:H:N.
2. Experimental 2.1. Specimen preparation Substrates of AISI 304 sheets of 1 mm thickness were bombarded with about 140 eV Ar, O2 or N2 ions from a gridless ion source (Commonwealth Scientific’s Mark II), and coated subsequently with a-C:H:N by introducing N2 gas into a discharge chamber of gridless ion source and injecting C2 H2 gas against the substrate holder through a gas distribution ring with a diameter of 10 cm. Substrates were held at the ambient temperature during deposition. The deposition conditions were: anode voltage, 130 V; anode current, 4 A; N2 gas flow, 8 sccm; C2 H2 gas flow, 35 sccm. The deposition time was 15 min. 2.2. Evaluation of film properties and adhesion testing Renishaw Raman spectroscopy with He–Ne laser excitation (6328 Å) was carried out over the spectral range 1000– 2000 cm−1 to study the nature of the films. An SCI thin film
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 4 2 7 - 8
D.-J. Jan, C.-F. Ai / Materials Chemistry and Physics 72 (2001) 158–162
159
Fig. 1. Raman spectra of (a) undoped and (b) N-doped DLC films.
metrology system was used to measure the refractive index and extinction coefficients of the films. Several tests were used to evaluate the adhesion of the a-C:H:N coatings. The tests were Scotch tape, thermal shock, saltwater immersion, chemical resistance and nanoindentor hardness tests. In the thermal shock test, the substrate was immersed in a bath of boiling water for 2 min, then
removed and plunged into a bath of ice water for 2 min. This cycle was repeated five times. The wear resistance of the films was assessed using a Falex#6 Abrasion Tester. The total number of rotations was 900 and the wheel load was 45 N. After the completion of each test, the surface of the test samples was examined under an optical microscope at 200×.
Table 1 Fitted Raman parameters of a-C:H and a-C:H:N films Film
a-C:H a-C:H:N
Type of fitting
Gaussian Gaussian
D band
G band
I(D)/I(G)
Position (ω0 )
Width (ω)
Position (ω0 )
Width (ω)
1293 1383
239 291
1506 1561
187 133
0.69 1.48
160
D.-J. Jan, C.-F. Ai / Materials Chemistry and Physics 72 (2001) 158–162
Fig. 2. Plots of the refractive indices and extinction coefficients of (a) undoped and (b) N-doped DLC films.
D.-J. Jan, C.-F. Ai / Materials Chemistry and Physics 72 (2001) 158–162
161
Table 2 Adhesion tests of N-doped DLC coatings with different ion treatments Sample
Defect inspection
Tape test
Thermal shock
Saltwater immersion
Chemical resistance
Untreated N2 ion O2 ion Ar ion
Many Rare Few Few
Partial lifting Pass Pass Pass
Some cracks Pass Pass Pass
Some cracks Pass Pass Pass
Some cracks Pass Few cracks at the edge Rare cracks at the edge
3. Results and discussion 3.1. Raman spectroscopy and optical properties The Raman spectra of undoped and N-doped DLC films are shown in Fig. 1. The spectra were deconvoluted with Gaussian line shapes. Table 1 gives the results of fitted parameters. The I(D)/I(G) ratio of the a-C:H:N film is twice as
high as that of the a-C:H film. The a-C:H:N exhibits D band and G band positions at 1383 and 1561 cm−1 , respectively, rather than at 1293 and 1506 cm−1 as for the a-C:H film. The full width at half maximum (FWHM) of the G band was 133 cm−1 for a-C:H:N rather than 187 cm−1 as for a-C:H. These results agree with those of Freire and Franceschini [9] and indicate that nitrogen incorporation caused an increase in the size and/or the number of graphitic domains in the
Fig. 3. 200× optical micrographs of (a) uncoated substrate and (b) N-doped DLC coating, after abrasion test.
162
D.-J. Jan, C.-F. Ai / Materials Chemistry and Physics 72 (2001) 158–162
films. Lenardi et al. [5] demonstrated that an increase in sp2 bonding in a-C:H:N film resulted in a G band shift to a higher wave number. Fig. 2 shows refractive index and extinction coefficient versus wavelengths for undoped and N-doped DLC films. The figure shows that doping leads to a drop of nearly 0.25 in refractive index. Wood et al. [2] stated that the reduction in refractive index appears to be the result of compositional changes which also significantly affects the atom number density of the films.
to those results, the film hardness decreased gradually under different ion treatments in the order, N2 > Ar > O2 . The N2 ion treated sample showed a hardness of 8 GPa, about 4 times higher than that of uncoated one. The photomicrographs in Fig. 3 display the results of the abrasion tests. The uncoated substrate was severely abraded, while the N-doped DLC coating showed just a little abrasion, indicating that N-doped DLC coating better protects the surface of the metal from scratching and chemical attack than does undoped DLC coating.
3.2. Adhesion tests 4. Conclusions The undoped DLC coating peeled away from the substrates after exposure to the atmosphere for a few days due to the high compressive stress. The N-doped DLC coatings did not show the same problem. Table 2 summarizes the results of adhesion tests for N-doped DLC coatings with three kinds of ion treatments. It is clear that many local spots were present in the untreated DLC since the residual contaminants could not be removed effectively without ion bombardments [10]. Adhesion tests reveal the influence of different ion treatments on the interface bond durability. The N-doped films passed exhibited the best adhesion performance under N2 ion treatment. The literature [8] has established that improvement of the film/metal interface quality can be explained by that N2 ion treatment leads to the formation of metal-nitride groups which in turn induce a larger cohesion of the substrate surface, as compared to the native oxide layer. The creation of metal–N–C linkages after N2 ion treatment clearly contributes to the improved adhesion. 3.3. Measurement of hardness and the abrasion testing The film hardness was measured by using a nanoindenter. The penetration depth was 60 nm. The substrate effect would be significant for a high penetration depth, reducing the observed hardness value [11] due to the fact that the thickness of the films was in the order of only 120 nm. Measurement of the hardness of the uncoated substrate was taken as a baseline to compare with that of the coated substrates. According
This work has successfully developed a novel process that combines nitrogen-containing DLC films from a mixture of C2 H2 and N2 using the ion beam technique with N2 ions bombarding the substrate surface previous to coating the DLC films. The nitrogen incorporated into the films reduced the film stress, and the N2 ion pretreatment induces the formation of metal–N–C linkages. Both changes significantly enhance the film’s adhesion strength. References [1] J.H. Kim, D.A. Ahn, Y.H. Kim, H.K. Baik, J. Appl. Phys. 82 (1997) 658. [2] P. Wood, T. Wydeven, O. Tsuji, Thin Solid Films 258 (1995) 151. [3] Y.H. Cheng, Y.P. Wu, J.G. Chen, X.L. Qiao, C.S. Xie, Diamond Relat. Mater. 8 (1999) 1214. [4] F.L. Freire Jr., Jpn. J. Appl. Phys. 36 (1997) 4886. [5] C. Lenardi, M.A. Baker, V. Briois, L. Nobili, P. Piseri, W. Gissler, Diamond Relat. Mater. 8 (1999) 595. [6] D.F. Franceschini, C.A. Achete, F.L. Freire Jr., Appl. Phys. Lett. 60 (1992) 3229. [7] J. Narayan, R.D. Vispute, K. Jagannadham, J. Adhesion Sci. Technol. 9 (1995) 753. [8] N. Bertrand, B. Drevillon, A. Gheorghiu, C. Senemaud, L. Martinu, J.E. Klemberg-Sapieha, J. Vac. Sci. Technol. A 16 (1998) 6. [9] F.L. Freire Jr., D.F. Franceschini, Thin Solid Films 293 (1997) 236. [10] F. Mild, K. Goedicke, M. Fahland, Thin Solid Films 279 (1996) 169. [11] H.C. Ong, R.P.H. Chang, N. Baker, W.C. Oliver, Surf. Coat. Technol. 89 (1997) 38.