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Microstructure and chemical bonding of DLC films deposited on ACM rubber by PACVD
Surface & Coatings Technology 205 (2011) S75–S78
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Microstructure and chemical bonding of DLC films deposited on ACM rubber by PACVD D. Martinez-Martinez a,⁎, M. Schenkel a, Y.T. Pei a, J.C. Sánchez-López b, J.Th.M. De Hosson a a b
Department of Applied Physics, Materials Innovation Institute M2i, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla), Américo Vespucio 49, 41092, Sevilla, Spain
a r t i c l e
i n f o
Available online 5 March 2011 Keywords: Scanning electron microscopy (SEM) Plasma-assisted chemical vapor deposition (PACVD) Rubber Raman spectroscopy
a b s t r a c t The microstructure and chemical bonding of DLC films prepared by plasma assisted chemical vapor deposition on acrylic rubber (ACM) are studied in this paper. The temperature variation produced by the ion impingement during plasma cleaning and subsequent film deposition was used to modify the film microstructure by controlling the different degrees of strain applied to the substrate. The film microstructure is studied by top view and cross sectional SEM. The observed patch sizes are correlated with the variation of temperature that occurred during deposition. Finally, the chemical bonding of the samples is studied by Raman spectroscopy. All the samples show similar spectra regardless the bias voltage used. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Ball bearings are widely used in many technical fields, like aerospace or automotive industries [1]. In many cases, a rubber seal is incorporated in order to prevent the leakage of lubricant and the entrance of dirt. However, under these conditions, dynamic rubber seals suffer from severe wear and cause high friction, leading to an ultimate failure of the bearing. Diamond-like carbon (DLC) films are a potential solution as protective coating, due to the combination of relatively high hardness, chemical inertness, low friction coefficient and low wear rates [2]. Further requirements are film flexibility and an excellent adhesion to the rubber substrate. The chemical nature of the rubber, which is mainly composed by C and H, suggests a good compatibility with a carbonaceous film. A tile-like structure of DLC film has been proposed forward to improve its flexibility and reduce the stresses under deformation [3]. This structure can be obtained by using a grid, which results in uncoated strips on a rubber substrate and the size of film patches cannot be reduced too much. This kind of structure appears also when growing the film by filtered arc deposition [4,5] but an explanation regarding the origin of this structure and therefore a way for controlling the patch size are missing. The aim of this work is to study the microstructure and chemical bonding of DLC films deposited on acrylic rubber (ACM) by plasmaassisted chemical vapor deposition (PACVD). A correlation between the deposition characteristics and the patch size, which determines the flexibility of the films, will be explored.
DLC thin films were deposited on acrylic rubber (alkyl acrylate copolymer, ACM) of 2 mm thickness by means of PACVD in a Teer UDP/400 close field unbalanced magnetron sputtering rig [6], with all the magnetrons powered off. The rubber substrates were clamped at one corner over metallic plates and mounted on the substrate carrousel rotating at 3 rpm to ensure film homogeneity. A pulsed DC (p-DC) power unit (Advanced Energy) was used as substrate bias source, operating at 250 kHz with a pulse off time of 500 ns. The nominal voltages were varied between 300 and 600 V, which are the minimum value to obtain a reasonable deposition rate and stable plasma conditions, and a value close to the maximum achievable by the power unit, respectively. Before deposition, the rubber substrates were cleaned by two subsequent wash procedures using a detergent solution and boiling water, in order to remove oil contamination and wax present on the rubber, respectively [7,8]. The deposition process was composed of three steps. First, the ACM samples were etched for 30–40 min in Ar plasma (15 sccm, 5 × 10 −3 mbar), in order to further clean the surface from contaminations. Then, a second treatment in a plasma mixture of Ar and H2 (10 min, flow ratio 15:10 sccm, 6 × 10−3 mbar) was used in order to improve the adhesion of the forthcoming carbonaceous layer. These treatments did not cause any changes in the rubber microstructure. These two steps are labeled as ‘etching’ steps, in contrast to the final one where H2 is replaced by C2H2 (10 sccm) and then DLC film was deposited. However, the residual hydrogen in the inlet pipeline used also for acetylene might last till the first 10 min of deposition, during which the growth of DLC film is only minor.
⁎ Corresponding author. E-mail address: [email protected] (D. Martinez-Martinez). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.02.067
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D. Martinez-Martinez et al. / Surface & Coatings Technology 205 (2011) S75–S78
Fig. 1. Examples of temperature–time protocols. a) Single protocol. b) Double protocol. The voltages used in each process and the values of ΔT (before and after the ‘purge time’ correction) are also included.
During the etching and deposition processes, the temperature of the rubber substrate varied as a consequence of ion impingement. The temperature variations during deposition can be tailored depending on the bias voltage and treatment time [8]. Thus, depending on the deposition protocol the film microstructure can be modified due to the different stresses on the substrate and the film. Two types of deposition protocols were studied; in the single ones, the voltage during the deposition is kept constant and only one change of temperature is obtained (e.g. ΔT = −94 °C in Fig. 1a). In the double ones, the voltage is changed during deposition, and two temperature variations are obtained (e.g. in Fig. 1b ΔT1 = 90 °C and ΔT2 = −80 °C, which is shortened as ΔT = 90/−80). To keep a constant film thickness of ~ 300 nm, the deposition durations were adjusted according to the bias voltages applied. The synthesis conditions of the samples under study are summarized in Table 1. The microstructure was characterized by scanning electron microscopy (SEM), using a Philips FEG-XL30s operating at 3 kV acceleration voltage. Cross sections of DLC film coated ACM were obtained by fracture after immersion in liquid nitrogen for 10 min. Raman spectra were acquired to investigate the chemical bonding of these films by using a LabRAM Horiba Jobin Yvon spectrometer equipped with a CCD detector and a He–Ne laser (532 nm) at 0.2 or 2 mW in order to avoid any damaging of the rubber substrate. All measurements were recorded for the wavelength range of 100– 2000 cm−1 under the same conditions (10 s of integration time and 10 accumulations) using a 100× magnification objective and a 100 μm pinhole. 3. Results and discussion Fig. 2 shows the typical microstructure of DLC films deposited on ACM rubber. At low magnification, a cracked appearance of the film is
observed as a result of the substrate strain during the film growth. These cracks can be also identified in the cross section views shown in Fig. 2d and e. In fact, a clear curvature can be observed in each patch indicative of the large compressive strain induced by the shrinkage of rubber substrate. In addition, the columnar growth of the DLC film can be distinguished in the micrograph taken at higher magnification (Fig. 2e). The globular microstructure observed in the top view micrograph at highest magnification (Fig. 2c) is a consequence of this columnar growth. This microstructure formed by columns and patches improves the film flexibility. The patch size was measured as the average length of straight lines connecting opposite sides of the patch. It depends on the sign and magnitude of the temperature difference used during growth, as it can be seen in Fig. 3. In the case of samples prepared by double protocols, the value of ΔT1 is used for plotting, since it has been observed that the first procedure has a higher influence on the final microstructure [8]. For single protocols, it can be seen that the patch size decreases as the value of |ΔT| increases due to the larger deformations caused to the substrate. In addition, the samples prepared using negative values of ΔT show smaller patch sizes, since compression has an influence from the beginning of the deposition. In contrast, in the case of ΔT N 0, the tensile stress hinders the formation of a continuous film, which can be formed only when the expansion of rubber substrate slows down. Thus, the formation of crack network is delayed [8]. Regarding the samples prepared by double protocols, both of them prepared with such a protocol that ΔT1 N 0 and ΔT2 b 0 (like the one depicted in Fig. 1b) show a smaller patch size than both samples prepared using single protocols with ΔT N 0. This can be attributed to an additional decrease of patch size caused by the compressive stress induced during that cooling. However, the sample prepared using ΔT = 49/−42 °C shows a very small patch size (in the range of those samples prepared with ΔT b 0 protocols). That sample was prepared using an etching step of 300 V during 50 min and two deposition steps, the first one of ‘heating’ at 600 V during 10 min, and a second one of ‘cooling’ at 300 V during 90 min (see Table 1). We have stated previously the existence of a ‘purge time’, which is needed to empty the H2 from the line and let the C2H2 entering the chamber. Therefore, no deposition would take place during this period, which was determined to be around 10 min while doing purging of the gas line. Thus, the nominal values of ΔT need to be corrected in order to account for this fact. The correction can be seen in Fig. 1, where the values of ΔT are re-calculated by shifting the beginning of deposition by 10 min. It has to be noted that this correction does not influence the value of ΔT2 in the double protocols (cf. Fig. 1b). The effect of correction is displayed by arrows in Fig. 3. As a result, the aforementioned film (ΔT = 49/−42 °C) is displaced to the region of ΔT b 0, since during the first step (ΔT1 = 49, ‘heating’ for 10 min. at 600 V) no deposition took place. In fact, this sample is actually prepared by a single protocol of ΔT = −42 °C. Moreover, it can be seen that all the samples prepared at ΔT b 0 can be represented by a common linear trend, where the sample prepared at ΔT = − 43/82 °C
Table 1 Synthesis conditions and patch sizes of the samples under study. Etching
Deposition step 1
ΔT1 (°C)
Deposition step 2
Voltage (V)
Time (min)
Voltage (V)
Time (min)
Voltage (V)
Time (min)
Nominal
300
50
600
40
600 462 600 600 300 400 462 300
45 55 30 10 120 60 55 40
— — 300 300 — — — 600
— — 40 90 — — — 30
103 46 90 49 − 94 − 68 − 46 − 85
After correction 53 25 42 — − 53 − 35 − 25 − 43
ΔT2 (°C)
Patch size (μm)
— — − 80 − 47 — — — 82
124 164 100 54 38 77 128 80
D. Martinez-Martinez et al. / Surface & Coatings Technology 205 (2011) S75–S78
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Fig. 2. Typical microstructure of DLC films deposited on ACM; top views (a–c) and cross sections (d,e).
also appears to be well correlated. This can be explained because the expansion caused during the second step (heating) cannot reduce further the patch size (nor increase it, of course). This behavior is different from the one observed for the sample prepared at ΔT = 42/ −80 °C, which has a lower patch size than the other samples prepared at ΔT N 0 due to the compressive stress caused during the second part of the deposition. The chemical bonding of the films has been studied by means of Raman spectroscopy (see Fig. 4) using a 532 nm excitation wavelength. Fig. 4a represents the full spectra for three samples prepared using single protocols at different voltages (300, 400 and 600 V, see Table 1), together with the spectrum of the ACM substrate recorded on the same conditions. It can be seen that the spectra of the films differ from the rubber spectrum in the appearance of the so-called D and G peaks characteristic of amorphous carbon films and an increasing photoluminescence background, as commonly observed in visible Raman spectra of hydrogenated samples. In order to obtain a
Fig. 3. Patch size depending on the temperature difference during growth (ΔT) before and after ‘purge time’ correction (full and hollow symbols, respectively). In the case of samples prepared using double protocols (labeled and represented by circles), the first value of ΔT is used for plotting. The black line represents a linear trend. See text for details.
better comparison among them, we have subtracted the spectrum of the bare ACM rubber from the spectra of each film (see Fig. 4b). The shape of the spectra looks very similar in all of the cases, and it is also similar to the spectra of samples prepared by double protocols, i.e. where different voltages were used during the film deposition (results not shown). Qualitative information about the bonding structure in the DLC coatings can be obtained by performing a fitting analysis using a combination of a Lorentzian and a Breit–Wigner–Fano functions for the D and G peaks respectively as suggested by Ferrari et al. [9,10]. The G position and the intensity ratios (ID/IG) are found in the range of 1528–1539 cm−1 and 0.4–0.5 respectively by changing the bias from 300 to 600 V. The low ID/IG ratio and G position arise from a low sp2 clustering and high structural disorder of the carbon network as it is generally observed in many amorphous hydrogenated and free-H carbon films [11]. Thus, it can be concluded that the change of bias voltage does not have a significant influence on the sp2 content of the samples but rather in the structural and topological disorder. This result agrees with previous observations, which show that the final microstructure of the samples was mainly controlled by the value of ΔT used and not by the bias voltage [8].
Fig. 4. Raman spectra of films prepared at different voltages of 300, 400 and 600 V. a) Full range, including the ACM substrate. b) Detail of the region from 900 to 1900 cm−1, after subtraction of the spectrum of uncovered ACM rubber and normalization.
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4. Conclusions DLC films were deposited on ACM rubber with PACVD using different values of ΔT during growth by controlling the applied bias. They showed a columnar growth, and a tile-like microstructure when prepared at ΔT ≠ 0. The patch size can be linearly correlated with ΔT, when the ‘purge time’ correction is included. The patch size is lower at higher values of |ΔT| due to the larger stresses applied to the substrate. Besides, protocols with ΔT b 0 lead to smaller patch sizes. All films showed very close Raman spectra regardless the bias voltage used for deposition, indicating a similar chemical bonding typical of disordered sp2 carbon-bonded structures. Acknowledgements This research was carried out under project number MC7.06247 in the framework of the Research Program of the Materials innovation institute M2i (www.m2i.nl). The financial support from projects Nos.
MAT2010-21597-C02-01 and FUNCOAT CSD2008-00023 is also acknowledged.
References [1] B.J. Hamrock, D. Dowson, Ball bearing lubrication, The Elastohydrodynamics of Elliptical Contacts., John Wiley & Sons Inc, 1981. [2] J. Robertson, Mater. Sci. Eng. R 37 (2002) 129. [3] Y. Aoki, N. Ohtake, Tribol. Int. 37 (2004) 941. [4] N. Miyakawa, S. Minamisawa, H. Takikawa, T. Sakakibara, Vacuum 73 (2004) 611. [5] H. Takikawa, N. Miyakawa, S. Minamisawa, T. Sakakibara, Thin Solid Films 457 (2004) 143. [6] B. Window, Surf. Coat. Technol. 81 (1996) 92. [7] X.L. Bui, Y.T. Pei, E.D.G. Mulder, J.T.M. De Hosson, Surf. Coat. Technol. 203 (2009) 1964. [8] D. Martinez-Martinez, M. Schenkel, Y.T. Pei, J.T.M. De Hosson, Thin Solid Films 519 (2011) 2213. [9] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [10] D. Martinez-Martinez, C. Lopez-Cartes, A. Fernandez, J.C. Sanchez-Lopez, Thin Solid Films 517 (2009) 1662. [11] C. Casiraghi, A.C. Ferrari, J. Robertson, Phys. Rev. B 72 (2005) 085401.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Microstructure and chemical bonding of DLC films deposited on ACM rubber by PACVD D. Martinez-Martinez a,⁎, M. Schenkel a, Y.T. Pei a, J.C. Sánchez-López b, J.Th.M. De Hosson a a b
Department of Applied Physics, Materials Innovation Institute M2i, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla), Américo Vespucio 49, 41092, Sevilla, Spain
a r t i c l e
i n f o
Available online 5 March 2011 Keywords: Scanning electron microscopy (SEM) Plasma-assisted chemical vapor deposition (PACVD) Rubber Raman spectroscopy
a b s t r a c t The microstructure and chemical bonding of DLC films prepared by plasma assisted chemical vapor deposition on acrylic rubber (ACM) are studied in this paper. The temperature variation produced by the ion impingement during plasma cleaning and subsequent film deposition was used to modify the film microstructure by controlling the different degrees of strain applied to the substrate. The film microstructure is studied by top view and cross sectional SEM. The observed patch sizes are correlated with the variation of temperature that occurred during deposition. Finally, the chemical bonding of the samples is studied by Raman spectroscopy. All the samples show similar spectra regardless the bias voltage used. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
Ball bearings are widely used in many technical fields, like aerospace or automotive industries [1]. In many cases, a rubber seal is incorporated in order to prevent the leakage of lubricant and the entrance of dirt. However, under these conditions, dynamic rubber seals suffer from severe wear and cause high friction, leading to an ultimate failure of the bearing. Diamond-like carbon (DLC) films are a potential solution as protective coating, due to the combination of relatively high hardness, chemical inertness, low friction coefficient and low wear rates [2]. Further requirements are film flexibility and an excellent adhesion to the rubber substrate. The chemical nature of the rubber, which is mainly composed by C and H, suggests a good compatibility with a carbonaceous film. A tile-like structure of DLC film has been proposed forward to improve its flexibility and reduce the stresses under deformation [3]. This structure can be obtained by using a grid, which results in uncoated strips on a rubber substrate and the size of film patches cannot be reduced too much. This kind of structure appears also when growing the film by filtered arc deposition [4,5] but an explanation regarding the origin of this structure and therefore a way for controlling the patch size are missing. The aim of this work is to study the microstructure and chemical bonding of DLC films deposited on acrylic rubber (ACM) by plasmaassisted chemical vapor deposition (PACVD). A correlation between the deposition characteristics and the patch size, which determines the flexibility of the films, will be explored.
DLC thin films were deposited on acrylic rubber (alkyl acrylate copolymer, ACM) of 2 mm thickness by means of PACVD in a Teer UDP/400 close field unbalanced magnetron sputtering rig [6], with all the magnetrons powered off. The rubber substrates were clamped at one corner over metallic plates and mounted on the substrate carrousel rotating at 3 rpm to ensure film homogeneity. A pulsed DC (p-DC) power unit (Advanced Energy) was used as substrate bias source, operating at 250 kHz with a pulse off time of 500 ns. The nominal voltages were varied between 300 and 600 V, which are the minimum value to obtain a reasonable deposition rate and stable plasma conditions, and a value close to the maximum achievable by the power unit, respectively. Before deposition, the rubber substrates were cleaned by two subsequent wash procedures using a detergent solution and boiling water, in order to remove oil contamination and wax present on the rubber, respectively [7,8]. The deposition process was composed of three steps. First, the ACM samples were etched for 30–40 min in Ar plasma (15 sccm, 5 × 10 −3 mbar), in order to further clean the surface from contaminations. Then, a second treatment in a plasma mixture of Ar and H2 (10 min, flow ratio 15:10 sccm, 6 × 10−3 mbar) was used in order to improve the adhesion of the forthcoming carbonaceous layer. These treatments did not cause any changes in the rubber microstructure. These two steps are labeled as ‘etching’ steps, in contrast to the final one where H2 is replaced by C2H2 (10 sccm) and then DLC film was deposited. However, the residual hydrogen in the inlet pipeline used also for acetylene might last till the first 10 min of deposition, during which the growth of DLC film is only minor.
⁎ Corresponding author. E-mail address: [email protected] (D. Martinez-Martinez). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.02.067
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D. Martinez-Martinez et al. / Surface & Coatings Technology 205 (2011) S75–S78
Fig. 1. Examples of temperature–time protocols. a) Single protocol. b) Double protocol. The voltages used in each process and the values of ΔT (before and after the ‘purge time’ correction) are also included.
During the etching and deposition processes, the temperature of the rubber substrate varied as a consequence of ion impingement. The temperature variations during deposition can be tailored depending on the bias voltage and treatment time [8]. Thus, depending on the deposition protocol the film microstructure can be modified due to the different stresses on the substrate and the film. Two types of deposition protocols were studied; in the single ones, the voltage during the deposition is kept constant and only one change of temperature is obtained (e.g. ΔT = −94 °C in Fig. 1a). In the double ones, the voltage is changed during deposition, and two temperature variations are obtained (e.g. in Fig. 1b ΔT1 = 90 °C and ΔT2 = −80 °C, which is shortened as ΔT = 90/−80). To keep a constant film thickness of ~ 300 nm, the deposition durations were adjusted according to the bias voltages applied. The synthesis conditions of the samples under study are summarized in Table 1. The microstructure was characterized by scanning electron microscopy (SEM), using a Philips FEG-XL30s operating at 3 kV acceleration voltage. Cross sections of DLC film coated ACM were obtained by fracture after immersion in liquid nitrogen for 10 min. Raman spectra were acquired to investigate the chemical bonding of these films by using a LabRAM Horiba Jobin Yvon spectrometer equipped with a CCD detector and a He–Ne laser (532 nm) at 0.2 or 2 mW in order to avoid any damaging of the rubber substrate. All measurements were recorded for the wavelength range of 100– 2000 cm−1 under the same conditions (10 s of integration time and 10 accumulations) using a 100× magnification objective and a 100 μm pinhole. 3. Results and discussion Fig. 2 shows the typical microstructure of DLC films deposited on ACM rubber. At low magnification, a cracked appearance of the film is
observed as a result of the substrate strain during the film growth. These cracks can be also identified in the cross section views shown in Fig. 2d and e. In fact, a clear curvature can be observed in each patch indicative of the large compressive strain induced by the shrinkage of rubber substrate. In addition, the columnar growth of the DLC film can be distinguished in the micrograph taken at higher magnification (Fig. 2e). The globular microstructure observed in the top view micrograph at highest magnification (Fig. 2c) is a consequence of this columnar growth. This microstructure formed by columns and patches improves the film flexibility. The patch size was measured as the average length of straight lines connecting opposite sides of the patch. It depends on the sign and magnitude of the temperature difference used during growth, as it can be seen in Fig. 3. In the case of samples prepared by double protocols, the value of ΔT1 is used for plotting, since it has been observed that the first procedure has a higher influence on the final microstructure [8]. For single protocols, it can be seen that the patch size decreases as the value of |ΔT| increases due to the larger deformations caused to the substrate. In addition, the samples prepared using negative values of ΔT show smaller patch sizes, since compression has an influence from the beginning of the deposition. In contrast, in the case of ΔT N 0, the tensile stress hinders the formation of a continuous film, which can be formed only when the expansion of rubber substrate slows down. Thus, the formation of crack network is delayed [8]. Regarding the samples prepared by double protocols, both of them prepared with such a protocol that ΔT1 N 0 and ΔT2 b 0 (like the one depicted in Fig. 1b) show a smaller patch size than both samples prepared using single protocols with ΔT N 0. This can be attributed to an additional decrease of patch size caused by the compressive stress induced during that cooling. However, the sample prepared using ΔT = 49/−42 °C shows a very small patch size (in the range of those samples prepared with ΔT b 0 protocols). That sample was prepared using an etching step of 300 V during 50 min and two deposition steps, the first one of ‘heating’ at 600 V during 10 min, and a second one of ‘cooling’ at 300 V during 90 min (see Table 1). We have stated previously the existence of a ‘purge time’, which is needed to empty the H2 from the line and let the C2H2 entering the chamber. Therefore, no deposition would take place during this period, which was determined to be around 10 min while doing purging of the gas line. Thus, the nominal values of ΔT need to be corrected in order to account for this fact. The correction can be seen in Fig. 1, where the values of ΔT are re-calculated by shifting the beginning of deposition by 10 min. It has to be noted that this correction does not influence the value of ΔT2 in the double protocols (cf. Fig. 1b). The effect of correction is displayed by arrows in Fig. 3. As a result, the aforementioned film (ΔT = 49/−42 °C) is displaced to the region of ΔT b 0, since during the first step (ΔT1 = 49, ‘heating’ for 10 min. at 600 V) no deposition took place. In fact, this sample is actually prepared by a single protocol of ΔT = −42 °C. Moreover, it can be seen that all the samples prepared at ΔT b 0 can be represented by a common linear trend, where the sample prepared at ΔT = − 43/82 °C
Table 1 Synthesis conditions and patch sizes of the samples under study. Etching
Deposition step 1
ΔT1 (°C)
Deposition step 2
Voltage (V)
Time (min)
Voltage (V)
Time (min)
Voltage (V)
Time (min)
Nominal
300
50
600
40
600 462 600 600 300 400 462 300
45 55 30 10 120 60 55 40
— — 300 300 — — — 600
— — 40 90 — — — 30
103 46 90 49 − 94 − 68 − 46 − 85
After correction 53 25 42 — − 53 − 35 − 25 − 43
ΔT2 (°C)
Patch size (μm)
— — − 80 − 47 — — — 82
124 164 100 54 38 77 128 80
D. Martinez-Martinez et al. / Surface & Coatings Technology 205 (2011) S75–S78
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Fig. 2. Typical microstructure of DLC films deposited on ACM; top views (a–c) and cross sections (d,e).
also appears to be well correlated. This can be explained because the expansion caused during the second step (heating) cannot reduce further the patch size (nor increase it, of course). This behavior is different from the one observed for the sample prepared at ΔT = 42/ −80 °C, which has a lower patch size than the other samples prepared at ΔT N 0 due to the compressive stress caused during the second part of the deposition. The chemical bonding of the films has been studied by means of Raman spectroscopy (see Fig. 4) using a 532 nm excitation wavelength. Fig. 4a represents the full spectra for three samples prepared using single protocols at different voltages (300, 400 and 600 V, see Table 1), together with the spectrum of the ACM substrate recorded on the same conditions. It can be seen that the spectra of the films differ from the rubber spectrum in the appearance of the so-called D and G peaks characteristic of amorphous carbon films and an increasing photoluminescence background, as commonly observed in visible Raman spectra of hydrogenated samples. In order to obtain a
Fig. 3. Patch size depending on the temperature difference during growth (ΔT) before and after ‘purge time’ correction (full and hollow symbols, respectively). In the case of samples prepared using double protocols (labeled and represented by circles), the first value of ΔT is used for plotting. The black line represents a linear trend. See text for details.
better comparison among them, we have subtracted the spectrum of the bare ACM rubber from the spectra of each film (see Fig. 4b). The shape of the spectra looks very similar in all of the cases, and it is also similar to the spectra of samples prepared by double protocols, i.e. where different voltages were used during the film deposition (results not shown). Qualitative information about the bonding structure in the DLC coatings can be obtained by performing a fitting analysis using a combination of a Lorentzian and a Breit–Wigner–Fano functions for the D and G peaks respectively as suggested by Ferrari et al. [9,10]. The G position and the intensity ratios (ID/IG) are found in the range of 1528–1539 cm−1 and 0.4–0.5 respectively by changing the bias from 300 to 600 V. The low ID/IG ratio and G position arise from a low sp2 clustering and high structural disorder of the carbon network as it is generally observed in many amorphous hydrogenated and free-H carbon films [11]. Thus, it can be concluded that the change of bias voltage does not have a significant influence on the sp2 content of the samples but rather in the structural and topological disorder. This result agrees with previous observations, which show that the final microstructure of the samples was mainly controlled by the value of ΔT used and not by the bias voltage [8].
Fig. 4. Raman spectra of films prepared at different voltages of 300, 400 and 600 V. a) Full range, including the ACM substrate. b) Detail of the region from 900 to 1900 cm−1, after subtraction of the spectrum of uncovered ACM rubber and normalization.
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4. Conclusions DLC films were deposited on ACM rubber with PACVD using different values of ΔT during growth by controlling the applied bias. They showed a columnar growth, and a tile-like microstructure when prepared at ΔT ≠ 0. The patch size can be linearly correlated with ΔT, when the ‘purge time’ correction is included. The patch size is lower at higher values of |ΔT| due to the larger stresses applied to the substrate. Besides, protocols with ΔT b 0 lead to smaller patch sizes. All films showed very close Raman spectra regardless the bias voltage used for deposition, indicating a similar chemical bonding typical of disordered sp2 carbon-bonded structures. Acknowledgements This research was carried out under project number MC7.06247 in the framework of the Research Program of the Materials innovation institute M2i (www.m2i.nl). The financial support from projects Nos.
MAT2010-21597-C02-01 and FUNCOAT CSD2008-00023 is also acknowledged.
References [1] B.J. Hamrock, D. Dowson, Ball bearing lubrication, The Elastohydrodynamics of Elliptical Contacts., John Wiley & Sons Inc, 1981. [2] J. Robertson, Mater. Sci. Eng. R 37 (2002) 129. [3] Y. Aoki, N. Ohtake, Tribol. Int. 37 (2004) 941. [4] N. Miyakawa, S. Minamisawa, H. Takikawa, T. Sakakibara, Vacuum 73 (2004) 611. [5] H. Takikawa, N. Miyakawa, S. Minamisawa, T. Sakakibara, Thin Solid Films 457 (2004) 143. [6] B. Window, Surf. Coat. Technol. 81 (1996) 92. [7] X.L. Bui, Y.T. Pei, E.D.G. Mulder, J.T.M. De Hosson, Surf. Coat. Technol. 203 (2009) 1964. [8] D. Martinez-Martinez, M. Schenkel, Y.T. Pei, J.T.M. De Hosson, Thin Solid Films 519 (2011) 2213. [9] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (2000) 14095. [10] D. Martinez-Martinez, C. Lopez-Cartes, A. Fernandez, J.C. Sanchez-Lopez, Thin Solid Films 517 (2009) 1662. [11] C. Casiraghi, A.C. Ferrari, J. Robertson, Phys. Rev. B 72 (2005) 085401.