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Nanoindentation and nanoscratch behaviors of DLC films growth on different thickness of Cr nanolayers
Diamond & Related Materials 70 (2016) 76–82
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Nanoindentation and nanoscratch behaviors of DLC films growth on different thickness of Cr nanolayers Fatemeh Shahsavari a,⁎, Maryam Ehteshamzadeh b, M. Reza Naimi-Jamal c, Ahmad Irannejad b a b c
Young Researchers Society, Mineral Industries Research Center, Shahid Bahonar University of Kerman, Kerman, Iran Department of Materials Engineering and Metallurgy, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran Research Laboratory of Green Organic Synthesis and Polymers, Department of Chemistry, Iran University of Science and Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 18 July 2016 Received in revised form 18 September 2016 Accepted 9 October 2016 Available online 11 October 2016 Keywords: Diamond-like carbon Chromium nanolayer Plasma enhancement CVD Atomic force microscopy Nanoindentation and nanoscratch test
a b s t r a c t Metal interlayer is a promising method for improving the adhesion of DLC film to the substrate. In this study, Cr thin film, with different thickness of 10, 20, 40 and 80 nm was applied as an interlayer, to investigate the influences on the tribological behavior of DLC films. Cr nanolayers were deposited by DC magnetron sputtering and DLC films were deposited by plasma enhancement chemical vapor deposition (PECVD) using methane and argon as precursors. Surface roughness and particle size distribution of Cr nanolayers were investigated by atomic force microscopy (AFM), which showed increasing the roughness from 0.17 nm to 0.69 nm by increasing Cr nanolayer thickness. Thickness and microstructure of DLC films obtained by cross-sectional field emission scanning electron microscopy images and Raman spectroscopy, respectively. Nanotribological behaviors of DLC films such as scratch resistance, adhesion strength, friction coefficient, hardness and wear resistance were investigated by nanoscratch and nanoindentation tests. Good adhesion for all samples was observed in which samples with Cr thickness of 10 and 20 nm had the best hardness of 17 GPa and 24 GPa, respectively. The results indicated lower surface roughness and better particle distribution in Cr interlayer cause better tribological behavior. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Diamond like carbon is a type of amorphous carbon with significant amount of sp3 bonding which causes them to have many properties of diamond such as high hardness, high elastic modulus, low friction coefficient and chemical inertness [1–4]. DLC films can be synthesized by various methods such as magnetron sputtering [5–7], cathodic vacuum arc [8,9], laser ablation [10,11] and plasma chemical vapor deposition [12–14]. It is much cheaper to produce DLC than to synthesize diamond itself which is a great advantage for many application [2]. The major problem for their industrial applications is poor adhesion of DLC films to the substrates which causes delamination of the films [4,9,15,16]. Intermediate metals such as Cr, Ni, Ti, Al,W, Zr can either be doped to DLC during the growth process [6] [5,17,18]or can be deposited on the substrate as an interlayer [11] [13,19] to reduce the internal stress of the film and improve the adhesion of the films. In our recent study, different nickel thickness was used to synthesize the adherent of DLC films and results showed the interlayer thickness is effective in microstructure and wear behavior of the DLC film [20]. Another study was about the effect of different metallic nanolayer (Ni, Cr and Cu) on the growth and
⁎ Corresponding author. E-mail address: [email protected] (F. Shahsavari).
http://dx.doi.org/10.1016/j.diamond.2016.10.003 0925-9635/© 2016 Elsevier B.V. All rights reserved.
properties of DLC films which showed better results for Cr interlayers. Cr has the most half-filled d shell in period 4, which causes better carbon diffusion to the substrate and formation of carbide phase which can promote the adhesion of diamond films to the substrate [21]. In this study, different thickness of Cr interlayer was used as an interlayer to investigate structural, hardness and adhesion of DLC films. The critical load in which delamination of the films occurs was analyzed by nanoscratch test. 2. Experimental details Cr interlayer with thickness of 10, 20, 40 and 80 nm was placed on ptype silicon wafers by DC sputtering magnetron system (Pishtaz Engineering Co. High Vacuum Technology Center-ACECR-Sharif University). Before Cr deposition, the silicon substrates were ultrasonically cleaned in acetone, ethanol and deionized water, sequentially. For sputtering system the base pressure was 10− 5 Torr with argon flow rate of 6 sccm. Details of sample condition were listed in Table 1 in which the thickness of chromium was controlled by quartz crystal monitoring. A DC plasma enhancement Chemical vapor deposition (PECVD) system which was described in details elsewhere [16] was used for growth of DLC films. Details of deposition condition of Cr interlayer and DLC films were presented in Table 2.
F. Shahsavari et al. / Diamond & Related Materials 70 (2016) 76–82 Table 1 The given names and conditions of each sample. Sample
Cr thickness
Temperature
Sputter rate
Cr1
10 nm
80 °C
Cr2
20 nm
78 °C
_ 12.5 A=s _ 11.7 A=s
Cr3
40 nm
69 °C
Cr4
80nm
67 °C
_ 8.2 A=s _ 6.2 A=s
Table 2 Details of deposition conditions of Cr interlayer and DLC film.
Method Temperature Working pressure deposition time Gas/gases
Interlayer deposition
DLC deposition
DC magnetron sputtering 67–80 °C 10−2 Torr Depends on thickness Ar
DC plasma enhancement CVD 300 °C 3 Torr 60 min CH4 + Ar (=20 vol% + 80 vol%)
The morphology of Cr nanoparticles on the substrates was studied using atomic force microscopy (AFM, XE-NSOM, in contact mode). A field-emission scanning electron microscope (FE-SEM, MIRA 3, TESCAN) was applied to determine the thickness of the films. The chemical bonding and the composition of the films were investigated by Raman spectroscopy (Nicolet Almega, Nd:YAG laser with wavelength of 532 nm) and X-ray photoelectron spectroscopy (XPS: Specs model EA10 plus, Bestec Co.). The nanoscratch test was performed by using a triboscope instrument (Hysitron Inc. TriboScope® Nanomechanical Test) with a Berkovich diamond indenter to investigate the adhesion of the films. Different normal loads increases from 0 to 1000, 2000, 3000 and 4500 μN in 30 s in a distance of 4 μm were applied over the surface of the samples. The lateral displacement of the tip and also its force of the tip of the samples were measured, simultaneously. An in-situ AFM (NanoScope E, Digital Instruments, USA) was utilized to investigate the damage of the samples after each scratch. A nanoindentation tester (Triboscope system, Hysitron Inc. USA) with a Berkovich indenter tip was employed to determine the film hardness and Young's modulus. Each indent consists of a five second loading, a five-second hold, and a five-second unloading. 3. Results and discussion 3.1. AFM studies of Cr nanolayers Atomic force microscopy was applied to investigate the morphology of Cr thin films such as surface roughness and nanoparticles
77
distribution on the surface. The three-dimensional (3D) AFM images in a scanning area of 3 μm × 3 μm was illustrated in Fig. 1 which showed the lowest surface roughness in Cr 1 and C2, including the lowest Z-scale for images. Root mean square (RMS) roughness of the substrate is an important parameter which can affect final properties of DLC films such as adhesion strength and formation of smooth DLC films [22,23]. Surface roughness of the Cr thin film can be influenced by sputtering deposition parameters. Results from AFM images demonstrate increasing in RMS roughness by increasing the thickness of Cr nanolayer which could be duo to agglomeration of Cr nanoparticles by increasing the sputtering time. Fig. 2(a) illustrates the diagram of changing the roughness from 0.17 nm to 0.69nm in which the value is low for all the samples. The histograms of Cr nanoparticles distribution on each samples were given in Fig. 2(b) which can be used to interpret the distributional function of particle size [13]. There are Gaussian and homogeneous distribution for Cr 1 and Cr2 but for Cr3 and Cr4 , the distribution of Cr particle size tended to higher values which was distributed abnormally. Normal or Gaussian distribution can cause the homogenous nucleation and formation of carbon atoms on the substrate and therefore a better character of DLC films. The AFM results demonstrate the lowest roughness and the best distribution of particle size corresponding to Cr2. 3.2. Thickness measurement of the films The thickness evaluation of the samples was investigated by observation of the cross-section FE-SEM micrographs as showed in Fig. 3. The thickness of DLC films increased in Cr 1 to Cr 3 from 131 nm to 221 nm which is related to increase of Cr interlayer thickness. However, in Cr 4 DLC films thickness decreased to 191 nm. The cross-section FE-SEM image of DLC films synthesized on the bare silicon with the same deposition condition, as the reference sample which was reported before [20], showed formation of a very thin (~ 15 nm) film. This shows the effective role of Cr in adsorption and nucleation of DLC films and also could indicates decreasing in the internal stress of the film which cause to formation of thicker DLC films. In Fig. 3(a–d), the thickness of DLC films was estimated 131, 180, 221 and 191 nm for Cr1, Cr2, Cr3 and Cr4, respectively. Fig. 3(c–d) also displays the Cr nanolayers between silicon substrates and DLC films. 3.3. Microstructure of DLC films Raman spectroscopy and XPS are applied for characterization of bonding states of elements. Raman spectra in DLC film is consist of two peaks; a peak around 1350 cm−1 is related to D band and a peak around 1580 cm−1 which is related to G band [24–26]. The Raman spectra of all samples were illustrated in Fig.4, clearly demonstrate the D band and G band.
Fig. 1. AFM image of Cr nanolayer in a) Cr1, b) Cr2, c) Cr3 and d) Cr4 samples in 3 μm × 3 μm area.
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F. Shahsavari et al. / Diamond & Related Materials 70 (2016) 76–82
Fig. 2. (a) Variation of surface roughness of Cr nanolayers (b) Distribiton of Cr nanopartocles on the surface.
In hydrogenated amorphous carbon, the intensity ratio of D band to G band (I D/IG ) is corresponded to sp2/sp3 ratio [7,24]. The results of Raman analysis of the samples were listed in Table 3. As it can be seen from the both Fig. 4 and Table 3, all the samples had almost the same value of ID/IG varied from 0.83 to 0.86. Another important factor is G band position. The G peak position in all samples shifted toward higher wavenumber which indicated lower residual stress in the films and could be duo to formation of carbide [10,27, 28]. To study the chemical bonding states of Cr and C, analysis of XPS spectra was employed. The original spectra of sample Cr1 in Fig. 5(a) showed peaks located around 284.8, 575 and 532.8 eV which are due to the photoelectrons excited from the C1s, Cr2p and O1s core levels, respectively [8,10]. Fig. 5(b) illustrated that the C1s XPS spectra which was deconvoluted into four Gaussian peaks centered at 282.9, 284.3, 285.2 and 286.6 eV. These peaks were assigned to the
Fig. 4. Raman spectra of DLC films of all samples.
Fig. 3. Cross-sectional of FE-SEM images of (a) Cr1, (b) Cr2, (c) Cr3 and (d) Cr4.
F. Shahsavari et al. / Diamond & Related Materials 70 (2016) 76–82 Table 3 Results of Raman spectra of the all samples.
Cr1 Cr2 Cr3 Cr4
D band position (cm−1)
G band position (cm−1)
ID
1333 1348 1342 1348
1593 1589 1591 1595
0.88 0.86 0.83 0.85
IG
Cr\\C, sp2 C\\C, sp3 C\\C, and C\\O (or C_O) bonds, respectively [8, 10,17,29,30]. Carburization can promote the adhesion and is an important factor in formation of DLC films which was recognized by the XPS spectra [15,21]. In order to estimate sp3 fraction of the sample, the ratio of corresponding peak area (285.2 eV) to total C1s peak area was estimated. The percentage of sp3 hybridized carbon atom obtained about 43% for DLC sample [10].
3.4. Nanoscratch and nanoindentation studies The nanoscratch tests were performed on the samples to characterize their tribological behavior and investigate the adhesion of DLC films to the substrates. The nanoscratch tests were performed at variable progressive normal load from 0 to 1000, 2000, 3000 and 4500 μN for each samples. Fig. 6 shows the AFM images of nanoscratch tests after ramping force from 0 to 3000 μN. For Cr1 (Fig. 6(a)) deleterious effect of delamination can be observed as the tip penetrated into Cr nanolayer and silicon substrate. In Cr2 as it can be seen in the Fig. 6(b) no delamination was observed. The image shows DLC layer and Cr layer, indicating the penetration into silicon substrate. However, the behavior of Cr 3 and Cr 4 after nanoscratch test (Fig. 6(c–d)) was different, no cracks and fractures of the film were observed but there was a film blister and a huge plastic deformation which can be the beginning of the film spallation. This bulge deformation in sample Cr 3 is more than in Cr4, indicated lower wear behavior and adhesion of the film. Thus, the nanoscratch on Cr 1 and Cr2 showed good adhesion and despite penetration of the tip to the substrate, no delamination was occurred. Friction coefficients of the films as a function of scratch distance for ramping force of 3000 were illustrate in Fig. 7. In Fig. 7(a), variation of friction coefficient of Cr1 is duo to penetration of the tip to
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the substrate and the difference between values of the DLC and Si. For determining the friction coefficient of DLC, the average mean measured in scratch distance of 0.1 to 0.5 μm were considered and 0.10 was obtained. The COF also can determine with ramping force of 1000 μN before penetration of the tip into substrate. The COF before distance of 0.1 μm was neglected duo to instability of the tip and high fluctuations of the values. The COF for other samples were estimated by measuring the mean values in the distance of 0.1 to 0.5 μm in which no delamination or penetration in the substrate were occurred and the values of 0.15, 0.15 and 0.12 for Cr 2 , Cr3 and Cr4 were obtained, respectively. The fluctuation in friction coefficient, especially in Cr 1 after scratch distance of 3 μm and Cr3 at 3 μm related to the delaminated film. The wear track create new forces, which increase more the lateral force in a very short period of time [31]. The nanoindentation technique was used to characterize the mechanical properties of the samples. The hardness and modules of Cr1 and Cr2 were investigated which showed very better wear behavior and adhesion strength than Cr3 and Cr4. Fig. 8 presented load versus displacement curves at maximum indentation load of 500 μN. This load was applied to avoid substrate effect regarding penetration depth and thickness of the film. Fig. 8 shows nanoindentation curves shifted to the left and maximum depth reduced by increasing the Cr thickness, indicates the increasing hardness. On other hand, the samples reveal low hysteresis between loading–unloading curves. Hardness (H) and Young modulus (E) for Cr1 found to be 16.6 GPa and 166 GPa, respectively. These values enhanced in Cr2 to 23.9 GPa and 213 GPa for H and E, respectively. This improvement in mechanical properties related to increasing of sp3 content in DLC layer [8,32,33]. The I (D) /I (G) ratio obtained from Raman spectroscopy revealed the lower ratio for Cr 2 than Cr 1 which means higher sp3 bonding in the structure and confirms the nanoindentation results. Another important parameter related to good wear resistance of amorphous carbon is the ratio H/E (or H3/E2). The ratio H/E or so-called “plasticity index” was obtained 0.10 for Cr1 and 0.11 for Cr2, which are high values for amorphous carbon and indicated elastic behavior of the film under contact events. Increasing H/E reduces plasticity which is good because many mechanism of film failure begin with or directly involve plastic deformation as reported in literature [10,34–36]. The result confirmed AFM image of the nanoscratch of Cr1 and Cr2 which presented better elastic behavior for Cr2 (Fig. 6). It can be indicated the lowest ratio H/E for Cr3 which showed high plastic deformation in Fig. 6(c).
Fig. 5. (a) Typical survey scan spectrum and (b) Deconvoluted spectra of C1s peak for Cr1 sample.
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F. Shahsavari et al. / Diamond & Related Materials 70 (2016) 76–82
Fig. 6. AFM images after nanoscratch tests with ramping force of 3000 μN for (a) Cr1, (b) Cr2, (c) Cr3 and (d) Cr4.
3.5. Residual stress The stress of DLC films can be measured by Raman shift. The Eq. (1) shows the related of residual stress σ and Raman shift as follows [28,37, 38]: 1 þ v Δω σ ¼ 2G 1−v ω0
ð1Þ
In which, σ , v , and G are residual stress, Poisson's ratio and shear modulus, respectively. ω0 is the G-peak position and Δω is the shift of G-peak position (Δ ω = ω − ω0). For pure DLC the G-band position was obtained 1580 cm−1 (ω0). The shear modulus (G) can measured from Eq. (2) where E is Young's modulus and obtained from nanoindentation results. G¼
E 2ð1 þ vÞ
ð2Þ
For Cr1, G-peak position and Young modulus were 1593 cm−1 and 166 GPa, respectively and for Cr2, they were 1589 cm−1 and 213 GPa, respectively. Therefore, the residual stress of Cr1 was obtained 1.59 GPa and the value for Cr2 was obtained 1.37 GPa, which shows the lower stress of Cr2. Lower stress in Cr2 causes better adhesion of DLC film to the substrate. 4. Conclusions In this study, different thicknesses of Cr nanolayers applied for deposition of DLC films. Results showed various surface roughness and Cr nanoparticles distribution on the substrates obtained,
increasing by deposition time or thickness of Cr film. Lower surface roughness with a normal distribution of Cr particles caused better formation of DLC films which had the best adhesion and hardness. The mechanism of mechanical interlocking between Cr and DLC improve the adhesion. The mechanisms of adhesion can be mainly divided into two groups: 1) mechanical interlocking 2) chemical bonding [13, 37]. Mechanical interlocking can be divided into locking by friction and locking by dovetailing [13]. Khalaj et al., 2012 reported a typical image, in which by decreasing the RMS roughness, the surface becomes smoother. Due to the high degree of chemical bonding and suitable mechanical interlocking, good friction coefficient of nanoparticles layer and smooth surface in this case, a higher adherence to the substrate will occur in the substrate with lower RMS roughness. This is exactly in confirming with our results. The model of the dependence between roughness and adhesion is reported by Khalaj et al. elsewhere [13]. Another study by Wei et al. [39] was about the effect of interlayer in reducing the stress in the DLC films. They showed that the thermal stress in the DLC film is linearly affected by the roughness and a rougher surface induces higher stress and is detrimental for deposition [39,40]. Therefore, a Cr interlayer with lower roughness shows lower stress and better adhesion. Thickness of DLC films increased by thickness of Cr interlayer and hardly DLC film formed on silicon substrate without Cr interlayer. This is related to chemical nature of Cr which has five half-filled d orbitals in its atomic structure and therefore a high diffusion of carbon atoms which causes the rapid formation of the DLC film. Nanoscratch and nanoindentation tests showed the best wear resistance and hardness for DLC film grow on 20 nm Cr interlayer which has the best and lowest surface roughness and distribution of Cr particles on the Si substrate.
F. Shahsavari et al. / Diamond & Related Materials 70 (2016) 76–82
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Acknowledgments The authors would like to thank Dr. Zahra Khalaj, Prof. Mahmood Ghoranneviss, Prof. Mircea V. Diudea and Mr. Saeed Nasiri Laheghi for their valuable assistance and cooperation.
References
Fig. 7. Variations of the COFs with ramping force of 3000 μN as the function of scratch distance in (a) Cr1, (b) Cr2, (c) Cr3 and (d) Cr4.
Fig. 8. Typical load-displacement curves obtained for Cr1 and Cr2 samples.
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Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Nanoindentation and nanoscratch behaviors of DLC films growth on different thickness of Cr nanolayers Fatemeh Shahsavari a,⁎, Maryam Ehteshamzadeh b, M. Reza Naimi-Jamal c, Ahmad Irannejad b a b c
Young Researchers Society, Mineral Industries Research Center, Shahid Bahonar University of Kerman, Kerman, Iran Department of Materials Engineering and Metallurgy, Faculty of Engineering, Shahid Bahonar University of Kerman, Kerman, Iran Research Laboratory of Green Organic Synthesis and Polymers, Department of Chemistry, Iran University of Science and Technology, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 18 July 2016 Received in revised form 18 September 2016 Accepted 9 October 2016 Available online 11 October 2016 Keywords: Diamond-like carbon Chromium nanolayer Plasma enhancement CVD Atomic force microscopy Nanoindentation and nanoscratch test
a b s t r a c t Metal interlayer is a promising method for improving the adhesion of DLC film to the substrate. In this study, Cr thin film, with different thickness of 10, 20, 40 and 80 nm was applied as an interlayer, to investigate the influences on the tribological behavior of DLC films. Cr nanolayers were deposited by DC magnetron sputtering and DLC films were deposited by plasma enhancement chemical vapor deposition (PECVD) using methane and argon as precursors. Surface roughness and particle size distribution of Cr nanolayers were investigated by atomic force microscopy (AFM), which showed increasing the roughness from 0.17 nm to 0.69 nm by increasing Cr nanolayer thickness. Thickness and microstructure of DLC films obtained by cross-sectional field emission scanning electron microscopy images and Raman spectroscopy, respectively. Nanotribological behaviors of DLC films such as scratch resistance, adhesion strength, friction coefficient, hardness and wear resistance were investigated by nanoscratch and nanoindentation tests. Good adhesion for all samples was observed in which samples with Cr thickness of 10 and 20 nm had the best hardness of 17 GPa and 24 GPa, respectively. The results indicated lower surface roughness and better particle distribution in Cr interlayer cause better tribological behavior. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Diamond like carbon is a type of amorphous carbon with significant amount of sp3 bonding which causes them to have many properties of diamond such as high hardness, high elastic modulus, low friction coefficient and chemical inertness [1–4]. DLC films can be synthesized by various methods such as magnetron sputtering [5–7], cathodic vacuum arc [8,9], laser ablation [10,11] and plasma chemical vapor deposition [12–14]. It is much cheaper to produce DLC than to synthesize diamond itself which is a great advantage for many application [2]. The major problem for their industrial applications is poor adhesion of DLC films to the substrates which causes delamination of the films [4,9,15,16]. Intermediate metals such as Cr, Ni, Ti, Al,W, Zr can either be doped to DLC during the growth process [6] [5,17,18]or can be deposited on the substrate as an interlayer [11] [13,19] to reduce the internal stress of the film and improve the adhesion of the films. In our recent study, different nickel thickness was used to synthesize the adherent of DLC films and results showed the interlayer thickness is effective in microstructure and wear behavior of the DLC film [20]. Another study was about the effect of different metallic nanolayer (Ni, Cr and Cu) on the growth and
⁎ Corresponding author. E-mail address: [email protected] (F. Shahsavari).
http://dx.doi.org/10.1016/j.diamond.2016.10.003 0925-9635/© 2016 Elsevier B.V. All rights reserved.
properties of DLC films which showed better results for Cr interlayers. Cr has the most half-filled d shell in period 4, which causes better carbon diffusion to the substrate and formation of carbide phase which can promote the adhesion of diamond films to the substrate [21]. In this study, different thickness of Cr interlayer was used as an interlayer to investigate structural, hardness and adhesion of DLC films. The critical load in which delamination of the films occurs was analyzed by nanoscratch test. 2. Experimental details Cr interlayer with thickness of 10, 20, 40 and 80 nm was placed on ptype silicon wafers by DC sputtering magnetron system (Pishtaz Engineering Co. High Vacuum Technology Center-ACECR-Sharif University). Before Cr deposition, the silicon substrates were ultrasonically cleaned in acetone, ethanol and deionized water, sequentially. For sputtering system the base pressure was 10− 5 Torr with argon flow rate of 6 sccm. Details of sample condition were listed in Table 1 in which the thickness of chromium was controlled by quartz crystal monitoring. A DC plasma enhancement Chemical vapor deposition (PECVD) system which was described in details elsewhere [16] was used for growth of DLC films. Details of deposition condition of Cr interlayer and DLC films were presented in Table 2.
F. Shahsavari et al. / Diamond & Related Materials 70 (2016) 76–82 Table 1 The given names and conditions of each sample. Sample
Cr thickness
Temperature
Sputter rate
Cr1
10 nm
80 °C
Cr2
20 nm
78 °C
_ 12.5 A=s _ 11.7 A=s
Cr3
40 nm
69 °C
Cr4
80nm
67 °C
_ 8.2 A=s _ 6.2 A=s
Table 2 Details of deposition conditions of Cr interlayer and DLC film.
Method Temperature Working pressure deposition time Gas/gases
Interlayer deposition
DLC deposition
DC magnetron sputtering 67–80 °C 10−2 Torr Depends on thickness Ar
DC plasma enhancement CVD 300 °C 3 Torr 60 min CH4 + Ar (=20 vol% + 80 vol%)
The morphology of Cr nanoparticles on the substrates was studied using atomic force microscopy (AFM, XE-NSOM, in contact mode). A field-emission scanning electron microscope (FE-SEM, MIRA 3, TESCAN) was applied to determine the thickness of the films. The chemical bonding and the composition of the films were investigated by Raman spectroscopy (Nicolet Almega, Nd:YAG laser with wavelength of 532 nm) and X-ray photoelectron spectroscopy (XPS: Specs model EA10 plus, Bestec Co.). The nanoscratch test was performed by using a triboscope instrument (Hysitron Inc. TriboScope® Nanomechanical Test) with a Berkovich diamond indenter to investigate the adhesion of the films. Different normal loads increases from 0 to 1000, 2000, 3000 and 4500 μN in 30 s in a distance of 4 μm were applied over the surface of the samples. The lateral displacement of the tip and also its force of the tip of the samples were measured, simultaneously. An in-situ AFM (NanoScope E, Digital Instruments, USA) was utilized to investigate the damage of the samples after each scratch. A nanoindentation tester (Triboscope system, Hysitron Inc. USA) with a Berkovich indenter tip was employed to determine the film hardness and Young's modulus. Each indent consists of a five second loading, a five-second hold, and a five-second unloading. 3. Results and discussion 3.1. AFM studies of Cr nanolayers Atomic force microscopy was applied to investigate the morphology of Cr thin films such as surface roughness and nanoparticles
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distribution on the surface. The three-dimensional (3D) AFM images in a scanning area of 3 μm × 3 μm was illustrated in Fig. 1 which showed the lowest surface roughness in Cr 1 and C2, including the lowest Z-scale for images. Root mean square (RMS) roughness of the substrate is an important parameter which can affect final properties of DLC films such as adhesion strength and formation of smooth DLC films [22,23]. Surface roughness of the Cr thin film can be influenced by sputtering deposition parameters. Results from AFM images demonstrate increasing in RMS roughness by increasing the thickness of Cr nanolayer which could be duo to agglomeration of Cr nanoparticles by increasing the sputtering time. Fig. 2(a) illustrates the diagram of changing the roughness from 0.17 nm to 0.69nm in which the value is low for all the samples. The histograms of Cr nanoparticles distribution on each samples were given in Fig. 2(b) which can be used to interpret the distributional function of particle size [13]. There are Gaussian and homogeneous distribution for Cr 1 and Cr2 but for Cr3 and Cr4 , the distribution of Cr particle size tended to higher values which was distributed abnormally. Normal or Gaussian distribution can cause the homogenous nucleation and formation of carbon atoms on the substrate and therefore a better character of DLC films. The AFM results demonstrate the lowest roughness and the best distribution of particle size corresponding to Cr2. 3.2. Thickness measurement of the films The thickness evaluation of the samples was investigated by observation of the cross-section FE-SEM micrographs as showed in Fig. 3. The thickness of DLC films increased in Cr 1 to Cr 3 from 131 nm to 221 nm which is related to increase of Cr interlayer thickness. However, in Cr 4 DLC films thickness decreased to 191 nm. The cross-section FE-SEM image of DLC films synthesized on the bare silicon with the same deposition condition, as the reference sample which was reported before [20], showed formation of a very thin (~ 15 nm) film. This shows the effective role of Cr in adsorption and nucleation of DLC films and also could indicates decreasing in the internal stress of the film which cause to formation of thicker DLC films. In Fig. 3(a–d), the thickness of DLC films was estimated 131, 180, 221 and 191 nm for Cr1, Cr2, Cr3 and Cr4, respectively. Fig. 3(c–d) also displays the Cr nanolayers between silicon substrates and DLC films. 3.3. Microstructure of DLC films Raman spectroscopy and XPS are applied for characterization of bonding states of elements. Raman spectra in DLC film is consist of two peaks; a peak around 1350 cm−1 is related to D band and a peak around 1580 cm−1 which is related to G band [24–26]. The Raman spectra of all samples were illustrated in Fig.4, clearly demonstrate the D band and G band.
Fig. 1. AFM image of Cr nanolayer in a) Cr1, b) Cr2, c) Cr3 and d) Cr4 samples in 3 μm × 3 μm area.
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Fig. 2. (a) Variation of surface roughness of Cr nanolayers (b) Distribiton of Cr nanopartocles on the surface.
In hydrogenated amorphous carbon, the intensity ratio of D band to G band (I D/IG ) is corresponded to sp2/sp3 ratio [7,24]. The results of Raman analysis of the samples were listed in Table 3. As it can be seen from the both Fig. 4 and Table 3, all the samples had almost the same value of ID/IG varied from 0.83 to 0.86. Another important factor is G band position. The G peak position in all samples shifted toward higher wavenumber which indicated lower residual stress in the films and could be duo to formation of carbide [10,27, 28]. To study the chemical bonding states of Cr and C, analysis of XPS spectra was employed. The original spectra of sample Cr1 in Fig. 5(a) showed peaks located around 284.8, 575 and 532.8 eV which are due to the photoelectrons excited from the C1s, Cr2p and O1s core levels, respectively [8,10]. Fig. 5(b) illustrated that the C1s XPS spectra which was deconvoluted into four Gaussian peaks centered at 282.9, 284.3, 285.2 and 286.6 eV. These peaks were assigned to the
Fig. 4. Raman spectra of DLC films of all samples.
Fig. 3. Cross-sectional of FE-SEM images of (a) Cr1, (b) Cr2, (c) Cr3 and (d) Cr4.
F. Shahsavari et al. / Diamond & Related Materials 70 (2016) 76–82 Table 3 Results of Raman spectra of the all samples.
Cr1 Cr2 Cr3 Cr4
D band position (cm−1)
G band position (cm−1)
ID
1333 1348 1342 1348
1593 1589 1591 1595
0.88 0.86 0.83 0.85
IG
Cr\\C, sp2 C\\C, sp3 C\\C, and C\\O (or C_O) bonds, respectively [8, 10,17,29,30]. Carburization can promote the adhesion and is an important factor in formation of DLC films which was recognized by the XPS spectra [15,21]. In order to estimate sp3 fraction of the sample, the ratio of corresponding peak area (285.2 eV) to total C1s peak area was estimated. The percentage of sp3 hybridized carbon atom obtained about 43% for DLC sample [10].
3.4. Nanoscratch and nanoindentation studies The nanoscratch tests were performed on the samples to characterize their tribological behavior and investigate the adhesion of DLC films to the substrates. The nanoscratch tests were performed at variable progressive normal load from 0 to 1000, 2000, 3000 and 4500 μN for each samples. Fig. 6 shows the AFM images of nanoscratch tests after ramping force from 0 to 3000 μN. For Cr1 (Fig. 6(a)) deleterious effect of delamination can be observed as the tip penetrated into Cr nanolayer and silicon substrate. In Cr2 as it can be seen in the Fig. 6(b) no delamination was observed. The image shows DLC layer and Cr layer, indicating the penetration into silicon substrate. However, the behavior of Cr 3 and Cr 4 after nanoscratch test (Fig. 6(c–d)) was different, no cracks and fractures of the film were observed but there was a film blister and a huge plastic deformation which can be the beginning of the film spallation. This bulge deformation in sample Cr 3 is more than in Cr4, indicated lower wear behavior and adhesion of the film. Thus, the nanoscratch on Cr 1 and Cr2 showed good adhesion and despite penetration of the tip to the substrate, no delamination was occurred. Friction coefficients of the films as a function of scratch distance for ramping force of 3000 were illustrate in Fig. 7. In Fig. 7(a), variation of friction coefficient of Cr1 is duo to penetration of the tip to
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the substrate and the difference between values of the DLC and Si. For determining the friction coefficient of DLC, the average mean measured in scratch distance of 0.1 to 0.5 μm were considered and 0.10 was obtained. The COF also can determine with ramping force of 1000 μN before penetration of the tip into substrate. The COF before distance of 0.1 μm was neglected duo to instability of the tip and high fluctuations of the values. The COF for other samples were estimated by measuring the mean values in the distance of 0.1 to 0.5 μm in which no delamination or penetration in the substrate were occurred and the values of 0.15, 0.15 and 0.12 for Cr 2 , Cr3 and Cr4 were obtained, respectively. The fluctuation in friction coefficient, especially in Cr 1 after scratch distance of 3 μm and Cr3 at 3 μm related to the delaminated film. The wear track create new forces, which increase more the lateral force in a very short period of time [31]. The nanoindentation technique was used to characterize the mechanical properties of the samples. The hardness and modules of Cr1 and Cr2 were investigated which showed very better wear behavior and adhesion strength than Cr3 and Cr4. Fig. 8 presented load versus displacement curves at maximum indentation load of 500 μN. This load was applied to avoid substrate effect regarding penetration depth and thickness of the film. Fig. 8 shows nanoindentation curves shifted to the left and maximum depth reduced by increasing the Cr thickness, indicates the increasing hardness. On other hand, the samples reveal low hysteresis between loading–unloading curves. Hardness (H) and Young modulus (E) for Cr1 found to be 16.6 GPa and 166 GPa, respectively. These values enhanced in Cr2 to 23.9 GPa and 213 GPa for H and E, respectively. This improvement in mechanical properties related to increasing of sp3 content in DLC layer [8,32,33]. The I (D) /I (G) ratio obtained from Raman spectroscopy revealed the lower ratio for Cr 2 than Cr 1 which means higher sp3 bonding in the structure and confirms the nanoindentation results. Another important parameter related to good wear resistance of amorphous carbon is the ratio H/E (or H3/E2). The ratio H/E or so-called “plasticity index” was obtained 0.10 for Cr1 and 0.11 for Cr2, which are high values for amorphous carbon and indicated elastic behavior of the film under contact events. Increasing H/E reduces plasticity which is good because many mechanism of film failure begin with or directly involve plastic deformation as reported in literature [10,34–36]. The result confirmed AFM image of the nanoscratch of Cr1 and Cr2 which presented better elastic behavior for Cr2 (Fig. 6). It can be indicated the lowest ratio H/E for Cr3 which showed high plastic deformation in Fig. 6(c).
Fig. 5. (a) Typical survey scan spectrum and (b) Deconvoluted spectra of C1s peak for Cr1 sample.
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Fig. 6. AFM images after nanoscratch tests with ramping force of 3000 μN for (a) Cr1, (b) Cr2, (c) Cr3 and (d) Cr4.
3.5. Residual stress The stress of DLC films can be measured by Raman shift. The Eq. (1) shows the related of residual stress σ and Raman shift as follows [28,37, 38]: 1 þ v Δω σ ¼ 2G 1−v ω0
ð1Þ
In which, σ , v , and G are residual stress, Poisson's ratio and shear modulus, respectively. ω0 is the G-peak position and Δω is the shift of G-peak position (Δ ω = ω − ω0). For pure DLC the G-band position was obtained 1580 cm−1 (ω0). The shear modulus (G) can measured from Eq. (2) where E is Young's modulus and obtained from nanoindentation results. G¼
E 2ð1 þ vÞ
ð2Þ
For Cr1, G-peak position and Young modulus were 1593 cm−1 and 166 GPa, respectively and for Cr2, they were 1589 cm−1 and 213 GPa, respectively. Therefore, the residual stress of Cr1 was obtained 1.59 GPa and the value for Cr2 was obtained 1.37 GPa, which shows the lower stress of Cr2. Lower stress in Cr2 causes better adhesion of DLC film to the substrate. 4. Conclusions In this study, different thicknesses of Cr nanolayers applied for deposition of DLC films. Results showed various surface roughness and Cr nanoparticles distribution on the substrates obtained,
increasing by deposition time or thickness of Cr film. Lower surface roughness with a normal distribution of Cr particles caused better formation of DLC films which had the best adhesion and hardness. The mechanism of mechanical interlocking between Cr and DLC improve the adhesion. The mechanisms of adhesion can be mainly divided into two groups: 1) mechanical interlocking 2) chemical bonding [13, 37]. Mechanical interlocking can be divided into locking by friction and locking by dovetailing [13]. Khalaj et al., 2012 reported a typical image, in which by decreasing the RMS roughness, the surface becomes smoother. Due to the high degree of chemical bonding and suitable mechanical interlocking, good friction coefficient of nanoparticles layer and smooth surface in this case, a higher adherence to the substrate will occur in the substrate with lower RMS roughness. This is exactly in confirming with our results. The model of the dependence between roughness and adhesion is reported by Khalaj et al. elsewhere [13]. Another study by Wei et al. [39] was about the effect of interlayer in reducing the stress in the DLC films. They showed that the thermal stress in the DLC film is linearly affected by the roughness and a rougher surface induces higher stress and is detrimental for deposition [39,40]. Therefore, a Cr interlayer with lower roughness shows lower stress and better adhesion. Thickness of DLC films increased by thickness of Cr interlayer and hardly DLC film formed on silicon substrate without Cr interlayer. This is related to chemical nature of Cr which has five half-filled d orbitals in its atomic structure and therefore a high diffusion of carbon atoms which causes the rapid formation of the DLC film. Nanoscratch and nanoindentation tests showed the best wear resistance and hardness for DLC film grow on 20 nm Cr interlayer which has the best and lowest surface roughness and distribution of Cr particles on the Si substrate.
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Acknowledgments The authors would like to thank Dr. Zahra Khalaj, Prof. Mahmood Ghoranneviss, Prof. Mircea V. Diudea and Mr. Saeed Nasiri Laheghi for their valuable assistance and cooperation.
References
Fig. 7. Variations of the COFs with ramping force of 3000 μN as the function of scratch distance in (a) Cr1, (b) Cr2, (c) Cr3 and (d) Cr4.
Fig. 8. Typical load-displacement curves obtained for Cr1 and Cr2 samples.
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