Scientific paper
Deposition of DLC Film on Stainless Steel Substrates Coated by Nickel Using PECVD Method
Zahra Khalaj,1 Mahmood Ghoranneviss,*'1 Elnaz Vaghri,1 Amir Saghaleini1 and Mircea V. Diudea2
1 Plasma Physics Research Center, Science and Research Branch, Islamic Azad University,Tehran,Iran.P.O.Box:14665/678
2 Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, Cluj, Romania
* Corresponding author: E-mail: ghoranneviss@pprc.srbiau.ac.ir Tel: +98-21-44869624; Fax:+98-21-4869626
Received: 21-11-2012
Abstract
Research on diamond-like carbon (DLC) films has been devoted to find both optimized conditions and characteristics of the deposited films on various substrates. In the present work, we investigate the quality of the DLC films grown on stainless steel substrates using different thickness of the nickel nanoparticle layers on the surface. Nickel nanoparticles were sputtered on the stainless steel substrates at 200 °C by a DC-sputtering system to make a good adherence between DLC coating and steel substrates. Atomic Force Microscopy was used to characterize the surface roughness and distribution function of the nickel nanoparticles on the substrate surface. Diamond like carbon films were deposited on stainless steel substrates coated by nickel using pure acetylene and C2H2/H2 with 15% flow ratio by DC-Plasma Enhanced Chemical Vapor Deposition (PECVD) systems. Microstructural analysis by Raman spectroscopy showed a low intensity ratio ID/IG for DLC films by increasing the Ni layer thickness on the stainless steel substrates. Fourier Transforms Infrared spectroscopy (FTIR) evidenced the peaks attributed to C-H bending and stretching vibration modes in the range of 1300-1700 cm-1 and 2700-3100 cm-1, respectively, in good agreement with the Raman spectroscopy and confirmed the DLC growth in all samples.
Keywords: Diamond like carbon; RMS roughness; FTIR; Ni NP; stainless steel substrate.
1. Introduction
Diamond-like carbon (DLC) films are metastable amorphous hydrogenated or non-hydrogenated forms of carbon materials; they consist of a random network of sp2 and sp3 carbon sites with varying amounts of hydrogen dispersed within the lattice1. Since their first preparation by Aisenberg and Chabot in the early 70s,2 DLC films have attracted word-wide attention due to their excellent properties which make them reliable to use in various fields of science and technology. Some of the remarkable properties of DLC films are their high wear resistance,3 chemical inertness,4 low friction coefficient5 and particularly high hardness6 which makes them one of the most suitable materials for coating surfaces. DLC coating would increase the surface hardness and its longevity7 A wide variety of deposition techniques based on ion beam or plasma techniques have been developed for synthesizing DLC
films.8-11 DC- and RF-PECVD methods are two of the most popular deposition techniques of DLC films,12-16 which allow the growth at low substrate temperatures and provide ion bombardment of the surface.17 The PECVD process is based on the decomposition of a carbonaceous gas into species such as ions, radicals and atoms near the substrate surface. Using DC-PECVD technique, there is a lower stress value in comparison with RF-PECVD.7
Usually, there is a low adherence between DLC coatings and the metal substrate such as stainless steel. Many attempts have been made to increase the adherence of diamond like carbon coatings to the stainless steel substrates, since DLC coatings have a wide range of applications.18-19 The major techniques to obtain a good adhesive strength include the fabrication of an adhesive layer between the DLC coating and substrates, and the reduction of the internal stress of the DLC coating.18-20 In this work we report the growth of diamond like carbon films on
stainless steel substrates coated by nickel. The nickel nanoparticles would act as "glue" to make a good adherence between DLC coating and stainless steel substrate.21 In addition, we found that the quality of the DLC films would be enhanced by increasing the nickel thickness and reducing the RMS roughness of the substrate surfaces. Atomic Force Microscopy (AFM), Raman spectroscopy and Fourier Transform Infrared spectroscopy (FTIR) were used to characterize the samples.
2. Experimental Details and Methodology
2. 1. Substrate Pretreatment
In the present investigation stainless steel (AISI 304L, AMS Co., USA) in the size of 10 mm x 10 mm x 1 mm was used as substrate; it was cleaned ultrasonically in acetone and ethanol solutions to remove any residual contaminants prior to deposition. The samples introduced into the planar DC-sputtering system, which was then pumped down to a base pressure of 4 x 101 Pa. A nickel plate was used as a cathode and was placed in parallel with the oven which was grounded. The distance between cathode and anode was about 1cm. Argon was introduced into the chamber with a flow of 220 sccm. The nickel nanoparticles (Ni NP) were sputtered on stainless steel substrates when the substrate temperature increasing gradually up to 200 °C. Deposition time for nickel sputtering (ts) was 40, 80, 120 and 160 minutes for S1, S2, S3 and S4, respectively.
2. 2. DLC Preparation
DLC coatings were prepared by DC-Plasma Enhanced Chemical Vapor Deposition (PECVD) system
which manufactured by our group in Plasma physics Research Center. Figure 1 shows the schematic diagram of the PECVD system. Nickel-coated AISI 304L substrates were introduced into the PECVD chamber, which was then pumped down to a base pressure. All the samples were mounted in the furnace. Deposition of DLC films used C2H2 with a flow of 80 sccm for 50 minutes. Then a mixture of acetylene and hydrogen (15 vol. % C2H2/H2) was inserted to the system with a total flow of 250 sccm. The working pressure was kept at 1600 Pa and deposition time was 120 minutes.
3. Results and Discussion
3. 1. AFM and SEM Studies
Using an Atomic Force Microscopy (AFM, XE-NSOM) in contact mode, we analyzed the surface morphology of Ni films deposited on AISI 304L substrates at 40, 80, 120 and 160 minutes, respectively. The bare stainless steel (S0) used to compare the roughness of the surface with other sputtered samples (see Fig. 2). With the temperature rise, Ni nanoparticles are preferentially deposited on the bare AISI surface, resulting in the high-density nanoparticles layer, where the surface roughness reaches the comparatively minimum of about 1.17 nm for the growth time of 160 min. In Fig. 3, 2D and 3D topographies of the surfaces of the samples deposited at different time duration are shown. All images have been obtained in a scanning area of 3 pm x 3 pm. 2D and 3D AFM images show different RMS roughness of each sample's surface. Figure 3 (a-d) shows the morphology and roughness of stainless steel coated by nickel in the samples S1 to S4 respectively. Figure 4 shows the histograms of distribution of Ni particle size on the surface of stainless steel deposited at 40, 80,120 and 160 minutes, respectively. For all the samples, the Gaussian diagram showed a non-homoge-
Fig. 1. Schematic diagram of the PECVD system for DLC films deposition.
Fig. 2. 3D AFM image of the bare stainless steel
neous distribution of the nickel nanoparticles on substrates. This may be due to the Ni sputtering at temperature of 200 °C that led to more agglomeration of the nickel nanoparticles by increasing the deposition time. The histogram of the nanoparticles distribution can be used to interpret the distribution function of particle size.22 By the Gaussian diagram,22 we can easily observe that the average particle size distribution of all samples is less than 50
nm. The Root-Mean-Squared roughness (Rms) of surface is one of the most important parameters for the characterization of the surface structure.7 The results of MS studies are listed in Figure 5.
The AFM results show the RMS roughness of the samples decreasing from S1 to S4. The lowest RMS roughness was in S4, corresponding to a temperature and deposition time of 200 °C and 160 minutes, respectively.
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Fig. 4. Histograms of the size distribution of Ni nanoparticles on AISI 304L substrates deposited at: (a) 40 min (b) 80 min (c) 120 min (d) 160 min, respectively
Fig. 5. Variation in the roughness of the Ni coating on AISI 304L substrates for different time duration
To follow closely the evolution of the Ni nanoparti-cles growth on SIAI 304L substrate, a mechanism image of the surface morphology is shown in Fig. 6. The mechanisms of adhesion can be mainly divided into two groups: 1) mechanical interlocking 2) chemical bonding.23 Mechanical interlocking can be divided into locking by friction and locking by dovetailing.23 As the typical images show, by decreasing the RMS roughness, the surface becomes smoother. Due to the high degree of chemical bonding and good friction coefficient of Ni nanoparticles layer23 in this case, there is more adherence to the AISI substrate.
Scanning Electron Microscopy (SEM) images in Figure 7 shows the minimum and the maximum of the thickness of the Ni layer which was coated by DLC in the same condition. Figure 7(a) related to the S1 in which the thickness of the Ni layer and DLC film are 1.96 pm and 2.13 pm respectively. The thickness of the DLC becomes 970.7 nm in S4 when the Ni thickness rises to the 2.68 pm (see Fig 7(b)).
3. 2. Raman and FTIR Studies
Raman spectroscopy is an effective technique for the characterization of DLC structure due to its availability and non-destructive nature.24 The Raman spectra of the DLC deposited films, measured by 532 nm excitation
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Fig. 7. Typical cross section SEM images of the DLC deposited on Ni coated steel: (a) S1 (b) S4
wave length of Nd:YLF laser are shown in Fig. 8. All carbon materials such as DLC coatings indicate common features in Raman spectra, named as D and G peaks. The G peak (labeled 'G' for graphite) corresponds to any pair of sp2 sites in both rings and chains while the D peak (la-
beled 'D' for disorder) is assigned to the breathing modes of sp2 atoms only in aromatic rings not in chains.25-29 The Raman spectra of the DLC films grown on nickel coated
Fig. 6. The contact mechanism of a DLC sliding on a Ni coated stainless steel with different surface roughness
AISI 304L substrate display two major features for each sample separately, corresponding to D and G bands, respectively30-32 (see Table 1).
Table 1. The statistic data of DLC growth on different substrate
Sample no.	D band (cm1)	G band (cm1)	VIg
Si	1342.44	1539.74	1.45
S2	1339.25	1535.15	0.96
S3	1339.11	1536.41	0.94
S3	1339.48	1537.99	0.86
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Fig. 8. Raman spectra for DLC coating on: (a) S1 (b) S2 (C) S3 (d) S4.
In amorphous carbons, the intensity ratio ID/IG is related to the size of the sp2 phase organized in rings.29,32 The intensity ratio ID/IG for the DLC coating on and S2 substrates was 1.45 and 0.96, respectively and decreased to 0.94 and 0.86 case of S3 and S4 substrates, respectively, as resulted by Raman spectra analysis. If the ratio ID/IG becomes lower or zero, the sp2 phase is organized rather in chains whereas a higher ratio ID/IG indicates an increase of the sp2 phase in aromatic rings33 but also a higher overall sp3 content.29,33,34 These results suggest that the quality of DLC structures can be improved by reducing the Ni surface roughness. In addition, the sp2 phase was organized rather in chain structures in S4 compared with the other samples.
Fourier transform infrared spectroscopy (FTIR) was used for analyzing the bonding structure of the DLC films. The FTIR absorption spectra obtained from the DLC films deposited on S1 to S4 substrates recorded in the range of 400-4000 cm1, are shown in Fig. 9.
The peaks attributed to C-H bending and stretching vibration modes appeared in the range of 1300-1700 cm-1 and 2700-3100 cm-1, respectively.35-38 The peak appearing at about 912 cm-1 can be associated with the
Fig. 9. FTIR spectra of the C-H vibrational region for DLC grown on nickel coated AISI 304L substrates: (a) S1 (b) S2 (c) S3 (d) S4
olefinic sp2 C-H2 vibration mode whereas the peak at around 1451 cm-1 could be assigned to C-H bending mode (see Table 2).
Table 2. Summary of the characteristic spn CHm (n and m = 1, 2, 3) vibration modes of DLCs grown on Ni coated steel: (a) S1, (b) S2, (c) S3, (d) S4.
Sample	Wave number (cm-1)	Assignment
S1, S25 S^, S4	912	Olefinic sp2 -CH2
	1227	sp2/sp3 C-C
	1451	Olefinic sp2-CH2 and sp3 -CH2
	2922	asymmetrical sp3 C-H2
	2862	spn CHm (n and m= 1,2,3)
	2968	Asymmetric sp3 -CH3
These results suggest the films grown on Ni coated steel substrates have DLC structure, are in good agreement with the Raman analysis.
4. Conclusions
In this study, a DLC coating with an adhesive Ni layer was deposited onto AISI 304L substrates. The Ni nanoparticle layers were successfully deposited onto substrate surfaces for 40, 80, 120 and 160 minutes, respectively. The surface roughness of the Ni coated steel substrate in 40 min (S1) was higher than the others. The RMS roughness of the surface decreased and the surfaces become smoother with increasing the deposition time. Raman analysis enabled evaluation of the intensity ratio
ID/IG for the DLC coating on the realized substrates S1 to S4. The results showed that by reducing the RMS roughness and increasing the Ni thickness on the AISI 304L substrate a lower intensity ratio ID/IG can obtain which is connected to a higher overall sp3 content and also the sp2 phase organized rather in chain structure. This results in the production of high quality DLC films on stainless steel substrates.
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Povzetek
Raziskave prevlek na osnovi ogljika, podobnega diamantu (angl. Diamond-Like Carbon, DLC), so bile posvečene optimizaciji pogojev priprave in lastnosti prevlek na različnih podlagah. V delu so opisane raziskave prevlek DLC na podlagah nerjavnega jekla. Za izboljšanje adhezijesmona podlage napršili plasti nanodelcev niklja različnih debelin pri 200 oC. Hrapavost površine in porazdelitev velikosti delcev niklja smo analizirali z mikroskopom na atomsko silo. Prevleke DLC smo nanesli s kemijsko metodo nanosaiz parne faze (angl. DC-Plasma Enhanced Chemical Vapor Deposition, PECVD). Strukturo DLC v vseh vzorcih smo analizirali z Ramansko in infrardečo spektroskopijo s Fourierjevo transformacijo.