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Investigation of (Ti:N)-DLC coatings prepared by ion source assisted cathodic arc ion-plating with varying Ti target currents
Diamond & Related Materials 69 (2016) 183–190
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Investigation of (Ti:N)-DLC coatings prepared by ion source assisted cathodic arc ion-plating with varying Ti target currents Y. Cai a, R.Y. Wang b, H.D. Liu a, C. Luo a, Q. Wan a, Y. Liu a, H. Chen a, Y.M. Chen a, Q.S. Mei a, B. Yang a,⁎ a b
School of Power and Mechanical Engineering, Wuhan University, 430072 Wuhan, China National Laboratory of Solid State Microstructures, Nanjing University, 210093 Nanjing, China
a r t i c l e
i n f o
Article history: Received 5 May 2016 Received in revised form 1 August 2016 Accepted 2 September 2016 Available online 4 September 2016 Keywords: (Ti:N)-DLC nanomultilayered coatings Composite Microstructure Properties
a b s t r a c t Ti and N co-doped diamond-like carbon ((Ti:N)-DLC) nanomultilayered composite coatings were synthesized by ion source assisted cathodic arc ion-plating at C2H2 and N2 ambient with varying Ti target currents. The compositions, morphologies and microstructures of the coatings were characterized by energy dispersive spectrometer (EDS), scanning electronic microscopy (SEM), atomic force microscope (AFM), X-Ray photoelectron spectrometer (XPS), Raman spectroscopy, X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Hardness and friction coefficient were tested by a hardness tester and a ball-on-disc tribometer, respectively. The (Ti:N)-DLC coating consists of uniform TiN, TiC, Ti(C,N) nanocrystallines embedded into Ndoped DLC (N-DLC) matrix. With the increase of Ti target current from 20 to 60 A, the sp2/sp3 ratio and the contents of TiN, TiC increase obviously, but the Ti(C,N) content sharply decreases. It was revealed that the coating deposited at Ti target current of 50 A exhibits the highest hardness (27.8 GPa), elastic modulus (491 GPa) and low friction coefficient (0.107). Prime novelty statement: In this study, we provide the (Ti:N)-DLC coating consisting of uniform TiN, TiC, Ti(C,N) nanocrystallines embedded into N-DLC matrix using a direct current ion source assisted cathodic arc ion-plating system, and have a detailed study about the influences of Ti target currents on the composition, phase content, microstructure and mechanical properties of (Ti:N)-DLC nanomultilayered composite coatings. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) coatings have been extensively studied for decades due to their excellent properties such as high hardness, low friction, superior wear resistance, which are highly consistent with a wide range of mechanical application as protective coatings [1]. However, the major drawbacks of DLC coating such as high residual stress, poor thermal stability have a bad effect on the workability of the coating. To optimize the properties of DLC coating, an effective way is to generate nanolayers or multiphases structure into amorphous carbon matrix [2]. Thereby, most of the scientific studies have been concentrated in metal-containing DLC (Me-DLC) coatings consisting of hard crystalline carbide and amorphous carbon phase in an effort to stabilize the coating structure, relax internal stress and improve mechanical and tribological performances [3,4]. For instance, the works of Ti [5–7], W [8,9], Cr [4] and Nb [10] containing DLC coatings have been devoted to systematically investigate the influence of incorporate metals. Mosayebi et al. deposited the TiC-DLC coating by RCAE-PVD and declared that the doped Ti increased the fracture toughness and maintained the hardness of DLC coating [7]. Furthermore, the incorporate N in DLC (N-DLC) coatings ⁎ Corresponding author. E-mail address: [email protected] (B. Yang).
http://dx.doi.org/10.1016/j.diamond.2016.09.003 0925-9635/© 2016 Elsevier B.V. All rights reserved.
can also positively improve the properties of DLC coatings. In the NDLC coatings, the C atoms can be replaced by N atoms in the aromatic ring or coalesce with N atoms to form the CNx coating which possesses many good properties such as low friction, residual stress and high corrosion resistance [11–14]. Recently, Zhou et al. reported the improved properties of N-DLC coatings in relation to carbon coatings and discussed the potential of these materials for mechanical, optical applications [15]. However, though both of the incorporate metal and N can enhance the performance of DLC coatings and have different influencing mechanism, few works have provided a detailed study about the metal and N co-doped DLC ((Me:N)-DLC) coatings. Particularly, Ti\\C\\N coatings that combine the high hardness and low fiction coefficient of TiC or CNx phases and the high toughness of TiN phases have been extensively studied as protective coatings due to their excellent properties [16,17]. Therefore, the Ti and N co-doped DLC ((Ti:N)-DLC) coating worths to own a high expectation. In this work, we fabricated (Ti:N)-DLC nanomultilayered composite coatings by incorporating Ti and N into DLC matrix with varying Ti target currents using a direct current ion source assisted cathodic arc ionplating system. The influences of Ti target currents on the composition, phase content, microstructure and mechanical properties of the coatings were systematically investigated. The aim of this work is to deposit the (Ti:N)-DLC nanomultilayered composite coating and initiate a
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systematic study of the microstructure–property relationship in (Ti:N)DLC coatings as a function of Ti target current. 2. Experimental details 2.1. Coatings deposition The (Ti:N)-DLC nanomultilayered composite coatings were deposited by a home-made hollow cathode ion source assisted cathode arc ionplating system as shown in Fig. 1. The use of hollow cathode ion source is to ensure the full ionization of gases and the restrictions of the plasma. In order to improve the adhesion of the coatings, the CrN bottom layer and TiN buffer layer were successively deposited onto silicon (100) wafers, cemented carbides substrates at N2 and Ar ambient with a Cr, Ti target, respectively. Then the (Ti:N)-DLC coatings were synthesized onto the substrates with a hollow cathode ion source and a Ti target. All the substrates were ultrasonically cleaned by acetone for 10 min, rinsed by deionized water and dried with a heater. Then the substrates were vertically placed on a rotating sample holder which was located in the center of the chamber. Prior to coating deposition, the chamber was pumped down to the pressure of 7 × 10−3 Pa and the substrates were heated up to 300 °C. A high bias voltage of −900 V was applied to the substrates and the Cr target current was set at 80 A to offer high energy ions to clean the substrates. The deposition parameters of (Ti:N)-DLC coatings are shown in Table 1. The C2H2 gas was introduced through the hollow cathode ion source and ionized by the ion source plasma. For all coating deposition process, the chamber ambient temperature was maintained at 300 °C, while the pressure of the chamber was kept at 0.5 Pa by adjusting the Ar flow rate. The deposition time of CrN, TiN and (Ti:N)-DLC layers were 5, 5, 30 min, respectively. To study the effect of Ti target current on the properties of the coatings, the Ti target currents were varied from 20 to 60 A. 2.2. Coatings characterization The surface and cross-sectional morphologies of the (Ti:N)-DLC coatings were observed by scanning electronic microscopy (SEM, FEI Sirion IMP SEM). The surface topography was probed using an atomic force microscope (AFM, Shimadzu SPM-9500J3) operated in the tapping
Table 1 Detailed deposition parameters of (Ti:N)-DLC coatings. Parameters value
Value
All target material Bias voltage (V) Reactive gas Working pressure (Pa) C2H2, N2 (sccm), respectively. Cr target current (A) (the CrN layer deposition process) Ion source current (A) Substrate temperature (°C) Rotation speed (rpm) Deposition time (min) Ti target current (A)
99.99% −200 80% duty cycle C2H2, N2 0.5 250, 100 50 60 300 4 5, 5, 30, respectively 20, 30, 40, 50, 60
mode with a measuring area of 5 × 5 μm2. The chemical composition was measured by energy dispersive spectrometer (EDS, EDAX genesis 7000 EDS system). The microstructure was examined by X-ray diffraction (XRD) performed on D8 advanced X-ray diffractometer with a Cu Kα radiation (0.154 nm) and high resolution transmission electron microscope (HRTEM, JEM-2010 FEF TEM) operated at 200 kV. A Laser Confocal Raman Micro-spectroscopy (Lab RAM HR 800 UV) with Ar+ laser excitation (632.8 nm) was used to determine the content of sp3 hybrid bond and G, D peak position. The chemical bonding state was investigated by an X-ray photoelectron spectroscopy (XPS, Kratos2AXIS2HS) using Mg Kα (1253.6 eV) X-ray radiation. The microhardness was measured on the surface of the coatings with a HXD-1000TM/LCD Vickers indenter using a Knoop hardness penetrator at a load of 100 gf. Ten points of each sample were indented and the average was taken as the hardness value. The micrographs of the indentations of different samples were observed by SEM (Zeiss Merlin Compact). The theoretical background on determining microhardness and elastic modulus of a material using the Knoop indentation testing is given by Marshall et al. [18] and Leigh et al. [19]. The elastic modulus can be calculated using Eq. (1): αH E¼ 0 b b − 0 a a
Fig. 1. Schematic diagram of the experimental setup.
ð1Þ
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Fig. 2. Surface morphologies of the coatings deposited at different Ti target currents: (a) 20 A, (b) 30 A, (c) 40 A, (d) 50 A, (e) 60 A.
where α is a constant and was taken as 0.45, H is Knoop hardness (in GPa), a′ and b′ are the lengths of the major and minor diagonals of the indentation impression, a and b are the major and minor diagonals of the ideal Knoop indentation (b/a = 1/7.11). The tribological performance was evaluated at room temperature in ambient air during dry sliding experiments conducted with a conventional ball-on-disc wear tester (MS-T3000). Here, the Si3N4 ball (99.9% purity) with 3 mm in diameter was used as mated materials. A load of 500 g and a sliding speed of 100 round per minute for 30 min were used in all experiments. The radius of the tribometer was set to 3 mm.
3. Results and discussions 3.1. Surface and cross-sectional morphologies Fig. 2 shows the surface SEM images of (Ti:N)-DLC coatings deposited at different target currents. The surfaces of the coatings contain obvious micro-droplets, which are characteristic of cathodic arc ion-plating. With the increase of the Ti target current, the distribution density and size of micro-droplets increase obviously. The representative AFM images are observed at Fig. 3, in which the rough morphologies can be
Fig. 3. AFM images of the coatings deposited at different Ti target currents: (a) 40 A, (b) 50 A, (c) 60 A.
Fig. 4. Cross-sectional images of the coatings deposited at different Ti target currents: (a) 20 A, (b) 40 A, (c) 60 A.
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Fig. 5. Chemical composition of the coatings deposited at different Ti target currents.
Fig. 6. XRD patterns of the coatings deposited at different Ti target currents.
3.3. Microstructure found on the surface. The root mean square (RMS) roughness of coatings deposited at 40, 50, 60 A are 13.1, 15.9, 25.8 nm, respectively. The cross-sectional images of (Ti:N)-DLC coating are shown in Fig. 4, in which the coatings exhibit very dense structure and homogeneous morphologies. At the Ti target current of 40 A, the thickness of total buffer layer and function layer are 0.6, 1.1 um, respectively.
3.2. Chemical composition The chemical compositions of the as-deposited coatings analyzed by EDS are shown in Fig. 5. As the Ti target current increases from 20 to 60 A, the C content in the (Ti:N)-DLC coatings decreases gradiently from 76.8 to 57.4 at.% with a simultaneous increase of N content from 16.0 to 20.8 at.%. However, with an increase in the Ti target current from 20 to 50 A, the Ti content first increases from 7.2 to 22.4 at.%, and then maintains the almost same value at higher Ti target current of 60 A. This indicates the (Ti:N)-DLC coatings deposited at the higher Ti target current contain higher Ti content when the Ti target current less than 50 A, and as the Ti target current increases further, the Ti content will reach saturation. The variation of Ti, C and N contents shows that the higher Ti target current will promote the formation of Ti\\N, which can be explained by that Ti atoms are more likely to bond with N rather than C to generate TiN compound [20]. These can be proved by the following XPS analysis.
Fig. 6 shows the XRD patterns of the coatings deposited under different Ti target currents. The characteristic peaks of TiC (100) and TiC (101) is identified at 2θ = 39.9°, 44.4°, respectively. Moreover, the peaks of TiN (111) and/or Ti(C,N) (111) are identified in the coatings at 2θ = 37.1°. It is difficult to distinguish TiN and Ti(C,N) since the atom radius of C and N are quite similar [21]. A certain amount of C atoms exist as interstitial atoms in the TiN structure to form Ti(C,N), and the same situation is reflected in the research [17]. The TiN (111) and/or Ti(C,N) (111) peaks in the diffraction patterns are unconspicuous that can be attributed to several factors such as small grain size or the influence of intense substrate peaks [22]. Fig. 7a shows the Raman spectra of (Ti:N)-DLC coatings deposited at different Ti target currents. The Raman spectra of the coatings can be divided into two regions. One from 300 to 800 cm−1 corresponds to titanium carbide/nitride while another from 900 to 1800 cm− 1 corresponds to amorphous carbon [16]. In the range of Raman shift from 300 to 800 cm−1, there are three peaks at ~ 423, ~ 547, ~634 cm−1 that can be assigned to the Ti(C,N), TiN and TiC, respectively [23–25]. When the Ti target current is higher than 40 A, the Ti(C,N) peak obviously shifts to lower frequencies with higher Ti target currents, which could be due to that the reduction of the C interstitial atoms in Ti(C,N) phase results in the decreasing formation of Ti(C,N) [23]. Furthermore, the region of Raman shift from 300 to 800 cm−1 shows significant characteristic bands (D and G bands), which indicates the
Fig. 7. (a) Raman spectra and (b) ID/IG ratio and Raman shifts of G, D peaks of the coatings.
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Fig. 8. XPS core level spectra of the coatings deposited at different Ti target currents: (a) Ti 2p, (b) C 1s, (c) N 1s.
presence of amorphous carbon phase in the coatings [26]. The G band centered at ~1576 cm−1 corresponds to symmetric E2g C\\C stretching mode in graphite-like materials, while the D peak at ~1360 cm−1 is to the bond angle disorder in the graphite-like microdomains [27]. As seen in Fig. 7a, the intensities of D and G peaks decrease drastically when the Ti target current is beyond 30 A. This indicates that the
Table 2 The binding energies of the Ti 2p, C 1s, N 1s. Element/Transition
Binding energy (eV)
Compound
Ti 2p
454.9/460.6 455.5/461.4 456.8/462.9 485.6/464.4 281.9 284.9 284.6 285.5 286.6 287.2 288.2 396.8 397.2 398.4 400.2 402.4
TiC TiN Ti(C,N) Ti\ \O TiC Ti(C,N) \C sp2 C\ \C sp3 C\ \N sp2 C\ 3 \N sp C\ C\ \O Ti(C,N) TiN N-sp3 C N-sp2 C N\ \O
C 1s
N 1s
variation of Ti target current will influence the formation of sp2 and sp3 bondings in the amorphous carbon host matrix [28]. In order to perform more detailed analysis of Raman spectra, Raman spectra of the coatings were deconvoluted using Gaussian function to calculate the peak positions of G, D and the ratio of the D and G band intensity (ID/ IG) presented in the different (Ti:N)-DLC coatings. Fig. 7b presents the curves-fitted Raman data. It is obvious that the D and G peaks shift slightly to lower wave numbers for all coatings with the increase of Ti target current from 20 to 60 A. This phenomenon was also reported in Ref. [28]. In addition, the sp3/sp2 ratio can be characterized by the ID/IG ratio, and the increasing sp3 fraction in the amorphous carbon coatings will decrease the ID/IG ratio [29,30]. The ratio of ID/IG increases from 4.74 to 9.49 with the increase of Ti target current from 20 to 60 A, which indicates that the increase of Ti target current will led to an increase in the formation of sp2 phase and reveal the graphitic characteristic of the (Ti:N)-DLC coatings. The chemical bonding states of (Ti:N)-DLC coatings were characterized by XPS. In order to further analyse the effect of Ti target current on the bonding structure of the coatings, the XPS data were fitted with Gaussian function in an attempt to determine the fraction of each bonding type, and the deconvolution results of the Ti 2p, C 1s and N 1s XPS core level spectra of the (Ti:N)-DLC coatings are shown in Fig. 8 and Table 2. In the Ti 2p spectra as shown in Fig. 8a, the coatings exhibit similar Ti 2p3/2 and Ti 2p1/2 peaks at about 453–460 eV and 460–466 eV, respectively. The Ti 2p spectra can be peak-fitted into eight Gaussian functions ascribed to TiC, TiN, Ti(C,N) and Ti\\O bonds [16,28,31]. The
Fig. 9. (a) TiN, TiC and Ti(C,N) content from fitting Ti 2p spectra and (b) sp3/sp2 ratio from fitting C 1s spectra.
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Fig. 10. Cross-sectional TEM images (a), the corresponding SAED pattern (b) and HRTEM images (c), (d) of the coating deposited at 50 A.
strong Ti\\O peaks at ~458.6 and 464.4 eV can be due to a gradual oxidation taking place in the first few days once the coatings were exposed to air. As seen in Fig. 8b, all C 1s XPS spectra consist of seven peaks at the binding energy range of 280–292 eV, which can be assigned to TiC, Ti(C,N), sp2 C\\C, sp3 C\\C, sp2 C\\N, sp3 C\\N and C\\O bonds, respectively [32,33]. In the meanwhile, for all the N 1s spectra, the presence of five peaks can be attributed to TiN, Ti(C,N), N-sp2 C, N-sp3 C and N\\O bonds, respectively [33,34] . Fig. 9 is the content of the TiN, TiC and Ti(C,N) phase derived from the Ti 2p spectra, and the ratio of sp3/sp2 determined from the C 1s spectra. It reveals that when the Ti target current increases from 20 to 60 A, the content of TiN apparently increases from 9.7 to 31.6 at.% with a simultaneous increase of TiC from 8.2 to 19.8 at.% while that of Ti(C,N) content becomes low obviously. It is confirmed that the increase of Ti target current will prevent the C atoms to substitute the N atoms in the TiN lattice to form Ti(C,N) and can be a favor to obtain TiN and TiC phases. Since less Gibbs free energy is needed of Ti reacting with N than C, the Ti atom is more likely to bond with N atom not C atom to generate TiN compound [35]. Furthermore, as the Ti target current increases from 20 to 60 A, the ratio of sp3/sp2 decreases from 1.1 to 0.33. It can be explained that higher target current supplies
more Ti ions incorporated in the coatings which can break some of the sp3 bonds, and lead to the formation of a stable sp2-bonded phase [36]. Fig. 10 shows the cross-sectional TEM images and the corresponding selected area electron diffraction (SAED) pattern of the (Ti:N)-DLC coating deposited on a Si substrate at 50 A. The nanomultilayered composite structure consisting of the uniform nanocrystallines and amorphous NDLC can be clearly observed in Fig. 10a and c, in which the thickness of each layer is found to be approximately 4 nm. The formation of the nanomultilayered composite structure is due to the rotation of the substrates during the deposition process. The distinct rings displayed in the SAED patterns as shown in Fig. 10b are identified to be the (100), (101), (112) diffractions of the TiC, and the (111), (220), (331), (422) diffractions of the TiN or Ti(C,N). The lattice spacings in the HRTEM image as shown in Fig. 10c are 0.197 and 0.222 nm, which are close to 0.203 and 0.229 nm for the (101) and (100) lattice spacing in Joint Committee on Powder Diffraction Standards (JCPDS) data (No. 51-0628), respectively. While the lattice fringes with d = 0.239 nm for the (111) direction can be assigned to TiN and/or Ti(C,N) (JCPDS card No. 38-1420, No. 42-1488, respectively). It is difficult to distinguish exactly TiN and Ti(C,N) nanocrystallines, since the lattice spacings of TiN and Ti(C,N)
Fig. 11. SEM micrographs of the indentations on the coatings deposited at Ti target currents of: (a) 20 A, (b) 40 A, (c) 60 A.
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Fig. 12. Microhardness and elastic modulus of the coatings deposited at different Ti target currents.
are very close because of the close atomic radius and electronegativity of N and C [37]. Fig. 10d presents more clear the lattice fringes with d = 0.210 nm assigned to TiC (101) and d = 0.246 nm assigned to TiN (111) and/or Ti(C,N) (111).
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lower, but in this study the (Ti:N)-DLC coating shows the opposite result. This mainly due to that the higher total content of tough Ti-containing phases is obtained with an increase in the Ti target current as confirmed by the EDS and XPS analysis. Fig. 13 shows the average friction coefficient and friction curves of (Ti:N)-DLC coatings sliding against Si3N4 balls in ambient air. The average friction coefficient increases from 0.086 to 0.145 with the increasing Ti target current and maintains in a relatively lower value. During each friction test, the friction coefficient of the coating maintains steady till the end of tribotest. The average friction coefficient and the friction curves of the coatings demonstrate a clear dependence between the friction coefficient and the Ti target current. The lower friction coefficient can be attributed to the low-friction N-DLC compound and the increasing friction coefficient can be explained by the increasing content of micro-droplets and particle coarsening caused by higher Ti target current as confirmed by the surface SEM images. 4. Conclusions The (Ti:N)-DLC nanomultilayered composite coatings were synthesized using an ion source assisted cathodic arc ion-plating system under different Ti target currents. The influences of Ti target currents on the microstructure, composition and the relating properties were studied systemically. The results are summarized as:
3.4. Mechanical and tribological performance Indentation fracture method is a widely accepted technique for studying the mechanical properties of coatings. The Knoop indentation impressions of the coatings are shown in Fig. 11, in which the major and minor diagonals of these impressions have different dimensions. There exhibits obvious pile-ups and semicircular radial cracks in the coating deposited at lower Ti target current. Then the pile-ups gradually disappears and the cracks become puny with increasing Ti target current. The result means that the coating with higher Ti target current performs a better ability to prevent crack formation and exhibits a higher fracture toughness, which can be attributed to the lower content of Ti(C,N) and the increasing formations of TiN and TiC [17]. Fig. 12 presents the microhardness and corresponding elastic modulus calculated by Eq. (1) as a function of Ti target current. When the Ti target current is lower than 50 A, the hardness and elastic modulus show an obvious increasing trend, then become steady with further increase of Ti target current. The coating deposited at Ti target current of 50 A has the highest hardness and elastic modulus of 27.8 GPa and 491 GPa, respectively. As is known, the hardness and elastic modulus of the amorphous carbon coating with lower sp3 bond content will be
1. The (Ti:N)-DLC nanomultilayered composite coating consists of TiN, TiC and Ti(C,N) multiphase nanocrystallines embedded into the NDLC matrix. 2. With an increase in the Ti target current, the Ti concentration in the (Ti:N)-DLC coating will ascend and the formations of TiN and TiC are more likely to be obtained. There exists larger content of sp2 bond in the (Ti:N)-DLC coatings as deposited at higher Ti target current. 3. When the Ti target current is lower than 50 A, the increase of Ti target current will enlarge the hardness and elastic modulus, while the hardness and elastic modulus remain almost unchanged with further Ti target current. In addition, higher Ti target current results in larger friction coefficient of the (Ti:N)-DLC coatings. Under a Ti target current of 50 A, the (Ti:N)-DLC coating can achieve a combination of high hardness and elastic modulus, good toughness and low friction coefficient. Acknowledgements This work was supported by the National Natural Science Foundation of China under contract No. 11275141 and 11175133, the International Cooperation Program of the Ministry of Science and Technology of the People's Republic of China under contract No. 2015DFR00720, the Center for Electron Microscopy of Wuhan University, the Ctr. of Nanosci. & Nanotech. Research of Wuhan University, the Analysis & Test Center of Wuhan University. We would like to acknowledge them for the financial and technical supports. References
Fig. 13. Average friction coefficient and friction curves of the coatings sliding against Si3N4 balls in ambient air.
[1] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R. Rep. 37 (4–6) (2002) 129–281. [2] H. Rol, P. Jörg, K. Lukas, D. Matthieu, New coatings by nanostructuring, Adv. Mater. 11 (11) (1999) 175–177. [3] P. Guo, L. Sun, X. Li, S. Xu, P. Ke, A. Wang, Structural properties and surface wettability of Cu-containing diamond-like carbon films prepared by a hybrid linear ion beam deposition technique, Thin Solid Films 584 (2015) 289–293. [4] V. Singh, J.C. Jiang, E.I. Meletis, Cr-diamond like carbon nanocomposite films: synthesis, characterization and properties, Thin Solid Films 489 (1–2) (2005) 150–158. [5] C.P. Klages, R. Memming, Microstructure and physical properties of metal-containing hydrogenated carbon films, Environ. Toxicol. Chem. 28 (3) (2008) 509–515. [6] M. Fryda, K. Taube, C.P. Klages, Nanometer indentation measurements on metalcontaining amorphous hydrogenated carbon films (Me⋅C: H), Vacuum 41 (4–6) (1990) 1291–1293.
190
Y. Cai et al. / Diamond & Related Materials 69 (2016) 183–190
[7] M.J. Mosayebi, S.R. Hosseini, Structural and tribological properties of TiC-DLC coatings deposited by RCAE-PVD at various bias voltages, Surf. Eng. 31 (2) (2015) 96–102. [8] A.A. Voevodin, J.P. O'Neill, J.S. Zabinski, Nanocomposite tribological coatings for aerospace applications, Surf. Coat. Technol. 116–119 (99) (1999) 36–45. [9] K. Bewilogua, C.V. Cooper, C. Specht, J. Schröder, R. Wittorf, M. Grischke, Effect of target material on deposition and properties of metal-containing DLC (Me-DLC) coatings, Surf. Coat. Technol. 127 (2–3) (2000) 223–231. [10] N. Nedfors, O. Tengstrand, A. Flink, A.M. Andersson, P. Eklund, L. Hultman, U. Jansson, Reactive sputtering of NbCx-based nanocomposite coatings: an up-scaling study, Surf. Coat. Technol. 253 (9) (2014) 100–108. [11] S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, B. Rafferty, L.M. Brown, J. Schwan, D.F. Franceschini, G. Mariotto, Nitrogen modification of hydrogenated amorphous carbon films, J. Appl. Phys. 81 (6) (1997) 2626–2634. [12] K. Kato, N. Umehara, K. Adachi, Friction, wear and N2-lubrication of carbon nitride coatings: a review, Wear 254 (11) (2003) 1062–1069. [13] S. Wei, T. Shao, P. Ding, Study of CNx films on 316L stainless steel for orthodontic application, Diam. Relat. Mater. 19 (5–6) (2010) 648–653. [14] R.Y. Wang, L.L. Wang, H.D. Liu, S.J. Yan, Y.M. Chen, D.J. Fu, B. Yang, Synthesis and characterization of CrCN–DLC composite coatings by cathodic arc ion-plating, Nucl. Inst. Methods Phys. Res. A 307 (7) (2013) 185–188. [15] B. Zhou, X. Jiang, A.V. Rogachev, R. Shen, D. Sun, D.G. Piliptsou, L. Lu, A comparison study between atomic and ionic nitrogen doped carbon films prepared by ion beam assisted cathode arc deposition at various pulse frequencies, Appl. Surf. Sci. 287 (2) (2013) 150–158. [16] Q. Wang, F. Zhou, Z. Zhou, Y. Yang, C. Yan, C. Wang, W. Zhang, K.Y. Li, I. Bello, S.T. Lee, Influence of carbon content on the microstructure and tribological properties of TiN(C) coatings in water lubrication, Surf. Coat. Technol. 206 (18) (2012) 3777–3787. [17] R. Chen, J.P. Tu, D.G. Liu, Y.J. Mai, C.D. Gu, Microstructure, mechanical and tribological properties of TiCN nanocomposite films deposited by DC magnetron sputtering, Surf. Coat. Technol. 205 (21−22) (2011) 5228–5234. [18] D.B. Marshall, N. Tatsuo, A.G. Evans, A simple method for determining elastic-modulus–to-hardness ratios using Knoop indentation measurements, J. Am. Ceram. Soc. 65 (10) (1982) c175–c176. [19] S.H. Leigh, C.K. Lin, C.C. Berndt, Elastic response of thermal spray deposits under indentation tests, J. Am. Ceram. Soc. 80 (8) (1997) 2093–2099. [20] J.M. Schneider, A. Voevodin, C. Rebholz, A. Matthews, J.H.C. Hogg, D.B. Lewis, M. Ives, X-ray diffraction investigations of magnetron sputtered TiCN coatings, Surface & Coatings Technology s 74–75 (95) (1995) 312–319. [21] G. Levi, W.D. Kaplan, M. Bamberger, Structure refinement of titanium carbonitride (TiCN), Mater. Lett. 35 (35) (1998) 344–350.
[22] J. Lu, U. Jansson, Chemical vapour deposition of molybdenum carbides: aspects of nanocrystallinity, Thin Solid Films 396 (1) (2001) 53–61. [23] I. Dreiling, A. Haug, H. Holzschuh, T. Chassé, Raman spectroscopy as a tool to study cubic Ti–C–N CVD coatings, Surf. Coat. Technol. 204 (6–7) (2009) 1008–1012. [24] H.C. Barshilia, K.S. Rajam, Raman spectroscopy studies on the thermal stability of TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings, J. Mater. Res. 19 (11) (2004) 3196–3205. [25] Y.R. Xu, H.D. Liu, Y.M. Chen, M.I. Yousaf, C. Luo, Q. Wan, L.W. Hu, D.J. Fu, F. Ren, Z.G. Li, In situ synthesized TiC–DLC nanocomposite coatings on titanium surface in acetylene ambient, Appl. Surf. Sci. 349 (2015) 93–100. [26] C.Y. Cheng, C.N. Hong, Growth of hydrogen-free diamond-like carbon films by a particle-free hollow-cathode arc ion plating system, Thin Solid Films 498 (1) (2006) 206–211. [27] M. Swanson, V.V. Pushkarev, V.I. Kovalchuk, J.L. d'Itri, There's more than one way to skin Schrodinger cat, Physica D 59 (59) (1992) 1–24. [28] Q. Wang, F. Zhou, Z. Zhou, Y. Yang, C. Yan, C. Wang, W. Zhang, K.Y. Li, I. Bello, S.T. Lee, Influence of Ti content on the structure and tribological properties of Ti-DLC coatings in water lubrication, Diam. Relat. Mater. 25 (2012) 163–175. [29] N. Paik, Raman and XPS studies of DLC films prepared by a magnetron sputter-type negative ion source, Surf. Coat. Technol. 200 (7) (2005) 2170–2174. [30] K.W.R. Gilkes, H.S. Sands, D.N. Batchelder, J. Robertson, W.I. Milne, Direct observation of sp3 bonding in tetrahedral amorphous carbon using ultraviolet Raman spectroscopy, Appl. Phys. Lett. 70 (70) (1997) 1980–1982. [31] L.C. Agudelo, R. Ospina, H.A. Castillo, A. Devia, Synthesis of Ti/TiN/TiCN coatings grown in graded form by sputtering dc, Phys. Scr. 2008 (19) (2008) 2517–2530. [32] M. Ahmed, J.A. Byrne, J.A.D. Mclaughlin, Glycine adsorption onto DLC and N-DLC thin films studied by XPS and AFM, e-J. Surf. Sci. Nanotechnol. 7 (7) (2009) 217–224. [33] M.A. Baker, P. Hammer, Study of the chemical composition and microstructure of ion beam-deposited CNx films including an XPS C 1s peak simulation, Surf. Interface Anal. 25 (5) (1998) 301–314. [34] X. Yan, T. Xu, G. Chen, S. Yang, H. Liu, Study of structure, tribological properties and growth mechanism of DLC and nitrogen-doped DLC films deposited by electrochemical technique, Appl. Surf. Sci. 236 (1–4) (2004) 328–335. [35] D.G. Liu, C.D. Gu, R. Chen, J.P. Tu, Interface and superhardness of TiN/CN x multilayer films, Surf. Coat. Technol. 205 (7) (2010) 2386–2392. [36] Y.X. Wang, L.P. Wan, Influence of Ti target current on microstructure and properties of Ti-doped graphite-like carbon films, Trans. Nonferrous Metals Soc. China 22 (6) (2012) 1372–1380. [37] Y.H. Lu, J.P. Wang, Y.G. Shen, Effect of N content on phase configuration, nanostructure and mechanical behaviors in Ti–Cx–Ny thin films, Appl. Surf. Sci. 255 (18) (2009) 7858–7863.
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Investigation of (Ti:N)-DLC coatings prepared by ion source assisted cathodic arc ion-plating with varying Ti target currents Y. Cai a, R.Y. Wang b, H.D. Liu a, C. Luo a, Q. Wan a, Y. Liu a, H. Chen a, Y.M. Chen a, Q.S. Mei a, B. Yang a,⁎ a b
School of Power and Mechanical Engineering, Wuhan University, 430072 Wuhan, China National Laboratory of Solid State Microstructures, Nanjing University, 210093 Nanjing, China
a r t i c l e
i n f o
Article history: Received 5 May 2016 Received in revised form 1 August 2016 Accepted 2 September 2016 Available online 4 September 2016 Keywords: (Ti:N)-DLC nanomultilayered coatings Composite Microstructure Properties
a b s t r a c t Ti and N co-doped diamond-like carbon ((Ti:N)-DLC) nanomultilayered composite coatings were synthesized by ion source assisted cathodic arc ion-plating at C2H2 and N2 ambient with varying Ti target currents. The compositions, morphologies and microstructures of the coatings were characterized by energy dispersive spectrometer (EDS), scanning electronic microscopy (SEM), atomic force microscope (AFM), X-Ray photoelectron spectrometer (XPS), Raman spectroscopy, X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Hardness and friction coefficient were tested by a hardness tester and a ball-on-disc tribometer, respectively. The (Ti:N)-DLC coating consists of uniform TiN, TiC, Ti(C,N) nanocrystallines embedded into Ndoped DLC (N-DLC) matrix. With the increase of Ti target current from 20 to 60 A, the sp2/sp3 ratio and the contents of TiN, TiC increase obviously, but the Ti(C,N) content sharply decreases. It was revealed that the coating deposited at Ti target current of 50 A exhibits the highest hardness (27.8 GPa), elastic modulus (491 GPa) and low friction coefficient (0.107). Prime novelty statement: In this study, we provide the (Ti:N)-DLC coating consisting of uniform TiN, TiC, Ti(C,N) nanocrystallines embedded into N-DLC matrix using a direct current ion source assisted cathodic arc ion-plating system, and have a detailed study about the influences of Ti target currents on the composition, phase content, microstructure and mechanical properties of (Ti:N)-DLC nanomultilayered composite coatings. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) coatings have been extensively studied for decades due to their excellent properties such as high hardness, low friction, superior wear resistance, which are highly consistent with a wide range of mechanical application as protective coatings [1]. However, the major drawbacks of DLC coating such as high residual stress, poor thermal stability have a bad effect on the workability of the coating. To optimize the properties of DLC coating, an effective way is to generate nanolayers or multiphases structure into amorphous carbon matrix [2]. Thereby, most of the scientific studies have been concentrated in metal-containing DLC (Me-DLC) coatings consisting of hard crystalline carbide and amorphous carbon phase in an effort to stabilize the coating structure, relax internal stress and improve mechanical and tribological performances [3,4]. For instance, the works of Ti [5–7], W [8,9], Cr [4] and Nb [10] containing DLC coatings have been devoted to systematically investigate the influence of incorporate metals. Mosayebi et al. deposited the TiC-DLC coating by RCAE-PVD and declared that the doped Ti increased the fracture toughness and maintained the hardness of DLC coating [7]. Furthermore, the incorporate N in DLC (N-DLC) coatings ⁎ Corresponding author. E-mail address: [email protected] (B. Yang).
http://dx.doi.org/10.1016/j.diamond.2016.09.003 0925-9635/© 2016 Elsevier B.V. All rights reserved.
can also positively improve the properties of DLC coatings. In the NDLC coatings, the C atoms can be replaced by N atoms in the aromatic ring or coalesce with N atoms to form the CNx coating which possesses many good properties such as low friction, residual stress and high corrosion resistance [11–14]. Recently, Zhou et al. reported the improved properties of N-DLC coatings in relation to carbon coatings and discussed the potential of these materials for mechanical, optical applications [15]. However, though both of the incorporate metal and N can enhance the performance of DLC coatings and have different influencing mechanism, few works have provided a detailed study about the metal and N co-doped DLC ((Me:N)-DLC) coatings. Particularly, Ti\\C\\N coatings that combine the high hardness and low fiction coefficient of TiC or CNx phases and the high toughness of TiN phases have been extensively studied as protective coatings due to their excellent properties [16,17]. Therefore, the Ti and N co-doped DLC ((Ti:N)-DLC) coating worths to own a high expectation. In this work, we fabricated (Ti:N)-DLC nanomultilayered composite coatings by incorporating Ti and N into DLC matrix with varying Ti target currents using a direct current ion source assisted cathodic arc ionplating system. The influences of Ti target currents on the composition, phase content, microstructure and mechanical properties of the coatings were systematically investigated. The aim of this work is to deposit the (Ti:N)-DLC nanomultilayered composite coating and initiate a
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systematic study of the microstructure–property relationship in (Ti:N)DLC coatings as a function of Ti target current. 2. Experimental details 2.1. Coatings deposition The (Ti:N)-DLC nanomultilayered composite coatings were deposited by a home-made hollow cathode ion source assisted cathode arc ionplating system as shown in Fig. 1. The use of hollow cathode ion source is to ensure the full ionization of gases and the restrictions of the plasma. In order to improve the adhesion of the coatings, the CrN bottom layer and TiN buffer layer were successively deposited onto silicon (100) wafers, cemented carbides substrates at N2 and Ar ambient with a Cr, Ti target, respectively. Then the (Ti:N)-DLC coatings were synthesized onto the substrates with a hollow cathode ion source and a Ti target. All the substrates were ultrasonically cleaned by acetone for 10 min, rinsed by deionized water and dried with a heater. Then the substrates were vertically placed on a rotating sample holder which was located in the center of the chamber. Prior to coating deposition, the chamber was pumped down to the pressure of 7 × 10−3 Pa and the substrates were heated up to 300 °C. A high bias voltage of −900 V was applied to the substrates and the Cr target current was set at 80 A to offer high energy ions to clean the substrates. The deposition parameters of (Ti:N)-DLC coatings are shown in Table 1. The C2H2 gas was introduced through the hollow cathode ion source and ionized by the ion source plasma. For all coating deposition process, the chamber ambient temperature was maintained at 300 °C, while the pressure of the chamber was kept at 0.5 Pa by adjusting the Ar flow rate. The deposition time of CrN, TiN and (Ti:N)-DLC layers were 5, 5, 30 min, respectively. To study the effect of Ti target current on the properties of the coatings, the Ti target currents were varied from 20 to 60 A. 2.2. Coatings characterization The surface and cross-sectional morphologies of the (Ti:N)-DLC coatings were observed by scanning electronic microscopy (SEM, FEI Sirion IMP SEM). The surface topography was probed using an atomic force microscope (AFM, Shimadzu SPM-9500J3) operated in the tapping
Table 1 Detailed deposition parameters of (Ti:N)-DLC coatings. Parameters value
Value
All target material Bias voltage (V) Reactive gas Working pressure (Pa) C2H2, N2 (sccm), respectively. Cr target current (A) (the CrN layer deposition process) Ion source current (A) Substrate temperature (°C) Rotation speed (rpm) Deposition time (min) Ti target current (A)
99.99% −200 80% duty cycle C2H2, N2 0.5 250, 100 50 60 300 4 5, 5, 30, respectively 20, 30, 40, 50, 60
mode with a measuring area of 5 × 5 μm2. The chemical composition was measured by energy dispersive spectrometer (EDS, EDAX genesis 7000 EDS system). The microstructure was examined by X-ray diffraction (XRD) performed on D8 advanced X-ray diffractometer with a Cu Kα radiation (0.154 nm) and high resolution transmission electron microscope (HRTEM, JEM-2010 FEF TEM) operated at 200 kV. A Laser Confocal Raman Micro-spectroscopy (Lab RAM HR 800 UV) with Ar+ laser excitation (632.8 nm) was used to determine the content of sp3 hybrid bond and G, D peak position. The chemical bonding state was investigated by an X-ray photoelectron spectroscopy (XPS, Kratos2AXIS2HS) using Mg Kα (1253.6 eV) X-ray radiation. The microhardness was measured on the surface of the coatings with a HXD-1000TM/LCD Vickers indenter using a Knoop hardness penetrator at a load of 100 gf. Ten points of each sample were indented and the average was taken as the hardness value. The micrographs of the indentations of different samples were observed by SEM (Zeiss Merlin Compact). The theoretical background on determining microhardness and elastic modulus of a material using the Knoop indentation testing is given by Marshall et al. [18] and Leigh et al. [19]. The elastic modulus can be calculated using Eq. (1): αH E¼ 0 b b − 0 a a
Fig. 1. Schematic diagram of the experimental setup.
ð1Þ
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Fig. 2. Surface morphologies of the coatings deposited at different Ti target currents: (a) 20 A, (b) 30 A, (c) 40 A, (d) 50 A, (e) 60 A.
where α is a constant and was taken as 0.45, H is Knoop hardness (in GPa), a′ and b′ are the lengths of the major and minor diagonals of the indentation impression, a and b are the major and minor diagonals of the ideal Knoop indentation (b/a = 1/7.11). The tribological performance was evaluated at room temperature in ambient air during dry sliding experiments conducted with a conventional ball-on-disc wear tester (MS-T3000). Here, the Si3N4 ball (99.9% purity) with 3 mm in diameter was used as mated materials. A load of 500 g and a sliding speed of 100 round per minute for 30 min were used in all experiments. The radius of the tribometer was set to 3 mm.
3. Results and discussions 3.1. Surface and cross-sectional morphologies Fig. 2 shows the surface SEM images of (Ti:N)-DLC coatings deposited at different target currents. The surfaces of the coatings contain obvious micro-droplets, which are characteristic of cathodic arc ion-plating. With the increase of the Ti target current, the distribution density and size of micro-droplets increase obviously. The representative AFM images are observed at Fig. 3, in which the rough morphologies can be
Fig. 3. AFM images of the coatings deposited at different Ti target currents: (a) 40 A, (b) 50 A, (c) 60 A.
Fig. 4. Cross-sectional images of the coatings deposited at different Ti target currents: (a) 20 A, (b) 40 A, (c) 60 A.
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Fig. 5. Chemical composition of the coatings deposited at different Ti target currents.
Fig. 6. XRD patterns of the coatings deposited at different Ti target currents.
3.3. Microstructure found on the surface. The root mean square (RMS) roughness of coatings deposited at 40, 50, 60 A are 13.1, 15.9, 25.8 nm, respectively. The cross-sectional images of (Ti:N)-DLC coating are shown in Fig. 4, in which the coatings exhibit very dense structure and homogeneous morphologies. At the Ti target current of 40 A, the thickness of total buffer layer and function layer are 0.6, 1.1 um, respectively.
3.2. Chemical composition The chemical compositions of the as-deposited coatings analyzed by EDS are shown in Fig. 5. As the Ti target current increases from 20 to 60 A, the C content in the (Ti:N)-DLC coatings decreases gradiently from 76.8 to 57.4 at.% with a simultaneous increase of N content from 16.0 to 20.8 at.%. However, with an increase in the Ti target current from 20 to 50 A, the Ti content first increases from 7.2 to 22.4 at.%, and then maintains the almost same value at higher Ti target current of 60 A. This indicates the (Ti:N)-DLC coatings deposited at the higher Ti target current contain higher Ti content when the Ti target current less than 50 A, and as the Ti target current increases further, the Ti content will reach saturation. The variation of Ti, C and N contents shows that the higher Ti target current will promote the formation of Ti\\N, which can be explained by that Ti atoms are more likely to bond with N rather than C to generate TiN compound [20]. These can be proved by the following XPS analysis.
Fig. 6 shows the XRD patterns of the coatings deposited under different Ti target currents. The characteristic peaks of TiC (100) and TiC (101) is identified at 2θ = 39.9°, 44.4°, respectively. Moreover, the peaks of TiN (111) and/or Ti(C,N) (111) are identified in the coatings at 2θ = 37.1°. It is difficult to distinguish TiN and Ti(C,N) since the atom radius of C and N are quite similar [21]. A certain amount of C atoms exist as interstitial atoms in the TiN structure to form Ti(C,N), and the same situation is reflected in the research [17]. The TiN (111) and/or Ti(C,N) (111) peaks in the diffraction patterns are unconspicuous that can be attributed to several factors such as small grain size or the influence of intense substrate peaks [22]. Fig. 7a shows the Raman spectra of (Ti:N)-DLC coatings deposited at different Ti target currents. The Raman spectra of the coatings can be divided into two regions. One from 300 to 800 cm−1 corresponds to titanium carbide/nitride while another from 900 to 1800 cm− 1 corresponds to amorphous carbon [16]. In the range of Raman shift from 300 to 800 cm−1, there are three peaks at ~ 423, ~ 547, ~634 cm−1 that can be assigned to the Ti(C,N), TiN and TiC, respectively [23–25]. When the Ti target current is higher than 40 A, the Ti(C,N) peak obviously shifts to lower frequencies with higher Ti target currents, which could be due to that the reduction of the C interstitial atoms in Ti(C,N) phase results in the decreasing formation of Ti(C,N) [23]. Furthermore, the region of Raman shift from 300 to 800 cm−1 shows significant characteristic bands (D and G bands), which indicates the
Fig. 7. (a) Raman spectra and (b) ID/IG ratio and Raman shifts of G, D peaks of the coatings.
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Fig. 8. XPS core level spectra of the coatings deposited at different Ti target currents: (a) Ti 2p, (b) C 1s, (c) N 1s.
presence of amorphous carbon phase in the coatings [26]. The G band centered at ~1576 cm−1 corresponds to symmetric E2g C\\C stretching mode in graphite-like materials, while the D peak at ~1360 cm−1 is to the bond angle disorder in the graphite-like microdomains [27]. As seen in Fig. 7a, the intensities of D and G peaks decrease drastically when the Ti target current is beyond 30 A. This indicates that the
Table 2 The binding energies of the Ti 2p, C 1s, N 1s. Element/Transition
Binding energy (eV)
Compound
Ti 2p
454.9/460.6 455.5/461.4 456.8/462.9 485.6/464.4 281.9 284.9 284.6 285.5 286.6 287.2 288.2 396.8 397.2 398.4 400.2 402.4
TiC TiN Ti(C,N) Ti\ \O TiC Ti(C,N) \C sp2 C\ \C sp3 C\ \N sp2 C\ 3 \N sp C\ C\ \O Ti(C,N) TiN N-sp3 C N-sp2 C N\ \O
C 1s
N 1s
variation of Ti target current will influence the formation of sp2 and sp3 bondings in the amorphous carbon host matrix [28]. In order to perform more detailed analysis of Raman spectra, Raman spectra of the coatings were deconvoluted using Gaussian function to calculate the peak positions of G, D and the ratio of the D and G band intensity (ID/ IG) presented in the different (Ti:N)-DLC coatings. Fig. 7b presents the curves-fitted Raman data. It is obvious that the D and G peaks shift slightly to lower wave numbers for all coatings with the increase of Ti target current from 20 to 60 A. This phenomenon was also reported in Ref. [28]. In addition, the sp3/sp2 ratio can be characterized by the ID/IG ratio, and the increasing sp3 fraction in the amorphous carbon coatings will decrease the ID/IG ratio [29,30]. The ratio of ID/IG increases from 4.74 to 9.49 with the increase of Ti target current from 20 to 60 A, which indicates that the increase of Ti target current will led to an increase in the formation of sp2 phase and reveal the graphitic characteristic of the (Ti:N)-DLC coatings. The chemical bonding states of (Ti:N)-DLC coatings were characterized by XPS. In order to further analyse the effect of Ti target current on the bonding structure of the coatings, the XPS data were fitted with Gaussian function in an attempt to determine the fraction of each bonding type, and the deconvolution results of the Ti 2p, C 1s and N 1s XPS core level spectra of the (Ti:N)-DLC coatings are shown in Fig. 8 and Table 2. In the Ti 2p spectra as shown in Fig. 8a, the coatings exhibit similar Ti 2p3/2 and Ti 2p1/2 peaks at about 453–460 eV and 460–466 eV, respectively. The Ti 2p spectra can be peak-fitted into eight Gaussian functions ascribed to TiC, TiN, Ti(C,N) and Ti\\O bonds [16,28,31]. The
Fig. 9. (a) TiN, TiC and Ti(C,N) content from fitting Ti 2p spectra and (b) sp3/sp2 ratio from fitting C 1s spectra.
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Fig. 10. Cross-sectional TEM images (a), the corresponding SAED pattern (b) and HRTEM images (c), (d) of the coating deposited at 50 A.
strong Ti\\O peaks at ~458.6 and 464.4 eV can be due to a gradual oxidation taking place in the first few days once the coatings were exposed to air. As seen in Fig. 8b, all C 1s XPS spectra consist of seven peaks at the binding energy range of 280–292 eV, which can be assigned to TiC, Ti(C,N), sp2 C\\C, sp3 C\\C, sp2 C\\N, sp3 C\\N and C\\O bonds, respectively [32,33]. In the meanwhile, for all the N 1s spectra, the presence of five peaks can be attributed to TiN, Ti(C,N), N-sp2 C, N-sp3 C and N\\O bonds, respectively [33,34] . Fig. 9 is the content of the TiN, TiC and Ti(C,N) phase derived from the Ti 2p spectra, and the ratio of sp3/sp2 determined from the C 1s spectra. It reveals that when the Ti target current increases from 20 to 60 A, the content of TiN apparently increases from 9.7 to 31.6 at.% with a simultaneous increase of TiC from 8.2 to 19.8 at.% while that of Ti(C,N) content becomes low obviously. It is confirmed that the increase of Ti target current will prevent the C atoms to substitute the N atoms in the TiN lattice to form Ti(C,N) and can be a favor to obtain TiN and TiC phases. Since less Gibbs free energy is needed of Ti reacting with N than C, the Ti atom is more likely to bond with N atom not C atom to generate TiN compound [35]. Furthermore, as the Ti target current increases from 20 to 60 A, the ratio of sp3/sp2 decreases from 1.1 to 0.33. It can be explained that higher target current supplies
more Ti ions incorporated in the coatings which can break some of the sp3 bonds, and lead to the formation of a stable sp2-bonded phase [36]. Fig. 10 shows the cross-sectional TEM images and the corresponding selected area electron diffraction (SAED) pattern of the (Ti:N)-DLC coating deposited on a Si substrate at 50 A. The nanomultilayered composite structure consisting of the uniform nanocrystallines and amorphous NDLC can be clearly observed in Fig. 10a and c, in which the thickness of each layer is found to be approximately 4 nm. The formation of the nanomultilayered composite structure is due to the rotation of the substrates during the deposition process. The distinct rings displayed in the SAED patterns as shown in Fig. 10b are identified to be the (100), (101), (112) diffractions of the TiC, and the (111), (220), (331), (422) diffractions of the TiN or Ti(C,N). The lattice spacings in the HRTEM image as shown in Fig. 10c are 0.197 and 0.222 nm, which are close to 0.203 and 0.229 nm for the (101) and (100) lattice spacing in Joint Committee on Powder Diffraction Standards (JCPDS) data (No. 51-0628), respectively. While the lattice fringes with d = 0.239 nm for the (111) direction can be assigned to TiN and/or Ti(C,N) (JCPDS card No. 38-1420, No. 42-1488, respectively). It is difficult to distinguish exactly TiN and Ti(C,N) nanocrystallines, since the lattice spacings of TiN and Ti(C,N)
Fig. 11. SEM micrographs of the indentations on the coatings deposited at Ti target currents of: (a) 20 A, (b) 40 A, (c) 60 A.
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Fig. 12. Microhardness and elastic modulus of the coatings deposited at different Ti target currents.
are very close because of the close atomic radius and electronegativity of N and C [37]. Fig. 10d presents more clear the lattice fringes with d = 0.210 nm assigned to TiC (101) and d = 0.246 nm assigned to TiN (111) and/or Ti(C,N) (111).
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lower, but in this study the (Ti:N)-DLC coating shows the opposite result. This mainly due to that the higher total content of tough Ti-containing phases is obtained with an increase in the Ti target current as confirmed by the EDS and XPS analysis. Fig. 13 shows the average friction coefficient and friction curves of (Ti:N)-DLC coatings sliding against Si3N4 balls in ambient air. The average friction coefficient increases from 0.086 to 0.145 with the increasing Ti target current and maintains in a relatively lower value. During each friction test, the friction coefficient of the coating maintains steady till the end of tribotest. The average friction coefficient and the friction curves of the coatings demonstrate a clear dependence between the friction coefficient and the Ti target current. The lower friction coefficient can be attributed to the low-friction N-DLC compound and the increasing friction coefficient can be explained by the increasing content of micro-droplets and particle coarsening caused by higher Ti target current as confirmed by the surface SEM images. 4. Conclusions The (Ti:N)-DLC nanomultilayered composite coatings were synthesized using an ion source assisted cathodic arc ion-plating system under different Ti target currents. The influences of Ti target currents on the microstructure, composition and the relating properties were studied systemically. The results are summarized as:
3.4. Mechanical and tribological performance Indentation fracture method is a widely accepted technique for studying the mechanical properties of coatings. The Knoop indentation impressions of the coatings are shown in Fig. 11, in which the major and minor diagonals of these impressions have different dimensions. There exhibits obvious pile-ups and semicircular radial cracks in the coating deposited at lower Ti target current. Then the pile-ups gradually disappears and the cracks become puny with increasing Ti target current. The result means that the coating with higher Ti target current performs a better ability to prevent crack formation and exhibits a higher fracture toughness, which can be attributed to the lower content of Ti(C,N) and the increasing formations of TiN and TiC [17]. Fig. 12 presents the microhardness and corresponding elastic modulus calculated by Eq. (1) as a function of Ti target current. When the Ti target current is lower than 50 A, the hardness and elastic modulus show an obvious increasing trend, then become steady with further increase of Ti target current. The coating deposited at Ti target current of 50 A has the highest hardness and elastic modulus of 27.8 GPa and 491 GPa, respectively. As is known, the hardness and elastic modulus of the amorphous carbon coating with lower sp3 bond content will be
1. The (Ti:N)-DLC nanomultilayered composite coating consists of TiN, TiC and Ti(C,N) multiphase nanocrystallines embedded into the NDLC matrix. 2. With an increase in the Ti target current, the Ti concentration in the (Ti:N)-DLC coating will ascend and the formations of TiN and TiC are more likely to be obtained. There exists larger content of sp2 bond in the (Ti:N)-DLC coatings as deposited at higher Ti target current. 3. When the Ti target current is lower than 50 A, the increase of Ti target current will enlarge the hardness and elastic modulus, while the hardness and elastic modulus remain almost unchanged with further Ti target current. In addition, higher Ti target current results in larger friction coefficient of the (Ti:N)-DLC coatings. Under a Ti target current of 50 A, the (Ti:N)-DLC coating can achieve a combination of high hardness and elastic modulus, good toughness and low friction coefficient. Acknowledgements This work was supported by the National Natural Science Foundation of China under contract No. 11275141 and 11175133, the International Cooperation Program of the Ministry of Science and Technology of the People's Republic of China under contract No. 2015DFR00720, the Center for Electron Microscopy of Wuhan University, the Ctr. of Nanosci. & Nanotech. Research of Wuhan University, the Analysis & Test Center of Wuhan University. We would like to acknowledge them for the financial and technical supports. References
Fig. 13. Average friction coefficient and friction curves of the coatings sliding against Si3N4 balls in ambient air.
[1] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R. Rep. 37 (4–6) (2002) 129–281. [2] H. Rol, P. Jörg, K. Lukas, D. Matthieu, New coatings by nanostructuring, Adv. Mater. 11 (11) (1999) 175–177. [3] P. Guo, L. Sun, X. Li, S. Xu, P. Ke, A. Wang, Structural properties and surface wettability of Cu-containing diamond-like carbon films prepared by a hybrid linear ion beam deposition technique, Thin Solid Films 584 (2015) 289–293. [4] V. Singh, J.C. Jiang, E.I. Meletis, Cr-diamond like carbon nanocomposite films: synthesis, characterization and properties, Thin Solid Films 489 (1–2) (2005) 150–158. [5] C.P. Klages, R. Memming, Microstructure and physical properties of metal-containing hydrogenated carbon films, Environ. Toxicol. Chem. 28 (3) (2008) 509–515. [6] M. Fryda, K. Taube, C.P. Klages, Nanometer indentation measurements on metalcontaining amorphous hydrogenated carbon films (Me⋅C: H), Vacuum 41 (4–6) (1990) 1291–1293.
190
Y. Cai et al. / Diamond & Related Materials 69 (2016) 183–190
[7] M.J. Mosayebi, S.R. Hosseini, Structural and tribological properties of TiC-DLC coatings deposited by RCAE-PVD at various bias voltages, Surf. Eng. 31 (2) (2015) 96–102. [8] A.A. Voevodin, J.P. O'Neill, J.S. Zabinski, Nanocomposite tribological coatings for aerospace applications, Surf. Coat. Technol. 116–119 (99) (1999) 36–45. [9] K. Bewilogua, C.V. Cooper, C. Specht, J. Schröder, R. Wittorf, M. Grischke, Effect of target material on deposition and properties of metal-containing DLC (Me-DLC) coatings, Surf. Coat. Technol. 127 (2–3) (2000) 223–231. [10] N. Nedfors, O. Tengstrand, A. Flink, A.M. Andersson, P. Eklund, L. Hultman, U. Jansson, Reactive sputtering of NbCx-based nanocomposite coatings: an up-scaling study, Surf. Coat. Technol. 253 (9) (2014) 100–108. [11] S.R.P. Silva, J. Robertson, G.A.J. Amaratunga, B. Rafferty, L.M. Brown, J. Schwan, D.F. Franceschini, G. Mariotto, Nitrogen modification of hydrogenated amorphous carbon films, J. Appl. Phys. 81 (6) (1997) 2626–2634. [12] K. Kato, N. Umehara, K. Adachi, Friction, wear and N2-lubrication of carbon nitride coatings: a review, Wear 254 (11) (2003) 1062–1069. [13] S. Wei, T. Shao, P. Ding, Study of CNx films on 316L stainless steel for orthodontic application, Diam. Relat. Mater. 19 (5–6) (2010) 648–653. [14] R.Y. Wang, L.L. Wang, H.D. Liu, S.J. Yan, Y.M. Chen, D.J. Fu, B. Yang, Synthesis and characterization of CrCN–DLC composite coatings by cathodic arc ion-plating, Nucl. Inst. Methods Phys. Res. A 307 (7) (2013) 185–188. [15] B. Zhou, X. Jiang, A.V. Rogachev, R. Shen, D. Sun, D.G. Piliptsou, L. Lu, A comparison study between atomic and ionic nitrogen doped carbon films prepared by ion beam assisted cathode arc deposition at various pulse frequencies, Appl. Surf. Sci. 287 (2) (2013) 150–158. [16] Q. Wang, F. Zhou, Z. Zhou, Y. Yang, C. Yan, C. Wang, W. Zhang, K.Y. Li, I. Bello, S.T. Lee, Influence of carbon content on the microstructure and tribological properties of TiN(C) coatings in water lubrication, Surf. Coat. Technol. 206 (18) (2012) 3777–3787. [17] R. Chen, J.P. Tu, D.G. Liu, Y.J. Mai, C.D. Gu, Microstructure, mechanical and tribological properties of TiCN nanocomposite films deposited by DC magnetron sputtering, Surf. Coat. Technol. 205 (21−22) (2011) 5228–5234. [18] D.B. Marshall, N. Tatsuo, A.G. Evans, A simple method for determining elastic-modulus–to-hardness ratios using Knoop indentation measurements, J. Am. Ceram. Soc. 65 (10) (1982) c175–c176. [19] S.H. Leigh, C.K. Lin, C.C. Berndt, Elastic response of thermal spray deposits under indentation tests, J. Am. Ceram. Soc. 80 (8) (1997) 2093–2099. [20] J.M. Schneider, A. Voevodin, C. Rebholz, A. Matthews, J.H.C. Hogg, D.B. Lewis, M. Ives, X-ray diffraction investigations of magnetron sputtered TiCN coatings, Surface & Coatings Technology s 74–75 (95) (1995) 312–319. [21] G. Levi, W.D. Kaplan, M. Bamberger, Structure refinement of titanium carbonitride (TiCN), Mater. Lett. 35 (35) (1998) 344–350.
[22] J. Lu, U. Jansson, Chemical vapour deposition of molybdenum carbides: aspects of nanocrystallinity, Thin Solid Films 396 (1) (2001) 53–61. [23] I. Dreiling, A. Haug, H. Holzschuh, T. Chassé, Raman spectroscopy as a tool to study cubic Ti–C–N CVD coatings, Surf. Coat. Technol. 204 (6–7) (2009) 1008–1012. [24] H.C. Barshilia, K.S. Rajam, Raman spectroscopy studies on the thermal stability of TiN, CrN, TiAlN coatings and nanolayered TiN/CrN, TiAlN/CrN multilayer coatings, J. Mater. Res. 19 (11) (2004) 3196–3205. [25] Y.R. Xu, H.D. Liu, Y.M. Chen, M.I. Yousaf, C. Luo, Q. Wan, L.W. Hu, D.J. Fu, F. Ren, Z.G. Li, In situ synthesized TiC–DLC nanocomposite coatings on titanium surface in acetylene ambient, Appl. Surf. Sci. 349 (2015) 93–100. [26] C.Y. Cheng, C.N. Hong, Growth of hydrogen-free diamond-like carbon films by a particle-free hollow-cathode arc ion plating system, Thin Solid Films 498 (1) (2006) 206–211. [27] M. Swanson, V.V. Pushkarev, V.I. Kovalchuk, J.L. d'Itri, There's more than one way to skin Schrodinger cat, Physica D 59 (59) (1992) 1–24. [28] Q. Wang, F. Zhou, Z. Zhou, Y. Yang, C. Yan, C. Wang, W. Zhang, K.Y. Li, I. Bello, S.T. Lee, Influence of Ti content on the structure and tribological properties of Ti-DLC coatings in water lubrication, Diam. Relat. Mater. 25 (2012) 163–175. [29] N. Paik, Raman and XPS studies of DLC films prepared by a magnetron sputter-type negative ion source, Surf. Coat. Technol. 200 (7) (2005) 2170–2174. [30] K.W.R. Gilkes, H.S. Sands, D.N. Batchelder, J. Robertson, W.I. Milne, Direct observation of sp3 bonding in tetrahedral amorphous carbon using ultraviolet Raman spectroscopy, Appl. Phys. Lett. 70 (70) (1997) 1980–1982. [31] L.C. Agudelo, R. Ospina, H.A. Castillo, A. Devia, Synthesis of Ti/TiN/TiCN coatings grown in graded form by sputtering dc, Phys. Scr. 2008 (19) (2008) 2517–2530. [32] M. Ahmed, J.A. Byrne, J.A.D. Mclaughlin, Glycine adsorption onto DLC and N-DLC thin films studied by XPS and AFM, e-J. Surf. Sci. Nanotechnol. 7 (7) (2009) 217–224. [33] M.A. Baker, P. Hammer, Study of the chemical composition and microstructure of ion beam-deposited CNx films including an XPS C 1s peak simulation, Surf. Interface Anal. 25 (5) (1998) 301–314. [34] X. Yan, T. Xu, G. Chen, S. Yang, H. Liu, Study of structure, tribological properties and growth mechanism of DLC and nitrogen-doped DLC films deposited by electrochemical technique, Appl. Surf. Sci. 236 (1–4) (2004) 328–335. [35] D.G. Liu, C.D. Gu, R. Chen, J.P. Tu, Interface and superhardness of TiN/CN x multilayer films, Surf. Coat. Technol. 205 (7) (2010) 2386–2392. [36] Y.X. Wang, L.P. Wan, Influence of Ti target current on microstructure and properties of Ti-doped graphite-like carbon films, Trans. Nonferrous Metals Soc. China 22 (6) (2012) 1372–1380. [37] Y.H. Lu, J.P. Wang, Y.G. Shen, Effect of N content on phase configuration, nanostructure and mechanical behaviors in Ti–Cx–Ny thin films, Appl. Surf. Sci. 255 (18) (2009) 7858–7863.