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DLC coated carburized layer on steel
Diamond & Related Materials 70 (2016) 18–25
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
One-step plasma-assisted method for functionally graded Fe3O4/DLC coated carburized layer on steel Y. Yang a, M.F. Yan a,⁎, S.D. Zhang a, Y.X. Zhang a, H.T. Chen a,b, X.A. Wang a a b
National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 30 June 2016 Received in revised form 4 August 2016 Accepted 23 September 2016 Available online 24 September 2016 Keywords: Diamond-like carbon Wear Surface characterization Tribology
a b s t r a c t A Fe3O4/Diamond-like carbon (DLC) coated carburized layer on M50NiL steel surface was fabricated by onestep plasma-assisted method in gaseous acetone at 550 °C. The surface layer showed high hardness (13.94 ± 0.92 GPa) and elastic modulus (314.18 ± 9.25 GPa) due to synergistic effect of rigid DLC and mild Fe 3O 4 . The carburized layer with gradual hardness distribution served as supporting layer. The Fe3O4/DLC-containing carburized layer showed lower friction coefficient (about 0.25) and 97.6% reduction in wear rate (1.53 × 10−6 mm3 N−1 m−1) compared with untreated specimen due to the lubricating effect and gradient multilayer structure. The tribological properties demonstrated that the treatment in present work has great advantages over other surface treatment. Moreover, the formation mechanism of the functionally graded carburized layer was discussed. The results pointed out a simple way to greatly improve wear resistance for steels. © 2016 Published by Elsevier B.V.
1. Introduction The diamond-like carbon (DLC) coatings have elicited considerable attention owing to their potential as advanced solid lubricant coatings with low friction coefficient and chemical inertness. The DLC coatings are used to describe a range of amorphous carbon coatings which include the tetrahedral DLC coating (ta-C, ta-C:H) with significant fraction of sp3 bonds [1], the DLC coating (a-C, a-C:H) containing high percentage of sp2 bonds [2] and those containing non-metal [3] or metal dopants [4–6]. Although DLC coatings have broad industrial applications, especially in optical [7] and electronic areas [8], there exist unresolved problems that have to be tackled. One of the critical issues is that the coating thickness is limited by the high residual stress [6,9,10]. Thus, thin and hard carbon coatings exhibit poor adhesion to the substrate and consequent limited load-bearing capacity. Many efforts have been made to decrease the intrinsic residual stress of carbon coatings and improve their adhesion to substrates, such as elements doping [6], interlayers incorporating [11] and graded multilayer structure [12–15]. For example, strategies combining chemical heat treatment and DLC deposition have been employed to improve adhesion and fatigue strength [12], decrease friction coefficient and wear rate [14], prevent the delamination failure and further extend coating life [15]. However, the multilayer or multicomponent technique is limited due to complicated process. Therefore, it will be of great significance if the multilayer or multicomponent layer could be formed in one-step process. Moreover, ⁎ Corresponding author. E-mail address: [email protected] (M.F. Yan).
http://dx.doi.org/10.1016/j.diamond.2016.09.020 0925-9635/© 2016 Published by Elsevier B.V.
previous investigation shows that Fe3O4 nanoparticles have good lubricating effects [16]. Until now, reports about fabrication of Fe3O4/DLCcontaining carburized layer by one-step method are not available. Quenching and tempering are traditional heat treatment for steels due to excellent combination of hardness, toughness and wear properties. Therefore, quenching is the pretreatment for the steel in the present work. Then, we innovatively applied gaseous acetone to obtain a Fe3O4/DLC-containing carburized layer on the steel by plasma-assisted method in one-step at 550 °C for 4 h, resulting in effects of surface treatment and tempering simultaneously. The microstructures, chemical composition, mechanical and tribological properties of the functionally graded layer were analyzed. Moreover, the formation mechanism of the composite layer was discussed.
2. Experimental procedure 2.1. Materials The as-received M50NiL steel with the following chemical composition of 0.13C, 4.1Cr, 3.4Ni, 4.2Mo, 1.2 V, 0.13Mn, 0.18Si, 0.012P, 0.002S and balance Fe (wt.%) was used. The samples were firstly solution treated at 1150o for 1 h and then quenched in oil. The quenched specimens were identified as untreated one. Prior to plasma treatment, the flat surfaces of the specimens with 12 mm × 12 mm × 5 mm dimensions were ground from 240 to 800 grade using SiC papers, and then ultrasonically cleaned with alcohol and acetone in succession to clean the surface of the steel.
Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25
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hydrogen (0.2 L/min) and a trace of argon were introduced into the chamber when the temperature achieves 550 ± 5 °C. The hydrogen and argon were used to increase temperature and prevent soot formation. The gaseous acetone acted as the source of active carbon atoms. After the treatment for 4 h, the treated specimens were directly cooled to room temperature in the furnace under a flow of acetone gas.
2.3. Characterization
Fig. 1. Schematic of plasma treatment system.
2.2. Plasma treatment The specimens were placed in a pulsed glow discharge plasma unit (LDMC-30, 30 kW), as shown in Fig. 1. Sputtering pre-treatment using hydrogen under a pulsed dc voltage (U = 660 V) during the heatingup stage was carried out to remove the passive film and activate the surface. Pure acetone was heated in water bath to 40 °C to increase the rate of evaporation. A gas mixture containing gaseous acetone (0.3 L/min),
The phase structure of the modified layer was analyzed by X-ray diffractometer (XRD, type D/max-rB) with Cu-Kα radiation (λ = 0.15406 nm) in the range of glancing angles 20–100o. The elemental compositions and chemical binding states were analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250). Raman measurements were performed on an Invia Raman microscope (Renishaw, UK) with excitation laser beam wavelength of 532 nm. Transmission electron microscopy (TEM, type Philips CM12) and scanning electron microscope (SEM, type S-4700) and optical microscope (OM, CMM-33E) were employed to observe the morphology of surface layer. The microhardness of the modified layer was measured inward from 2 um underneath surface to the core by Vickers hardness tester (type HV-1000) under a load of 100 g and a dwelling time of 15 s. Three indentations at the same depth were taken and the mean value was used to ensure a high accuracy. Nano-indentation tests were performed in the surface of the samples using a nano-indenter XP (MTS) equipped with a Berkovich diamond indenter. Five indentations depth were taken and the mean value was used to ensure a high accuracy. The dry wear properties of the specimens were evaluated by pin-on-disc tribometer (type POD1). The test was carried out in ambient air of 60% RH at ambient temperature (28 °C). During the test, the specimens were rotated against a stationary WC ball of 5 mm diameter at the speed of 200 r/min for 1800 s
Fig. 2. (a) SEM and OM micrographs of the cross-section (b) XRD patterns of the surface layers (c)(d) TEM micrographs and SAED patterns of the surface layer.
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Fig. 3. Characterization of the surface layer (a) Raman spectrum (b) Typical XPS profile (c) C 1s XPS spectra (d) Fe 2p XPS spectra.
under 10 N load. The wear rate η (mm3 N−1 m−1) was calculated by according to the following Eq. (1): η¼
V FL
ð1Þ
where F (N) is the normal contact load and L (m) is the total sliding distance. V (mm3) is the wear volume measured by the Eq. (2): V ¼ πDS
ð2Þ
D(mm) is the diameter of the circular wear scar. S(mm2) is the crosssectional area of the wear scar profile measured by a stylus
profilometer (CCI MP, Taylor Hobson). The morphologies of worn surface were observed by SEM equipped with energy dispersive X-ray analyzer (EDS). 3. Results and discussion 3.1. Microstructure and phase structures of surface layer The OM micrograph of the etched cross-sectional microstructure for the plasma treated specimen is shown in Fig. 2 (a). The lath martensite is the main phase for the surface layer. Most importantly, a thin and continuous layer appears ‘white’ on the treated surface, which is confirmed
Fig. 4. (a) Microhardness profile of the carburized layer (b) The load-displacement curves of the untreated and plasma treated specimen under nano-indentation tests.
Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25 Table 1 Hardness H, elastic modulus E, total deformation work Wt, plastic deformation work Wp, plasticity factor ηp and the ratios of H/E of the surface of the untreated and plasma treated samples. Sample
H (GPa)
E (GPa)
Wt (nJ)
Wp (nJ)
ηp (%)
H/E
Untreated Plasma treated
5.94 13.94
221.41 314.18
155.55 371.23
127.65 248.39
82.06 66.91
0.0268 0.0444
by SEM micrograph shown in the top right corner of Fig. 2 (a). It indicates that the layer is resistant to the corrosive attack by the 4% Nital solution. According to the X-ray diffraction patterns shown in Fig. 2 (b), the surface layer is mainly composed of Fe3C phase (JCPDS file 652412), carbon expanded martensite (α'C) (JCPDS file 44-1292) and a few of Fe3O4 phase (JCPDS file 65-3107). Compared with the untreated sample, the α'C phase peaks are fairly broadened, which may due to residual stress [17] or the large amount of carbon incorporated in the phase. Similar results by introducing nitrogen in martensite phase were shown in [18–20]. In fact, Fe3C layer is supposed to be compound layer while α'C phase attributes to the subsurface, i.e., diffusion layer because the compound layer is too thin (about 1 μm) (Fig. 2 (a)) to block the X-ray diffraction. However, the elements analysis show high carbon content (45 at.%) on the surface which imply a carbon-rich layer, rather than a single Fe3C (25 at.%) layer. The non-reported result of grazing incidence XRD (GIXRD) at 1° incidence angle proves existence of the amorphous carbon. Hence, the white layer on the surface is actually a Fe3O4/amorphous carbon coated Fe3C layer. The amorphous structure and selected-area electron diffraction (SAED) patterns of DLC on the surface are shown in Fig. 2 (c) and confirmed by the corresponding SAED pattern with a diffuse halo. Three diffraction rings correspond to d-spacing of 0.102 nm, 0.124 nm and 0.209 nm, which is consistent with the previous reports [21,22]. (311), (220) and (111) crystal planes are the strongest diffraction for crystal diamond and correspond to d = 0.107, d = 0.126 and 0.206 nm,
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indicating a diamond-like structure. Combining the SAED pattern given in the inset and EDS composition analysis, the nanoparticles on the surface layer shown in Fig. 2 (d) can be identified as Fe3C and Fe3O4 phase. Diffraction rings corresponding to d-spacing of 0.210 nm, and 0.135 nm is consistent with the diffraction rings of (211) and (123) crystal planes for orthorhombic Fe3C. Other diffraction rings corresponding to d-spacing of 0.253 nm and 0.296 nm may be ascribed to (311) and (220) crystal planes of Fe3O4. Thus, the SAED patterns are in good agreement with results of XRD. 3.2. Phase constituents and bonding states of surface layer Raman measurements are carried out to characterize the surface layer, shown in Fig. 3 (a). The Raman spectra peaks at around 1350 cm-1 and 1600 cm-1 corresponds to the D and G bands, respectively, which can be attributed to the breathing mode of sp2 atoms in rings and the stretching mode of all pairs of sp2 carbon atoms in both rings and chains, respectively [23]. The D band and G band shown in Fig. 3 (a) for plasma treated specimen indicate the presence of the amorphous carbon on the surface [24,25]. Moreover, the peak located at 670 cm− 1 is corresponding to the mixture of FeO(OH) and Fe3O4 phases [26], which is in consistent with the XRD result. The XPS analysis was employed to estimate the surface bonding states in detail. Fig. 3 (b) show the typical XPS profile for the surface of plasma treated specimen. The peaks located at around 284.8, 532.8 and 714.1 eV are related to the photoelectrons excited from the C1s O1s and Fe2p core levels, respectively [25,27]. Fig. 3 (c) exhibits the XPS carbon core level spectra (C 1 s) of the surface layer, which could be used to analyze the sp3 fraction in the carbon coating. The full width at half maximum (FWHM) of the C1s spectra of the surface is about 1.5 eV, larger than that of graphite (FWHM = 0.6 eV) and diamond samples (FWHM = 1.0 eV) [6]. Therefore, the C1s XPS spectra are deconvoluted into three distinct Gaussian–Lorentzian peaks, which are located at binding energy of 286.6 ± 0.2, 285.1 ± 0.1,
Fig. 5. (a) Frictional coefficient curves of the untreated and plasma treated specimens (b) (c) (d) Wear scars profile of the untreated and plasma treated specimens.
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Table 2 Summary of frictional and wear test results for M50NiL steel or specimens treated by carburizing. Materials
Treatment process
Counter surface
Contact load
COF
Wear rate (mm3 m−1 N−1)
Conditions
Ref.
M50NiL M50NiL M50NiL Fe T8 AISI410 AISI 304 M50NiL
Plasma nitrocarburizing Plasma nitrocarburizing with rare earths Plasma nitrocarburizing Plasma electrolytic carburizing Plasma electrolytic carburizing Plasma carburizing Plasma carburizing + DLC Present treatment
WC WC WC Al2O3 ZrO2 WC AISI 52100 WC
10 10 10 10 5 10 1.96 10
About 0.5 About 0.44 0.58 ~ 0.65 0.6 ~ 1.0 0.3 ~ 0.35 – 0.2 0.25
4.333 × 10−5 4.056 × 10−5 5.95 ~ 8.25 × 10−5 1 ~ 7 × 10−5 1.13 × 10−5 4.4 × 10−6 8.29 × 10−5 1.53 × 10−6
Air, RT Air, RT Air, RT Air, RT Air, RT Air, RT Air, RT Air, RT
[35] [35–36] [37] [38] [25] [39] [13] Present work
284.3 ± 0.1 eV, representing C\\O bond, sp3 hybridization of carbon bond and sp2 hybridization of carbon bond, respectively [6,27]. The sp3 hybridized carbon atom fraction is estimated as the ratio of the corresponding peak area to the total C1s peak area. The percentage of sp3 hybridized carbon atom is about 53% for the amorphous carbon, which could be termed as diamond-like carbon. Fig. 3 (d) exhibits the XPS Fe core level spectra (Fe2p) of the surface layer. Fe 2p peaks located at binding energy of 707.0 ± 0.5 and 708.2 ± 0.5 eV corresponds to Fe metal and Fe3C, respectively [28]. The binding energy of 710.0– 712.7 eV mainly represents Fe\\O bonds [28], in consistent with Fe3O4 shown in XRD and TEM. 3.3. Mechanical properties of Fe3O4/DLC-containing carburized layer Fig. 4 (a) shows the microhardness profile for the carburized layers of the specimen. A average hardness gradient from surface layer to matrix is observed, which is beneficial to eliminating stress concentration and improving the fatigue performance [12]. The hardness of the carburized layer at distance of 2 μm from the surface is above 1000 HV, which is beneficial to wear resistance. Moreover, the carburized layer could be considered as supporting layer for diamond-like carbon, which benefits to adhesion with diamond-like carbon and thus improve wear
resistance performance and load-bearing capacity [12]. The substrate hardness is about 405 HV and is lower than the untreated specimen (491 HV) because of tempering effect simultaneously with plasma treatment. It is known that the martensite hardness is a function of carbon content. Hence, the gradient distribution of microhardness is corresponding to graded distribution of carbon concentration. In order to investigate the mechanical properties of the surface layer, load-displacement curves of the plasma treated and untreated specimen were obtained by nano-indentation tests, as shown in Fig. 4 (b). The L1 and UL1 represent the loading and unloading curves for the plasma treated specimen, respectively, the L2 and UL2 for the untreated specimen. As shown in Table 1, the hardness of plasma treated specimen is higher than untreated specimen. Moreover, the Fe3O4/DLC-containing carburized layer possess higher hardness (13.94 ± 0.92 GPa) and elastic modulus (314.18 ± 9.25 GPa) than the traditional DLC (13.53 GPa, 139.24 GPa) and GLC (8.87 GPa, 113.56 GPa) coating [29]. Moreover, it possesses better elastic recovery ability (76%) than the DLC (62%) and GLC (58%) coating [29] which may attribute to synergistic effect between hard DLC and ductile Fe3O4 phase. Furthermore, we adopt the plastic factor ηp as the ratio of plastic deformation work to total deformation work to evaluate the resistance to plastic deformation. The lower value of ηp implies better resistance to plastic
Fig. 6. SEM micrographs of wear scars for the specimens (a) untreated; (b) magnification of the area outlined in white in (a); (c) plasma treated; (d) magnification of the area outlined in white in (c).
Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25
Fe
O
W
Plasma treated
Untreated
C
23
Fig. 7. Features of the wear tracks and the distributions of elements (C, Fe, O and W) by EDS.
deformation [30–32]. Therefore, the ηp(untreated) and ηp(coating) can be calculated by the formulas in Eq. (3). ηpðuntreatedÞ ¼
W p AOP2 C ¼ W t AOP2 D
ηpðcoatingÞ ¼
W p AOP1 B ¼ W t AOP1 E
ð3Þ
where AOP2C is the area enclosed by L2, UL2 and OC. AOP2D is the area enclosed by L2, P2D and OD. AOP1B is the area enclosed by L1, UL1 and OB. AOP1E is the area enclosed by L1, P1E and OE. Although the ηp(coating) (66.91%) is lower than the ηp(untreated) (82.06%), the ηp(coating) is more than 60%, indicating good plasticity in view of intrinsic brittleness [30,31]. The wear resistance of materials can be evaluated by the ratio of hardness H to elastic modulus E. which basically describes the elastic strain-to-failure of materials [33]. Materials with a higher H/E ratio indicate better wear resistance. The H/E ratio of plasma treated specimen is much higher than that of untreated specimen (shown in Table 1), which demonstrates that the wear resistance of M50NiL steel has been greatly improved by Fe3O4/ DLC-containing carburized layer. 3.4. Tribological properties of Fe3O4/DLC-containing carburized layer The friction coefficients versus sliding time of the untreated specimen and the plasma treated specimens are shown in Fig. 5 (a). The oscillation of the friction coefficient for the untreated specimen may be caused by the cushioning and self-lubricating of the abrasive debris (such as Fe3O4) in the wear track [34]. The friction coefficients of steady wear for untreated and plasma treated specimens are 0.54 and 0.25, respectively. The marked low friction coefficient for the plasma treated
specimen is attributed to DLC and Fe3O4 [16] owing to their lubrication effect and synergistic effect. The wear scar profile of untreated and plasma treated specimens are demonstrated in Fig. 5. (b), (c) and (d). For the untreated specimen, a deep and broad wear scar indicates poor wear resistance. In contrast, the plasma treated specimen exhibits a shallower and narrower wear scar, which indicates excellent wear resistance. We calculate the wear rate and find that there is 97.6% reduction in the wear rate for Fe3O4/DLC-containing carburized layer (1.53 × 10− 6 mm3 N− 1 m− 1) compared with untreated specimen (6.3 × 10−5 mm3 N−1 m−1). The excellent wear resistance is comprehensive effects of functionally graded carburized layer: self-lubricating Fe3O4/DLC-containing surface layer with high hardness and supporting carburized layer with gradual hardness distribution. An overview of frictional and wear test results for M50NiL steel or specimens treated by carburizing from various sources is given in Table 2 [13,25,35–39]. The coefficient of friction (COF) and wear rate of M50NiL treated in present work are much less than M50NiL treated by plasma nitrocarburizing with or without rare earths [35–37], which indicates that the present treatment has great advantages over other treatments for M50NiL. Moreover, present treatment shows lower COF and wear rate compared with traditional carburizing [25,38,39] because it innovatively introduces Fe3O4 and DLC on carburized layer by one-step. Most importantly, the Fe3O4/DLC-containing carburized layer by one step shows obvious advantages in comparison to duplex treatment (plasma carburizing and DLC deposition [13]) because of lower wear rate and simpler process. The worn surface morphologies of untreated and plasma treated specimens are shown in Fig. 6. It can be seen that the wear for untreated sample occurs in a severe adhesive behavior and fatigue wear characterized by plate-like debris, tearing, delamination and fatigue cracks of the
Fig. 8. Schematic illustration of formation mechanism of the carburized layer (A color version of this figure can be viewed online).
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surface, as shown in Fig. 6 (a) and (b). The severe adhesive wear mechanism is verified by the high content of tungsten transferred from WC ball according to the distribution picture of tungsten by EDS analysis (shown in Fig. 7). Besides, the distribution picture of oxygen demonstrate severe oxidation wear mechanism. The plate-like debris on the wear scar are mainly mixture of oxide and tungsten according to the wear scar and distributions of W and O. The poor wear scar is consistent with high wear rate shown in Fig. 5. By comparison, the wear track of plasma treated specimen is obviously narrower than the untreated one (Fig. 6 (c) (d)), corresponding to the wear scar profile shown in Fig. 5. The wear scar is smooth with a few of lamellate debris, which is also corresponding to oxide film as Fig. 7 shown. The distribution of O reveals that the mild oxidation wear mechanism. The mild wear groove and the crack-free wear scar without obvious tearing imply that the abrasive wear and adhesive wear mechanism is negligible. Furthermore, the difference of distribution for carbon (Fig. 7) indicates that the diamond-like carbon coated carburized layer plays important role in excellent wear performance owing to its high hardness and low friction coefficient.
3.5. Formation mechanism of Fe3O4/DLC-containing carburized layer Based on above discussion, the formation mechanism of Fe3O4/DLCcontaining carburized layer on steel could be divided into four steps (shown in Fig. 8): (a) Carbon diffusion: A great deal of carbon diffuse into the M50NiL steel after the surface is activated by plasma. The diffusion layer with mainly α'C phase is formed on the surface. (b) Fe3C layer formation: More carbon atoms transfer into the carbon-supersaturated steel to form a cementite layer or compound layer which acts as a barrier for further carbon diffusion [40]. (c) Carbon deposition: The Fe3C phase is metastable and could be decomposed into graphic carbon and iron. Then, the iron acts as catalyst for further graphic carbon deposition [41]. Besides, the iron may react with residual oxygen in the furnace. (d) DLC coating formation: The high plasma density (high ionization effect) [42] and active hydrogen atoms [21,22,25] play important role in promoting the formation of sp3 bonds, which are believed to be responsible for high hardness.
4. Conclusions In summary, a functionally graded Fe3O4/DLC-containing carburized layer on M50NiL steel with excellent wear resistance was obtained by one-step plasma-assisted method. The composite layer possesses high surface hardness (13.94 ± 0.92 GPa), high elastic modulus (314.18 ± 9.25 GPa), good elastic recovery ability (76%) and gradual hardness distribution. Combination self-lubricating Fe3O4 doped DLC and supporting carburized layer gives the specimen lower friction coefficient (0.25) and lower wear rate (1.53 × 10−6 mm3 N−1 m−1, 97.6% reduction) than untreated one. The formation mechanism is combination of carbon diffusion, Fe3C layer formation, carbon deposition and DLC coating formation. The results point out a simple way to greatly improve wear resistance for steel by introducing a functionally graded Fe3O4/DLC-containing carburized layer in one step.
Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51371070) for the financial support of this research work.
References [1] H.A. Tasdemir, M. Wakayama, T. Tokoroyama, H. Kousaka, N. Umehara, Y. Mabuchi, T. Higuchi, Ultra-low friction of tetrahedral amorphous diamond-like carbon (ta-C DLC) under boundary lubrication in poly alpha-olefin (PAO) with additives, Tribol. Int. 65 (2013) 286–294. [2] S. Field, M. Jarratt, D. Teer, Tribological properties of graphite-like and diamond-like carbon coatings, Tribol. Int. 37 (2004) 949–956. [3] S. Akaike, D. Kobayashi, Y. Aono, M. Hiratsuka, A. Hirata, T. Hayakawa, Y. Nakamura, Relationship between static friction and surface wettability of orthodontic brackets coated with diamond-like carbon (DLC), fluorine-or silicone-doped DLC coatings, Diam. Relat. Mater. 61 (2016) 109–114. [4] B. Kržan, F. Novotny-Farkas, J. Vižintin, Tribological behavior of tungsten-doped DLC coating under oil lubrication, Tribol. Int. 42 (2009) 229–235. [5] T. Takeno, T. Sugawara, H. Miki, T. Takagi, Deposition of DLC film with adhesive WDLC layer on stainless steel and its tribological properties, Diam. Relat. Mater. 18 (2009) 1023–1027. [6] W. Bai, L. Li, X. Wang, F. He, D. Liu, G. Jin, J. Tu, Effects of Ti content on microstructure, mechanical and tribological properties of Ti-doped amorphous carbon multilayer films, Surf. Coat. Technol. 266 (2015) 70–78. [7] K. Honglertkongsakul, P.W. May, B. Paosawatyanyong, Electrical and optical properties of diamond-like carbon films deposited by pulsed laser ablation, Diam. Relat. Mater. 19 (2010) 999–1002. [8] R. Bandorf, T. Staedler, H. Lüthje, Influencing factors on microtribology of DLC films for MEMS and microactuators, Diam. Relat. Mater. 13 (2004) 1491–1493. [9] H. Cho, S. Kim, H. Ki, Pulsed laser deposition of functionally gradient diamond-like carbon (DLC) films using a 355 nm picosecond laser, Acta Mater. 60 (2012) 6237–6246. [10] S.V. Hainsworth, N. Uhure, Diamond like carbon coatings for tribology: production techniques, characterisation methods and applications, Int. Mater. Rev. (2013) (DOI). [11] F. Cemin, L. Bim, C. Menezes, M.M. da Costa, I. Baumvol, F. Alvarez, C. Figueroa, The influence of different silicon adhesion interlayers on the tribological behavior of DLC thin films deposited on steel by EC-PECVD, Surf. Coat. Technol. 283 (2015) 115–121. [12] T. Morita, Y. Hirano, K. Asakura, T. Kumakiri, M. Ikenaga, C. Kagaya, Effects of plasma carburizing and DLC coating on friction-wear characteristics, mechanical properties and fatigue strength of stainless steel, Mater. Sci. Eng. A 558 (2012) 349–355. [13] N. Ueda, N. Yamauchi, T. Sone, A. Okamoto, M. Tsujikawa, DLC film coating on plasma-carburized austenitic stainless steel, Surf. Coat. Technol. 201 (2007) 5487–5492. [14] A. Yerokhin, A. Leyland, C. Tsotsos, A. Wilson, X. Nie, A. Matthews, Duplex surface treatments combining plasma electrolytic nitrocarburising and plasma-immersion ion-assisted deposition, Surf. Coat. Technol. 142 (2001) 1129–1136. [15] I. Boromei, L. Ceschini, A. Marconi, C. Martini, A duplex treatment to improve the sliding behavior of AISI 316L: low-temperature carburizing with a DLC (a-C: H) topcoat, Wear 302 (2013) 899–908. [16] G. Zhou, Y. Zhu, X. Wang, M. Xia, Y. Zhang, H. Ding, Sliding tribological properties of 0.45% carbon steel lubricated with Fe3O4 magnetic nano-particle additives in baseoil, Wear 301 (2013) 753–757. [17] C. Scheuer, R. Cardoso, R. Pereira, M. Mafra, S. Brunatto, Low temperature plasma carburizing of martensitic stainless steel, Mater. Sci. Eng. A 539 (2012) 369–372. [18] W. Liang, Surface modification of AISI 304 austenitic stainless steel by plasma nitriding, Appl. Surf. Sci. 211 (2003) 308–314. [19] P.-C. Chen, C.-G. Chao, T.-F. Liu, A novel high-strength, high-ductility and high-corrosion-resistance FeAlMnC low-density alloy, Scr. Mater. 68 (2013) 380–383. [20] M. Olzon-Dionysio, S. De Souza, R. Basso, S. De Souza, Application of Mössbauer spectroscopy to the study of corrosion resistance in NaCl solution of plasma nitrided AISI 316L stainless steel, Surf. Coat. Technol. 202 (2008) 3607–3614. [21] Y. Liu, A. Erdemir, E. Meletis, An investigation of the relationship between graphitization and frictional behavior of DLC coatings, Surf. Coat. Technol. 86 (1996) 564–568. [22] Y. Liu, A. Erdemir, E. Meletis, A study of the wear mechanism of diamond-like carbon films, Surf. Coat. Technol. 82 (1996) 48–56. [23] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (2000) 14095. [24] S. Chiu, S.-C. Lee, C. Wang, F. Tai, C. Chu, D. Gan, Electrical and mechanical properties of DLC coatings modified by plasma immersion ion implantation, J. Alloys Compd. 449 (2008) 379–383. [25] J. Wu, W. Xue, B. Wang, X. Jin, J. Du, Y. Li, Characterization of carburized layer on T8 steel fabricated by cathodic plasma electrolysis, Surf. Coat. Technol. 245 (2014) 9–15. [26] A. Chattopadhyay, N. Bandyopadhyay, A. Das, M. Panigrahi, Oxide scale characterization of hot rolled coils by Raman spectroscopy technique, Scr. Mater. 52 (2005) 211–215. [27] A. Modabberasl, P. Kameli, M. Ranjbar, H. Salamati, R. Ashiri, Fabrication of DLC thin films with improved diamond-like carbon character by the application of external magnetic field, Carbon 94 (2015) 485–493. [28] P. Ghods, O. Isgor, J. Brown, F. Bensebaa, D. Kingston, XPS depth profiling study on the passive oxide film of carbon steel in saturated calcium hydroxide solution and the effect of chloride on the film properties, Appl. Surf. Sci. 257 (2011) 4669–4677. [29] D. Du, D. Liu, Z. Ye, X. Zhang, F. Li, Z. Zhou, L. Yu, Fretting wear and fretting fatigue behaviors of diamond-like carbon and graphite-like carbon films deposited on Ti6Al-4 V alloy, Appl. Surf. Sci. 313 (2014) 462–469. [30] M. Yan, Y. Wu, R. Liu, Plasticity and ab initio characterizations on Fe4N produced on the surface of nanocrystallized 18Ni-maraging steel plasma nitrided at lower temperature, Appl. Surf. Sci. 255 (2009) 8902–8906. [31] Y. Wu, M. Yan, Plasticity characterization of the modified layer produced by plasma nitrocarburizing of nanocrystallized 18Ni maraging steel, Vacuum 86 (2011) 119–123.
Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25 [32] F. Zhang, M. Yan, Performance characterization on a novel intermetallic layer produced by interdiffusion between Ti film and 5083 Al alloy, Vacuum 103 (2014) 87–92. [33] W. Ni, Y.-T. Cheng, M.J. Lukitsch, A.M. Weiner, L.C. Lev, D.S. Grummon, Effects of the ratio of hardness to Young's modulus on the friction and wear behavior of bilayer coatings, Appl. Phys. Lett. 85 (2004) 4028–4030. [34] Y. Yang, M.F. Yan, Y.X. Zhang, C.S. Zhang, X.A. Wang, Self-lubricating and anti-corrosion amorphous carbon/Fe3C composite coating on M50NiL steel by low temperature plasma carburizing, Surf. Coat. Technol. 304 (2016) 142–149. [35] Z. Sun, C. Zhang, M. Yan, Microstructure and mechanical properties of M50NiL steel plasma nitrocarburized with and without rare earths addition, Mater. Des. 55 (2014) 128–136. [36] M. Yan, C. Zhang, Z. Sun, Study on depth-related microstructure and wear property of rare earth nitrocarburized layer of M50NiL steel, Appl. Surf. Sci. 289 (2014) 370–377. [37] C. Zhang, M. Yan, Z. Sun, Y. Wang, Y. You, B. Bai, L. Chen, Z. Long, R. Li, Optimizing the mechanical properties of M50NiL steel by plasma nitrocarburizing, Appl. Surf. Sci. 315 (2014) 28–35.
25
[38] F. Çavuşlu, M. Usta, Kinetics and mechanical study of plasma electrolytic carburizing for pure iron, Appl. Surf. Sci. 257 (2011) 4014–4020. [39] C. Li, T. Bell, A comparative study of low temperature plasma nitriding, carburising and nitrocarburising of AISI 410 martensitic stainless steel, Mater. Sci. Technol. 23 (2007) 355–361. [40] X. Li, L. He, Y. Li, Q. Yang, A. Hirose, TEM interfacial characterization of CVD diamond film grown on Al inter-layered steel substrate, Diam. Relat. Mater. 50 (2014) 103–109. [41] A. Moritani, T. Yamada, T. Kitamura, H. Katayama, Y. Noda, N. Kanayama, In situ spectroscopic ellipsometry for plasma-carburising process, Thin Solid Films 374 (2000) 298–302. [42] S. Nakao, K. Yukimura, H. Ogiso, S. Nakano, T. Sonoda, Effects of Ar gas pressure on microstructure of DLC films deposited by high-power pulsed magnetron sputtering, Vacuum 89 (2013) 261–266.
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
One-step plasma-assisted method for functionally graded Fe3O4/DLC coated carburized layer on steel Y. Yang a, M.F. Yan a,⁎, S.D. Zhang a, Y.X. Zhang a, H.T. Chen a,b, X.A. Wang a a b
National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China School of Materials Science and Engineering, Harbin University of Science and Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 30 June 2016 Received in revised form 4 August 2016 Accepted 23 September 2016 Available online 24 September 2016 Keywords: Diamond-like carbon Wear Surface characterization Tribology
a b s t r a c t A Fe3O4/Diamond-like carbon (DLC) coated carburized layer on M50NiL steel surface was fabricated by onestep plasma-assisted method in gaseous acetone at 550 °C. The surface layer showed high hardness (13.94 ± 0.92 GPa) and elastic modulus (314.18 ± 9.25 GPa) due to synergistic effect of rigid DLC and mild Fe 3O 4 . The carburized layer with gradual hardness distribution served as supporting layer. The Fe3O4/DLC-containing carburized layer showed lower friction coefficient (about 0.25) and 97.6% reduction in wear rate (1.53 × 10−6 mm3 N−1 m−1) compared with untreated specimen due to the lubricating effect and gradient multilayer structure. The tribological properties demonstrated that the treatment in present work has great advantages over other surface treatment. Moreover, the formation mechanism of the functionally graded carburized layer was discussed. The results pointed out a simple way to greatly improve wear resistance for steels. © 2016 Published by Elsevier B.V.
1. Introduction The diamond-like carbon (DLC) coatings have elicited considerable attention owing to their potential as advanced solid lubricant coatings with low friction coefficient and chemical inertness. The DLC coatings are used to describe a range of amorphous carbon coatings which include the tetrahedral DLC coating (ta-C, ta-C:H) with significant fraction of sp3 bonds [1], the DLC coating (a-C, a-C:H) containing high percentage of sp2 bonds [2] and those containing non-metal [3] or metal dopants [4–6]. Although DLC coatings have broad industrial applications, especially in optical [7] and electronic areas [8], there exist unresolved problems that have to be tackled. One of the critical issues is that the coating thickness is limited by the high residual stress [6,9,10]. Thus, thin and hard carbon coatings exhibit poor adhesion to the substrate and consequent limited load-bearing capacity. Many efforts have been made to decrease the intrinsic residual stress of carbon coatings and improve their adhesion to substrates, such as elements doping [6], interlayers incorporating [11] and graded multilayer structure [12–15]. For example, strategies combining chemical heat treatment and DLC deposition have been employed to improve adhesion and fatigue strength [12], decrease friction coefficient and wear rate [14], prevent the delamination failure and further extend coating life [15]. However, the multilayer or multicomponent technique is limited due to complicated process. Therefore, it will be of great significance if the multilayer or multicomponent layer could be formed in one-step process. Moreover, ⁎ Corresponding author. E-mail address: [email protected] (M.F. Yan).
http://dx.doi.org/10.1016/j.diamond.2016.09.020 0925-9635/© 2016 Published by Elsevier B.V.
previous investigation shows that Fe3O4 nanoparticles have good lubricating effects [16]. Until now, reports about fabrication of Fe3O4/DLCcontaining carburized layer by one-step method are not available. Quenching and tempering are traditional heat treatment for steels due to excellent combination of hardness, toughness and wear properties. Therefore, quenching is the pretreatment for the steel in the present work. Then, we innovatively applied gaseous acetone to obtain a Fe3O4/DLC-containing carburized layer on the steel by plasma-assisted method in one-step at 550 °C for 4 h, resulting in effects of surface treatment and tempering simultaneously. The microstructures, chemical composition, mechanical and tribological properties of the functionally graded layer were analyzed. Moreover, the formation mechanism of the composite layer was discussed.
2. Experimental procedure 2.1. Materials The as-received M50NiL steel with the following chemical composition of 0.13C, 4.1Cr, 3.4Ni, 4.2Mo, 1.2 V, 0.13Mn, 0.18Si, 0.012P, 0.002S and balance Fe (wt.%) was used. The samples were firstly solution treated at 1150o for 1 h and then quenched in oil. The quenched specimens were identified as untreated one. Prior to plasma treatment, the flat surfaces of the specimens with 12 mm × 12 mm × 5 mm dimensions were ground from 240 to 800 grade using SiC papers, and then ultrasonically cleaned with alcohol and acetone in succession to clean the surface of the steel.
Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25
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hydrogen (0.2 L/min) and a trace of argon were introduced into the chamber when the temperature achieves 550 ± 5 °C. The hydrogen and argon were used to increase temperature and prevent soot formation. The gaseous acetone acted as the source of active carbon atoms. After the treatment for 4 h, the treated specimens were directly cooled to room temperature in the furnace under a flow of acetone gas.
2.3. Characterization
Fig. 1. Schematic of plasma treatment system.
2.2. Plasma treatment The specimens were placed in a pulsed glow discharge plasma unit (LDMC-30, 30 kW), as shown in Fig. 1. Sputtering pre-treatment using hydrogen under a pulsed dc voltage (U = 660 V) during the heatingup stage was carried out to remove the passive film and activate the surface. Pure acetone was heated in water bath to 40 °C to increase the rate of evaporation. A gas mixture containing gaseous acetone (0.3 L/min),
The phase structure of the modified layer was analyzed by X-ray diffractometer (XRD, type D/max-rB) with Cu-Kα radiation (λ = 0.15406 nm) in the range of glancing angles 20–100o. The elemental compositions and chemical binding states were analyzed by X-ray photoelectron spectroscopy (XPS, Escalab 250). Raman measurements were performed on an Invia Raman microscope (Renishaw, UK) with excitation laser beam wavelength of 532 nm. Transmission electron microscopy (TEM, type Philips CM12) and scanning electron microscope (SEM, type S-4700) and optical microscope (OM, CMM-33E) were employed to observe the morphology of surface layer. The microhardness of the modified layer was measured inward from 2 um underneath surface to the core by Vickers hardness tester (type HV-1000) under a load of 100 g and a dwelling time of 15 s. Three indentations at the same depth were taken and the mean value was used to ensure a high accuracy. Nano-indentation tests were performed in the surface of the samples using a nano-indenter XP (MTS) equipped with a Berkovich diamond indenter. Five indentations depth were taken and the mean value was used to ensure a high accuracy. The dry wear properties of the specimens were evaluated by pin-on-disc tribometer (type POD1). The test was carried out in ambient air of 60% RH at ambient temperature (28 °C). During the test, the specimens were rotated against a stationary WC ball of 5 mm diameter at the speed of 200 r/min for 1800 s
Fig. 2. (a) SEM and OM micrographs of the cross-section (b) XRD patterns of the surface layers (c)(d) TEM micrographs and SAED patterns of the surface layer.
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Fig. 3. Characterization of the surface layer (a) Raman spectrum (b) Typical XPS profile (c) C 1s XPS spectra (d) Fe 2p XPS spectra.
under 10 N load. The wear rate η (mm3 N−1 m−1) was calculated by according to the following Eq. (1): η¼
V FL
ð1Þ
where F (N) is the normal contact load and L (m) is the total sliding distance. V (mm3) is the wear volume measured by the Eq. (2): V ¼ πDS
ð2Þ
D(mm) is the diameter of the circular wear scar. S(mm2) is the crosssectional area of the wear scar profile measured by a stylus
profilometer (CCI MP, Taylor Hobson). The morphologies of worn surface were observed by SEM equipped with energy dispersive X-ray analyzer (EDS). 3. Results and discussion 3.1. Microstructure and phase structures of surface layer The OM micrograph of the etched cross-sectional microstructure for the plasma treated specimen is shown in Fig. 2 (a). The lath martensite is the main phase for the surface layer. Most importantly, a thin and continuous layer appears ‘white’ on the treated surface, which is confirmed
Fig. 4. (a) Microhardness profile of the carburized layer (b) The load-displacement curves of the untreated and plasma treated specimen under nano-indentation tests.
Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25 Table 1 Hardness H, elastic modulus E, total deformation work Wt, plastic deformation work Wp, plasticity factor ηp and the ratios of H/E of the surface of the untreated and plasma treated samples. Sample
H (GPa)
E (GPa)
Wt (nJ)
Wp (nJ)
ηp (%)
H/E
Untreated Plasma treated
5.94 13.94
221.41 314.18
155.55 371.23
127.65 248.39
82.06 66.91
0.0268 0.0444
by SEM micrograph shown in the top right corner of Fig. 2 (a). It indicates that the layer is resistant to the corrosive attack by the 4% Nital solution. According to the X-ray diffraction patterns shown in Fig. 2 (b), the surface layer is mainly composed of Fe3C phase (JCPDS file 652412), carbon expanded martensite (α'C) (JCPDS file 44-1292) and a few of Fe3O4 phase (JCPDS file 65-3107). Compared with the untreated sample, the α'C phase peaks are fairly broadened, which may due to residual stress [17] or the large amount of carbon incorporated in the phase. Similar results by introducing nitrogen in martensite phase were shown in [18–20]. In fact, Fe3C layer is supposed to be compound layer while α'C phase attributes to the subsurface, i.e., diffusion layer because the compound layer is too thin (about 1 μm) (Fig. 2 (a)) to block the X-ray diffraction. However, the elements analysis show high carbon content (45 at.%) on the surface which imply a carbon-rich layer, rather than a single Fe3C (25 at.%) layer. The non-reported result of grazing incidence XRD (GIXRD) at 1° incidence angle proves existence of the amorphous carbon. Hence, the white layer on the surface is actually a Fe3O4/amorphous carbon coated Fe3C layer. The amorphous structure and selected-area electron diffraction (SAED) patterns of DLC on the surface are shown in Fig. 2 (c) and confirmed by the corresponding SAED pattern with a diffuse halo. Three diffraction rings correspond to d-spacing of 0.102 nm, 0.124 nm and 0.209 nm, which is consistent with the previous reports [21,22]. (311), (220) and (111) crystal planes are the strongest diffraction for crystal diamond and correspond to d = 0.107, d = 0.126 and 0.206 nm,
21
indicating a diamond-like structure. Combining the SAED pattern given in the inset and EDS composition analysis, the nanoparticles on the surface layer shown in Fig. 2 (d) can be identified as Fe3C and Fe3O4 phase. Diffraction rings corresponding to d-spacing of 0.210 nm, and 0.135 nm is consistent with the diffraction rings of (211) and (123) crystal planes for orthorhombic Fe3C. Other diffraction rings corresponding to d-spacing of 0.253 nm and 0.296 nm may be ascribed to (311) and (220) crystal planes of Fe3O4. Thus, the SAED patterns are in good agreement with results of XRD. 3.2. Phase constituents and bonding states of surface layer Raman measurements are carried out to characterize the surface layer, shown in Fig. 3 (a). The Raman spectra peaks at around 1350 cm-1 and 1600 cm-1 corresponds to the D and G bands, respectively, which can be attributed to the breathing mode of sp2 atoms in rings and the stretching mode of all pairs of sp2 carbon atoms in both rings and chains, respectively [23]. The D band and G band shown in Fig. 3 (a) for plasma treated specimen indicate the presence of the amorphous carbon on the surface [24,25]. Moreover, the peak located at 670 cm− 1 is corresponding to the mixture of FeO(OH) and Fe3O4 phases [26], which is in consistent with the XRD result. The XPS analysis was employed to estimate the surface bonding states in detail. Fig. 3 (b) show the typical XPS profile for the surface of plasma treated specimen. The peaks located at around 284.8, 532.8 and 714.1 eV are related to the photoelectrons excited from the C1s O1s and Fe2p core levels, respectively [25,27]. Fig. 3 (c) exhibits the XPS carbon core level spectra (C 1 s) of the surface layer, which could be used to analyze the sp3 fraction in the carbon coating. The full width at half maximum (FWHM) of the C1s spectra of the surface is about 1.5 eV, larger than that of graphite (FWHM = 0.6 eV) and diamond samples (FWHM = 1.0 eV) [6]. Therefore, the C1s XPS spectra are deconvoluted into three distinct Gaussian–Lorentzian peaks, which are located at binding energy of 286.6 ± 0.2, 285.1 ± 0.1,
Fig. 5. (a) Frictional coefficient curves of the untreated and plasma treated specimens (b) (c) (d) Wear scars profile of the untreated and plasma treated specimens.
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Table 2 Summary of frictional and wear test results for M50NiL steel or specimens treated by carburizing. Materials
Treatment process
Counter surface
Contact load
COF
Wear rate (mm3 m−1 N−1)
Conditions
Ref.
M50NiL M50NiL M50NiL Fe T8 AISI410 AISI 304 M50NiL
Plasma nitrocarburizing Plasma nitrocarburizing with rare earths Plasma nitrocarburizing Plasma electrolytic carburizing Plasma electrolytic carburizing Plasma carburizing Plasma carburizing + DLC Present treatment
WC WC WC Al2O3 ZrO2 WC AISI 52100 WC
10 10 10 10 5 10 1.96 10
About 0.5 About 0.44 0.58 ~ 0.65 0.6 ~ 1.0 0.3 ~ 0.35 – 0.2 0.25
4.333 × 10−5 4.056 × 10−5 5.95 ~ 8.25 × 10−5 1 ~ 7 × 10−5 1.13 × 10−5 4.4 × 10−6 8.29 × 10−5 1.53 × 10−6
Air, RT Air, RT Air, RT Air, RT Air, RT Air, RT Air, RT Air, RT
[35] [35–36] [37] [38] [25] [39] [13] Present work
284.3 ± 0.1 eV, representing C\\O bond, sp3 hybridization of carbon bond and sp2 hybridization of carbon bond, respectively [6,27]. The sp3 hybridized carbon atom fraction is estimated as the ratio of the corresponding peak area to the total C1s peak area. The percentage of sp3 hybridized carbon atom is about 53% for the amorphous carbon, which could be termed as diamond-like carbon. Fig. 3 (d) exhibits the XPS Fe core level spectra (Fe2p) of the surface layer. Fe 2p peaks located at binding energy of 707.0 ± 0.5 and 708.2 ± 0.5 eV corresponds to Fe metal and Fe3C, respectively [28]. The binding energy of 710.0– 712.7 eV mainly represents Fe\\O bonds [28], in consistent with Fe3O4 shown in XRD and TEM. 3.3. Mechanical properties of Fe3O4/DLC-containing carburized layer Fig. 4 (a) shows the microhardness profile for the carburized layers of the specimen. A average hardness gradient from surface layer to matrix is observed, which is beneficial to eliminating stress concentration and improving the fatigue performance [12]. The hardness of the carburized layer at distance of 2 μm from the surface is above 1000 HV, which is beneficial to wear resistance. Moreover, the carburized layer could be considered as supporting layer for diamond-like carbon, which benefits to adhesion with diamond-like carbon and thus improve wear
resistance performance and load-bearing capacity [12]. The substrate hardness is about 405 HV and is lower than the untreated specimen (491 HV) because of tempering effect simultaneously with plasma treatment. It is known that the martensite hardness is a function of carbon content. Hence, the gradient distribution of microhardness is corresponding to graded distribution of carbon concentration. In order to investigate the mechanical properties of the surface layer, load-displacement curves of the plasma treated and untreated specimen were obtained by nano-indentation tests, as shown in Fig. 4 (b). The L1 and UL1 represent the loading and unloading curves for the plasma treated specimen, respectively, the L2 and UL2 for the untreated specimen. As shown in Table 1, the hardness of plasma treated specimen is higher than untreated specimen. Moreover, the Fe3O4/DLC-containing carburized layer possess higher hardness (13.94 ± 0.92 GPa) and elastic modulus (314.18 ± 9.25 GPa) than the traditional DLC (13.53 GPa, 139.24 GPa) and GLC (8.87 GPa, 113.56 GPa) coating [29]. Moreover, it possesses better elastic recovery ability (76%) than the DLC (62%) and GLC (58%) coating [29] which may attribute to synergistic effect between hard DLC and ductile Fe3O4 phase. Furthermore, we adopt the plastic factor ηp as the ratio of plastic deformation work to total deformation work to evaluate the resistance to plastic deformation. The lower value of ηp implies better resistance to plastic
Fig. 6. SEM micrographs of wear scars for the specimens (a) untreated; (b) magnification of the area outlined in white in (a); (c) plasma treated; (d) magnification of the area outlined in white in (c).
Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25
Fe
O
W
Plasma treated
Untreated
C
23
Fig. 7. Features of the wear tracks and the distributions of elements (C, Fe, O and W) by EDS.
deformation [30–32]. Therefore, the ηp(untreated) and ηp(coating) can be calculated by the formulas in Eq. (3). ηpðuntreatedÞ ¼
W p AOP2 C ¼ W t AOP2 D
ηpðcoatingÞ ¼
W p AOP1 B ¼ W t AOP1 E
ð3Þ
where AOP2C is the area enclosed by L2, UL2 and OC. AOP2D is the area enclosed by L2, P2D and OD. AOP1B is the area enclosed by L1, UL1 and OB. AOP1E is the area enclosed by L1, P1E and OE. Although the ηp(coating) (66.91%) is lower than the ηp(untreated) (82.06%), the ηp(coating) is more than 60%, indicating good plasticity in view of intrinsic brittleness [30,31]. The wear resistance of materials can be evaluated by the ratio of hardness H to elastic modulus E. which basically describes the elastic strain-to-failure of materials [33]. Materials with a higher H/E ratio indicate better wear resistance. The H/E ratio of plasma treated specimen is much higher than that of untreated specimen (shown in Table 1), which demonstrates that the wear resistance of M50NiL steel has been greatly improved by Fe3O4/ DLC-containing carburized layer. 3.4. Tribological properties of Fe3O4/DLC-containing carburized layer The friction coefficients versus sliding time of the untreated specimen and the plasma treated specimens are shown in Fig. 5 (a). The oscillation of the friction coefficient for the untreated specimen may be caused by the cushioning and self-lubricating of the abrasive debris (such as Fe3O4) in the wear track [34]. The friction coefficients of steady wear for untreated and plasma treated specimens are 0.54 and 0.25, respectively. The marked low friction coefficient for the plasma treated
specimen is attributed to DLC and Fe3O4 [16] owing to their lubrication effect and synergistic effect. The wear scar profile of untreated and plasma treated specimens are demonstrated in Fig. 5. (b), (c) and (d). For the untreated specimen, a deep and broad wear scar indicates poor wear resistance. In contrast, the plasma treated specimen exhibits a shallower and narrower wear scar, which indicates excellent wear resistance. We calculate the wear rate and find that there is 97.6% reduction in the wear rate for Fe3O4/DLC-containing carburized layer (1.53 × 10− 6 mm3 N− 1 m− 1) compared with untreated specimen (6.3 × 10−5 mm3 N−1 m−1). The excellent wear resistance is comprehensive effects of functionally graded carburized layer: self-lubricating Fe3O4/DLC-containing surface layer with high hardness and supporting carburized layer with gradual hardness distribution. An overview of frictional and wear test results for M50NiL steel or specimens treated by carburizing from various sources is given in Table 2 [13,25,35–39]. The coefficient of friction (COF) and wear rate of M50NiL treated in present work are much less than M50NiL treated by plasma nitrocarburizing with or without rare earths [35–37], which indicates that the present treatment has great advantages over other treatments for M50NiL. Moreover, present treatment shows lower COF and wear rate compared with traditional carburizing [25,38,39] because it innovatively introduces Fe3O4 and DLC on carburized layer by one-step. Most importantly, the Fe3O4/DLC-containing carburized layer by one step shows obvious advantages in comparison to duplex treatment (plasma carburizing and DLC deposition [13]) because of lower wear rate and simpler process. The worn surface morphologies of untreated and plasma treated specimens are shown in Fig. 6. It can be seen that the wear for untreated sample occurs in a severe adhesive behavior and fatigue wear characterized by plate-like debris, tearing, delamination and fatigue cracks of the
Fig. 8. Schematic illustration of formation mechanism of the carburized layer (A color version of this figure can be viewed online).
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Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25
surface, as shown in Fig. 6 (a) and (b). The severe adhesive wear mechanism is verified by the high content of tungsten transferred from WC ball according to the distribution picture of tungsten by EDS analysis (shown in Fig. 7). Besides, the distribution picture of oxygen demonstrate severe oxidation wear mechanism. The plate-like debris on the wear scar are mainly mixture of oxide and tungsten according to the wear scar and distributions of W and O. The poor wear scar is consistent with high wear rate shown in Fig. 5. By comparison, the wear track of plasma treated specimen is obviously narrower than the untreated one (Fig. 6 (c) (d)), corresponding to the wear scar profile shown in Fig. 5. The wear scar is smooth with a few of lamellate debris, which is also corresponding to oxide film as Fig. 7 shown. The distribution of O reveals that the mild oxidation wear mechanism. The mild wear groove and the crack-free wear scar without obvious tearing imply that the abrasive wear and adhesive wear mechanism is negligible. Furthermore, the difference of distribution for carbon (Fig. 7) indicates that the diamond-like carbon coated carburized layer plays important role in excellent wear performance owing to its high hardness and low friction coefficient.
3.5. Formation mechanism of Fe3O4/DLC-containing carburized layer Based on above discussion, the formation mechanism of Fe3O4/DLCcontaining carburized layer on steel could be divided into four steps (shown in Fig. 8): (a) Carbon diffusion: A great deal of carbon diffuse into the M50NiL steel after the surface is activated by plasma. The diffusion layer with mainly α'C phase is formed on the surface. (b) Fe3C layer formation: More carbon atoms transfer into the carbon-supersaturated steel to form a cementite layer or compound layer which acts as a barrier for further carbon diffusion [40]. (c) Carbon deposition: The Fe3C phase is metastable and could be decomposed into graphic carbon and iron. Then, the iron acts as catalyst for further graphic carbon deposition [41]. Besides, the iron may react with residual oxygen in the furnace. (d) DLC coating formation: The high plasma density (high ionization effect) [42] and active hydrogen atoms [21,22,25] play important role in promoting the formation of sp3 bonds, which are believed to be responsible for high hardness.
4. Conclusions In summary, a functionally graded Fe3O4/DLC-containing carburized layer on M50NiL steel with excellent wear resistance was obtained by one-step plasma-assisted method. The composite layer possesses high surface hardness (13.94 ± 0.92 GPa), high elastic modulus (314.18 ± 9.25 GPa), good elastic recovery ability (76%) and gradual hardness distribution. Combination self-lubricating Fe3O4 doped DLC and supporting carburized layer gives the specimen lower friction coefficient (0.25) and lower wear rate (1.53 × 10−6 mm3 N−1 m−1, 97.6% reduction) than untreated one. The formation mechanism is combination of carbon diffusion, Fe3C layer formation, carbon deposition and DLC coating formation. The results point out a simple way to greatly improve wear resistance for steel by introducing a functionally graded Fe3O4/DLC-containing carburized layer in one step.
Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51371070) for the financial support of this research work.
References [1] H.A. Tasdemir, M. Wakayama, T. Tokoroyama, H. Kousaka, N. Umehara, Y. Mabuchi, T. Higuchi, Ultra-low friction of tetrahedral amorphous diamond-like carbon (ta-C DLC) under boundary lubrication in poly alpha-olefin (PAO) with additives, Tribol. Int. 65 (2013) 286–294. [2] S. Field, M. Jarratt, D. Teer, Tribological properties of graphite-like and diamond-like carbon coatings, Tribol. Int. 37 (2004) 949–956. [3] S. Akaike, D. Kobayashi, Y. Aono, M. Hiratsuka, A. Hirata, T. Hayakawa, Y. Nakamura, Relationship between static friction and surface wettability of orthodontic brackets coated with diamond-like carbon (DLC), fluorine-or silicone-doped DLC coatings, Diam. Relat. Mater. 61 (2016) 109–114. [4] B. Kržan, F. Novotny-Farkas, J. Vižintin, Tribological behavior of tungsten-doped DLC coating under oil lubrication, Tribol. Int. 42 (2009) 229–235. [5] T. Takeno, T. Sugawara, H. Miki, T. Takagi, Deposition of DLC film with adhesive WDLC layer on stainless steel and its tribological properties, Diam. Relat. Mater. 18 (2009) 1023–1027. [6] W. Bai, L. Li, X. Wang, F. He, D. Liu, G. Jin, J. Tu, Effects of Ti content on microstructure, mechanical and tribological properties of Ti-doped amorphous carbon multilayer films, Surf. Coat. Technol. 266 (2015) 70–78. [7] K. Honglertkongsakul, P.W. May, B. Paosawatyanyong, Electrical and optical properties of diamond-like carbon films deposited by pulsed laser ablation, Diam. Relat. Mater. 19 (2010) 999–1002. [8] R. Bandorf, T. Staedler, H. Lüthje, Influencing factors on microtribology of DLC films for MEMS and microactuators, Diam. Relat. Mater. 13 (2004) 1491–1493. [9] H. Cho, S. Kim, H. Ki, Pulsed laser deposition of functionally gradient diamond-like carbon (DLC) films using a 355 nm picosecond laser, Acta Mater. 60 (2012) 6237–6246. [10] S.V. Hainsworth, N. Uhure, Diamond like carbon coatings for tribology: production techniques, characterisation methods and applications, Int. Mater. Rev. (2013) (DOI). [11] F. Cemin, L. Bim, C. Menezes, M.M. da Costa, I. Baumvol, F. Alvarez, C. Figueroa, The influence of different silicon adhesion interlayers on the tribological behavior of DLC thin films deposited on steel by EC-PECVD, Surf. Coat. Technol. 283 (2015) 115–121. [12] T. Morita, Y. Hirano, K. Asakura, T. Kumakiri, M. Ikenaga, C. Kagaya, Effects of plasma carburizing and DLC coating on friction-wear characteristics, mechanical properties and fatigue strength of stainless steel, Mater. Sci. Eng. A 558 (2012) 349–355. [13] N. Ueda, N. Yamauchi, T. Sone, A. Okamoto, M. Tsujikawa, DLC film coating on plasma-carburized austenitic stainless steel, Surf. Coat. Technol. 201 (2007) 5487–5492. [14] A. Yerokhin, A. Leyland, C. Tsotsos, A. Wilson, X. Nie, A. Matthews, Duplex surface treatments combining plasma electrolytic nitrocarburising and plasma-immersion ion-assisted deposition, Surf. Coat. Technol. 142 (2001) 1129–1136. [15] I. Boromei, L. Ceschini, A. Marconi, C. Martini, A duplex treatment to improve the sliding behavior of AISI 316L: low-temperature carburizing with a DLC (a-C: H) topcoat, Wear 302 (2013) 899–908. [16] G. Zhou, Y. Zhu, X. Wang, M. Xia, Y. Zhang, H. Ding, Sliding tribological properties of 0.45% carbon steel lubricated with Fe3O4 magnetic nano-particle additives in baseoil, Wear 301 (2013) 753–757. [17] C. Scheuer, R. Cardoso, R. Pereira, M. Mafra, S. Brunatto, Low temperature plasma carburizing of martensitic stainless steel, Mater. Sci. Eng. A 539 (2012) 369–372. [18] W. Liang, Surface modification of AISI 304 austenitic stainless steel by plasma nitriding, Appl. Surf. Sci. 211 (2003) 308–314. [19] P.-C. Chen, C.-G. Chao, T.-F. Liu, A novel high-strength, high-ductility and high-corrosion-resistance FeAlMnC low-density alloy, Scr. Mater. 68 (2013) 380–383. [20] M. Olzon-Dionysio, S. De Souza, R. Basso, S. De Souza, Application of Mössbauer spectroscopy to the study of corrosion resistance in NaCl solution of plasma nitrided AISI 316L stainless steel, Surf. Coat. Technol. 202 (2008) 3607–3614. [21] Y. Liu, A. Erdemir, E. Meletis, An investigation of the relationship between graphitization and frictional behavior of DLC coatings, Surf. Coat. Technol. 86 (1996) 564–568. [22] Y. Liu, A. Erdemir, E. Meletis, A study of the wear mechanism of diamond-like carbon films, Surf. Coat. Technol. 82 (1996) 48–56. [23] A.C. Ferrari, J. Robertson, Interpretation of Raman spectra of disordered and amorphous carbon, Phys. Rev. B 61 (2000) 14095. [24] S. Chiu, S.-C. Lee, C. Wang, F. Tai, C. Chu, D. Gan, Electrical and mechanical properties of DLC coatings modified by plasma immersion ion implantation, J. Alloys Compd. 449 (2008) 379–383. [25] J. Wu, W. Xue, B. Wang, X. Jin, J. Du, Y. Li, Characterization of carburized layer on T8 steel fabricated by cathodic plasma electrolysis, Surf. Coat. Technol. 245 (2014) 9–15. [26] A. Chattopadhyay, N. Bandyopadhyay, A. Das, M. Panigrahi, Oxide scale characterization of hot rolled coils by Raman spectroscopy technique, Scr. Mater. 52 (2005) 211–215. [27] A. Modabberasl, P. Kameli, M. Ranjbar, H. Salamati, R. Ashiri, Fabrication of DLC thin films with improved diamond-like carbon character by the application of external magnetic field, Carbon 94 (2015) 485–493. [28] P. Ghods, O. Isgor, J. Brown, F. Bensebaa, D. Kingston, XPS depth profiling study on the passive oxide film of carbon steel in saturated calcium hydroxide solution and the effect of chloride on the film properties, Appl. Surf. Sci. 257 (2011) 4669–4677. [29] D. Du, D. Liu, Z. Ye, X. Zhang, F. Li, Z. Zhou, L. Yu, Fretting wear and fretting fatigue behaviors of diamond-like carbon and graphite-like carbon films deposited on Ti6Al-4 V alloy, Appl. Surf. Sci. 313 (2014) 462–469. [30] M. Yan, Y. Wu, R. Liu, Plasticity and ab initio characterizations on Fe4N produced on the surface of nanocrystallized 18Ni-maraging steel plasma nitrided at lower temperature, Appl. Surf. Sci. 255 (2009) 8902–8906. [31] Y. Wu, M. Yan, Plasticity characterization of the modified layer produced by plasma nitrocarburizing of nanocrystallized 18Ni maraging steel, Vacuum 86 (2011) 119–123.
Y. Yang et al. / Diamond & Related Materials 70 (2016) 18–25 [32] F. Zhang, M. Yan, Performance characterization on a novel intermetallic layer produced by interdiffusion between Ti film and 5083 Al alloy, Vacuum 103 (2014) 87–92. [33] W. Ni, Y.-T. Cheng, M.J. Lukitsch, A.M. Weiner, L.C. Lev, D.S. Grummon, Effects of the ratio of hardness to Young's modulus on the friction and wear behavior of bilayer coatings, Appl. Phys. Lett. 85 (2004) 4028–4030. [34] Y. Yang, M.F. Yan, Y.X. Zhang, C.S. Zhang, X.A. Wang, Self-lubricating and anti-corrosion amorphous carbon/Fe3C composite coating on M50NiL steel by low temperature plasma carburizing, Surf. Coat. Technol. 304 (2016) 142–149. [35] Z. Sun, C. Zhang, M. Yan, Microstructure and mechanical properties of M50NiL steel plasma nitrocarburized with and without rare earths addition, Mater. Des. 55 (2014) 128–136. [36] M. Yan, C. Zhang, Z. Sun, Study on depth-related microstructure and wear property of rare earth nitrocarburized layer of M50NiL steel, Appl. Surf. Sci. 289 (2014) 370–377. [37] C. Zhang, M. Yan, Z. Sun, Y. Wang, Y. You, B. Bai, L. Chen, Z. Long, R. Li, Optimizing the mechanical properties of M50NiL steel by plasma nitrocarburizing, Appl. Surf. Sci. 315 (2014) 28–35.
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[38] F. Çavuşlu, M. Usta, Kinetics and mechanical study of plasma electrolytic carburizing for pure iron, Appl. Surf. Sci. 257 (2011) 4014–4020. [39] C. Li, T. Bell, A comparative study of low temperature plasma nitriding, carburising and nitrocarburising of AISI 410 martensitic stainless steel, Mater. Sci. Technol. 23 (2007) 355–361. [40] X. Li, L. He, Y. Li, Q. Yang, A. Hirose, TEM interfacial characterization of CVD diamond film grown on Al inter-layered steel substrate, Diam. Relat. Mater. 50 (2014) 103–109. [41] A. Moritani, T. Yamada, T. Kitamura, H. Katayama, Y. Noda, N. Kanayama, In situ spectroscopic ellipsometry for plasma-carburising process, Thin Solid Films 374 (2000) 298–302. [42] S. Nakao, K. Yukimura, H. Ogiso, S. Nakano, T. Sonoda, Effects of Ar gas pressure on microstructure of DLC films deposited by high-power pulsed magnetron sputtering, Vacuum 89 (2013) 261–266.