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MoS2 composite coating on Ti6Al4V alloy by plasma electrolytic oxidation and its tribological properties
Surface & Coatings Technology 214 (2013) 124–130
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
One-step preparation of TiO2/MoS2 composite coating on Ti6Al4V alloy by plasma electrolytic oxidation and its tribological properties Ming Mu a, b, Jun Liang a,⁎, Xinjian Zhou b, Qian Xiao b a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China The Ministry of Education Key Laboratory, Conveyance Tools and Equipment, East China Jiaotong University, Nanchang 330013, PR China
a r t i c l e
i n f o
Article history: Received 26 June 2012 Accepted in revised form 31 October 2012 Available online 13 November 2012 Keywords: Titanium alloy Plasma electrolytic oxidation Oxide coating MoS2 Friction and wear
a b s t r a c t A MoS2-containing oxide coating on Ti6Al4V alloy was prepared by one-step plasma electrolytic oxidation (PEO) process in a MoS2-dispersed phosphate electrolyte. The composition and microstructure of the oxide coatings produced in the electrolytes with and without the addition of MoS2 were analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Results showed that the MoS2 particles can be successfully incorporated into the oxide coating during the PEO process and were preferentially located in the micropores. The ball-on-disk sliding tests indicated that MoS2-containing oxide coating registered much lower friction coefficient and wear rate than the oxide coating without MoS2 under dry sliding condition. The improved tribological property of the MoS2-containing oxide coating was also discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Titanium alloys exhibit an ever increasing interest in the fields of automotive, aviation industry, exploitation of ocean, artificial implants of human body and so on, due to their high strength-to-weight, good corrosion resistance and excellent biocompatibility. However, the titanium alloys are characterized by low surface hardness, high friction coefficient and poor wear resistance, which has been a severe barrier to tribological applications [1,2]. The poor tribological properties of the titanium alloys can be overcome by means of loading specific surface treatments [3–5]. Many surface treatments, such as nitriding [6], thermal oxidation [7], ion implantation [8], spraying [9], laser modification [10] and plasma electrolytic oxidation (PEO) [11] have been conventionally used to improve the tribological properties of the titanium alloys. Among them, plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), has been proven to be an effective and economical method [12]. Through the PEO treatment, a thick, hard and well adherent ceramic-like coating could be produced on titanium alloys to provide applicable wear resistance [13]. The tribological properties of the PEO coatings prepared on titanium alloys have been studied by many scholars in recent years [14–19]. Most of these studies reported that the PEO coatings have been shown to remarkably improve the wear resistance of the titanium alloys but
⁎ Corresponding author. Tel.: +86 931 4968381; fax: +86 931 4968163. E-mail address: [email protected] (J. Liang). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.079
normally exhibit higher friction coefficients under unlubricated conditions. In many tribological applications, high friction coefficient could lead not only to the wear of the slider, but also to the wear damage of the counter material. Thus, PEO coatings with good wear resistance and low friction coefficient are necessary to provide better tribological properties. Many attempts have been made to reduce the friction coefficient of the PEO coatings on titanium alloys. For example, duplex approaches, in which low friction topcoats (e.g., graphite and PTFE) were deposited onto the PEO coatings, have been considered by some authors [20–22]. Notwithstanding, these approaches were surely effective in reducing the friction coefficient and improving the overall tribological properties of the PEO coatings, while, in any case, it was usual to obtain the coatings by multi-step. In addition, these coatings were subjected to losing the low friction property in case the low friction topcoats were worn out. An alternative approach to obtain PEO coatings with low friction property was to introduce low friction materials into the coating by modifying the electrolytes with the solid lubricant additives [23–25]. This approach was more effective than the duplex methods in practical applications, due to the PEO coatings contained low friction materials could be obtained by only one step. Besides, the coatings were expected to integrate the advantages of wear resistance of the PEO coating and low friction property of solid lubricants. To date, however, few investigations have been carried out concerning the preparation and properties of such coatings on titanium alloys. MoS2 is well-known for its low friction property and it is widely used as a solid lubricant. In the present work, we developed a method to introduce MoS2 particles into the PEO coatings to achieve low friction PEO coatings through a one-step PEO process. The composition, microstructure and tribological properties of the MoS2-containing PEO coatings were studied.
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2.3. Tribological evaluation A reciprocating friction and wear test-rig of UMT-2MT (CETR, USA) was used to evaluate the tribological properties of the PEO coatings prepared in the phosphate electrolytes without and with the addition of MoS2 particles under a ball-on-disk contact configuration. AISI52100 steel balls with a diameter of 3 mm were used as the counter materials. The hardness (HRC) of the steel ball was 62–63 and the average surface Ra was about 0.01 μm. All the friction and wear tests were performed using a constant normal load of 2 N with a frequency of 5 Hz and an oscillating amplitude of 5 mm for 30 min at ambient temperature and humidity (relative humidity is about 30%). During the test, the friction
Fig. 1. XRD patterns of PEO coatings prepared in the phosphate electrolytes (a) without and (b) with the addition of MoS2 particles.
2. Experimental 2.1. Preparation of PEO coatings Ti6Al4V alloy (6.020% Al, 4.100% V, 0.168% Fe, 0.160% O, 0.043% C and balance Ti, mass fraction) was used as the substrate in this study. The alloy was machined to a size of 8 mm × Ø24 mm and then polished to a surface roughness Ra of approximately 0.85 μm. Prior to the PEO process, all specimens were ultrasonically cleaned in the acetone and thoroughly dried in the air. The electrolyte for PEO process was prepared using 20.0 g/l Na3PO4 and 2.0 g/l KOH in distilled water with the addition of 20.0 g/l submicron sized MoS2 particles (particle size 0.5–1.0 μm), 100 ml/l ethanol and 0.5 g/l additive. The zeta potential of the MoS2 particles in the electrolyte was examined by a Zetasizer Nano ZS ZEN 3600 dynamic laser scattering particle size analyzer to determine its surface charge. The PEO processes were conducted on a bi-polar pulsed power source. The Ti6Al4V specimens and the stainless steel were used as the anode and the cathode, respectively. Coatings were produced at a constant current density of 8.0 A·dm −2 for 90 min. The temperature of the electrolytes was kept at 20 ± 2 °C by a water cooling system during the PEO processes. For comparison purposes, the Ti6Al4V specimens were also PEO treated using the same parameters in the phosphate electrolyte without the addition of MoS2 particles and other additives. The coated specimens were rinsed thoroughly with distilled water and dried in warm air after the PEO processes.
2.2. Composition and structure analyses The compositions of the PEO coatings prepared in the phosphate electrolytes without and with the addition of MoS2 particles were determined by a Siemens D5000 X-Ray Diffractometer (XRD), using Cu-Kα radiation as the excitation source with grazing angle of 2°. A JSM-5600LV scanning electron microscope (SEM) was employed to observe the surface and cross-sectional morphologies of the PEO coatings. The PEO coatings prepared in the phosphate electrolyte with the addition of MoS2 particles were further examined by energy dispersive X-ray spectroscopy (EDS) device attached to the JSM-6700F SEMHitachi S520 system to determine the distribution of Mo and S elements in the coating.
Fig. 2. Surface morphologies of (a) TiO2 coating, (b) TiO2/MoS2 composite coating and (c) MoS2 particles (arrows in b indicate MoS2 particles in the composite coating).
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coefficient can be continuously and automatically recorded as a function of sliding time. The sliding tests were repeated three times for reliability and reproducibility. After the sliding tests, a Micro-XAM 3D non-contact surface profilometer was used to measure the morphologies and depth profiles of wear tracks. The wear volumes were then obtained from the area of wear track multiplied by the oscillating amplitude. Wear rates of specimens can be calculated from the equation: ω= V /(L ×N) (where ω is wear rate, V is wear volume, L is sliding distance and N is load). The worn surface morphologies of the specimens and the counterpart (steel balls) were examined using JSM-5600LV scanning electron microscope (SEM). A Thermon Scientific X-ray photoelectron spectroscope (XPS) with monochromatic Al-Kα radiation of energy 1486.6 eV was used to analyze the worn surface of steel ball counterpart after sliding against the PEO coating produced in the phosphate electrolyte with the addition of MoS2 particles. In the XPS analysis, the pass energy and spot size are 50 eV and 400 μm, respectively. 3. Results and discussion 3.1. Composition and microstructure Fig. 1 presents the XRD patterns of the PEO coatings produced in the electrolyte without and with the addition of MoS2 particles. Meanwhile, the pattern of pure MoS2 particles is also shown in the figure. Diffractions from rutile and anatase TiO2 were observed in the XRD patterns (Fig. 1a and b) for both PEO coatings. However, new diffraction peaks at 2θ of 14.14° and 39.92° can be defined for the PEO coating prepared from the electrolyte with the addition of MoS2 particles. These diffraction peaks corresponded to the MoS2
Fig. 4. EDS mapping of the elements of (a) Mo and (b) S in the TiO2/MoS2 composite coating.
crystals (JCPDS card no. 65–0160), indicating the presence of the MoS2 phase in the PEO coating. The XRD results clearly demonstrated that MoS2 particles dispersed in the electrolyte were successfully incorporated into the PEO coatings. According to the zeta potential measurement, the MoS2 suspensions in the phosphate electrolyte solution with the addition of the anionic surfactant showed a reasonably high zeta potential of − 18.1 mV and the zeta potential was found to be almost constant during the PEO process. Thus, it was
Fig. 3. Cross-sectional morphologies of (a) TiO2 coating and (b) TiO2/MoS2 composite coatings.
Fig. 5. Friction coefficients of (a) TiO2 coating and (b) TiO2/MoS2 composite coatings.
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expected that the negatively charged MoS2 particles could be drawn toward the anode (the Ti6Al4V alloy specimen) by the electric field (E) force between the poles of the electrochemical cell and was eventually incorporated into the TiO2 coating. Similar result was reported for the incorporation of graphite particles during the PEO process of titanium alloys [26]. On the basis of the compositions of the PEO coatings determined by the XRD analyses, the specimens treated in the phosphate electrolytes without and with the addition of MoS2 particles are addressed as “TiO2 coating” and “TiO2/MoS2 composite coating”, respectively, in this work. Fig. 2 demonstrates the surface morphologies of the TiO2 coating, TiO2/MoS2 composite coating and MoS2 particles. It can be seen that the surfaces of both coatings were characterized by micropores, irrespective of MoS2 particle addition. The size of the micropores ranged from a few micrometers to more than 15 μm and the shape of many micropores were irregular (Fig. 2a). There were no significant differences concerning the size and shape of the micropores on the surface between the TiO2 coating and TiO2/MoS2 composite coating. However, many particles can be observed on the surface of the TiO2/MoS2 composite coating (marked by arrows in Fig. 2b). The particles are flaky textured with the size around 1 μm, which are similar with the morphology of MoS2 particles added into the electrolyte (Fig. 2c). This observation further confirmed the presence of the MoS2 particles in the TiO2/MoS2 composite coating. It should also be noted that these particles were nonuniformly distributed over the surface and many of them appeared to concentrate in the micropores. Fig. 3 shows the cross-sectional morphologies of the TiO2 coating and TiO2/MoS2 composite coating. It revealed that both coatings were
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well bonded with the substrate and there were some micropores presented in the cross-section of the coatings. The TiO2 coating registered approximately 25 μm in thickness (Fig. 3a), while the TiO2/MoS2 composite coating was thinner with the thickness of around 15 μm (Fig. 3b). The EDS analysis of the cross-section of the TiO2/MoS2 composite coating was performed to investigate the distribution of the MoS2 particles in the coating. The EDS mapping shown in Fig. 4 demonstrated that the entire cross-section of coating contained the elements of Mo and S but the distribution of them was not uniform. High concentrations of Mo and S were detected in some micropores in the coating. The EDS analysis indicated that the distribution of the MoS2 particles in the coating was not uniform and enriched in the micropores. 3.2. Tribological properties 3.2.1. Friction and wear Fig. 5 shows the typical evolution of friction coefficient with sliding time under dry sliding condition for the TiO2 coating and TiO2/MoS2 composite coating. Distinctly different friction behaviors were recorded in 30 min sliding test. It can be seen from Fig. 5a that the friction coefficient of the TiO2 coating increased quickly along with the sliding time from 0.15 to 0.6 in 5 min. After that, the friction coefficient increased gradually to nearly 0.8 and then remained unchanged. Therefore, the TiO2 coating registered a high friction coefficient as sliding against steel ball under dry condition. For the TiO2/MoS2 composite coating, however, a low and stable friction coefficient of approximately 0.12 was recorded during the whole sliding period (Fig. 5b), showing a significant improvement in friction behavior compared to the TiO2 coating.
Fig. 6. 3D morphologies and corresponding depth profiles of the wear tracks of (a) TiO2 coating and (b) TiO2/MoS2 composite coatings.
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The improvement in friction behavior can be attributed to the presence of the MoS2 particles in the TiO2/MoS2 composite coating which acted as lubricants during the sliding tests. The 3D morphologies and depth profiles of the wear tracks for the TiO2 coating and TiO2/MoS2 composite coating after the dry sliding tests are shown in Fig. 6. A clear wear track was observed for the TiO2 coating (Fig. 6a). The width and depth of the wear track was around 500 μm and 3.4 μm, respectively. While the TiO2/MoS2 composite coating exhibited much shallower (around 160 μm in width) and narrower (less than 2.0 μm in depth) wear track (Fig. 6b) compared to the TiO2 coating. With the depth profiles, the specific wear rates of the TiO2 coating and TiO2/MoS2 composite coating were calculated to be 1.7 × 10 −5 mm 3/N m and 5.5 × 10 −6 mm 3/N m, respectively. It was clear that the wear rate of the TiO2/MoS2 composite coating was more than three times lower than that of the TiO2 coating. Therefore, on the basis of measured friction coefficients and wear rates, it can be concluded that the TiO2/MoS2 composite coating exhibited better tribological property under dry sliding condition in comparison with the TiO2 coating. 3.2.2. Worn surfaces analysis The difference in the tribological properties for the TiO2 coating and TiO2/MoS2 composite coating was further investigated by comparing the SEM morphologies of their worn surfaces as shown in Fig. 7. The detailed information of typical areas in the wear tracks are also on display. It can be seen from Fig. 7a that the TiO2 coating suffered from severe wear characterized by a wide and rough wear track. These observations suggested that the main wear mechanism of the TiO2 coating was abrasive wear [27]. Particularly, many large
fragments were observed on the worn surface (Fig. 7b), resulting from the detachment of the TiO2 coating in the sliding test. It was believed that the detachment was mainly ascribed to the brittle fracture of the TiO2 coating sliding against the steel ball. The brittle fracture was caused by the high frictional shear stress (resulted from high friction coefficient) between the coating and counterpart steel ball. Therefore, the abrasive wear and brittle fracture were the main wear damage for the TiO2 coating and caused its higher wear rate. Fig. 7c and d shows the SEM morphologies of worn surface of the TiO2/MoS2 composite coating sliding against steel ball. The wear track of the TiO2/MoS2 composite coating was much smaller (Fig. 7c) than that of the TiO2 coating. The entire worn surface appeared quite smooth and showed no evidence of appreciable detachment of the coating (Fig. 7d), indicating that only slight wear damage occurred in the sliding test. The decrease of the wear damage can be attributed to the presence of the MoS2 particles in the TiO2/MoS2 composite coating. The MoS2 particles distributed in the TiO2 coating (especially in the micropores) could be exposed to the sliding surface during the wear process. These exposed MoS2 particles were then involved in the wear process and partially transferred to the counterpart surface. The transferred MoS2 particles, acting as solid lubricants, decreased significantly the frictional shear stress between the coating and counterpart steel ball, thus reducing the friction coefficient and wear rate of the TiO2/MoS2 composite coating in the sliding test. The worn surfaces of the counterpart steel balls after sliding tests examined by SEM are shown in Fig. 8. The steel ball sliding against the TiO2 coating showed a large wear scar on the surface (Fig. 8a). Many grooves and fatigue spalling debris appeared on the worn surface. This indicated that the sliding test caused severe wear of the counterpart
Fig. 7. SEM micrographs of the worn surfaces of (a,b) TiO2 coating and (c,d) TiO2/MoS2 composite coatings (b and d are detailed information of typical areas in a and c, respectively).
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Fig. 8. SEM micrographs of the worn surfaces of (a) the counterpart steel ball sliding against the TiO2 coating and (b) the counterpart steel ball sliding against the TiO2/ MoS2 composite coating.
except for the TiO2 coating itself under dry sliding condition. For the steel ball sliding against the TiO2/MoS2 composite coating, however, the wear scar was relatively small (Fig. 8b), suggesting that only slight wear occurred for the counterpart ball in the sliding test. In addition, the worn surface was smooth and no obvious grooves and debris were observed on it. XPS analysis was also carried out to clarify the existence of the transfer material on the worn surface of the steel ball sliding against the TiO2/MoS2 composite coating. The XPS spectra presented in Fig. 9a and b exhibited that the elements of Mo and S can be detected on the worn surface, which confirmed that the MoS2 particles in the coating transferred to the counterpart surface in the sliding test.
4. Conclusions In this work, a MoS2-containing TiO2 composite coating on Ti6Al4V alloy was successfully produced by one-step PEO process in MoS2 particles-dispersed phosphate electrolyte. SEM and EDS examinations revealed that the MoS2 particles were distributed nonuniformly in the coating and many of them were concentrated into the micropores. The TiO2/MoS2 composite coating exhibited improved tribological property compared with the TiO2 coating under dry sliding condition, which reduced the friction coefficient from 0.8 to about 0.12 and decreased the wear rate from 1.7 ×10−5 mm3/N m to 5.5× 10−6 mm3/N m. The improvement of the tribological properties was attributed to the MoS2 particles in the coating transferred to the counterpart surface and acted as lubricants in the sliding test.
Fig. 9. XPS spectra of the worn surface of the counterpart steel ball sliding against the TiO2/MoS2 composite coating: (a) Mo3d and (b) S2p.
Acknowledgments The authors express their sincere thanks to “Hundred Talents Program” of Chinese Academy of Sciences (J. Liang), the Graduate Student Innovation Special Foundation Jiangxi province (YC2011-S078) and the National Natural Science Foundation of China (Grant no. 51005075) for the funding. References [1] H. Dong, in: K. Funatani, G.E. Totten (Eds.), Heat Treating: Proceedings of the 20th Conference, vols 1 & 2, ASM International, St. Louis, 2000, p. 170. [2] H.J. Spies, Surf. Eng. 26 (2010) 126. [3] A. Zhecheva, W. Sha, S. Malinov, A. Long, Surf. Coat.Technol. 200 (2005) 2192. [4] A.S. Korhonen, E. Harju, J. Mater. Eng. Perform. 9 (2000) 302. [5] X.Y. Liu, P.K. Chu, C.X. Ding, Mater. Sci. Eng. R 47 (2004) 49. [6] H.J. Brading, P.-H. Morton, T. Bell, L.G. Earwaker, Surf. Eng. 8 (1992) 206. [7] Z.X. Zhang, H. Dong, T. Bell, Surf. Coat.Technol. 200 (2006) 5237. [8] T.R. Rautray, R. Narayanan, T.Y. Kwon, K.H. Kim, J. Biomed. Mater. Res. 93B (2010) 581. [9] C. Bartuli, T. Valente, F. Casadei, M. Tului, Proc. Inst. Mech. Eng. Part L J. Mater. Des. Appl. 221 (2007) 175. [10] B.G. Guo, J.S. Zhou, S.T. Zhang, H.H. Zhou, Y.P. Pu, J.M. Chen, Mater. Sci. Eng. A 480 (2008) 404. [11] F.C. Walsh, C.T.J. Low, R.J.K. Wood, K.T. Stevens, J. Archer, A.R. Poeton, A. Ryder, Trans. Inst. Met. Finish. 87 (2009) 122. [12] P. Gupta, G. Tenhundfeld, E.O. Daigle, D. Ryabkov, Surf. Coat.Technol. 201 (2007) 8746. [13] M.D. Klapkiv, N.Y. Povstyana, H.M. Nykyforchyn, Mater. Sci. 42 (2006) 277. [14] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat.Technol. 130 (2000) 195. [15] Y.M. Wang, B.L. Jiang, L.X. Guo, T.C. Lei, Mater. Sci. Technol. 20 (2004) 1590.
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[16] F. Chen, H. Zhou, C. Chen, Y.J. Xia, Prog. Org. Coat. 64 (2009) 264. [17] X.Z. Lin, M.H. Zhu, J.F. Zheng, J. Luo, J.L. Mo, Trans. Nonferrous Met. Soc. China 20 (2010) 537. [18] S. Tsunekawa, Y. Aoki, H. Habazaki, Surf. Coat.Technol. 205 (2011) 4732. [19] M. Khorasanian, A. Dehghan, M.H. Shariat, M.E. Bahrololoom, S. Javadpour, Surf. Coat.Technol. 206 (2011) 1495. [20] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Appl. Surf. Sci. 246 (2005) 214. [21] C. Martini, L. Ceschini, F. Tartertini, J.M. Paillard, J.A. Curran, Wear 269 (2010) 747.
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Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
One-step preparation of TiO2/MoS2 composite coating on Ti6Al4V alloy by plasma electrolytic oxidation and its tribological properties Ming Mu a, b, Jun Liang a,⁎, Xinjian Zhou b, Qian Xiao b a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China The Ministry of Education Key Laboratory, Conveyance Tools and Equipment, East China Jiaotong University, Nanchang 330013, PR China
a r t i c l e
i n f o
Article history: Received 26 June 2012 Accepted in revised form 31 October 2012 Available online 13 November 2012 Keywords: Titanium alloy Plasma electrolytic oxidation Oxide coating MoS2 Friction and wear
a b s t r a c t A MoS2-containing oxide coating on Ti6Al4V alloy was prepared by one-step plasma electrolytic oxidation (PEO) process in a MoS2-dispersed phosphate electrolyte. The composition and microstructure of the oxide coatings produced in the electrolytes with and without the addition of MoS2 were analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Results showed that the MoS2 particles can be successfully incorporated into the oxide coating during the PEO process and were preferentially located in the micropores. The ball-on-disk sliding tests indicated that MoS2-containing oxide coating registered much lower friction coefficient and wear rate than the oxide coating without MoS2 under dry sliding condition. The improved tribological property of the MoS2-containing oxide coating was also discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Titanium alloys exhibit an ever increasing interest in the fields of automotive, aviation industry, exploitation of ocean, artificial implants of human body and so on, due to their high strength-to-weight, good corrosion resistance and excellent biocompatibility. However, the titanium alloys are characterized by low surface hardness, high friction coefficient and poor wear resistance, which has been a severe barrier to tribological applications [1,2]. The poor tribological properties of the titanium alloys can be overcome by means of loading specific surface treatments [3–5]. Many surface treatments, such as nitriding [6], thermal oxidation [7], ion implantation [8], spraying [9], laser modification [10] and plasma electrolytic oxidation (PEO) [11] have been conventionally used to improve the tribological properties of the titanium alloys. Among them, plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), has been proven to be an effective and economical method [12]. Through the PEO treatment, a thick, hard and well adherent ceramic-like coating could be produced on titanium alloys to provide applicable wear resistance [13]. The tribological properties of the PEO coatings prepared on titanium alloys have been studied by many scholars in recent years [14–19]. Most of these studies reported that the PEO coatings have been shown to remarkably improve the wear resistance of the titanium alloys but
⁎ Corresponding author. Tel.: +86 931 4968381; fax: +86 931 4968163. E-mail address: [email protected] (J. Liang). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.10.079
normally exhibit higher friction coefficients under unlubricated conditions. In many tribological applications, high friction coefficient could lead not only to the wear of the slider, but also to the wear damage of the counter material. Thus, PEO coatings with good wear resistance and low friction coefficient are necessary to provide better tribological properties. Many attempts have been made to reduce the friction coefficient of the PEO coatings on titanium alloys. For example, duplex approaches, in which low friction topcoats (e.g., graphite and PTFE) were deposited onto the PEO coatings, have been considered by some authors [20–22]. Notwithstanding, these approaches were surely effective in reducing the friction coefficient and improving the overall tribological properties of the PEO coatings, while, in any case, it was usual to obtain the coatings by multi-step. In addition, these coatings were subjected to losing the low friction property in case the low friction topcoats were worn out. An alternative approach to obtain PEO coatings with low friction property was to introduce low friction materials into the coating by modifying the electrolytes with the solid lubricant additives [23–25]. This approach was more effective than the duplex methods in practical applications, due to the PEO coatings contained low friction materials could be obtained by only one step. Besides, the coatings were expected to integrate the advantages of wear resistance of the PEO coating and low friction property of solid lubricants. To date, however, few investigations have been carried out concerning the preparation and properties of such coatings on titanium alloys. MoS2 is well-known for its low friction property and it is widely used as a solid lubricant. In the present work, we developed a method to introduce MoS2 particles into the PEO coatings to achieve low friction PEO coatings through a one-step PEO process. The composition, microstructure and tribological properties of the MoS2-containing PEO coatings were studied.
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2.3. Tribological evaluation A reciprocating friction and wear test-rig of UMT-2MT (CETR, USA) was used to evaluate the tribological properties of the PEO coatings prepared in the phosphate electrolytes without and with the addition of MoS2 particles under a ball-on-disk contact configuration. AISI52100 steel balls with a diameter of 3 mm were used as the counter materials. The hardness (HRC) of the steel ball was 62–63 and the average surface Ra was about 0.01 μm. All the friction and wear tests were performed using a constant normal load of 2 N with a frequency of 5 Hz and an oscillating amplitude of 5 mm for 30 min at ambient temperature and humidity (relative humidity is about 30%). During the test, the friction
Fig. 1. XRD patterns of PEO coatings prepared in the phosphate electrolytes (a) without and (b) with the addition of MoS2 particles.
2. Experimental 2.1. Preparation of PEO coatings Ti6Al4V alloy (6.020% Al, 4.100% V, 0.168% Fe, 0.160% O, 0.043% C and balance Ti, mass fraction) was used as the substrate in this study. The alloy was machined to a size of 8 mm × Ø24 mm and then polished to a surface roughness Ra of approximately 0.85 μm. Prior to the PEO process, all specimens were ultrasonically cleaned in the acetone and thoroughly dried in the air. The electrolyte for PEO process was prepared using 20.0 g/l Na3PO4 and 2.0 g/l KOH in distilled water with the addition of 20.0 g/l submicron sized MoS2 particles (particle size 0.5–1.0 μm), 100 ml/l ethanol and 0.5 g/l additive. The zeta potential of the MoS2 particles in the electrolyte was examined by a Zetasizer Nano ZS ZEN 3600 dynamic laser scattering particle size analyzer to determine its surface charge. The PEO processes were conducted on a bi-polar pulsed power source. The Ti6Al4V specimens and the stainless steel were used as the anode and the cathode, respectively. Coatings were produced at a constant current density of 8.0 A·dm −2 for 90 min. The temperature of the electrolytes was kept at 20 ± 2 °C by a water cooling system during the PEO processes. For comparison purposes, the Ti6Al4V specimens were also PEO treated using the same parameters in the phosphate electrolyte without the addition of MoS2 particles and other additives. The coated specimens were rinsed thoroughly with distilled water and dried in warm air after the PEO processes.
2.2. Composition and structure analyses The compositions of the PEO coatings prepared in the phosphate electrolytes without and with the addition of MoS2 particles were determined by a Siemens D5000 X-Ray Diffractometer (XRD), using Cu-Kα radiation as the excitation source with grazing angle of 2°. A JSM-5600LV scanning electron microscope (SEM) was employed to observe the surface and cross-sectional morphologies of the PEO coatings. The PEO coatings prepared in the phosphate electrolyte with the addition of MoS2 particles were further examined by energy dispersive X-ray spectroscopy (EDS) device attached to the JSM-6700F SEMHitachi S520 system to determine the distribution of Mo and S elements in the coating.
Fig. 2. Surface morphologies of (a) TiO2 coating, (b) TiO2/MoS2 composite coating and (c) MoS2 particles (arrows in b indicate MoS2 particles in the composite coating).
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coefficient can be continuously and automatically recorded as a function of sliding time. The sliding tests were repeated three times for reliability and reproducibility. After the sliding tests, a Micro-XAM 3D non-contact surface profilometer was used to measure the morphologies and depth profiles of wear tracks. The wear volumes were then obtained from the area of wear track multiplied by the oscillating amplitude. Wear rates of specimens can be calculated from the equation: ω= V /(L ×N) (where ω is wear rate, V is wear volume, L is sliding distance and N is load). The worn surface morphologies of the specimens and the counterpart (steel balls) were examined using JSM-5600LV scanning electron microscope (SEM). A Thermon Scientific X-ray photoelectron spectroscope (XPS) with monochromatic Al-Kα radiation of energy 1486.6 eV was used to analyze the worn surface of steel ball counterpart after sliding against the PEO coating produced in the phosphate electrolyte with the addition of MoS2 particles. In the XPS analysis, the pass energy and spot size are 50 eV and 400 μm, respectively. 3. Results and discussion 3.1. Composition and microstructure Fig. 1 presents the XRD patterns of the PEO coatings produced in the electrolyte without and with the addition of MoS2 particles. Meanwhile, the pattern of pure MoS2 particles is also shown in the figure. Diffractions from rutile and anatase TiO2 were observed in the XRD patterns (Fig. 1a and b) for both PEO coatings. However, new diffraction peaks at 2θ of 14.14° and 39.92° can be defined for the PEO coating prepared from the electrolyte with the addition of MoS2 particles. These diffraction peaks corresponded to the MoS2
Fig. 4. EDS mapping of the elements of (a) Mo and (b) S in the TiO2/MoS2 composite coating.
crystals (JCPDS card no. 65–0160), indicating the presence of the MoS2 phase in the PEO coating. The XRD results clearly demonstrated that MoS2 particles dispersed in the electrolyte were successfully incorporated into the PEO coatings. According to the zeta potential measurement, the MoS2 suspensions in the phosphate electrolyte solution with the addition of the anionic surfactant showed a reasonably high zeta potential of − 18.1 mV and the zeta potential was found to be almost constant during the PEO process. Thus, it was
Fig. 3. Cross-sectional morphologies of (a) TiO2 coating and (b) TiO2/MoS2 composite coatings.
Fig. 5. Friction coefficients of (a) TiO2 coating and (b) TiO2/MoS2 composite coatings.
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expected that the negatively charged MoS2 particles could be drawn toward the anode (the Ti6Al4V alloy specimen) by the electric field (E) force between the poles of the electrochemical cell and was eventually incorporated into the TiO2 coating. Similar result was reported for the incorporation of graphite particles during the PEO process of titanium alloys [26]. On the basis of the compositions of the PEO coatings determined by the XRD analyses, the specimens treated in the phosphate electrolytes without and with the addition of MoS2 particles are addressed as “TiO2 coating” and “TiO2/MoS2 composite coating”, respectively, in this work. Fig. 2 demonstrates the surface morphologies of the TiO2 coating, TiO2/MoS2 composite coating and MoS2 particles. It can be seen that the surfaces of both coatings were characterized by micropores, irrespective of MoS2 particle addition. The size of the micropores ranged from a few micrometers to more than 15 μm and the shape of many micropores were irregular (Fig. 2a). There were no significant differences concerning the size and shape of the micropores on the surface between the TiO2 coating and TiO2/MoS2 composite coating. However, many particles can be observed on the surface of the TiO2/MoS2 composite coating (marked by arrows in Fig. 2b). The particles are flaky textured with the size around 1 μm, which are similar with the morphology of MoS2 particles added into the electrolyte (Fig. 2c). This observation further confirmed the presence of the MoS2 particles in the TiO2/MoS2 composite coating. It should also be noted that these particles were nonuniformly distributed over the surface and many of them appeared to concentrate in the micropores. Fig. 3 shows the cross-sectional morphologies of the TiO2 coating and TiO2/MoS2 composite coating. It revealed that both coatings were
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well bonded with the substrate and there were some micropores presented in the cross-section of the coatings. The TiO2 coating registered approximately 25 μm in thickness (Fig. 3a), while the TiO2/MoS2 composite coating was thinner with the thickness of around 15 μm (Fig. 3b). The EDS analysis of the cross-section of the TiO2/MoS2 composite coating was performed to investigate the distribution of the MoS2 particles in the coating. The EDS mapping shown in Fig. 4 demonstrated that the entire cross-section of coating contained the elements of Mo and S but the distribution of them was not uniform. High concentrations of Mo and S were detected in some micropores in the coating. The EDS analysis indicated that the distribution of the MoS2 particles in the coating was not uniform and enriched in the micropores. 3.2. Tribological properties 3.2.1. Friction and wear Fig. 5 shows the typical evolution of friction coefficient with sliding time under dry sliding condition for the TiO2 coating and TiO2/MoS2 composite coating. Distinctly different friction behaviors were recorded in 30 min sliding test. It can be seen from Fig. 5a that the friction coefficient of the TiO2 coating increased quickly along with the sliding time from 0.15 to 0.6 in 5 min. After that, the friction coefficient increased gradually to nearly 0.8 and then remained unchanged. Therefore, the TiO2 coating registered a high friction coefficient as sliding against steel ball under dry condition. For the TiO2/MoS2 composite coating, however, a low and stable friction coefficient of approximately 0.12 was recorded during the whole sliding period (Fig. 5b), showing a significant improvement in friction behavior compared to the TiO2 coating.
Fig. 6. 3D morphologies and corresponding depth profiles of the wear tracks of (a) TiO2 coating and (b) TiO2/MoS2 composite coatings.
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The improvement in friction behavior can be attributed to the presence of the MoS2 particles in the TiO2/MoS2 composite coating which acted as lubricants during the sliding tests. The 3D morphologies and depth profiles of the wear tracks for the TiO2 coating and TiO2/MoS2 composite coating after the dry sliding tests are shown in Fig. 6. A clear wear track was observed for the TiO2 coating (Fig. 6a). The width and depth of the wear track was around 500 μm and 3.4 μm, respectively. While the TiO2/MoS2 composite coating exhibited much shallower (around 160 μm in width) and narrower (less than 2.0 μm in depth) wear track (Fig. 6b) compared to the TiO2 coating. With the depth profiles, the specific wear rates of the TiO2 coating and TiO2/MoS2 composite coating were calculated to be 1.7 × 10 −5 mm 3/N m and 5.5 × 10 −6 mm 3/N m, respectively. It was clear that the wear rate of the TiO2/MoS2 composite coating was more than three times lower than that of the TiO2 coating. Therefore, on the basis of measured friction coefficients and wear rates, it can be concluded that the TiO2/MoS2 composite coating exhibited better tribological property under dry sliding condition in comparison with the TiO2 coating. 3.2.2. Worn surfaces analysis The difference in the tribological properties for the TiO2 coating and TiO2/MoS2 composite coating was further investigated by comparing the SEM morphologies of their worn surfaces as shown in Fig. 7. The detailed information of typical areas in the wear tracks are also on display. It can be seen from Fig. 7a that the TiO2 coating suffered from severe wear characterized by a wide and rough wear track. These observations suggested that the main wear mechanism of the TiO2 coating was abrasive wear [27]. Particularly, many large
fragments were observed on the worn surface (Fig. 7b), resulting from the detachment of the TiO2 coating in the sliding test. It was believed that the detachment was mainly ascribed to the brittle fracture of the TiO2 coating sliding against the steel ball. The brittle fracture was caused by the high frictional shear stress (resulted from high friction coefficient) between the coating and counterpart steel ball. Therefore, the abrasive wear and brittle fracture were the main wear damage for the TiO2 coating and caused its higher wear rate. Fig. 7c and d shows the SEM morphologies of worn surface of the TiO2/MoS2 composite coating sliding against steel ball. The wear track of the TiO2/MoS2 composite coating was much smaller (Fig. 7c) than that of the TiO2 coating. The entire worn surface appeared quite smooth and showed no evidence of appreciable detachment of the coating (Fig. 7d), indicating that only slight wear damage occurred in the sliding test. The decrease of the wear damage can be attributed to the presence of the MoS2 particles in the TiO2/MoS2 composite coating. The MoS2 particles distributed in the TiO2 coating (especially in the micropores) could be exposed to the sliding surface during the wear process. These exposed MoS2 particles were then involved in the wear process and partially transferred to the counterpart surface. The transferred MoS2 particles, acting as solid lubricants, decreased significantly the frictional shear stress between the coating and counterpart steel ball, thus reducing the friction coefficient and wear rate of the TiO2/MoS2 composite coating in the sliding test. The worn surfaces of the counterpart steel balls after sliding tests examined by SEM are shown in Fig. 8. The steel ball sliding against the TiO2 coating showed a large wear scar on the surface (Fig. 8a). Many grooves and fatigue spalling debris appeared on the worn surface. This indicated that the sliding test caused severe wear of the counterpart
Fig. 7. SEM micrographs of the worn surfaces of (a,b) TiO2 coating and (c,d) TiO2/MoS2 composite coatings (b and d are detailed information of typical areas in a and c, respectively).
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Fig. 8. SEM micrographs of the worn surfaces of (a) the counterpart steel ball sliding against the TiO2 coating and (b) the counterpart steel ball sliding against the TiO2/ MoS2 composite coating.
except for the TiO2 coating itself under dry sliding condition. For the steel ball sliding against the TiO2/MoS2 composite coating, however, the wear scar was relatively small (Fig. 8b), suggesting that only slight wear occurred for the counterpart ball in the sliding test. In addition, the worn surface was smooth and no obvious grooves and debris were observed on it. XPS analysis was also carried out to clarify the existence of the transfer material on the worn surface of the steel ball sliding against the TiO2/MoS2 composite coating. The XPS spectra presented in Fig. 9a and b exhibited that the elements of Mo and S can be detected on the worn surface, which confirmed that the MoS2 particles in the coating transferred to the counterpart surface in the sliding test.
4. Conclusions In this work, a MoS2-containing TiO2 composite coating on Ti6Al4V alloy was successfully produced by one-step PEO process in MoS2 particles-dispersed phosphate electrolyte. SEM and EDS examinations revealed that the MoS2 particles were distributed nonuniformly in the coating and many of them were concentrated into the micropores. The TiO2/MoS2 composite coating exhibited improved tribological property compared with the TiO2 coating under dry sliding condition, which reduced the friction coefficient from 0.8 to about 0.12 and decreased the wear rate from 1.7 ×10−5 mm3/N m to 5.5× 10−6 mm3/N m. The improvement of the tribological properties was attributed to the MoS2 particles in the coating transferred to the counterpart surface and acted as lubricants in the sliding test.
Fig. 9. XPS spectra of the worn surface of the counterpart steel ball sliding against the TiO2/MoS2 composite coating: (a) Mo3d and (b) S2p.
Acknowledgments The authors express their sincere thanks to “Hundred Talents Program” of Chinese Academy of Sciences (J. Liang), the Graduate Student Innovation Special Foundation Jiangxi province (YC2011-S078) and the National Natural Science Foundation of China (Grant no. 51005075) for the funding. References [1] H. Dong, in: K. Funatani, G.E. Totten (Eds.), Heat Treating: Proceedings of the 20th Conference, vols 1 & 2, ASM International, St. Louis, 2000, p. 170. [2] H.J. Spies, Surf. Eng. 26 (2010) 126. [3] A. Zhecheva, W. Sha, S. Malinov, A. Long, Surf. Coat.Technol. 200 (2005) 2192. [4] A.S. Korhonen, E. Harju, J. Mater. Eng. Perform. 9 (2000) 302. [5] X.Y. Liu, P.K. Chu, C.X. Ding, Mater. Sci. Eng. R 47 (2004) 49. [6] H.J. Brading, P.-H. Morton, T. Bell, L.G. Earwaker, Surf. Eng. 8 (1992) 206. [7] Z.X. Zhang, H. Dong, T. Bell, Surf. Coat.Technol. 200 (2006) 5237. [8] T.R. Rautray, R. Narayanan, T.Y. Kwon, K.H. Kim, J. Biomed. Mater. Res. 93B (2010) 581. [9] C. Bartuli, T. Valente, F. Casadei, M. Tului, Proc. Inst. Mech. Eng. Part L J. Mater. Des. Appl. 221 (2007) 175. [10] B.G. Guo, J.S. Zhou, S.T. Zhang, H.H. Zhou, Y.P. Pu, J.M. Chen, Mater. Sci. Eng. A 480 (2008) 404. [11] F.C. Walsh, C.T.J. Low, R.J.K. Wood, K.T. Stevens, J. Archer, A.R. Poeton, A. Ryder, Trans. Inst. Met. Finish. 87 (2009) 122. [12] P. Gupta, G. Tenhundfeld, E.O. Daigle, D. Ryabkov, Surf. Coat.Technol. 201 (2007) 8746. [13] M.D. Klapkiv, N.Y. Povstyana, H.M. Nykyforchyn, Mater. Sci. 42 (2006) 277. [14] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat.Technol. 130 (2000) 195. [15] Y.M. Wang, B.L. Jiang, L.X. Guo, T.C. Lei, Mater. Sci. Technol. 20 (2004) 1590.
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