- Email: [email protected]
A novel assembly of MoS2-PTFE solid lubricants into wear-resistant micro-hole array template and corresponding tribological performance
Optics and Laser Technology 116 (2019) 171–179
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
A novel assembly of MoS2-PTFE solid lubricants into wear-resistant microhole array template and corresponding tribological performance
T
A.H. Wanga, , J. Xiaa, Z.X. Yanga,b, D.H. Xiongb ⁎
a
State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China b Wuhan Huagong Laser Engineering Co., Ltd., Wuhan 430223, PR China
HIGHLIGHTS
templates were produced by laser drilling on the laser-clad wear-resistant coatings, mixtures of MoS and PTFE powders were uniformly deposited • Micro-hole into the laser-drilled micro-hole templates by electrophoretic deposition. sintering at 300 °C densified the EPD coatings without oxidization of MoS , but increasing sintering temperature to above 350 °C resulted in oxidi• Microwave zation of MoS into MoO . friction coefficient of the MoS -10wt.%PTFE coating with 20% micro-hole density sintered at 300 °C presented the lowest friction coefficient amongst the all • The coatings. 2
2
2
2
2
ARTICLE INFO
ABSTRACT
Keywords: Laser-drilled micro-hole template MoS2 and PTFE Electrophoretic deposition Microwave sintering Tribological performance
Special blind micro-hole array templates were constructed by fiber laser drilling on laser-clad wear-resistant Febased alloy coatings with aim to carry and to preserve solid lubricants into micro-hole reservoir. The influence of laser parameters on the morphology of blind holes was investigated. The blind U-shape micro-hole array templates, which are the most suitable for subsequent electrophoretic deposition (EDP), were fabricated by the optimized laser drilling parameters. Different lubricant mixtures of MoS2 and PTFE powder were assembled into the blind micro-hole array templates with different micro-hole density by EDP. The deposited mixtures of MoS2 and PTFE powder were densified by microwave sintering. The tribological performance evaluated by a ring-onblock wear tester indicated that the 20% micro-hole density template assembled with MoS2-10wt%PTFE powder presented the lowest friction coefficient.
1. Introduction Molybdenum disulfide (MoS2) has been widely considered as an effective solid lubricant due to its strong covalent intralayer and weak van der Waals interlayer interactions [1]. A lot of work based on functional MoS2 has been mainly produced in form of MoS2 film or coating through RF PVD, thermal spraying and laser cladding methods [2–4]. However, MoS2 lubricant films produced by PVD or thermal spraying method are readily to fail under high surface loading in many cases because of their low adhesion strength with substrates [5]. A duplex technique based on the use of laser texturing together to the application of a solid lubricant on the textured surfaces has been proposed in order to reduce the friction coefficient and enlarge the lifetime of the alloy in service [6,7]. The generation of grooves or
⁎
Corresponding author. E-mail address: [email protected] (A.H. Wang).
https://doi.org/10.1016/j.optlastec.2019.03.033 Received 8 March 2019; Accepted 17 March 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
dimples patterns on the alloy surface can act as lubricant reservoirs because the lubricant can be stored in textured cavities and then released during the friction process, effectively reducing the friction coefficient of titanium alloys [7–9]. However, the loose lubricants dispersing in the textured zone were hard to be kept in the grooves, which resulted in uneven surface and deteriorated wear resistant. Consequently, the maintenance of lubricants in the textured zone becomes particularly important for increasing lifetime of the lubricants. Polytetrafluoroethylene (PTFE) is a thermoplastic polymer with a melting point of 600 K, maintains high strength, toughness and selflubrication at low temperatures down to 5 K, and good flexibility at temperatures above 194 K. The frictional coefficient of PTFE against polished steel is usually 0.05–0.10, which is the third-lowest of any known solid material. However, PTFE exhibits poor wear and abrasion
Optics and Laser Technology 116 (2019) 171–179
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Fig. 1. Microstructure of the laser-clad Fe-Cr-Ni-C coating.
60 V for 20 min. Microwave sintering of the deposited templates was carried out by a microwave oven with frequency 2.45 GHz and power 1.4 kW (Changsha SYNO-THERM Corporation, China) in order to densify the deposited lubricants. The deposited micro-hole templates were buried by silicon carbide and alumina powder mixture for rapid and uniform heating. Microwave sintering was conducted at four different temperatures (300 °C, 350 °C, 400 °C and 500 °C) with a heating rate of 3 °C/min in high pure argon atmosphere (99.99%). Accurate temperature of samples was measured by an infrared thermometer and controlled by changing microwave power. The deposited micro-hole templates were heated to the above each sintering temperature and kept warm for 120 min, finally, the sintered samples were cooled down to room temperature in air. Frictional behavior of the sintered samples was evaluated using a MM-U10G ring-on-block wear tester at room temperature. The sliding was performed at the laser-clad Fe-0.1C-15.0Cr-0.5Ni coating block (with dimension of 30 mm × 30 mm × 12 mm) sliding against the deposited and sintered composite template ring. The sliding tests were carried in air out under applied load of 100 N, rotation speed of 50 rpm and test time of 180 min. The friction coefficient was continuously recorded by the computer connected to the sensor of the tester. The ring surfaces were all ground and polished to remove the deposited lubricants outside the micro-holes and then washed in acetone prior to sliding test.
Table 1 Different ratio of the suspension (g/L). No.
MoS2
PTFE
Weight percent of PTFE
1 2 3 4 5
12.5 11.875 11.25 10.625 10
0 0.625 1.25 1.875 2.5
0 5 10 15 20
resistance, leading to early failure and leakage problems in the seals. The wear resistance of PTFE can be significantly improved by addition of suitable filler materials [10]. Therefore, the complementation of both WS2 and PTFE probably exhibits good friction and wear performance. In the present paper, a special blind micro-hole array template produced by laser drilling on the laser-clad wear-resistant coating was deposited with lubricant mixtures of MoS2 and PTFE by electrophoretic deposition (EDP) method, and then densified by microwave sintering. The tribological performance was evaluated by a ringon-block wear tester. 2. Experimental procedure Wear resistant coatings with a composition of Fe-0.1C-15.0Cr-0.5Ni (at wt%) and a microhardness of about HV0.1 650 were produced by laser cladding onto a medium-carbon steel (0.45C) using following laser parameters, i.e., laser power 1.8 kW, scanning velocity 15 mm·s−1, beam diameter 5 mm, coating thickness 1.5 mm, overlapping ratio 50%. The cross-section of the template substrate is shown in Fig. 1. The surfaces of the laser-clad samples were ground down to 1500grit, ultrasonically cleaned and then dried in an oven before laser drilling. A LMC2011 type IPG fiber laser with wavelength of 1064 nm was adopted to drill blind micro-hole arrays on the surfaces of the laser-clad coatings. Laser drilling experiment was carried out in air atmosphere by changing laser beam defocusing distance with an aim to optimize the micro-hole profile. The optimized laser parameters were then used to produce blind micro-hole array templates for subsequent electrophoretic deposition. Different ratio of MoS2 powder (about 1 μm) and PTFE powder (about 0.5 μm, molecular weight 104–105) listed in Table 1 were firstly surface activated by C16H26O2 with a solid content of 10 g/L and then dispersed by distilled water with a solid content of 5 g/L. The suspension was stirred for 1 h by a magnetic stirring apparatus and then dispersed for 1 h by an ultrasonic device. Two parallel stainless steel plates with a distance of 15 mm were used as the anode while the laser-clad substrates were used as the cathode. The micro-hole templates were deposited by electrophoretic deposition (EPD) at an applied voltage of
3. Results and discussion Under the condition of average power 20 W, pulse width 200 ns, frequency 25 kHz and laser irradiation time 2 ms, the profile of the laser-drilled blind hole influenced by laser beam defocusing distance is shown in Fig. 2. The cross-sections of the laser-drilled micro-holes at different laser beam defocusing distance are displayed in Fig. 2. Three typical profiles were observed, i.e., V-shape micro-hole at a variation of defocusing distance from −0.2 mm to −0.1 mm (Fig. 2a and b), Ushape micro-hole at the defocusing distance at 0 mm (Fig. 2c), and drum-shape micro-hole at the defocusing distance ranging from 1 mm to 2 mm (Fig. 2d and f). Amongst three kinds of profiles, the V-shape blind micro-holes are easy to be deposited by EPD, but the V-shape wall has poor restriction to the deposited solid lubricant particles during frictional wear process, the drum-shape micro-holes are hard to be deposited because of their small entrance size. Therefore, the ideal profile for this special application is the U-shape micro-hole, by which the particles were easier to be deposited into the whole blind microholes, since the entrance of the U-shape micro-hole had no shadowing effect to the deposited particles because of lager entrance size in comparison with the drum-shape micro-holes. At the same time, the Ushape micro-hole can trap and hold the deposited solid lubricant
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Fig. 2. Optical micrograph showing the shape of the micro-hole drilled at different defocusing distance. (a) −2 mm, (b) −1 mm, (c) 0 mm, (d) 1 mm, (e) 2 mm.
HNO3 = 1:3) for about 25 s for cleaning the spatter around the laserdrilled holes. Fig. 3 shows the typical surface and cross-section of the EPD coating on the micro-hole template. Uniform and dense deposition coatings into the laser-drilled micro-holes and onto the surface of the template were produced by the optimized EDP parameters, i.e., the distance of two parallel stainless steel plates 15 mm, applied voltage 60 V and deposition time 20 min. Fig. 4 shows the EPD samples sintered by microwave at different sintering temperatures for 2 h. No change in surface color of the EPD coatings at the sintering temperature less than 300 °C indicates that there was no chemical reaction during sintering process. The surface
Table 2 Micro-hole density and area ratio at different pitch. Pitch (μm) Area ratio (%)
196 10
138 20
113 30
98 40
particles in the afterward-sintering process and frictional wear applications. Consequently, the U-shape blind micro-hole templates were chosen for the subsequent EPD. The templates with different micro-hole density and different pitch were produced for EPD process, as listed in Table 2. Prior to EPD process, the templates were immersed in a acidic solution (HF/
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Fig. 3. Morphologies of the EPD coating. (a) Top view, (b) Cross-section view.
Fig. 4. Macroscopic morphologies of the sintered samples. (1) Non-sintering, (2) 300 °C, (3) 350 °C, (4) 400 °C, (5) 500 °C.
color became dark at the sintering temperature of both 350 °C and 400 °C, the higher the sintering temperature, the deeper the surface color. The surface color becoming brown at the sintering temperature of 500 °C indicates that obvious chemical reaction happened. Fig. 5 shows the surface morphologies of the EPD coatings on the laser-drilled microhole templates. It also supports that the surface morphologies at the sintering temperature of 300 °C and 350 °C changed slightly, became uneven and granuliform at the sintering temperature of 400 °C, and turned to porous at the sintering temperature of 500 °C. Fig. 6 shows XRD patterns of the EPD coatings sintered at different sintering temperature. Only MoS2 peaks found at 300 °C indicates no oxidization occurred at this sintering temperature, and weak MoO2 phase peak found at 350 °C indicates that slight oxidization happened at this temperature. The intensity of MoO2 phase peaks increases with an increase in sintering temperature from 350 °C to 500 °C and only MoO2 phase peaks at 500 °C indicates that nearly all MoS2 was oxidized. Normally, the oxidization process can be described as following Eqs. (1) and (2), MoS2 was firstly oxidized into MoO2 and SO2, then SO2 was vaporized, finally the MoO2 was oxidized into MoO3 furthermore. No MoO3 peaks found at all sintering temperatures means further oxidization of MoO2 was prohibited because of low oxygen partial pressure in the microwave oven chamber. MoS2 + O2 → MoO2 + SO2
(1)
MoO2 + O2 → MoO3
(2)
environment, oxygen source introduced from three aspects resulted in low oxygen partial pressure in the microwave oven chamber: (1) the oxygen adsorption of MoS2 particles; (2) the low vacuum degree of the microwave; (3) the low purity of Ar gas 99.99%. Fig. 7 shows the influence of sintering temperature on the friction coefficient of the MoS2-10wt%PTFE coating with 20% micro-hole density. The phenomena that the friction coefficient increases with an increase in sintering temperature from 300 °C to 400 °C could be explained by the oxidization of MoS2 at both 350 °C and 400 °C (see Fig. 6). The lower friction coefficient at sintering temperature of 300 °C in comparison with that of the unsintered sample verifies that the densification by microwave sintering could reduce the friction coefficient. The SEM observation shown in Fig. 8 displays that the microstructure before and after sintering is characterized by a mixture of particulate MoS2 and fibrous PTFE, fuzzy MoS2 particles surrounded by flocculent PTFE, respectively. The PTFE becoming viscous flow state at 300 °C could diffuse between MoS2 particle gaps and interconnect MoS2 particles at the same time, which improved the bond strength between lubricant particles and reduced friction coefficient. Fig. 9 shows the influence of PTFE content on the friction coefficient of the samples with 20% micro-hole density at sintering temperature of 300 °C. The pure MoS2 coating presents very low friction coefficient (about 0.15) at the preliminary stage and high friction coefficient (about 0.7) after testing time of 4500 s, which means the loss of lubrication function. The friction coefficient increases from about 0.25 to about 0.42 with an increase in PTFE content from 5 wt% to 20 wt% except the lowest friction coefficient (from 0.07 to 0.15) at the PTFE
The sintering process was carried out in Ar atmosphere
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Fig. 5. Surface morphologies of the sintered EPD coatings on the templates. (1) Non-sintering, (2) 300 °C, (3) 350 °C, (4) 400 °C, (5) 500 °C.
content of 10%. The pure MoS2 coating possesses very low friction coefficient (about 0.15) at the preliminary stage because of its lamellar structure and weak Van der Waal’s force between lamels. The poor binding interaction amongst MoS2 particles in the micro-holes due to lack of PTFE resulted in spallation and loss of MoS2 particles from the micro-holes, as shown in Fig. 10(a), so high friction coefficient (about 0.7) was measured after testing time of 4500 s. Addition of PTFE content to 5 wt% into the coating could both form discontinuous lubricant film and stabilize the friction coefficient to some extent in comparison
with the pure MoS2 coating because of the binding interaction amongst MoS2, as shown in Fig. 10(b). Addition of PTFE content to 10 wt%, 15 wt% and 20 wt% into the coating generated uniform lubricant films (see Fig. 10(c)–(e)) and prevented spallation of MoS2 particles in the micro-holes, owing to the reticular structure (see Fig. 8(b)) sintered at 300 °C and good binding interaction amongst MoS2 particles, which slowed down the loss of solid lubrication materials in the process of friction and maintained constant friction coefficient for long time (see Fig. 9). The MoS2-10 wt% coating presented the lowest friction
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Fig. 9. Influence of PTFE content on friction coefficient.
coefficient amongst all the MoS2-PTFE coatings, but the influence mechanism of PTFE content on friction coefficient is not clear up to now. Fig. 11 displays the influence of micro-hole density on the friction coefficient of the EPD MoS2-10wt%PTFE coating sintered at 300 °C for 2 h. An increase in micro-hole density from 10% to 20% and from 20% to 40% resulted in reduce of friction coefficient and increase of friction coefficient, respectively. It can be understood that the friction coefficient of the 20% micro-hole density sample is much lower than that of the 10% micro-hole density sample since solid lubricant film with higher micro-hole density was readily to be generated owing to larger original area fraction of solid lubricants, as shown in Fig. 12(a) and (b). However, an increase in micro-hole density up to 30% and 40% led to an increase in friction coefficient, and the higher the micro-hole density, the larger the friction coefficient, as shown in Fig. 11. Observation of the worn surface morphologies indicates that, friction scratches were found for the samples with 10%, 30% and 40% micro-hole density, and the higher the micro-hole density, the severer the friction scratches, as shown in Fig. 12(a), (c) and (d). This phenomenon could be explained by the surface strength of the micro-hole samples, i.e., increasing the micro-hole density is equivalent to decreasing surface strength and macroscopic hardness, which resulted in an increase in friction coefficient and number of friction scratches, even larger fraction of solid lubricants existed.
Fig. 6. XRD patterns of the EPD coatings at different sintering temperature.
Fig. 7. Influence of sintering temperature on friction coefficient.
Fig. 8. SEM morphologies of the MoS2-10wt.%PTFE in the micro-hole zone. (a) unsintered (b) sintered at 300 °C for 2 h.
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Fig. 10. Influence of PTFE content on the worn surface morphologies. (a) 0%, (b) 5%, (c) 10%, (d) 15%, (e) 20%.
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4. Conclusions 1. Laser-clad wear-resistant Fe-based coatings were laser-drilled by optimized laser drilling parameters, i.e., average power 20 W, pulse width 200 ns, frequency 25 kHz, laser irradiation time 2 ms and defocusing distance 0 mm, to produce U-shape micro-hole templates, which are the most suitable for subsequent EPD. 2. Mixtures of MoS2 and PTFE powders were uniformly deposited into the laser-drilled micro-hole templates by electrophoretic deposition. 3. Microwave sintering at 300 °C could densify the EPD coatings without oxidization of MoS2, but increasing sintering temperature to above 350 °C resulted in oxidization of MoS2 into MoO2. 4. The friction coefficient of MoS2-10wt.%PTFE coating with 20% micro-hole density sintered at 300 °C presented the lowest friction coefficient amongst the unsintered coating, the coatings sintered at 350 °C and 400 °C owing to reticular structure. 5. The friction coefficient increases from about 0.25 to about 0.42 with an increase in PTFE content from 5 wt% to 20 wt% except the lowest friction coefficient (about 0.15) at the PTFE content of 10%, which was proved by the worn surface morphologies.
Fig. 11. Influence of the micro-hole density on the friction coefficient.
Fig. 12. Influence of the micro-hole density on the worn surface morphologies. (a) 10%, (b) 20%, (c) 30%, (d) 40%.
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Acknowledgement
Ceram. Int. 44 (2018) 17864–17872. [4] H. Torres, T. Vuchkov, M. Rodríguez Ripoll, B. Prakash, Tribological behaviour of MoS2based self-lubricating laser cladding for use in high temperature applications, Tribol. Int. 126 (2018) 153–165. [5] C. Donneta, A.E., Solid lubricant coatings: recent developments and future trends, Tribol. Lett. 3 (2004) 389–397. [6] T. Hu, Y. Zhang, L. Hu, Tribological investigation of MoS2 coatings deposited on the laser textured surface, Wear 278–279 (2012) 77–82. [7] M.A. Arenas, J.I. Ahuir-Torres, I. García, H. Carvajal, J. de Damborenea, Tribological behaviour of laser textured Ti6Al4V alloy coated with MoS2 and graphene, Tribol. Int. 128 (2018) 240–247. [8] Z. Wu, Y. Xing, P. Huang, L. Liu, Tribological properties of dimple-textured titanium alloys under dry sliding contact, Surf. Coat. Technol. 3092 (2017) 21–28. [9] T.C. Hu, L.L. Hu, Q. Ding, The effect of laser surface texturing on the tribological behavior of Ti-6Al-4V, Proc. Inst. Mech. Eng. J., J. Eng. Tribol. 226 (2012) 854–863. [10] Jaydeep Khedkar, Ioan Negulescu, Efstathios I. Meletis, Sliding wear behavior of PTFE composites, Wear 252 (2002) 361–369.
The authors would like to thank the financial support provided by National Key Research and Development Program of China under contract No. 2016YFB1102700. References [1] K. Gong, X. Wu, G. Zhao, X. Wang, Nanosized MoS2 deposited on graphene as lubricant additive in polyalkylene glycol for steel/steel contact at elevated temperature, Tribol. Int. 110 (2017) 1–7. [2] Elisabetta Serpini, Alberto Rota, Antonio Ballestrazzi, Diego Marchetto, Enrico Gualtieri, Sergio Valeri, The role of humidity and oxygen on MoS2 thin films deposited by RF PVD magnetron sputtering, Surf. Coat. Tech. 319 (2017) 345–352. [3] Shuangjian Li, Xiaoqin Zhao, Yulong An, Duanduan Liu, Huidi Zhou, Jianmin Chen, YSZ/ MoS2 self-lubricating coating fabricated by thermal spraying and hydrothermal reaction,
179
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
A novel assembly of MoS2-PTFE solid lubricants into wear-resistant microhole array template and corresponding tribological performance
T
A.H. Wanga, , J. Xiaa, Z.X. Yanga,b, D.H. Xiongb ⁎
a
State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science & Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China b Wuhan Huagong Laser Engineering Co., Ltd., Wuhan 430223, PR China
HIGHLIGHTS
templates were produced by laser drilling on the laser-clad wear-resistant coatings, mixtures of MoS and PTFE powders were uniformly deposited • Micro-hole into the laser-drilled micro-hole templates by electrophoretic deposition. sintering at 300 °C densified the EPD coatings without oxidization of MoS , but increasing sintering temperature to above 350 °C resulted in oxidi• Microwave zation of MoS into MoO . friction coefficient of the MoS -10wt.%PTFE coating with 20% micro-hole density sintered at 300 °C presented the lowest friction coefficient amongst the all • The coatings. 2
2
2
2
2
ARTICLE INFO
ABSTRACT
Keywords: Laser-drilled micro-hole template MoS2 and PTFE Electrophoretic deposition Microwave sintering Tribological performance
Special blind micro-hole array templates were constructed by fiber laser drilling on laser-clad wear-resistant Febased alloy coatings with aim to carry and to preserve solid lubricants into micro-hole reservoir. The influence of laser parameters on the morphology of blind holes was investigated. The blind U-shape micro-hole array templates, which are the most suitable for subsequent electrophoretic deposition (EDP), were fabricated by the optimized laser drilling parameters. Different lubricant mixtures of MoS2 and PTFE powder were assembled into the blind micro-hole array templates with different micro-hole density by EDP. The deposited mixtures of MoS2 and PTFE powder were densified by microwave sintering. The tribological performance evaluated by a ring-onblock wear tester indicated that the 20% micro-hole density template assembled with MoS2-10wt%PTFE powder presented the lowest friction coefficient.
1. Introduction Molybdenum disulfide (MoS2) has been widely considered as an effective solid lubricant due to its strong covalent intralayer and weak van der Waals interlayer interactions [1]. A lot of work based on functional MoS2 has been mainly produced in form of MoS2 film or coating through RF PVD, thermal spraying and laser cladding methods [2–4]. However, MoS2 lubricant films produced by PVD or thermal spraying method are readily to fail under high surface loading in many cases because of their low adhesion strength with substrates [5]. A duplex technique based on the use of laser texturing together to the application of a solid lubricant on the textured surfaces has been proposed in order to reduce the friction coefficient and enlarge the lifetime of the alloy in service [6,7]. The generation of grooves or
⁎
Corresponding author. E-mail address: [email protected] (A.H. Wang).
https://doi.org/10.1016/j.optlastec.2019.03.033 Received 8 March 2019; Accepted 17 March 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
dimples patterns on the alloy surface can act as lubricant reservoirs because the lubricant can be stored in textured cavities and then released during the friction process, effectively reducing the friction coefficient of titanium alloys [7–9]. However, the loose lubricants dispersing in the textured zone were hard to be kept in the grooves, which resulted in uneven surface and deteriorated wear resistant. Consequently, the maintenance of lubricants in the textured zone becomes particularly important for increasing lifetime of the lubricants. Polytetrafluoroethylene (PTFE) is a thermoplastic polymer with a melting point of 600 K, maintains high strength, toughness and selflubrication at low temperatures down to 5 K, and good flexibility at temperatures above 194 K. The frictional coefficient of PTFE against polished steel is usually 0.05–0.10, which is the third-lowest of any known solid material. However, PTFE exhibits poor wear and abrasion
Optics and Laser Technology 116 (2019) 171–179
A.H. Wang, et al.
Fig. 1. Microstructure of the laser-clad Fe-Cr-Ni-C coating.
60 V for 20 min. Microwave sintering of the deposited templates was carried out by a microwave oven with frequency 2.45 GHz and power 1.4 kW (Changsha SYNO-THERM Corporation, China) in order to densify the deposited lubricants. The deposited micro-hole templates were buried by silicon carbide and alumina powder mixture for rapid and uniform heating. Microwave sintering was conducted at four different temperatures (300 °C, 350 °C, 400 °C and 500 °C) with a heating rate of 3 °C/min in high pure argon atmosphere (99.99%). Accurate temperature of samples was measured by an infrared thermometer and controlled by changing microwave power. The deposited micro-hole templates were heated to the above each sintering temperature and kept warm for 120 min, finally, the sintered samples were cooled down to room temperature in air. Frictional behavior of the sintered samples was evaluated using a MM-U10G ring-on-block wear tester at room temperature. The sliding was performed at the laser-clad Fe-0.1C-15.0Cr-0.5Ni coating block (with dimension of 30 mm × 30 mm × 12 mm) sliding against the deposited and sintered composite template ring. The sliding tests were carried in air out under applied load of 100 N, rotation speed of 50 rpm and test time of 180 min. The friction coefficient was continuously recorded by the computer connected to the sensor of the tester. The ring surfaces were all ground and polished to remove the deposited lubricants outside the micro-holes and then washed in acetone prior to sliding test.
Table 1 Different ratio of the suspension (g/L). No.
MoS2
PTFE
Weight percent of PTFE
1 2 3 4 5
12.5 11.875 11.25 10.625 10
0 0.625 1.25 1.875 2.5
0 5 10 15 20
resistance, leading to early failure and leakage problems in the seals. The wear resistance of PTFE can be significantly improved by addition of suitable filler materials [10]. Therefore, the complementation of both WS2 and PTFE probably exhibits good friction and wear performance. In the present paper, a special blind micro-hole array template produced by laser drilling on the laser-clad wear-resistant coating was deposited with lubricant mixtures of MoS2 and PTFE by electrophoretic deposition (EDP) method, and then densified by microwave sintering. The tribological performance was evaluated by a ringon-block wear tester. 2. Experimental procedure Wear resistant coatings with a composition of Fe-0.1C-15.0Cr-0.5Ni (at wt%) and a microhardness of about HV0.1 650 were produced by laser cladding onto a medium-carbon steel (0.45C) using following laser parameters, i.e., laser power 1.8 kW, scanning velocity 15 mm·s−1, beam diameter 5 mm, coating thickness 1.5 mm, overlapping ratio 50%. The cross-section of the template substrate is shown in Fig. 1. The surfaces of the laser-clad samples were ground down to 1500grit, ultrasonically cleaned and then dried in an oven before laser drilling. A LMC2011 type IPG fiber laser with wavelength of 1064 nm was adopted to drill blind micro-hole arrays on the surfaces of the laser-clad coatings. Laser drilling experiment was carried out in air atmosphere by changing laser beam defocusing distance with an aim to optimize the micro-hole profile. The optimized laser parameters were then used to produce blind micro-hole array templates for subsequent electrophoretic deposition. Different ratio of MoS2 powder (about 1 μm) and PTFE powder (about 0.5 μm, molecular weight 104–105) listed in Table 1 were firstly surface activated by C16H26O2 with a solid content of 10 g/L and then dispersed by distilled water with a solid content of 5 g/L. The suspension was stirred for 1 h by a magnetic stirring apparatus and then dispersed for 1 h by an ultrasonic device. Two parallel stainless steel plates with a distance of 15 mm were used as the anode while the laser-clad substrates were used as the cathode. The micro-hole templates were deposited by electrophoretic deposition (EPD) at an applied voltage of
3. Results and discussion Under the condition of average power 20 W, pulse width 200 ns, frequency 25 kHz and laser irradiation time 2 ms, the profile of the laser-drilled blind hole influenced by laser beam defocusing distance is shown in Fig. 2. The cross-sections of the laser-drilled micro-holes at different laser beam defocusing distance are displayed in Fig. 2. Three typical profiles were observed, i.e., V-shape micro-hole at a variation of defocusing distance from −0.2 mm to −0.1 mm (Fig. 2a and b), Ushape micro-hole at the defocusing distance at 0 mm (Fig. 2c), and drum-shape micro-hole at the defocusing distance ranging from 1 mm to 2 mm (Fig. 2d and f). Amongst three kinds of profiles, the V-shape blind micro-holes are easy to be deposited by EPD, but the V-shape wall has poor restriction to the deposited solid lubricant particles during frictional wear process, the drum-shape micro-holes are hard to be deposited because of their small entrance size. Therefore, the ideal profile for this special application is the U-shape micro-hole, by which the particles were easier to be deposited into the whole blind microholes, since the entrance of the U-shape micro-hole had no shadowing effect to the deposited particles because of lager entrance size in comparison with the drum-shape micro-holes. At the same time, the Ushape micro-hole can trap and hold the deposited solid lubricant
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Fig. 2. Optical micrograph showing the shape of the micro-hole drilled at different defocusing distance. (a) −2 mm, (b) −1 mm, (c) 0 mm, (d) 1 mm, (e) 2 mm.
HNO3 = 1:3) for about 25 s for cleaning the spatter around the laserdrilled holes. Fig. 3 shows the typical surface and cross-section of the EPD coating on the micro-hole template. Uniform and dense deposition coatings into the laser-drilled micro-holes and onto the surface of the template were produced by the optimized EDP parameters, i.e., the distance of two parallel stainless steel plates 15 mm, applied voltage 60 V and deposition time 20 min. Fig. 4 shows the EPD samples sintered by microwave at different sintering temperatures for 2 h. No change in surface color of the EPD coatings at the sintering temperature less than 300 °C indicates that there was no chemical reaction during sintering process. The surface
Table 2 Micro-hole density and area ratio at different pitch. Pitch (μm) Area ratio (%)
196 10
138 20
113 30
98 40
particles in the afterward-sintering process and frictional wear applications. Consequently, the U-shape blind micro-hole templates were chosen for the subsequent EPD. The templates with different micro-hole density and different pitch were produced for EPD process, as listed in Table 2. Prior to EPD process, the templates were immersed in a acidic solution (HF/
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Fig. 3. Morphologies of the EPD coating. (a) Top view, (b) Cross-section view.
Fig. 4. Macroscopic morphologies of the sintered samples. (1) Non-sintering, (2) 300 °C, (3) 350 °C, (4) 400 °C, (5) 500 °C.
color became dark at the sintering temperature of both 350 °C and 400 °C, the higher the sintering temperature, the deeper the surface color. The surface color becoming brown at the sintering temperature of 500 °C indicates that obvious chemical reaction happened. Fig. 5 shows the surface morphologies of the EPD coatings on the laser-drilled microhole templates. It also supports that the surface morphologies at the sintering temperature of 300 °C and 350 °C changed slightly, became uneven and granuliform at the sintering temperature of 400 °C, and turned to porous at the sintering temperature of 500 °C. Fig. 6 shows XRD patterns of the EPD coatings sintered at different sintering temperature. Only MoS2 peaks found at 300 °C indicates no oxidization occurred at this sintering temperature, and weak MoO2 phase peak found at 350 °C indicates that slight oxidization happened at this temperature. The intensity of MoO2 phase peaks increases with an increase in sintering temperature from 350 °C to 500 °C and only MoO2 phase peaks at 500 °C indicates that nearly all MoS2 was oxidized. Normally, the oxidization process can be described as following Eqs. (1) and (2), MoS2 was firstly oxidized into MoO2 and SO2, then SO2 was vaporized, finally the MoO2 was oxidized into MoO3 furthermore. No MoO3 peaks found at all sintering temperatures means further oxidization of MoO2 was prohibited because of low oxygen partial pressure in the microwave oven chamber. MoS2 + O2 → MoO2 + SO2
(1)
MoO2 + O2 → MoO3
(2)
environment, oxygen source introduced from three aspects resulted in low oxygen partial pressure in the microwave oven chamber: (1) the oxygen adsorption of MoS2 particles; (2) the low vacuum degree of the microwave; (3) the low purity of Ar gas 99.99%. Fig. 7 shows the influence of sintering temperature on the friction coefficient of the MoS2-10wt%PTFE coating with 20% micro-hole density. The phenomena that the friction coefficient increases with an increase in sintering temperature from 300 °C to 400 °C could be explained by the oxidization of MoS2 at both 350 °C and 400 °C (see Fig. 6). The lower friction coefficient at sintering temperature of 300 °C in comparison with that of the unsintered sample verifies that the densification by microwave sintering could reduce the friction coefficient. The SEM observation shown in Fig. 8 displays that the microstructure before and after sintering is characterized by a mixture of particulate MoS2 and fibrous PTFE, fuzzy MoS2 particles surrounded by flocculent PTFE, respectively. The PTFE becoming viscous flow state at 300 °C could diffuse between MoS2 particle gaps and interconnect MoS2 particles at the same time, which improved the bond strength between lubricant particles and reduced friction coefficient. Fig. 9 shows the influence of PTFE content on the friction coefficient of the samples with 20% micro-hole density at sintering temperature of 300 °C. The pure MoS2 coating presents very low friction coefficient (about 0.15) at the preliminary stage and high friction coefficient (about 0.7) after testing time of 4500 s, which means the loss of lubrication function. The friction coefficient increases from about 0.25 to about 0.42 with an increase in PTFE content from 5 wt% to 20 wt% except the lowest friction coefficient (from 0.07 to 0.15) at the PTFE
The sintering process was carried out in Ar atmosphere
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Fig. 5. Surface morphologies of the sintered EPD coatings on the templates. (1) Non-sintering, (2) 300 °C, (3) 350 °C, (4) 400 °C, (5) 500 °C.
content of 10%. The pure MoS2 coating possesses very low friction coefficient (about 0.15) at the preliminary stage because of its lamellar structure and weak Van der Waal’s force between lamels. The poor binding interaction amongst MoS2 particles in the micro-holes due to lack of PTFE resulted in spallation and loss of MoS2 particles from the micro-holes, as shown in Fig. 10(a), so high friction coefficient (about 0.7) was measured after testing time of 4500 s. Addition of PTFE content to 5 wt% into the coating could both form discontinuous lubricant film and stabilize the friction coefficient to some extent in comparison
with the pure MoS2 coating because of the binding interaction amongst MoS2, as shown in Fig. 10(b). Addition of PTFE content to 10 wt%, 15 wt% and 20 wt% into the coating generated uniform lubricant films (see Fig. 10(c)–(e)) and prevented spallation of MoS2 particles in the micro-holes, owing to the reticular structure (see Fig. 8(b)) sintered at 300 °C and good binding interaction amongst MoS2 particles, which slowed down the loss of solid lubrication materials in the process of friction and maintained constant friction coefficient for long time (see Fig. 9). The MoS2-10 wt% coating presented the lowest friction
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Fig. 9. Influence of PTFE content on friction coefficient.
coefficient amongst all the MoS2-PTFE coatings, but the influence mechanism of PTFE content on friction coefficient is not clear up to now. Fig. 11 displays the influence of micro-hole density on the friction coefficient of the EPD MoS2-10wt%PTFE coating sintered at 300 °C for 2 h. An increase in micro-hole density from 10% to 20% and from 20% to 40% resulted in reduce of friction coefficient and increase of friction coefficient, respectively. It can be understood that the friction coefficient of the 20% micro-hole density sample is much lower than that of the 10% micro-hole density sample since solid lubricant film with higher micro-hole density was readily to be generated owing to larger original area fraction of solid lubricants, as shown in Fig. 12(a) and (b). However, an increase in micro-hole density up to 30% and 40% led to an increase in friction coefficient, and the higher the micro-hole density, the larger the friction coefficient, as shown in Fig. 11. Observation of the worn surface morphologies indicates that, friction scratches were found for the samples with 10%, 30% and 40% micro-hole density, and the higher the micro-hole density, the severer the friction scratches, as shown in Fig. 12(a), (c) and (d). This phenomenon could be explained by the surface strength of the micro-hole samples, i.e., increasing the micro-hole density is equivalent to decreasing surface strength and macroscopic hardness, which resulted in an increase in friction coefficient and number of friction scratches, even larger fraction of solid lubricants existed.
Fig. 6. XRD patterns of the EPD coatings at different sintering temperature.
Fig. 7. Influence of sintering temperature on friction coefficient.
Fig. 8. SEM morphologies of the MoS2-10wt.%PTFE in the micro-hole zone. (a) unsintered (b) sintered at 300 °C for 2 h.
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Fig. 10. Influence of PTFE content on the worn surface morphologies. (a) 0%, (b) 5%, (c) 10%, (d) 15%, (e) 20%.
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4. Conclusions 1. Laser-clad wear-resistant Fe-based coatings were laser-drilled by optimized laser drilling parameters, i.e., average power 20 W, pulse width 200 ns, frequency 25 kHz, laser irradiation time 2 ms and defocusing distance 0 mm, to produce U-shape micro-hole templates, which are the most suitable for subsequent EPD. 2. Mixtures of MoS2 and PTFE powders were uniformly deposited into the laser-drilled micro-hole templates by electrophoretic deposition. 3. Microwave sintering at 300 °C could densify the EPD coatings without oxidization of MoS2, but increasing sintering temperature to above 350 °C resulted in oxidization of MoS2 into MoO2. 4. The friction coefficient of MoS2-10wt.%PTFE coating with 20% micro-hole density sintered at 300 °C presented the lowest friction coefficient amongst the unsintered coating, the coatings sintered at 350 °C and 400 °C owing to reticular structure. 5. The friction coefficient increases from about 0.25 to about 0.42 with an increase in PTFE content from 5 wt% to 20 wt% except the lowest friction coefficient (about 0.15) at the PTFE content of 10%, which was proved by the worn surface morphologies.
Fig. 11. Influence of the micro-hole density on the friction coefficient.
Fig. 12. Influence of the micro-hole density on the worn surface morphologies. (a) 10%, (b) 20%, (c) 30%, (d) 40%.
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Acknowledgement
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