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Preparation of flame sprayed poly(tetrafluoroethylene-co-hexafluoropropylene) coatings and their tribological properties under water lubrication
Applied Surface Science 255 (2008) 2408–2413
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of flame sprayed poly(tetrafluoroethylene-co-hexafluoropropylene) coatings and their tribological properties under water lubrication Zhizhong Feng a,b,*, Haiyan Xu a,b, Fengyuan Yan a,b a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui Middle Road No. 18, Lanzhou 730000, PR China Graduate School, Chinese Academy of Sciences, Beijing 100064, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 May 2008 Received in revised form 16 July 2008 Accepted 16 July 2008 Available online 29 July 2008
Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) coatings were prepared on AISI-1045 steel via flame spraying. The chemical changes of the FEP powder occurring during the spraying process were analyzed by means of Fourier transformation infrared spectroscopy. The flame spraying of the FEP powders under the chosen conditions did not lead to structural changes related to degradation and oxidation. The friction and wear behaviors of the FEP coatings sliding against AISI-52100 steel ball under dry- and water-lubricated conditions were investigated using a ball-on-disc test rig, and the worn surface morphologies of the coatings were also observed using the scanning electron microscope. The FEP coatings recorded smaller friction coefficients under water lubrication than under dry sliding. However, the wear rate of the coating under water lubrication was about two times of that under dry sliding. This indicated that water as a lubricant was able to effectively reduce the friction coefficient but it led to an increased wear rate of the FEP coatings/steel sliding pairs. X-ray photoelectron spectroscope (XPS) results illustrate that the transfer film did formed during the dry sliding but it is hindered under water lubrication, and it might be the major cause of the larger wear rate under the water lubrication. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Flame spraying FEP Polymer coatings Water lubrication Friction and wear behavior
1. Introduction Polymer coatings have been widely used for surface decoration, corrosion protection, and wear resistance based on their novel properties [1–4]. Many technologies are available for the deposition of polymer coatings [5,6], but most of them require volatile organic solvents and large equipments that result in environmental issues and high-cost. In this sense thermal spray as a process to prepare polymeric coatings is superior to many other processes, owing to its low-cost, low pollution, and simple equipment and procedures. So far, many thermal spray processes such as high velocity oxy-fuel spray (HVOF), plasma spray, and conventional flame spray have been available to produce polymer coatings, and the conventional flame spray is the most costeffective and physically portable process for thermoplastics [7]. Therefore, conventional thermal spraying has been used to prepare various kinds of polymeric coatings such as polyetheretherketone
* Corresponding author at: State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui Middle Road No. 18, Lanzhou 730000, PR China. Tel.: +86 931 4968078; fax: +86 931 8277088. E-mail address: [email protected] (Z. Feng). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.110
(PEEK), polyphenylenesulfide (PPS) [8,9], polyethylene (PE), PE copolymers [10,11], polymethylmethacrylate (PMMA) [12], polytetrafluoroethylene (PTFE) [13], and nylons [5]. However, little has been reported on the preparation of poly(tetrafluoroethylene-cohexafluoropropylene) (FEP) using conventional flame spraying technique, though it has excellent chemical resistance to acid, bases, and solvents, and good high temperature stability, release, and non-stick properties [14]. At the same time, most of the researchers on the friction and wear behaviors of bulk polymer materials and polymer coatings are limited to dry sliding conditions. In other words, little is available on the tribological study of polymeric materials under water lubrication or in aqueous medium, though it is of considerable importance in terms of the tribological application of polymeric materials in engineering [15,16]. With those perspectives in mind, FEP powder was thermally sprayed to form coating using conventional flame spray process and tribological properties of the coating in water is investigated. Friction of the coating under dry condition is conducted as comparison. And this work might be helpful to understand the wear mechanism of FEP coatings both under dry sliding and water lubrication, and will provide guidance to the tribological application of FEP coating and other polymer coatings.
Z. Feng et al. / Applied Surface Science 255 (2008) 2408–2413
2. Experimental A CMD-PS plastic spray system was used to prepare the FEP coatings, using acetylene as the fuel gas and compressed air to fluidize the FEP particles in the powder feeder and transport the powder to the torch. The pressure of the acetylene and compressed air was 0.05–0.08 MPa and 0.35–0.45 MPa, respectively. An air shroud at 0.35–0.40 MPa and powder carrier air at 0.35–0.40 MPa were used. The flame spraying was conducted at a standoff distance of 15–25 cm, traverse rate of 10 cm/s, using AISI-1045 steel disc of a size Ø25 mm 12 mm as the substrate. Before spraying, the substrate was sand-blasted using SiO2 grit and cleaned in an ultrasonic bath of ethanol. Those substrates used to prepare the FEP coatings for the subsequent thermal and structural analyses were polished instead of grit-blasted so as to allow an easier removal of the coating. The FEP powder of a size about 100 mm was supplied by Shanghai Synica LTD of China. The steel substrate was preheated to prepare the flame sprayed FEP coatings. The temperature for the preheating of the steel substrate was measured using a RayngerST device (Raytek Co., USA). The friction and wear behaviors of the FEP coatings sliding against AISI-52100 steel in a ball-on-disc configuration were evaluated using a THT07-135 tribometer. Fig. 1 shows the contact schematic diagram of the frictional pair. The upper ball made of AISI-52100 steel ball (composition: 0.95–1.05%C, 1.30–1.65%Cr, 0.15–0.35%Si, 0.20–0.40%Mn, <0.027%P, <0.020%S, <0.30%Ni, <0.25%Cu, balance Fe) had a hardness of 580 Hv. The lower disc made of AISI-1045 steel was coated with the FEP coating. Prior to commencing the friction and wear test, all the specimens were ultrasonically cleaned in ethanol and dried in air. Both the ball and the disc were immerged in distilled water and performed to slide at sliding velocities of 0.1 m/s, 0.2 m/s, 0.3 m/s, and 0.4 m/s, a sliding distance of 1.0 103 m, and normal load of 2 N and 5 N, in ambient atmosphere. A surface profilometer was used to determine the vertical cross-section area of the wear track on the disc, and the wear rate of the FEP coating was then automatically calculated by the computer connected with the friction and wear test rig. The friction coefficient was recorded continuously by the computer. A minimum of three replicate friction and wear tests were carried out, and the relative errors for measurement of the friction coefficient was about 10%.
Fig. 1. Schematic diagram of THT07-135 tribometer with a ball-on-disc configuration.
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The static contact angles were measured in ambient air (relative humidity 40%) using a CA-A contact angle measurement device (Kyowa Scientific Co., Ltd.). Distilled water was used as the spreading reagent. The values reported here are at least five measurements for each sample. The thermal behavior of the FEP powder was investigated using a PerkinElmer 7 series system, by means of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in flowing nitrogen at a scanning rate of 10 8C/min. The morphologies of the FEP coatings were observed using a scanning electron microscope (SEM). The compositions of the FEP raw material powder and the coating were analyzed using an infrared spectrometer (Bio-Rad FTS-165). The chemical states of the elements on the worn surface were determined using an X-ray photoelectron spectroscope (XPS). The XPS analysis was conducted at 400 W and pass energy of 29.35 eV, using Mg Ka radiation as the excitation source and the binding energy of contaminated carbon (C 1s = 284.6 eV) is used as a reference. 3. Results and discussion Fig. 2 shows the SEM micrograph of the surface of FEP coating. It is noted that the surface of the FEP coating was quite smooth, typical of the flame sprayed polymer coatings. Contact angle measurement is often used as a simple, useful and sensitive tool to gain insights into the chemical nature of material surfaces. The water contact angle for the FEP coating is in the range of 101–1108, which indicates that the surface of FEP coating is hydrophobic. However, the water contact angle for the FEP coating is lower than the molding FEP bulk material (1208). It could be rational to suppose that during the preparation process of the coating, few defects were formed and existed on the surface of the coating resulted in the lower contact angles of coatings. The TGA and DSC curves of the FEP powders are given in Fig. 3. It is seen that the FEP powder had a melting point about 261 8C, while it experienced initial weight loss and degradation at 408 8C. As reported by Brogan [17], the lower temperature threshold (261 8C) is the minimum temperature required to coalesce the polymer particles by viscous flow for maximum density, and the higher temperature threshold (408 8C) allows the maximization of the inter-splat equilibration process. Therefore, there exists a wide processing window between the melting point and the thermaldecomposition temperature of the FEP (261–408 8C), which is critical to the preparation of the FEP coatings. In other words, the FEP coatings should be prepared within 261–408 8C.
Fig. 2. The SEM photograph of FEP coating.
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Z. Feng et al. / Applied Surface Science 255 (2008) 2408–2413
Fig. 3. DSC and TGA curves of FEP powder.
Fig. 4. SEM morphologies of the FEP coatings prepared at various preheating temperatures of the steel substrates: (a) preheated at 100 8C and (b) preheated at 270 8C.
The SEM morphologies of the FEP coatings prepared at various preheating temperatures of the substrate steel are shown in Fig. 4. It is seen that the FEP coatings prepared at lower preheating temperature had many defects such as irregular holes and nonmolten powders (see Fig. 4(a)) and were of poor uniformity as well. Contrary to the above, the coating prepared at a higher preheating temperatures of 270 8C had fewer defects and was of good uniformity (see Fig. 4(b)). Moreover, when the preheating temperature rose to 300 8C, many little bubbles began to appear on the surface of the coating. In this case the coating was adhered to the substrate so strongly that it was hardly removed from the substrate to obtain the SEM specimen. The differences in the micro-morphologies of the flame sprayed FEP coatings were closely dependent on the different substrate preheating temperatures. When FEP powders were sprayed onto the substrate surface with lower preheating temperature, the heat exchange between the molten or semimolten FEP powders and the steel substrate would be severe owing to the large temperature difference. Subsequently, the molten or semimolten FEP powders would rapidly quench and solidify to form the FEP coating. Because of the large temperature difference between the molten or semimolten FEP powders and the steel substrate in this case, the melted droplets would have no enough time to spread over the substrate and hence many defects were generated in the coating. At a appropriate preheating temperature (270 8C) of the steel substrate, the melted droplets had enough time to overlap and spread on the substrate surface. Hence there were few defects were found in the FEP coatings and in this case the coatings strongly adhered to the
steel substrate owing to the small temperature difference between the melted FEP powders and the steel substrate. Fig. 5 shows the infrared spectra of the FEP powders, FEP coatings and wear debris obtained from the process of friction test. It is seen that there were no significant differences in the IR peak position and height of these curves. At the same time, no signs of degradation and oxidation of the FEP powders during the flame spraying process and friction test process were detected by the IR analysis. Therefore, it can be concluded that the combustion flame used in these experiments did not lead to a severe change in the molecular structure of the FEP powders.
Fig. 5. FTIR spectra of FEP powder, FEP coating and FEP coating wear debris (drysliding, 5 N, 0.4 m/s).
Z. Feng et al. / Applied Surface Science 255 (2008) 2408–2413
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Table 1 Variations in friction coefficient and wear rate of the FEP coating with different speeds under dry- and water-lubricated sliding (load 2 N, sliding distance 1 103 m)
Table 2 Variations in friction coefficient and wear rate of the FEP coating with different speeds under dry- and water-lubricated sliding (load 5 N, sliding distance 1 103 m)
Velocity (m/s)
Velocity (m/s)
0.1 0.2 0.3 0.4
Friction coefficient
Wear rate (10 13 m3(N m)
Friction coefficient
1
)
Dry sliding
Water lubrication
Dry sliding
Water lubrication
0.36 0.35 0.34 0.32
0.17 0.20 0.22 0.21
2.47 2.68 2.74 3.02
5.22 5.89 6.90 7.61
Table 1 shows the friction coefficient and wear rate of the FEP coating prepared at a substrate preheating temperature of 270 8C under dry- and water-lubricated conditions (sliding speed 0.1 m/s, 0.2 m/s, 0.3 m/s, and 0.4 m/s; sliding distance 1 103 m, normal load 2 N). It is seen that the friction coefficient of the FEP coating was almost independent of the sliding speed under dry-sliding condition. Similar to that under dry-sliding, the sliding speed also had little effect on the friction coefficient of the FEP coating under water-lubricated condition, except that a slightly smaller friction coefficient was recorded at a low sliding speed of 0.1 m/s. It was supposed that the relatively small friction coefficient of the FEP coat sliding against the steel under unlubricated condition was related to the transfer of the wear debris of FEP with a small shearing strength onto the counterpart steel surface. Similarly, the smaller friction coefficient of the FEP coating under water-lubricated condition than under dry-sliding was largely attributed to the lubricity of the water. In other words, water as a lubricant was able to effectively reduce the friction coefficient of the FEP coating/steel sliding pair. This was rational, since water was capable of cooling the frictional pair and decreasing the adhesion and plastic deformation of the FEP coating/steel pair. Unsurprisingly, the wear rate of the FEP coating increased with increasing sliding speed under both dry- and water-lubricated conditions. However, to our disappointment, the FEP coating had a much larger wear rate under water-lubricated condition than under dry sliding. This could be related to the reduction in the strength and elastic modulus of the coating in water, and water as the lubricating medium might facilitate the weakening of the inter-chain bonds of the FEP polymeric molecules, which gave rise to the wear rate of the polymeric coating in water environment [18]. Meanwhile, the FEP coating with surface defects was liable to be damaged by water, which led to a decrease in the mechanical strength of the coating and hence severe wear of the coating as well.
0.1 0.2 0.3 0.4
Wear rate (10 13 m3(N m)
1
)
Dry sliding
Water lubrication
Dry sliding
Water lubrication
0.23 0.21 0.23 0.20
0.17 0.16 0.16 0.15
2.94 2.87 3.14 3.21
6.87 6.89 7.65 8.33
Table 2 shows the friction coefficient and wear rate of the FEP coating under dry- and water-lubricated conditions at a normal load 5 N at different sliding speeds (0.1 m/s, 0.2 m/s, 0.3 m/s, and 0.4 m/s; sliding distance 1 103 m). Similarly, the friction coefficient of the FEP coating was almost independent of the sliding speed at a heavier load of 5 N under dry-sliding condition, as well as under waterlubricated condition. At the same time, friction coefficients were lower than that at a load of 2 N. Moreover, wear rate increased obviously along with the load increasing both under the dry-sliding and water lubrication conditions. To gain more insights into the friction and wear mechanisms of the coatings, the worn surface of the AISI-52100 steel ball under dry- and water-lubricated sliding at a normal load of 5 N with 0.1 m/s speed have been investigated using an XPS. The XPS spectra of some typical elements on the worn surface of the AISI52100 steel are given in Fig. 6. As shown in the figures the curve 1 belong to the dry sliding and curve 2 are for water lubrication. The binding energies from these plots were compared with the data listed in the Handbook of X-ray Photoelectron Spectroscopy [19]. The main peak of F 1s around 685.0 eV assigned to iron fluoride appears only on the worn steel surface under dry sliding (Fig. 6(a)), which indicates that iron fluoride (Fe-F) bonds are formed during the dry sliding process. Almost no F is observed on the worn steel surface under water lubrication. The peak of Fe 2p at 711.1 eV is assigned to iron fluoride (Fig. 6(b)). This indicated that iron fluoride bonds are formed during the dry sliding process, combining with the XPS spectra of F 1s, which illustrates that the transfer film did formed during the dry sliding but it is hindered under water lubrication, and it might be the major cause of the larger wear rate under the water lubrication. The similar Fe 2p spectra are observed on the worn steel ball surfaces under both water lubrication and dry sliding condition, which indicates that the steel ball surface is oxidized in both conditions. It is rationally supposed that such a
Fig. 6. XPS analyses on the worn surface of the AISI-52100 steel ball sliding at a normal load of 5 N with 0.1 m/s speed. (a) F 1s, (b) Fe 2p, (1) dry sliding, (2) water lubrication.
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Z. Feng et al. / Applied Surface Science 255 (2008) 2408–2413
Fig. 7. SEM morphologies of worn surface of FEP coating under water lubrication at (a) 0.1 m/s and (b) 0.4 m/s, and under dry sliding at (c) 0.1 m/s and (d) 0.4 m/s.
kind of the steel oxidation is largely dependent on the frictioninduced thermal effect under dry sliding while on the corrosiveness of the water under water-lubricated sliding. Fig. 7 shows the SEM morphologies of the worn surfaces of the FEP coatings sliding against the AISI-52100 steel under dry- and water-lubricated conditions at a normal load of 5 N, 0.4 m/s speed. It is seen that the FEP coating under water lubrication against the steel at a low speed was characterized by slight scuffing, and a few wear debris appeared at the edges of the wear track in this case (Fig. 7(a)). This observation was seemingly contradictive to the much larger wear rate of the FEP coating under water lubrication than under dry sliding, which could be related to the easier removal of the wear debris in the presence of water. Surprisingly, similar features were observed for the worn surfaces of the coating under water lubrication at a high sliding speed of 0.4 m/s (see Fig. 7(b)) and under dry-sliding (see Fig. 7(c) and (d)), which also seemed to be contradictive to the differences in their wear rates. Namely, the worn surface of the FEP coating under dry sliding was characterized by severe adhesion, scuffing, and plastic deformation (see Fig. 7(c) and (d)), but the wear rate of the coating in this case was smaller than that under water lubrication, which might also imply that the distilled water as the lubricant contributed to enhancing the removal of the wear debris of the polymer coating abraded by the harder SAE-52100 steel counterpart and the water prohibited forming of the transfer film onto the steel ball surface. 4. Conclusions FEP coatings were prepared on steel substrate via flame spraying route. It was found that no chemical changes were involved in the flame spraying process at a relatively high temperature. The distilled water as the lubricating medium contributed to greatly decreasing
the friction coefficients of the coatings sliding against the AISI-52100 steel. However, it led to a significant increase in the wear rate of the FEP coating, possibly owing to the worsening in the mechanical strength of the coating in the presence of water. Further work should be conducted to introduce proper reinforcing phases into the flame sprayed FEP coating so as to increase the wear resistance. Acknowledgements The authors wish to acknowledge the financial support from the National High Technology Research and Development Program of China (‘‘863’’ Program) (Grant No. 2007AA03Z358). Prof. Jiazheng Zhao and Engineer Jinfang Zhou at the State Key Laboratory of Solid Lubrication are gratefully acknowledged for carrying out the SEM and FTIR analyses. References [1] L. Favre-Quattropani, P. Groening, P. Ramseyer, L. Schlapbach, Surf. Coat. Technol. 125 (2000) 377–382. [2] R. Kawase, A. Nakano, Thermal Spray: Practical Solutions for Engineering Problems, ASM International, Materials Park, 1996pp.257–261. [3] E. Petrovicova, R. Knight, L.S. Schadler, T.E. Twardowski, J. Appl. Polym. Sci. 78 (2000) 2272–2289. [4] S. Lathabai, M. Ottmuller, I. Fernandez, Wear 221 (1998) 93–108. [5] L.S. Schadler, K.O. Laul, R.W. Smith, E. Petrovicova, J. Therm. Spray Technol. 6 (1997) 475–485. [6] J.A. Brogan, C.C. Berndt, G.P. Simon, D. Hewitt, Polym. Eng. Sci. 38 (1998) 1873– 1881. [7] F.Y. Yan, K.A. Gross, G.P. Simon, C.C. Berndt, Polym. Eng. Sci. 44 (2004) 1448–1459. [8] H. Liao, E. Bechi, F. Berger, C. Coddet, in: Proceedings of the 15th International Thermal Spray Conference, Nice France, ASM International, Materials Park, OH, 1998, p. 25. [9] E. Lugscheider, C. Herbst, A. Fischer, in: Proceedings of the 15th International Thermal Spray Conference, Nice France, ASM International, Materials Park, OH, 1998, p. 19.
Z. Feng et al. / Applied Surface Science 255 (2008) 2408–2413 [10] P.J. Loustaunau, D. Horton, Mater. Perf. (1994) 32–35. [11] T.W. Glass, J.A. Depay, in: Proceedings of the Fourth National Thermal Spray Conference, ASM International, Materials Park, OH, (1991), p. 345. [12] T. Zhang, D.T. Gawne, Y. Bao, in: Proceedings of the First United Thermal Spray Conference, Indianapolis, USA, ASM International, Materials Park, OH, 1997 , p. 231. [13] J.A. Brogan, C.C. Berndt, J. Mater. Sci. 32 (1997) 2099–2106.
[14] [15] [16] [17] [18] [19]
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E. Leivo, T. Wilenius, T. Kinos, P. Vuoristo, T. Ma¨ntyla¨, Prog. Org. Coat. 49 (2004) 69–73. S. Bahadur, Wear 245 (2000) 92–95. E.L. Yang, J. Mater. Res. 7 (1992) 1–8. J.A. Brogan, MRS Bull. (2000) 48–53. M.D. Lutton, T.A. Stolarski, J. Appl. Polym. Sci. 54 (1994) 771–782. F.M. John, F.S. William, E.S. Peter, D.B. Kennrth, Handbook of X-ray Photoelectroscopy, Physical Electronics Press, Minnesota, 1995.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of flame sprayed poly(tetrafluoroethylene-co-hexafluoropropylene) coatings and their tribological properties under water lubrication Zhizhong Feng a,b,*, Haiyan Xu a,b, Fengyuan Yan a,b a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui Middle Road No. 18, Lanzhou 730000, PR China Graduate School, Chinese Academy of Sciences, Beijing 100064, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 May 2008 Received in revised form 16 July 2008 Accepted 16 July 2008 Available online 29 July 2008
Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) coatings were prepared on AISI-1045 steel via flame spraying. The chemical changes of the FEP powder occurring during the spraying process were analyzed by means of Fourier transformation infrared spectroscopy. The flame spraying of the FEP powders under the chosen conditions did not lead to structural changes related to degradation and oxidation. The friction and wear behaviors of the FEP coatings sliding against AISI-52100 steel ball under dry- and water-lubricated conditions were investigated using a ball-on-disc test rig, and the worn surface morphologies of the coatings were also observed using the scanning electron microscope. The FEP coatings recorded smaller friction coefficients under water lubrication than under dry sliding. However, the wear rate of the coating under water lubrication was about two times of that under dry sliding. This indicated that water as a lubricant was able to effectively reduce the friction coefficient but it led to an increased wear rate of the FEP coatings/steel sliding pairs. X-ray photoelectron spectroscope (XPS) results illustrate that the transfer film did formed during the dry sliding but it is hindered under water lubrication, and it might be the major cause of the larger wear rate under the water lubrication. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Flame spraying FEP Polymer coatings Water lubrication Friction and wear behavior
1. Introduction Polymer coatings have been widely used for surface decoration, corrosion protection, and wear resistance based on their novel properties [1–4]. Many technologies are available for the deposition of polymer coatings [5,6], but most of them require volatile organic solvents and large equipments that result in environmental issues and high-cost. In this sense thermal spray as a process to prepare polymeric coatings is superior to many other processes, owing to its low-cost, low pollution, and simple equipment and procedures. So far, many thermal spray processes such as high velocity oxy-fuel spray (HVOF), plasma spray, and conventional flame spray have been available to produce polymer coatings, and the conventional flame spray is the most costeffective and physically portable process for thermoplastics [7]. Therefore, conventional thermal spraying has been used to prepare various kinds of polymeric coatings such as polyetheretherketone
* Corresponding author at: State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui Middle Road No. 18, Lanzhou 730000, PR China. Tel.: +86 931 4968078; fax: +86 931 8277088. E-mail address: [email protected] (Z. Feng). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.110
(PEEK), polyphenylenesulfide (PPS) [8,9], polyethylene (PE), PE copolymers [10,11], polymethylmethacrylate (PMMA) [12], polytetrafluoroethylene (PTFE) [13], and nylons [5]. However, little has been reported on the preparation of poly(tetrafluoroethylene-cohexafluoropropylene) (FEP) using conventional flame spraying technique, though it has excellent chemical resistance to acid, bases, and solvents, and good high temperature stability, release, and non-stick properties [14]. At the same time, most of the researchers on the friction and wear behaviors of bulk polymer materials and polymer coatings are limited to dry sliding conditions. In other words, little is available on the tribological study of polymeric materials under water lubrication or in aqueous medium, though it is of considerable importance in terms of the tribological application of polymeric materials in engineering [15,16]. With those perspectives in mind, FEP powder was thermally sprayed to form coating using conventional flame spray process and tribological properties of the coating in water is investigated. Friction of the coating under dry condition is conducted as comparison. And this work might be helpful to understand the wear mechanism of FEP coatings both under dry sliding and water lubrication, and will provide guidance to the tribological application of FEP coating and other polymer coatings.
Z. Feng et al. / Applied Surface Science 255 (2008) 2408–2413
2. Experimental A CMD-PS plastic spray system was used to prepare the FEP coatings, using acetylene as the fuel gas and compressed air to fluidize the FEP particles in the powder feeder and transport the powder to the torch. The pressure of the acetylene and compressed air was 0.05–0.08 MPa and 0.35–0.45 MPa, respectively. An air shroud at 0.35–0.40 MPa and powder carrier air at 0.35–0.40 MPa were used. The flame spraying was conducted at a standoff distance of 15–25 cm, traverse rate of 10 cm/s, using AISI-1045 steel disc of a size Ø25 mm 12 mm as the substrate. Before spraying, the substrate was sand-blasted using SiO2 grit and cleaned in an ultrasonic bath of ethanol. Those substrates used to prepare the FEP coatings for the subsequent thermal and structural analyses were polished instead of grit-blasted so as to allow an easier removal of the coating. The FEP powder of a size about 100 mm was supplied by Shanghai Synica LTD of China. The steel substrate was preheated to prepare the flame sprayed FEP coatings. The temperature for the preheating of the steel substrate was measured using a RayngerST device (Raytek Co., USA). The friction and wear behaviors of the FEP coatings sliding against AISI-52100 steel in a ball-on-disc configuration were evaluated using a THT07-135 tribometer. Fig. 1 shows the contact schematic diagram of the frictional pair. The upper ball made of AISI-52100 steel ball (composition: 0.95–1.05%C, 1.30–1.65%Cr, 0.15–0.35%Si, 0.20–0.40%Mn, <0.027%P, <0.020%S, <0.30%Ni, <0.25%Cu, balance Fe) had a hardness of 580 Hv. The lower disc made of AISI-1045 steel was coated with the FEP coating. Prior to commencing the friction and wear test, all the specimens were ultrasonically cleaned in ethanol and dried in air. Both the ball and the disc were immerged in distilled water and performed to slide at sliding velocities of 0.1 m/s, 0.2 m/s, 0.3 m/s, and 0.4 m/s, a sliding distance of 1.0 103 m, and normal load of 2 N and 5 N, in ambient atmosphere. A surface profilometer was used to determine the vertical cross-section area of the wear track on the disc, and the wear rate of the FEP coating was then automatically calculated by the computer connected with the friction and wear test rig. The friction coefficient was recorded continuously by the computer. A minimum of three replicate friction and wear tests were carried out, and the relative errors for measurement of the friction coefficient was about 10%.
Fig. 1. Schematic diagram of THT07-135 tribometer with a ball-on-disc configuration.
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The static contact angles were measured in ambient air (relative humidity 40%) using a CA-A contact angle measurement device (Kyowa Scientific Co., Ltd.). Distilled water was used as the spreading reagent. The values reported here are at least five measurements for each sample. The thermal behavior of the FEP powder was investigated using a PerkinElmer 7 series system, by means of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in flowing nitrogen at a scanning rate of 10 8C/min. The morphologies of the FEP coatings were observed using a scanning electron microscope (SEM). The compositions of the FEP raw material powder and the coating were analyzed using an infrared spectrometer (Bio-Rad FTS-165). The chemical states of the elements on the worn surface were determined using an X-ray photoelectron spectroscope (XPS). The XPS analysis was conducted at 400 W and pass energy of 29.35 eV, using Mg Ka radiation as the excitation source and the binding energy of contaminated carbon (C 1s = 284.6 eV) is used as a reference. 3. Results and discussion Fig. 2 shows the SEM micrograph of the surface of FEP coating. It is noted that the surface of the FEP coating was quite smooth, typical of the flame sprayed polymer coatings. Contact angle measurement is often used as a simple, useful and sensitive tool to gain insights into the chemical nature of material surfaces. The water contact angle for the FEP coating is in the range of 101–1108, which indicates that the surface of FEP coating is hydrophobic. However, the water contact angle for the FEP coating is lower than the molding FEP bulk material (1208). It could be rational to suppose that during the preparation process of the coating, few defects were formed and existed on the surface of the coating resulted in the lower contact angles of coatings. The TGA and DSC curves of the FEP powders are given in Fig. 3. It is seen that the FEP powder had a melting point about 261 8C, while it experienced initial weight loss and degradation at 408 8C. As reported by Brogan [17], the lower temperature threshold (261 8C) is the minimum temperature required to coalesce the polymer particles by viscous flow for maximum density, and the higher temperature threshold (408 8C) allows the maximization of the inter-splat equilibration process. Therefore, there exists a wide processing window between the melting point and the thermaldecomposition temperature of the FEP (261–408 8C), which is critical to the preparation of the FEP coatings. In other words, the FEP coatings should be prepared within 261–408 8C.
Fig. 2. The SEM photograph of FEP coating.
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Fig. 3. DSC and TGA curves of FEP powder.
Fig. 4. SEM morphologies of the FEP coatings prepared at various preheating temperatures of the steel substrates: (a) preheated at 100 8C and (b) preheated at 270 8C.
The SEM morphologies of the FEP coatings prepared at various preheating temperatures of the substrate steel are shown in Fig. 4. It is seen that the FEP coatings prepared at lower preheating temperature had many defects such as irregular holes and nonmolten powders (see Fig. 4(a)) and were of poor uniformity as well. Contrary to the above, the coating prepared at a higher preheating temperatures of 270 8C had fewer defects and was of good uniformity (see Fig. 4(b)). Moreover, when the preheating temperature rose to 300 8C, many little bubbles began to appear on the surface of the coating. In this case the coating was adhered to the substrate so strongly that it was hardly removed from the substrate to obtain the SEM specimen. The differences in the micro-morphologies of the flame sprayed FEP coatings were closely dependent on the different substrate preheating temperatures. When FEP powders were sprayed onto the substrate surface with lower preheating temperature, the heat exchange between the molten or semimolten FEP powders and the steel substrate would be severe owing to the large temperature difference. Subsequently, the molten or semimolten FEP powders would rapidly quench and solidify to form the FEP coating. Because of the large temperature difference between the molten or semimolten FEP powders and the steel substrate in this case, the melted droplets would have no enough time to spread over the substrate and hence many defects were generated in the coating. At a appropriate preheating temperature (270 8C) of the steel substrate, the melted droplets had enough time to overlap and spread on the substrate surface. Hence there were few defects were found in the FEP coatings and in this case the coatings strongly adhered to the
steel substrate owing to the small temperature difference between the melted FEP powders and the steel substrate. Fig. 5 shows the infrared spectra of the FEP powders, FEP coatings and wear debris obtained from the process of friction test. It is seen that there were no significant differences in the IR peak position and height of these curves. At the same time, no signs of degradation and oxidation of the FEP powders during the flame spraying process and friction test process were detected by the IR analysis. Therefore, it can be concluded that the combustion flame used in these experiments did not lead to a severe change in the molecular structure of the FEP powders.
Fig. 5. FTIR spectra of FEP powder, FEP coating and FEP coating wear debris (drysliding, 5 N, 0.4 m/s).
Z. Feng et al. / Applied Surface Science 255 (2008) 2408–2413
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Table 1 Variations in friction coefficient and wear rate of the FEP coating with different speeds under dry- and water-lubricated sliding (load 2 N, sliding distance 1 103 m)
Table 2 Variations in friction coefficient and wear rate of the FEP coating with different speeds under dry- and water-lubricated sliding (load 5 N, sliding distance 1 103 m)
Velocity (m/s)
Velocity (m/s)
0.1 0.2 0.3 0.4
Friction coefficient
Wear rate (10 13 m3(N m)
Friction coefficient
1
)
Dry sliding
Water lubrication
Dry sliding
Water lubrication
0.36 0.35 0.34 0.32
0.17 0.20 0.22 0.21
2.47 2.68 2.74 3.02
5.22 5.89 6.90 7.61
Table 1 shows the friction coefficient and wear rate of the FEP coating prepared at a substrate preheating temperature of 270 8C under dry- and water-lubricated conditions (sliding speed 0.1 m/s, 0.2 m/s, 0.3 m/s, and 0.4 m/s; sliding distance 1 103 m, normal load 2 N). It is seen that the friction coefficient of the FEP coating was almost independent of the sliding speed under dry-sliding condition. Similar to that under dry-sliding, the sliding speed also had little effect on the friction coefficient of the FEP coating under water-lubricated condition, except that a slightly smaller friction coefficient was recorded at a low sliding speed of 0.1 m/s. It was supposed that the relatively small friction coefficient of the FEP coat sliding against the steel under unlubricated condition was related to the transfer of the wear debris of FEP with a small shearing strength onto the counterpart steel surface. Similarly, the smaller friction coefficient of the FEP coating under water-lubricated condition than under dry-sliding was largely attributed to the lubricity of the water. In other words, water as a lubricant was able to effectively reduce the friction coefficient of the FEP coating/steel sliding pair. This was rational, since water was capable of cooling the frictional pair and decreasing the adhesion and plastic deformation of the FEP coating/steel pair. Unsurprisingly, the wear rate of the FEP coating increased with increasing sliding speed under both dry- and water-lubricated conditions. However, to our disappointment, the FEP coating had a much larger wear rate under water-lubricated condition than under dry sliding. This could be related to the reduction in the strength and elastic modulus of the coating in water, and water as the lubricating medium might facilitate the weakening of the inter-chain bonds of the FEP polymeric molecules, which gave rise to the wear rate of the polymeric coating in water environment [18]. Meanwhile, the FEP coating with surface defects was liable to be damaged by water, which led to a decrease in the mechanical strength of the coating and hence severe wear of the coating as well.
0.1 0.2 0.3 0.4
Wear rate (10 13 m3(N m)
1
)
Dry sliding
Water lubrication
Dry sliding
Water lubrication
0.23 0.21 0.23 0.20
0.17 0.16 0.16 0.15
2.94 2.87 3.14 3.21
6.87 6.89 7.65 8.33
Table 2 shows the friction coefficient and wear rate of the FEP coating under dry- and water-lubricated conditions at a normal load 5 N at different sliding speeds (0.1 m/s, 0.2 m/s, 0.3 m/s, and 0.4 m/s; sliding distance 1 103 m). Similarly, the friction coefficient of the FEP coating was almost independent of the sliding speed at a heavier load of 5 N under dry-sliding condition, as well as under waterlubricated condition. At the same time, friction coefficients were lower than that at a load of 2 N. Moreover, wear rate increased obviously along with the load increasing both under the dry-sliding and water lubrication conditions. To gain more insights into the friction and wear mechanisms of the coatings, the worn surface of the AISI-52100 steel ball under dry- and water-lubricated sliding at a normal load of 5 N with 0.1 m/s speed have been investigated using an XPS. The XPS spectra of some typical elements on the worn surface of the AISI52100 steel are given in Fig. 6. As shown in the figures the curve 1 belong to the dry sliding and curve 2 are for water lubrication. The binding energies from these plots were compared with the data listed in the Handbook of X-ray Photoelectron Spectroscopy [19]. The main peak of F 1s around 685.0 eV assigned to iron fluoride appears only on the worn steel surface under dry sliding (Fig. 6(a)), which indicates that iron fluoride (Fe-F) bonds are formed during the dry sliding process. Almost no F is observed on the worn steel surface under water lubrication. The peak of Fe 2p at 711.1 eV is assigned to iron fluoride (Fig. 6(b)). This indicated that iron fluoride bonds are formed during the dry sliding process, combining with the XPS spectra of F 1s, which illustrates that the transfer film did formed during the dry sliding but it is hindered under water lubrication, and it might be the major cause of the larger wear rate under the water lubrication. The similar Fe 2p spectra are observed on the worn steel ball surfaces under both water lubrication and dry sliding condition, which indicates that the steel ball surface is oxidized in both conditions. It is rationally supposed that such a
Fig. 6. XPS analyses on the worn surface of the AISI-52100 steel ball sliding at a normal load of 5 N with 0.1 m/s speed. (a) F 1s, (b) Fe 2p, (1) dry sliding, (2) water lubrication.
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Fig. 7. SEM morphologies of worn surface of FEP coating under water lubrication at (a) 0.1 m/s and (b) 0.4 m/s, and under dry sliding at (c) 0.1 m/s and (d) 0.4 m/s.
kind of the steel oxidation is largely dependent on the frictioninduced thermal effect under dry sliding while on the corrosiveness of the water under water-lubricated sliding. Fig. 7 shows the SEM morphologies of the worn surfaces of the FEP coatings sliding against the AISI-52100 steel under dry- and water-lubricated conditions at a normal load of 5 N, 0.4 m/s speed. It is seen that the FEP coating under water lubrication against the steel at a low speed was characterized by slight scuffing, and a few wear debris appeared at the edges of the wear track in this case (Fig. 7(a)). This observation was seemingly contradictive to the much larger wear rate of the FEP coating under water lubrication than under dry sliding, which could be related to the easier removal of the wear debris in the presence of water. Surprisingly, similar features were observed for the worn surfaces of the coating under water lubrication at a high sliding speed of 0.4 m/s (see Fig. 7(b)) and under dry-sliding (see Fig. 7(c) and (d)), which also seemed to be contradictive to the differences in their wear rates. Namely, the worn surface of the FEP coating under dry sliding was characterized by severe adhesion, scuffing, and plastic deformation (see Fig. 7(c) and (d)), but the wear rate of the coating in this case was smaller than that under water lubrication, which might also imply that the distilled water as the lubricant contributed to enhancing the removal of the wear debris of the polymer coating abraded by the harder SAE-52100 steel counterpart and the water prohibited forming of the transfer film onto the steel ball surface. 4. Conclusions FEP coatings were prepared on steel substrate via flame spraying route. It was found that no chemical changes were involved in the flame spraying process at a relatively high temperature. The distilled water as the lubricating medium contributed to greatly decreasing
the friction coefficients of the coatings sliding against the AISI-52100 steel. However, it led to a significant increase in the wear rate of the FEP coating, possibly owing to the worsening in the mechanical strength of the coating in the presence of water. Further work should be conducted to introduce proper reinforcing phases into the flame sprayed FEP coating so as to increase the wear resistance. Acknowledgements The authors wish to acknowledge the financial support from the National High Technology Research and Development Program of China (‘‘863’’ Program) (Grant No. 2007AA03Z358). Prof. Jiazheng Zhao and Engineer Jinfang Zhou at the State Key Laboratory of Solid Lubrication are gratefully acknowledged for carrying out the SEM and FTIR analyses. References [1] L. Favre-Quattropani, P. Groening, P. Ramseyer, L. Schlapbach, Surf. Coat. Technol. 125 (2000) 377–382. [2] R. Kawase, A. Nakano, Thermal Spray: Practical Solutions for Engineering Problems, ASM International, Materials Park, 1996pp.257–261. [3] E. Petrovicova, R. Knight, L.S. Schadler, T.E. Twardowski, J. Appl. Polym. Sci. 78 (2000) 2272–2289. [4] S. Lathabai, M. Ottmuller, I. Fernandez, Wear 221 (1998) 93–108. [5] L.S. Schadler, K.O. Laul, R.W. Smith, E. Petrovicova, J. Therm. Spray Technol. 6 (1997) 475–485. [6] J.A. Brogan, C.C. Berndt, G.P. Simon, D. Hewitt, Polym. Eng. Sci. 38 (1998) 1873– 1881. [7] F.Y. Yan, K.A. Gross, G.P. Simon, C.C. Berndt, Polym. Eng. Sci. 44 (2004) 1448–1459. [8] H. Liao, E. Bechi, F. Berger, C. Coddet, in: Proceedings of the 15th International Thermal Spray Conference, Nice France, ASM International, Materials Park, OH, 1998, p. 25. [9] E. Lugscheider, C. Herbst, A. Fischer, in: Proceedings of the 15th International Thermal Spray Conference, Nice France, ASM International, Materials Park, OH, 1998, p. 19.
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