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Transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage
Materials Chemistry and Physics 87 (2004) 102–108
Transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage Ming-Der Ger a,∗ , Kung-Hsu Hou b , Bing-Joe Hwang c a
b
Electrochemical Microfabrication Laboratory, Department of Applied Chemistry, Chung Cheng Institute of Technology, National Defense University, Ta-His, Tao-Yuan 335, Taiwan, ROC Department of Vehicle Engineering, Chung Cheng Institute of Technology, National Defense University, Ta-His, Tao-Yuan 335, Taiwan, ROC c Microelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology 43, Sec. 4, Keelung Road, Taipei 106, Taiwan, ROC Received 18 January 2004; received in revised form 27 April 2004; accepted 10 May 2004
Abstract The effect of surfactants on the transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage was investigated in this study. Two surfactants (CTAB and FC134) are utilized for comparison. The composition variation of the deposited layer is strongly related to the cathodic reactivity of the surfactants depending on the substrates at the early stage. When the cathodic reactivity of the surfactants is higher, correspondingly the PTFE particles are more easily embedded in the codeposition layer. The volume fraction of PTFE loading increases with the growth of the codeposition layer when the cathodic reactivity of surfactants on a substrate is less than that on the deposited layer. On the contrary, the volume fraction of PTFE loading decreases with the growth of the codeposition layer. Increasing the PTFE loading with the growth of the codeposition layer would provide good adhesion between the substrate and the codeposition layer. © 2004 Elsevier B.V. All rights reserved. Keywords: Ni–P/PTFE; Electroless; Codeposition; Surfactants
1. Introduction It is well known that the electroless Ni–P coating has a highly even plating capability, high bonding strength, excellent weldability, electrical conductivity, good antiwear properties, and controllable magnetic properties through suitable heat treatment. Furthermore, the mechanical and tribological properties of the electroless Ni–P coatings can be improved by the incorporation of PTFE particles. PTFE is chemically very inert and has a relatively high melting point (325 ◦ C). Its coefficient of friction is lower than that of almost any other polymers. Because of its extremely low surface energy (18.6 mN m−1 ), PTFE has excellent non-stick properties [1]. Due to its non-stick nature, non-galling, excellent dry-lubricity, low friction, good corrosion and wear resistance [2–5], the electroless Ni–P/PTFE composite coating becomes of great interest. It is well known that surfactants cannot only improve the stability of a suspension by increasing the wettability and the ∗ Corresponding author. Tel.: +886-3-3900714; fax: +886-3-3900714. E-mail address: [email protected] (M.-D. Ger).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.05.004
surface charge of suspended particles but also enhance the electrostatic adsorption of suspended particles on a cathode surface by increasing their net positive charge [6]. Although the role of surfactant is essential, it is usually a difficult task to choose a suitable surfactant for electrochemical deposition system such as PTFE/Ni–P system due to the mechanism still unclear in the codeposition of PTFE with electroless Ni–P coating. In the PTFE/Ni–P system, Matsuda et al. [7] and Hu et al. [8] have proposed that the zeta potential is the dominant factor for the codeposition of PTFE with electroless nickel. In other words, as long as the zeta potential of PTFE changes to a positive level, the codeposition would therefore be carried out without any difficulty. It means that the weak adsorption would dominate the codeposition behavior. However in our previous study [9], it was found that both the zeta potential of the PTFE particles and the cathodic reactivity of the surfactants play an important role in the codeposition process. It indicated that the zeta potential of the CTAB-modified PTFE particles is larger than that of the FC134-modified PTFE particles. But the amount of PTFE presents in the codeposition layer obtained in the bath of FC134 is significantly more
M.-D. Ger et al. / Materials Chemistry and Physics 87 (2004) 102–108
than that in the bath of CTAB. It is because that the cathodic reactivity of FC134 is significantly larger than that of CTAB on a PTFE/Ni–P composite substrate. Surfactants adsorbed on the inert particle provide itself a strong reduction reaction on the cathode would enhance the strong adsorption, therefore increase the possibility for the inert particle being embedded in the codeposition layer. Hovestad et al. [10] and Shrestha et al. [11] recently had also noticed that the importance of surfactant reactivity in electro–codeposition system. Although the composition gradient of a codeposition layer, which is strongly related to the transient phenomena at the early stage, is essential for its properties, no literature has been reported for this issue. The objective of this study is to investigate the effect of surfactants on the transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage. Two surfactants (CTAB and FC134) are utilized for comparison to provide a deeper insight into the transient behaviors in the codeposition system of PTFE with Ni–P matrix.
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PTFE powders (Du Pont Ltd., 30J) of average size 0.23 m were used. Two cationic surfactants, fluorinated alkyl quaternary ammonium iodides (FC134) from 3 M Co. and cetyltrimethylammonium bromide (CTAB) from TCI Co., were used. The electroless plating was carried out in the electrolyte involving the PTFE concentration of 4.5 g l−1 , and either FC134 (220 mol l−1 ) or CTAB (550 mol l−1 ) at the conditions of pH = 5, T = 89 ◦ C and stirring rate of 100 rpm. The substrate employed was low carbon steel with a dimension of 5 cm × 6.5 cm. one liters of electroless bath solution was used in each experiment. A field emission scanning electron microscope (FESEM, Hitachi S-4000) was used to observe the surface microstructure of the deposits. At least 70 particles were measured for each condition and the volume percentage of embedded particles as well as the average size in the deposits was determined with the image analysis technique. This technique employs an imaging and data processing software (Optimas 6×) to allow quantification of the portion of PTFE particles in the codeposits.
2. Experimental work 3. Results and discussion The electroless bath contains NiSO4 ·6H2 O (29.3 g l−1 ), NaH2 PO2 ·H2 O (30 g l−1 ), glycerin (10 g l−1 ), sodium lactate (41.3 g l−1 ), KIO3 (15m g l−1 ) and PbNO3 (0.38 mg l−1 ).
Fig. 1 shows the surface morphology of the PTFE/Ni–P composite layer at various deposition times in the
Fig. 1. The microstructures of composite surface morphology at various deposition times: (a) 1 min, (b) 3 min, (c) 5 min and (d) 10 min in the electrolyte involving the PTFE concentration of 4.5 g l−1 , CTAB (550 mol l−1 ) at the conditions of pH = 5, T = 89 ◦ C and stirring rate of 100 rpm.
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Table 1 The volume fraction of PTFE particles in the deposits at various deposition times for the CTAB system (conditions: pH = 5; T = 89 ◦ C; stirring rate = 100 rpm; CTAB = 550 mol l−1 )
Table 2 The volume fraction of PTFE particles in the deposits at various deposition times for the FC134 system (conditions: pH = 5; T = 89 ◦ C; stirring rate = 100 rpm; FC134 = 220 mol l−1 )
Deposition time (min)
Volume fraction of PTFE particles in deposit (%)
Deposition time (min)
Volume fraction of PTFE particles in deposit (%)
0 1 3 5 10
0 12.4 6.2 3.8 3.3
0 1 3 5 10
0 5.2 6.8 18.5 26.2
electrolyte involving the PTFE concentration of 4.5 g l−1 , CTAB (550 mol l−1 ) at the conditions of pH = 5, T = 89 ◦ C and stirring rate of 100 rpm. It appears that the amount of PTFE (as arrowed black spots in Fig. 1) in the deposit is decreased with an increase in the deposition time. Table 1 shows the volume fraction of PTFE particles in the deposits. It was found that initially the volume fraction of PTFE particles in deposits is 12.4% and then decreases to around 3%. The operation conditions in the FC134 system were same as those used in the CTAB system but surfactant FC134 instead of CTAB. The amount of PTFE in the deposit is increased with an increase in the deposition time, as shown in Fig. 2. It is worthy to note that the composition transient
in the FC134 system is different from that of the CTAB system. Table 2 shows the volume fraction of PTFE particles in the deposits. The volume fraction of PTFE particles in deposit was increased from initial 5.2 to 26.2%. It is noted that the volume fraction of PTFE particles in deposit in the CTAB system was larger than in the FC134 system at the earlier stage (t = 1 min). But later (t = 10 min) the PTFE loading increases in the FC134 system whereas decreases in the CTAB system. Further studies are carried out to compare the codeposition layer of CTAB system with that of FC134 systems. The morphologies of composite surface of two systems at 30 min deposition time are depicted in Fig. 3. It was found that some cracks are formed obviously in the codeposition
Fig. 2. The microstructures of composite surface morphology at various deposition times: (a) 1 min, (b) 3 min, (c) 5 min and (d) 10 min in the same electrolyte and operation condition mentioned above but with different surfactant FC134 (220 mol l−1 ).
M.-D. Ger et al. / Materials Chemistry and Physics 87 (2004) 102–108
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sorbed surfactants on the particle surface; K1 the adsorption constant of reductant; CR the concentration of reductant; K2 the equilibrium constant of loose adsorption, which depends on the zeta potential of the surfactant-modified PTFE particles; Cp the particle loading in suspension; θ 1 the reductant adsorption coverage, θ 2 the surfactant adsorption coverage and θ 3 the strong adsorption coverage. Due to the reaction time is very short in our electroless plating system, the consumption of reductant and surfactant in the bath can be negligible. In this case, θ 1 and θ 2 are nearly constant, therefore Eq. (1) can be simplified to K2 Cp K 1 CR VP = k exp(Bη)Cs (θk − θ3 )2 , 1 + K 1 CR 1 + K 2 Cp (2) θk = 1 − θ1 − θ2 In the present study, since the operation conditions and the composition of the electroless bath are fixed, Cs , K1 , CR and Cp in Eq. (2) can be considered as constants. The zeta potential of PTFE particles can be reasonably considered as the same (∼40 mV) in the experiments performed at FC134 of 220 mol l−1 in the FC134 system and at CTAB of 550 mol l−1 in the CTAB system, as shown in Fig. 4. Thus, the constant K2 can be regarded to be same for both the
Fig. 3. The microstructures of composite surface morphology in (a) CTAB-modified and (b) FC134-modified system at deposition time 30 min in the same electrolyte and operation condition mentioned above.
layer of the CTAB-modified system (as shown in Fig. 3a). However, the codeposition layer of the FC134-modified system shows smooth morphology (as shown in Fig. 3b). It might be ascribe to the composition changes abruptly at the interface of the substrate and the codeposition layer in the CTAB-modified system resulting in a high stress and a poor adhesion between the substrate and the codeposition layer. The non-crack deposit surface in the FC134-modified system might be the result of the gradual change of composition at the interface of the substrate and the codeposition layer. It leads to a strong adhesion and therewith lower stress. Therefore, the crack formation is retarded. In order to explain these phenomena, a proposed model has to be considered. In our previous study, the deposition rate of PTFE particles (VP ) in the codeposition layer in an electroless–codeposition system was established as [12]: K2 Cp K 1 CR VP = k exp(Bη)Cs 1 + K 1 CR 1 + K 2 Cp ×(1 − θ1 − θ2 − θ3 )2
(1)
where k and B are the constant in the Tafel equation, which depends on the surfactant and substrate; η is the overpotential of the electroless system; Cs the concentration of ad-
Fig. 4. Zeta potential of the dispersed PTFE particles at various concentrations of surfactants (a) FC134 and (b) CTAB [9].
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systems. Therefore the volume fraction of PTFE loading in the codeposition layer would be affected only by k exp(Bη) and (θk − θ3 ) which are related to the cathodic reactivity of surfactants and their strong adsorption on the deposited layer, respectively. Increasing the term of reactivity of the surfactants and decreasing the strong adsorption of PTFE particles leads to increase the PTFE loading in the codeposition layer. It is found that the cathodic reactivity of the surfactants depends on the substrates in our previous study [9]. It was observed that the cathodic reactivity of the surfactant CTAB on the substrate of carbon steel is larger than that of FC134. When the substrate is changed from carbon steel to Ni–P/PTFE, the reactivity of FC134 increased significantly but that of CTAB remains unchanged [9]. Since the cathodic reactivity of the surfactants on the carbon steel is CTAB > iFC134, the term of k exp(Bη) in the CTAB system is higher than that in FC134 system. Meanwhile, the term (θk − θ3 ) is nearly constant (∼θ k ) at the beginning. From Eq. (2), it gives that the volume fraction of PTFE loading in the codeposition layer in the CTAB system is more than that in the FC134 system at the beginning. It is consistent with the experimental results, as shown in Tables 1 and 2. As the reaction proceeds, the Ni–P and PTFE codeposited on carbon steel and the substrate would change to
a PTFE/Ni–P composite layer. The reactivity of FC134 on PTFE/Ni–P substrate increased gradually with an increase in the PTFE loading in deposit. This results in the term of (k exp(Bη))FC134 > C(k exp(Bηp))CTAB . Therefore the PTFE loading in the codeposition layer presents in the codeposition layer obtained in the FC134 system after 10 min is significantly more than that in the CTAB-modified system (as shown in Figs. 1d and 2d). In the CTAB-modified system, when the substrate in the electroless bath starts to reaction, no significant inert particles are adsorbed on the surface of substrate. The value of (k exp(Bη))CTAB (θk −θ3 ) at the beginning stage is larger than that of the finial stage. Therefore the PTFE loading in the codeposition layer was higher at this beginning stage. As the deposition proceeds, the value of (kp exp(Bηp ))CTAB (θk −θ3 ) becomes smaller and results in the lower PTFE loading. Nevertheless in the FC134-modified system (see Eq. (2)), the value of (k exp(B))FC134 increased significantly with increased PTFE amount in the deposit. Therefore the volume fraction of PTFE loading in the codeposition layer was lower at this beginning stage. As the deposition proceeds, the value of (k exp(Bη))FC134 becomes larger and the volume fraction of PTFE loading in the codeposition layer increases. However, the volume fraction of PTFE loading will reach a steady state value finally due to the decrease of the term of (θk − θ3 ).
Fig. 5. The distribution of PTFE particles corresponding to Fig. 1. At various times: (a) 1 min, (b) 3 min, (c) 5 min and (d) 10 min.
M.-D. Ger et al. / Materials Chemistry and Physics 87 (2004) 102–108
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Fig. 6. The distribution of PTFE particles corresponding to Fig. 2. At various times: (a) 1 min, (b) 3 min, (c) 5 min and (d) 10 min.
It should be noted that, whether it could be successful or not for codeposition of inner particle is determined by the competition between particle adhesion and removal forces at the cathode surface [13]. The adhesion forces of particle at the surface must surpass the removal forces then it would have the opportunity being embedded in the codeposition layer. In our system, it was proposed that the surfactants adsorbed on the particles surface would join the reduction therefore would offer a strong adsorption force [12]. Figs. 5 and 6 show the distribution of PTFE particles corresponding to Figs. 1 and 2 at various times. It shows that, for the CTAB-modified system, the average size of PTFE particles in deposit is about from 0.151 (Fig. 5a) to 0.116 m (Fig. 5d) decreasing with time increasing. However on the contrast, for the FC134-modified system, the average size of PTFE particles in deposit is about from 0.147 (Fig. 6a) to 0.254 m (Fig. 6d) increasing with time increasing. The results revealed that the average size of PTFE particles in deposit with CTAB-modified system was larger than FC134-modified system at the earlier stage (t = 1 min). But with the deposition time increased, the average size of PTFE particles in deposit is decreased in CTAB-modified system, and increased significantly in FC134-modified system. It also appears that the more volume fraction of PTFE particles in deposit, the larger the PTFE particles average size would present in the matrix. It is concluded that when the cathodic
reactivity of the surfactants is higher, larger particles would get sufficient strong adsorption force to be embedded in the codeposition layer. However, while the reactivity of the surfactants is weak, the strong adsorption force of larger particles to the cathode surface would therefore decrease and could not overcome the removal force resulting in only smaller particles present in the matrix (Fig. 1d).
4. Conclusion The effect of surfactants on the transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage was investigated. The reactivity of surfactant significantly correlated to the substrate resulting in the amount of PTFE particles varied at the early stage. In the CTAB-modified system, the volume fraction of PTFE loading decreases with the deposition time. The composition changes abruptly at the interface of the substrate and the codeposition layer. It would lead to generate the stress and to reduce adhesion between the substrate and the codeposition layer. In the FC134-modified system, the volume fraction of PTFE loading increases with the deposition time. The composition changes smoothly at the interface of the substrate and the codeposition layer. It would lead to improve the adhesion and reduce the stress between the substrate
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and the codeposition layer. Meanwhile, its hydrophobicity and lubricity would be improved if the PTFE loading on the top of the PTFE/Ni–P codeposition layer increased. The cathodic reactivity of the surfactants provides a strong adsorption force in the codeposition process. When the cathodic reactivity of the surfactants is higher, larger particles would be more easily embedded in the codeposition layer. However, while the reactivity of the surfactants is weak, the adhesion force of larger particles could not overcome the removal force resulting in only smaller particles present in the matrix.
Acknowledgements This work was financially supported by the National Science Council of the Republic of China (Grant NSC-87-2212-E-014-005) and National Taiwan University of Science and Technology.
]
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
J.S. Hadley, L.E. Harland, Met. Finish. 85 (12) (1987) 51. P.R. Ebdon, Plat. Surf. Finish. 75 (9) (1988) 65. M. Nishira, O. Takano, Plat. Surf. Finish. 81 (1994) 48. Q. Zhao, Y. Liu, H. Mueller-Steinhagen, G. Liu, Surf. Coat. Technol. 155 (2002) 279. M.D. Ger, K.H. Hou, L.M. Wang, B.J. Hwang, Mater. Chem. Phys. 77 (2003) 755. P.K.N. Bartlett, Industrial training report AKZO, Arnhem 10 (1980). H. Matsuda, Y. Kiyono, M. Nishira, O. Takano, Trans. I.M.F. 72 (2) (1994) 55. X. Hu, C. Dai, J. Li, D. Wang, Plat. Surf. Finish. 3 (1997) 51. M.D. Ger, B.J. Hwang, J. Chin. Inst. Eng. 32 (6) (2001) 503. A. Hovestad, R.J.C.H.L. Hessen, L.J.J. Janssen, J. Appl. Electrochem. 29 (1999) 331. N.K. Shrestha, I. Miwa, T. Saji, S. Tetsuo, J. Electrochem. Soc. 148 (2001) C106. M.D. Ger, B.J. Hwang, Mater. Chem. Phys. 76 (2002) 38. J. Fransear, J.P. Celis, J.R. Roos, J. Electrochem. Soc. 139 (1992) 413.
Transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage Ming-Der Ger a,∗ , Kung-Hsu Hou b , Bing-Joe Hwang c a
b
Electrochemical Microfabrication Laboratory, Department of Applied Chemistry, Chung Cheng Institute of Technology, National Defense University, Ta-His, Tao-Yuan 335, Taiwan, ROC Department of Vehicle Engineering, Chung Cheng Institute of Technology, National Defense University, Ta-His, Tao-Yuan 335, Taiwan, ROC c Microelectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology 43, Sec. 4, Keelung Road, Taipei 106, Taiwan, ROC Received 18 January 2004; received in revised form 27 April 2004; accepted 10 May 2004
Abstract The effect of surfactants on the transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage was investigated in this study. Two surfactants (CTAB and FC134) are utilized for comparison. The composition variation of the deposited layer is strongly related to the cathodic reactivity of the surfactants depending on the substrates at the early stage. When the cathodic reactivity of the surfactants is higher, correspondingly the PTFE particles are more easily embedded in the codeposition layer. The volume fraction of PTFE loading increases with the growth of the codeposition layer when the cathodic reactivity of surfactants on a substrate is less than that on the deposited layer. On the contrary, the volume fraction of PTFE loading decreases with the growth of the codeposition layer. Increasing the PTFE loading with the growth of the codeposition layer would provide good adhesion between the substrate and the codeposition layer. © 2004 Elsevier B.V. All rights reserved. Keywords: Ni–P/PTFE; Electroless; Codeposition; Surfactants
1. Introduction It is well known that the electroless Ni–P coating has a highly even plating capability, high bonding strength, excellent weldability, electrical conductivity, good antiwear properties, and controllable magnetic properties through suitable heat treatment. Furthermore, the mechanical and tribological properties of the electroless Ni–P coatings can be improved by the incorporation of PTFE particles. PTFE is chemically very inert and has a relatively high melting point (325 ◦ C). Its coefficient of friction is lower than that of almost any other polymers. Because of its extremely low surface energy (18.6 mN m−1 ), PTFE has excellent non-stick properties [1]. Due to its non-stick nature, non-galling, excellent dry-lubricity, low friction, good corrosion and wear resistance [2–5], the electroless Ni–P/PTFE composite coating becomes of great interest. It is well known that surfactants cannot only improve the stability of a suspension by increasing the wettability and the ∗ Corresponding author. Tel.: +886-3-3900714; fax: +886-3-3900714. E-mail address: [email protected] (M.-D. Ger).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.05.004
surface charge of suspended particles but also enhance the electrostatic adsorption of suspended particles on a cathode surface by increasing their net positive charge [6]. Although the role of surfactant is essential, it is usually a difficult task to choose a suitable surfactant for electrochemical deposition system such as PTFE/Ni–P system due to the mechanism still unclear in the codeposition of PTFE with electroless Ni–P coating. In the PTFE/Ni–P system, Matsuda et al. [7] and Hu et al. [8] have proposed that the zeta potential is the dominant factor for the codeposition of PTFE with electroless nickel. In other words, as long as the zeta potential of PTFE changes to a positive level, the codeposition would therefore be carried out without any difficulty. It means that the weak adsorption would dominate the codeposition behavior. However in our previous study [9], it was found that both the zeta potential of the PTFE particles and the cathodic reactivity of the surfactants play an important role in the codeposition process. It indicated that the zeta potential of the CTAB-modified PTFE particles is larger than that of the FC134-modified PTFE particles. But the amount of PTFE presents in the codeposition layer obtained in the bath of FC134 is significantly more
M.-D. Ger et al. / Materials Chemistry and Physics 87 (2004) 102–108
than that in the bath of CTAB. It is because that the cathodic reactivity of FC134 is significantly larger than that of CTAB on a PTFE/Ni–P composite substrate. Surfactants adsorbed on the inert particle provide itself a strong reduction reaction on the cathode would enhance the strong adsorption, therefore increase the possibility for the inert particle being embedded in the codeposition layer. Hovestad et al. [10] and Shrestha et al. [11] recently had also noticed that the importance of surfactant reactivity in electro–codeposition system. Although the composition gradient of a codeposition layer, which is strongly related to the transient phenomena at the early stage, is essential for its properties, no literature has been reported for this issue. The objective of this study is to investigate the effect of surfactants on the transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage. Two surfactants (CTAB and FC134) are utilized for comparison to provide a deeper insight into the transient behaviors in the codeposition system of PTFE with Ni–P matrix.
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PTFE powders (Du Pont Ltd., 30J) of average size 0.23 m were used. Two cationic surfactants, fluorinated alkyl quaternary ammonium iodides (FC134) from 3 M Co. and cetyltrimethylammonium bromide (CTAB) from TCI Co., were used. The electroless plating was carried out in the electrolyte involving the PTFE concentration of 4.5 g l−1 , and either FC134 (220 mol l−1 ) or CTAB (550 mol l−1 ) at the conditions of pH = 5, T = 89 ◦ C and stirring rate of 100 rpm. The substrate employed was low carbon steel with a dimension of 5 cm × 6.5 cm. one liters of electroless bath solution was used in each experiment. A field emission scanning electron microscope (FESEM, Hitachi S-4000) was used to observe the surface microstructure of the deposits. At least 70 particles were measured for each condition and the volume percentage of embedded particles as well as the average size in the deposits was determined with the image analysis technique. This technique employs an imaging and data processing software (Optimas 6×) to allow quantification of the portion of PTFE particles in the codeposits.
2. Experimental work 3. Results and discussion The electroless bath contains NiSO4 ·6H2 O (29.3 g l−1 ), NaH2 PO2 ·H2 O (30 g l−1 ), glycerin (10 g l−1 ), sodium lactate (41.3 g l−1 ), KIO3 (15m g l−1 ) and PbNO3 (0.38 mg l−1 ).
Fig. 1 shows the surface morphology of the PTFE/Ni–P composite layer at various deposition times in the
Fig. 1. The microstructures of composite surface morphology at various deposition times: (a) 1 min, (b) 3 min, (c) 5 min and (d) 10 min in the electrolyte involving the PTFE concentration of 4.5 g l−1 , CTAB (550 mol l−1 ) at the conditions of pH = 5, T = 89 ◦ C and stirring rate of 100 rpm.
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Table 1 The volume fraction of PTFE particles in the deposits at various deposition times for the CTAB system (conditions: pH = 5; T = 89 ◦ C; stirring rate = 100 rpm; CTAB = 550 mol l−1 )
Table 2 The volume fraction of PTFE particles in the deposits at various deposition times for the FC134 system (conditions: pH = 5; T = 89 ◦ C; stirring rate = 100 rpm; FC134 = 220 mol l−1 )
Deposition time (min)
Volume fraction of PTFE particles in deposit (%)
Deposition time (min)
Volume fraction of PTFE particles in deposit (%)
0 1 3 5 10
0 12.4 6.2 3.8 3.3
0 1 3 5 10
0 5.2 6.8 18.5 26.2
electrolyte involving the PTFE concentration of 4.5 g l−1 , CTAB (550 mol l−1 ) at the conditions of pH = 5, T = 89 ◦ C and stirring rate of 100 rpm. It appears that the amount of PTFE (as arrowed black spots in Fig. 1) in the deposit is decreased with an increase in the deposition time. Table 1 shows the volume fraction of PTFE particles in the deposits. It was found that initially the volume fraction of PTFE particles in deposits is 12.4% and then decreases to around 3%. The operation conditions in the FC134 system were same as those used in the CTAB system but surfactant FC134 instead of CTAB. The amount of PTFE in the deposit is increased with an increase in the deposition time, as shown in Fig. 2. It is worthy to note that the composition transient
in the FC134 system is different from that of the CTAB system. Table 2 shows the volume fraction of PTFE particles in the deposits. The volume fraction of PTFE particles in deposit was increased from initial 5.2 to 26.2%. It is noted that the volume fraction of PTFE particles in deposit in the CTAB system was larger than in the FC134 system at the earlier stage (t = 1 min). But later (t = 10 min) the PTFE loading increases in the FC134 system whereas decreases in the CTAB system. Further studies are carried out to compare the codeposition layer of CTAB system with that of FC134 systems. The morphologies of composite surface of two systems at 30 min deposition time are depicted in Fig. 3. It was found that some cracks are formed obviously in the codeposition
Fig. 2. The microstructures of composite surface morphology at various deposition times: (a) 1 min, (b) 3 min, (c) 5 min and (d) 10 min in the same electrolyte and operation condition mentioned above but with different surfactant FC134 (220 mol l−1 ).
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sorbed surfactants on the particle surface; K1 the adsorption constant of reductant; CR the concentration of reductant; K2 the equilibrium constant of loose adsorption, which depends on the zeta potential of the surfactant-modified PTFE particles; Cp the particle loading in suspension; θ 1 the reductant adsorption coverage, θ 2 the surfactant adsorption coverage and θ 3 the strong adsorption coverage. Due to the reaction time is very short in our electroless plating system, the consumption of reductant and surfactant in the bath can be negligible. In this case, θ 1 and θ 2 are nearly constant, therefore Eq. (1) can be simplified to K2 Cp K 1 CR VP = k exp(Bη)Cs (θk − θ3 )2 , 1 + K 1 CR 1 + K 2 Cp (2) θk = 1 − θ1 − θ2 In the present study, since the operation conditions and the composition of the electroless bath are fixed, Cs , K1 , CR and Cp in Eq. (2) can be considered as constants. The zeta potential of PTFE particles can be reasonably considered as the same (∼40 mV) in the experiments performed at FC134 of 220 mol l−1 in the FC134 system and at CTAB of 550 mol l−1 in the CTAB system, as shown in Fig. 4. Thus, the constant K2 can be regarded to be same for both the
Fig. 3. The microstructures of composite surface morphology in (a) CTAB-modified and (b) FC134-modified system at deposition time 30 min in the same electrolyte and operation condition mentioned above.
layer of the CTAB-modified system (as shown in Fig. 3a). However, the codeposition layer of the FC134-modified system shows smooth morphology (as shown in Fig. 3b). It might be ascribe to the composition changes abruptly at the interface of the substrate and the codeposition layer in the CTAB-modified system resulting in a high stress and a poor adhesion between the substrate and the codeposition layer. The non-crack deposit surface in the FC134-modified system might be the result of the gradual change of composition at the interface of the substrate and the codeposition layer. It leads to a strong adhesion and therewith lower stress. Therefore, the crack formation is retarded. In order to explain these phenomena, a proposed model has to be considered. In our previous study, the deposition rate of PTFE particles (VP ) in the codeposition layer in an electroless–codeposition system was established as [12]: K2 Cp K 1 CR VP = k exp(Bη)Cs 1 + K 1 CR 1 + K 2 Cp ×(1 − θ1 − θ2 − θ3 )2
(1)
where k and B are the constant in the Tafel equation, which depends on the surfactant and substrate; η is the overpotential of the electroless system; Cs the concentration of ad-
Fig. 4. Zeta potential of the dispersed PTFE particles at various concentrations of surfactants (a) FC134 and (b) CTAB [9].
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systems. Therefore the volume fraction of PTFE loading in the codeposition layer would be affected only by k exp(Bη) and (θk − θ3 ) which are related to the cathodic reactivity of surfactants and their strong adsorption on the deposited layer, respectively. Increasing the term of reactivity of the surfactants and decreasing the strong adsorption of PTFE particles leads to increase the PTFE loading in the codeposition layer. It is found that the cathodic reactivity of the surfactants depends on the substrates in our previous study [9]. It was observed that the cathodic reactivity of the surfactant CTAB on the substrate of carbon steel is larger than that of FC134. When the substrate is changed from carbon steel to Ni–P/PTFE, the reactivity of FC134 increased significantly but that of CTAB remains unchanged [9]. Since the cathodic reactivity of the surfactants on the carbon steel is CTAB > iFC134, the term of k exp(Bη) in the CTAB system is higher than that in FC134 system. Meanwhile, the term (θk − θ3 ) is nearly constant (∼θ k ) at the beginning. From Eq. (2), it gives that the volume fraction of PTFE loading in the codeposition layer in the CTAB system is more than that in the FC134 system at the beginning. It is consistent with the experimental results, as shown in Tables 1 and 2. As the reaction proceeds, the Ni–P and PTFE codeposited on carbon steel and the substrate would change to
a PTFE/Ni–P composite layer. The reactivity of FC134 on PTFE/Ni–P substrate increased gradually with an increase in the PTFE loading in deposit. This results in the term of (k exp(Bη))FC134 > C(k exp(Bηp))CTAB . Therefore the PTFE loading in the codeposition layer presents in the codeposition layer obtained in the FC134 system after 10 min is significantly more than that in the CTAB-modified system (as shown in Figs. 1d and 2d). In the CTAB-modified system, when the substrate in the electroless bath starts to reaction, no significant inert particles are adsorbed on the surface of substrate. The value of (k exp(Bη))CTAB (θk −θ3 ) at the beginning stage is larger than that of the finial stage. Therefore the PTFE loading in the codeposition layer was higher at this beginning stage. As the deposition proceeds, the value of (kp exp(Bηp ))CTAB (θk −θ3 ) becomes smaller and results in the lower PTFE loading. Nevertheless in the FC134-modified system (see Eq. (2)), the value of (k exp(B))FC134 increased significantly with increased PTFE amount in the deposit. Therefore the volume fraction of PTFE loading in the codeposition layer was lower at this beginning stage. As the deposition proceeds, the value of (k exp(Bη))FC134 becomes larger and the volume fraction of PTFE loading in the codeposition layer increases. However, the volume fraction of PTFE loading will reach a steady state value finally due to the decrease of the term of (θk − θ3 ).
Fig. 5. The distribution of PTFE particles corresponding to Fig. 1. At various times: (a) 1 min, (b) 3 min, (c) 5 min and (d) 10 min.
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Fig. 6. The distribution of PTFE particles corresponding to Fig. 2. At various times: (a) 1 min, (b) 3 min, (c) 5 min and (d) 10 min.
It should be noted that, whether it could be successful or not for codeposition of inner particle is determined by the competition between particle adhesion and removal forces at the cathode surface [13]. The adhesion forces of particle at the surface must surpass the removal forces then it would have the opportunity being embedded in the codeposition layer. In our system, it was proposed that the surfactants adsorbed on the particles surface would join the reduction therefore would offer a strong adsorption force [12]. Figs. 5 and 6 show the distribution of PTFE particles corresponding to Figs. 1 and 2 at various times. It shows that, for the CTAB-modified system, the average size of PTFE particles in deposit is about from 0.151 (Fig. 5a) to 0.116 m (Fig. 5d) decreasing with time increasing. However on the contrast, for the FC134-modified system, the average size of PTFE particles in deposit is about from 0.147 (Fig. 6a) to 0.254 m (Fig. 6d) increasing with time increasing. The results revealed that the average size of PTFE particles in deposit with CTAB-modified system was larger than FC134-modified system at the earlier stage (t = 1 min). But with the deposition time increased, the average size of PTFE particles in deposit is decreased in CTAB-modified system, and increased significantly in FC134-modified system. It also appears that the more volume fraction of PTFE particles in deposit, the larger the PTFE particles average size would present in the matrix. It is concluded that when the cathodic
reactivity of the surfactants is higher, larger particles would get sufficient strong adsorption force to be embedded in the codeposition layer. However, while the reactivity of the surfactants is weak, the strong adsorption force of larger particles to the cathode surface would therefore decrease and could not overcome the removal force resulting in only smaller particles present in the matrix (Fig. 1d).
4. Conclusion The effect of surfactants on the transient phenomena of the codeposition of PTFE with electroless Ni–P coating at the early stage was investigated. The reactivity of surfactant significantly correlated to the substrate resulting in the amount of PTFE particles varied at the early stage. In the CTAB-modified system, the volume fraction of PTFE loading decreases with the deposition time. The composition changes abruptly at the interface of the substrate and the codeposition layer. It would lead to generate the stress and to reduce adhesion between the substrate and the codeposition layer. In the FC134-modified system, the volume fraction of PTFE loading increases with the deposition time. The composition changes smoothly at the interface of the substrate and the codeposition layer. It would lead to improve the adhesion and reduce the stress between the substrate
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and the codeposition layer. Meanwhile, its hydrophobicity and lubricity would be improved if the PTFE loading on the top of the PTFE/Ni–P codeposition layer increased. The cathodic reactivity of the surfactants provides a strong adsorption force in the codeposition process. When the cathodic reactivity of the surfactants is higher, larger particles would be more easily embedded in the codeposition layer. However, while the reactivity of the surfactants is weak, the adhesion force of larger particles could not overcome the removal force resulting in only smaller particles present in the matrix.
Acknowledgements This work was financially supported by the National Science Council of the Republic of China (Grant NSC-87-2212-E-014-005) and National Taiwan University of Science and Technology.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
J.S. Hadley, L.E. Harland, Met. Finish. 85 (12) (1987) 51. P.R. Ebdon, Plat. Surf. Finish. 75 (9) (1988) 65. M. Nishira, O. Takano, Plat. Surf. Finish. 81 (1994) 48. Q. Zhao, Y. Liu, H. Mueller-Steinhagen, G. Liu, Surf. Coat. Technol. 155 (2002) 279. M.D. Ger, K.H. Hou, L.M. Wang, B.J. Hwang, Mater. Chem. Phys. 77 (2003) 755. P.K.N. Bartlett, Industrial training report AKZO, Arnhem 10 (1980). H. Matsuda, Y. Kiyono, M. Nishira, O. Takano, Trans. I.M.F. 72 (2) (1994) 55. X. Hu, C. Dai, J. Li, D. Wang, Plat. Surf. Finish. 3 (1997) 51. M.D. Ger, B.J. Hwang, J. Chin. Inst. Eng. 32 (6) (2001) 503. A. Hovestad, R.J.C.H.L. Hessen, L.J.J. Janssen, J. Appl. Electrochem. 29 (1999) 331. N.K. Shrestha, I. Miwa, T. Saji, S. Tetsuo, J. Electrochem. Soc. 148 (2001) C106. M.D. Ger, B.J. Hwang, Mater. Chem. Phys. 76 (2002) 38. J. Fransear, J.P. Celis, J.R. Roos, J. Electrochem. Soc. 139 (1992) 413.