Abstract
A new nickel activation process was developed for metalizing the carbon fiber (CF) surface with electroless nickel plating. The oxidation and activation processes were examined using X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectrometry (EDS), and X-ray Diffractometry (XRD). XPS analysis showed that HNO3 oxidation improved the O/C ratio of the CF surface, which resulted in an increase in the amount of oxygen-containing groups. As activation time increased, both Nio cluster and Ni-P deposits increased. More than 50 mg of Nio clusters formed on the surface of 1g CF. SEM observations illustrated that particles and island-like shapes were developed from the Nio cluster, which helped to initiate the electroless nickel-plating on the CF surface. A continuous and compact coating with a thickness of about 2 to 3 μm was obtained. EDS results showed that the nickel and phosphorus content in the deposits was 97.34 wt.% and 2.66 wt.%, respectively. XRD indicated that the Ni-P coating was crystalline and the maximum deposition on 1 g CFs was as high as 9000 mg. As the results of the observations reveal, the activation method is a feasible alternative to Pd activation.
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A Simple Nickel Activation Process for Electroless Nickel-Phosphorus Plating on Carbon Fiber
Tingguo Yan,a Leihong Li,b and Lijuan Wang*, a
A new nickel activation process was developed for metalizing the carbon fiber (CF) surface with electroless nickel plating. The oxidation and activation processes were examined using X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectrometry (EDS), and X-ray Diffractometry (XRD). XPS analysis showed that HNO3 oxidation improved the O/C ratio of the CF surface, which resulted in an increase in the amount of oxygen-containing groups. As activation time increased, both Nio cluster and Ni-P deposits increased. More than 50 mg of Nio clusters formed on the surface of 1g CF. SEM observations illustrated that particles and island-like shapes were developed from the Nio cluster, which helped to initiate the electroless nickel-plating on the CF surface. A continuous and compact coating with a thickness of about 2 to 3 μm was obtained. EDS results showed that the nickel and phosphorus content in the deposits was 97.34 wt.% and 2.66 wt.%, respectively. XRD indicated that the Ni-P coating was crystalline and the maximum deposition on 1 g CFs was as high as 9000 mg. As the results of the observations reveal, the activation method is a feasible alternative to Pd activation.
Keywords: Electroless nickel plating; Carbon fiber (CF); Ni activation; Coating
Contact information: a: Key Laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, 26 Hexing Road, Harbin 150040, P. R. China; b: Northeast Forestry University-Adult Education College; * Corresponding author: donglinwlj@163.com
INTRODUCTION
Due to its strength, stiffness, dimensional stability, low coefficient of thermal expansion, biological compatibility, and fatigue resistance, carbon fibers (CFs) are widely used in some special applications such as aerospace, automobile, chemical industry, general engineering, missile, nuclear field, reinforcement in composite material, and textiles (Xu et al. 2008；Seo et al. 2009; Rezaei et al. 2009; Dai et al. 2011). However, the bond between CFs and the polymer matrix in composite materials is weak because the surface of CFs is smooth, compact, and lacks polar functional groups (Pittman et al. 1997). Therefore, interest in the modification of the CF surface to improve the interfacial adhesion has recently arisen. Electroless plating is a cost-effective technology because it can make a complex surface harder and more uniform. The coating can serve as the medium for the adhesion and the transferring loads. As is well known, an activation process is necessary for electroless plating on a dielectric surface. Conven-tional activation always involves the metal Pd (Bosko et al. 2011; Cheon et al.2011; Park et al. 2011), which has high catalysis performance in the electroless plating process. However, the cost is so great that the alternative activation is required to decrease the cost of the surface modification. Ni activation has received more attention in the past decade in order to reduce the cost of the surface modification. Tang et al. (2008) employed chitosan to fix the nickel on the surface of ABS plastic as a means of initiating electroless nickel plating. Li et al. (2011) conducted a new Ni activation process in which nickel oxide particles were obtained on the surface of glass fiber through Ni2+ adsorption and heat treatment, separately. In our previous work (Li et al. 2010), the nickel coating was successfully deposited on a wood surface using the method that wood veneers loaded with NaBH4 were immersed in a plating solution to complete the activation and plating process in one bath due to the porous surfaces and good wettability of wood. It was found that only a small amount of Ni2+ ions can be directly absorbed and cannot form enough active sites for initiating the subsequent plating process owing to the smooth and hydrophobic surfaces on CFs.
In the present work, electroless nickel-plating on CFs surface was investigated using a new activation process in order to achieve uniform deposition. In this process, the plating solution was used as an activation solution for absorbing the Ni2+. The process involved lactic acid complexion on the CFs surface and reduction to form Nio clusters for initiating the plating reaction.
EXPERIMENTAL PROCEDURES
Materials
The polyacrylonitrile substrate CF used in the present study was obtained from Mitsubishi Company, Japan. Other chemical reagents used were all of analytical grade.
Pretreatment of CFs
Two steps, degreasing and roughening, were used in the pretreatment of CFs. After CFs were cut into a length of about 3.5 cm, the epoxy resin on the surfaces of CFs was removed by acetone in a Soxhlet device at 80oC for 24 h. Then CFs were immersed in a boiled 1.0 mol/L NaOH solution for 15 min and a following treatment in boiled 1.0 mol/L HCl solution for 15 min. Degreased CFs were ultrasonically cleaned in distilled water until the solution was neutral. Finally, they were dried at 100oC to a constant weight.
Degreased CFs were oxidized in nitric acid at 30oC or 80oC for a certain time, as specified. The roughened CFs were cleaned with distilled water and dried in oven at 100oC to a constant weight.
Activation and Electroless Plating
The surface activation was conducted by immersion of 1 g of pretreated CFs in the 200 mL or 500 mL plating solution for certain time followed by immersion in the NaBH4 solution for forming the catalytic sites (Nio clusters). After the activation process, the samples were rinsed with distilled water.
The electroless nickel plating was achieved by dipping the pre-nucleated CFs into the plating solution at 70oC for 20 min. Then, the coated CFs were cleaned with distilled water and dried to a constant weight at 50oC. The composition of the electroless nickel-plating solution used in both the activation process and the nickel-plating process can be found in Table 1. The pH value was adjusted by using ammonia solution. The whole process including pretreatment, activation, and plating is illustrated in Fig. 1.
Fig. 1. Electroless nickel plating process on carbon fiber using nickel activation
Measurement of Nickel Amount on CFs
Nio clusters or the coatings were redissolved into the nitric acid solution, and the obtained solution was diluted in a constant volume. The concentration of Ni2+ ions was analyzed by spectrophotography. The mass of Nio clusters or the coatings can be calculated from the concentration and the volume of the solution. The amount of Nio clusters and the deposition of the coating can be calculated as below,
Nio cluster amount = (C1×V1×MNi) /MCF (1)
Deposition = (C2×V2×MNi) /MCF (2)
where C1 and V1 are the concentration and the volume of the solution from Nio clusters; C2 and V2 are the concentration and the volume of the solution from the coatings; and MNi is atomic weight of nickel and MCF is the mass of the carbon fiber.
Characterization Methods
X-ray Photoelectron Spectroscopy (XPS) was used for analyzing the O/C ratio of CFs before and after the pretreatment. Scanning Electron Microscopy (SEM) was used to observe the morphologies of pretreated, activated, and plated CFs surfaces. The Energy Dispersive X-ray Spectrometry (EDS) and X-ray Diffractometry (XRD) methods were used for examining the nickel coating.
RESULTS AND DISCUSSION
HNO3 Oxidation
In order to obtain oxygen-containing groups, CFs were oxidized in nitric acid. XPS was used for measuring the O/C ratio to evaluate the amount of oxygen-containing groups on carbon fiber surfaces. As shown in Table 2, the element nitrogen was from the polyacrylonitrile substrate prior to carbonization. The O/C ratio was only 0.17 in degreased CF and increased after HNO3 oxidation. When the oxidation temperature increased from 30 to 80oC and time increased from 40 to 60 min, the O/C ratio remained unchanged. Moreover, high-resolution XPS C1s spectra of CF before and after HNO3 oxidation were analyzed, and results are shown in Fig. 2 and Table 3. It was found that the C 1s peaks could be fitted to four smooth distribution functions corresponding to different binding energies. The different binding energy peaks were assigned to C-C, C-O, C=O, and O-C=O groups. After HNO3 oxidation, the results showed that the relative amount of C-C groups decreased but that of C=O and O-C=O increased, which indicated that some C-C groups were transformed to C=O or O-C=O groups in the oxidation process. It has been suggested that HNO3 oxidation can lead to oxygen-containing groups on carbon fiber surface which has been proved carboxyl groups, phenolic groups, quinines, and lactones in previous work (Pittman et al. 1997). Such groups favor the adsorption of Ni2+ in the plating solution for the activation because of complex formation.
Fig. 2. High-resolution XPS C1s spectra of CF before (a) and after (b) HNO3 oxidation at 80 oC for 60 min
Activation Process
The adsorption process is very important because the aim of activation in this study is to form catalytic Ni0 sites. The reaction in the activation process can be written as:
(3)
Higher adsorption of Ni2+ can provide more active sites to initiate the plating process. The result of the activation can be influenced by the adsorption time and the ratio of CF mass to the volume of the solution. The amount of Nio clusters on carbon fiber was measured under various conditions. As shown in Fig. 3, the amount of Nio (as clusters) increased from 29.8 mg to 54.6 mg on 1g CF as the absorption time increased from 4 min to 12 min in a 200 mL solution. A slight change was observed when the absorption time was longer than 12 min. Under the same adsorption time, the more volume of solution can lead to more Nio clusters.
Fig. 3. The proportion of nickel deposits and CF changing along with adsorption time in activation process
Figure 4(a) shows that the many gaps and cracks on the carbon fiber surface were compact and clean. After activation, many particles or island-like materials were observed on the surface (Figs. 4b and c). Moreover, it was found that Nio particles were distributed uniformly in the cracks under the activation in a 200 mL solution for 12 min. However, when the solution volume was increased to 500 mL, some Nio particles bonded into island-like shapes, which worsened the uniformity.
Fig. 4. SEM photographs of CFs before activation (a) and after activation for 12 min in 200 mL (b) and 500 mL solution (c)
Electroless Nickel Plating
The effect of activation time on deposition amount is shown in Fig. 5. With an increase in activation time from 4 min to 12 min, the deposition increased from 6000 mg/g CF to 9000 mg/g CF. The rate of increase was slowed down after 12 min. The amount of auto-catalyst Nio increased with an increase in activation time. Therefore, more Nio can rapidly initiate and accelerate the electroless plating process, resulting in more deposition. Comparing the deposition under the different volumes of solution in the activation process, the effect of the 500 mL solution was a little better than that of 200 mL. When the activation time was up to 14 min, almost same amount of deposition was obtained for both solutions. This indicated that almost all the Ni2+ in the plating solution were reduced and deposited on CF surface because the volume of plating solution was fixed.
Fig. 5. The effect of activation time on deposition amount on CF surface
Figure 6 shows the SEM images of CF after electroless nickel plating. A uniform, smooth, and compact layer was observed in the photograph of the coating surface. The cross-section image reveals that the thickness of the coating was around 3 μm in Fig. 6 (b) and around 2 μm in Fig. 6 (d). This result is accordance with the conclusion from Fig. 5. The EDS spectrum of cross-section of the plated CF surface is shown in Fig. 7. The high content of the element carbon was from the CF itself. The contents of nickel and phosphorus in the coating were 97.34 wt.% and 2.66 wt.%, respectively.
Fig. 6. SEM images of CF after plating under activation in 500 mL and 200 mL solution for 12 min (a, c) the plated CF surface (5000×) and (b, d) the cross-section of the plated CF (5000×)
Fig. 7. EDS spectrum of the cross-section of plated CF
Figure 8 (a) shows a strong peak at 2θ= 26.18o, which is the characteristic peak of carbon in CF. After plating, a series of peaks were apparent. As shown in Fig. 8 (b), peaks at 2θ = 44.5o, 51.8o, and 76.3o, which are attributable to Ni(111), Ni(200), and Ni(220), respectively, indicated the presence of the face-centered cubic phase of nickel (JCPDS 04-0850) and the crystalline nature of the layer. This result can be largely attributed to the fact that there was little distortion of the crystalline of Ni caused by P atoms because of low P content. It was found that the characteristic peak intensity of CF significantly decreased after plating. In accordance with the results from SEM observation, it was found that the CF surface was entirely and compactly covered by the Ni-P coating and the thickness of the coating reached at least several micrometers.
Fig. 8. XRD patterns of CF before (a) and after (b) electroless nickel plating
CONCLUSIONS
In this study, electroless nickel-plating on CF surface catalyzed with a new activation was investigated. The plating solution was used for both activation and plating process. The hydrophilicity of CF surface was improved by HNO3 oxidation at 30oC for 40 min. By dipping 1 g CFs in a 500 mL plating solution for 12 min for activation and 20 min for plating, a uniform and continuous coating was deposited on the CF surface. The maximum deposition reached more than 9000 mg Ni-P alloy. The thickness of the coating was around 2 to 3 μm. Moreover, the film was crystalline and contained 97.34 wt.% and 2.66 wt.% of the nickel and phosphorus, respectively. It was confirmed that the method was an effective and promising alternative to the conventional Pd activation.
ACKNOWLEDGMENTS
The authors are grateful for the Program for NCET (08-0752).
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Article submitted: September 10, 2012; Peer review completed: November 15, 2012; Revised version received and accepted: November, 27, 2012; Published: November 28, 2012.