Study on new process of electroless copper plating pretreatment on carbon fiber surface

In general, conventional carbon fiber surface pretreatment, with palladium, silver, and other precious metals as activators, increases its surface activity by initiating redox reactions for metallization, whereas it exhibits a high cost and poor economy. In this study, copper-nickel mixed colloids with low cost served as the activators, and sodium borohydride (NaBH4) and sodium hydroxide (NaOH) were employed as sensitizing reducing agents to successfully plate copper on the surface of carbon fibers. Subsequently, the morphology of the treated samples was characterized through SEM and XPS to explore the pretreatment process of the carbon fiber surface. The results indicated that the carbon fiber exhibited the optimal debinding effect at 55°C and the heating time was longer than 120 min. The result indicated that coarsening effect of 65% concentrated nitric acid was the optimal. The activation effect of copper-nickel mixed colloid was the optimal, and the surface quality of the coating was the optimal.


Background
Carbon fiber composites primarily comprise structural materials made of carbon fiber, metal, ceramic, and resin matrix. However, the structure of carbon fiber is stable, and it is difficult to directly react with the above matrix. Notably, when it is compounded with metal, ceramic, and other substrates, the wettability is poor, and there is an obvious interface reaction and poor bonding force [1][2][3]. The surface of carbon fibers should be metalized to enhance the surface properties of carbon fibers. After metallization, the surface activity of carbon fibers can be increased to effectively combine with other materials, and electroless plating takes on a great significance to the metallization of carbon fiber surface [4,5]. The sensitization and activation of carbon fibers has been considered the critical step in the whole electroless plating, directly correlated with whether the electroless plating can be performed, whether the plating solution can be decomposed, whether the coating coverage is high or low, as well as whether the adhesion between the coating and the substrate is good. Thus, it is also the focus of the research of electroless plating on the surface of carbon fiber materials [6][7][8]. Numerous methods have been proposed to activate carbon fiber. For instance, Huang Yuanfei et al [9] used electroless plating to obtain carbon fibers with uniform and dense Ni coating, with a thickness of 0.5-0.7 μm. Zeng Wenqing et al [10] plated Ni on the surface of carbon fibers using iron mesh catalysis instead of the conventional palladium activation method, and the obtained coating was uniform and dense, which achieved the effect of the nickel layer prepared by the palladium activation method.
Precious metal pretreatment is characterized by a high cost and cumbersome process [11][12][13], whereas the preparation of copper-nickel mixed colloid catalyst for carbon fiber coating is simple. Compared with the existing technology, the cost of using precious metals (e.g., palladium, platinum, and silver), and other precious metals as catalysts is less than one-tenth of their cost. The copper coating can avoid the harmful interaction between carbon fiber and matrix, improve the wetting ability of the interface between carbon fiber and matrix, enhance the interface strength between carbon fiber and matrix, and substantively improve the performance of Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. the composite materials, thus providing ideas and methods for the preparation of carbon fiber composites. Accordingly, in this study, copper-nickel mixed colloid composed of copper sulfate, nickel sulfate, gelatin, and sodium hydroxide first served as the activator. Subsequently, sodium borohydride (NaBH 4 ) and sodium hydroxide (NaOH) were employed as the sensitizing reducing agent. After plating on the surface of carbon fibers, the quality of the prepared coating was examined, the activation effect and the sensitization effect were explored, and the mechanism was studied.

Experimental materials
The carbon fiber material employed in the experiment was PAN-based 3 K carbon fibers produced by Jiyan High-tech Fiber Co., Ltd It was characterized by small specific gravity, high specific strength, high specific modulus, small thermal conductivity, small thermal expansion coefficient (dimensional stability), good ablation resistance, as well as self-lubricating property. Table 1 lists the specific chemical and physical performance indexes.

Sample preparation and detection
2.2.1. Carbon fiber pretreatment process First, the surface of the carbon fiber was degummed by the organic solvent acetone. Subsequently, the degummed carbon fiber was coarsened in NaOH solution, 65% concentrated nitric acid, and ammonium persulfate solution for 60 min, respectively. After being fully neutralized, the sample was washed in ionic water and then dried. Next, the mixed colloids of low-cost Cu colloids, Ni colloids, and Cu-Ni served as the activators, sodium borohydride (NaBH 4 ) and sodium hydroxide (NaOH) were employed as the sensitizers, and a suitable copper plating process was selected.

Copper plating process on carbon fiber surface
In this study, formaldehyde (HCHO) was employed as the reducing agent of the plating solution, and CuSO4·5H 2 O was adopted as the main salt of the electroless plating solution. On that basis, the disodium ethylenediamine tetraacetate and potassium sodium tartrate served as the double complexing agent, and a certain proportion of K4Fe(CN) 6 ·3H 2 O was added as a stabilizer. Under alkaline conditions, copper was deposited on the surface of carbon fiber. During the plating process, NaOH solution was required to prevent the decrease of the pH value of the plating solution. Table 2 lists the specific composition and process parameters of the plating solution.

Organizational structure detection
The carbon fibers after degumming were characterized using Hitachi SU8010 scanning electron microscope (SEM) and Thermo Scientific K-Alpha photoelectron spectrometer (XPS). The morphology and element content of carbon fibers after copper plating on the surface were characterized and analyzed using SEM and EDS energy spectrum analyzer. Furthermore, the effect of electroless copper plating on the surface of carbon fibers activated by different colloids was examined.  Table 3 lists the changes in the carbon fiber at different temperatures and times. As depicted in the table, at the same temperature, the colloid on the surface of the carbon fiber gradually dissolved with the increase of time, and the dissolution of the colloid on the carbon fiber surface was significantly affected by temperature. The polarity of acetone increased, and the ability to remove glue was enhanced with the increase of acetone solution temperature. The boiling point of acetone was 56.53°C. The highest temperature of acetone heating was 55°C to prevent acetone from volatilizing too fast. As depicted in table 3, the carbon fiber exhibited the most weight loss at 55°C. When the heating time was longer than 120 min, the weight loss of carbon fiber changed slightly, such that the surface colloid of carbon fiber was completely dissolved by acetone after 120 min. It can be seen that the ability of acetone to dissolve colloids depends on the heating temperature and heating time. In the process of colloid removal test, as far as possible at the temperature close to the boiling point of acetone heating, and heating time should be greater than 120 min, the colloid of carbon fiber can be removed.

Carbon fiber coarsening
The surface of carbon fiber should be coarsened after degumming to increase the surface activity of carbon fiber and enhance hydrophilicity. There have been usually three ways of coarsening, including mechanical coarsening, organic solvent coarsening, as well as chemical coarsening. In general, the rank of the coarsening effect is presented as follows: chemical coarsening>organic solvent coarsening>mechanical coarsening [14,15]. Besides, the chemical roughening method is characterized by low cost, simple equipment, and easy operation. The roughness and activation energy of the carbon fiber surface can be better increased by reasonably selecting coarsening agents, which plays a vital role in the subsequent sensitization and activation [16,17]. As depicted in figure 1, the surface roughness of carbon fiber coarsened by different coarsening solutions increased, thus indicating a gully morphology. The surface texture of Figure (b) treated with 65% concentrated nitric acid was deeper, the specific surface area increased more significantly, and the surface activity was higher.
XPS analysis was conducted on the surface of the coarsened carbon fiber, and peak separation was performed to gain more insights into the coarsening effect of the three coarsening agents. As depicted in figure 2, the surface of the carbon fiber after roughening treatment primarily comprised C-C bonds, carboxyl groups (-COOH), and hydroxyl groups (-OH). As depicted in Figure (b), the number of functional groups on the surface of carbon fibers treated with 65% concentrated nitric acid was significantly higher than that of the other two groups since concentrated nitric acid exhibits strong oxidizing properties. Thus, the surface of carbon fiber contained more oxygen-containing functional groups. The above hydrophilic oxygen-containing functional groups are capable of effectively improving the wettability of carbon fibers and aqueous solution, thus that the carbon fiber can be fully dispersed in the plating solution and easy to be combined with other substances, thus providing powerful conditions for obtaining uniform coating.

Carbon fiber surface activation
The carbon fiber surface activation process takes on a great significance to the surface metallization of carbon fibers, and the core of the activation process is to form a certain number of catalysts on the surface. The precious metal activators (e.g., palladium, platinum, and gold) applied in the conventional process have been extensively used in the chemical industry for their excellent catalytic performance. However, in the catalytic activation process before electroless plating, the above precious metals are inevitably introduced into the plating solution and coating to cause the loss of mass, thus significantly increasing the production cost of electroless plating [18][19][20]. Accordingly, the preparation of non-noble metal colloidal catalysts (e.g., nickel and copper) with a low price and excellent catalytic performance to replace noble metal catalysts has become a vital research direction that has aroused wide attention [21][22][23].
In this study, nickel colloid, copper colloid, and copper-nickel colloid served as the activation catalysts, and sodium borohydride (NaBH 4 ) and sodium hydroxide (NaOH) were employed as the sensitizing reducing agents to activate carbon fibers. Table 4 lists the specific components.
The SEM morphology observation and EDS energy spectrum analysis of the prepared samples were conducted to gain insights into the effect of copper plating on the carbon fiber surface treated by the three  activators ( figure 3). Figure 3(a) presents the coating of carbon fiber after Ni colloid treatment. As depicted in the figure, only a few Cu atoms were deposited on the carbon fiber surface, and the weight percentage of Cu element only reached 11.91%, thus indicating that the activation effect of Ni colloid was not ideal. This is because the precipitation potential of the Ni element is low, and the attached carbon fiber surfaces of Ni colloid has a small number, such that there were fewer catalytic particles and smaller content of Cu atoms on the surface of deposited carbon fiber. Figure 3(b) depicts the carbon fiber coating after Cu colloid treatment. The content of Cu element accounted for 39.88%, whereas most of the carbon fiber surface was still not plated. This is because the precipitation potential of Cu element is high [22,23], which is easier to be precipitated than Ni element, thus resulting in the formation of more Cu colloids than Ni colloids. However, the catalytic effect of Cu colloids was slightly lower than that of Ni colloids. Accordingly, despite the large number of Cu colloids, Cu colloids did not completely make more Cu atoms be plated on carbon fibers. Figure 3(c) presents the copper plating of carbon fiber after Cu-Ni colloid treatment. As depicted in the figure, the Cu element completely covered the entire surface of the carbon fiber. The Cu element content was as high as 92.98%, and the coating was uniform and dense. Moreover, the activation effect of Cu-Ni mixed colloid was the optimal, since Cu colloid is easy to precipitate, which can catalyze and induce the precipitation of more Ni colloids. Ni colloids had a better catalytic effect and a capability to induce more Cu atoms to deposit on the carbon fiber, such that more Cu atom coating was obtained.

Copper plating on carbon fiber surface
The surface of the pre-treated carbon fiber was plated with copper using the plating solution parameters in table 2. Figure (4) presents the electron microscope morphology of copper plating for 3 min, 5 min, and 7 min, respectively. As depicted in the figure, the surface of the carbon fiber was not completely covered by copper in 3 min, indicating that the Cu atoms were not completely deposited on the carbon fiber surface within 3 min, and the surface of the carbon fiber was completely plated with copper after 5 min ( figure 4(b)). As also depicted in the figure, the copper plating surface was smooth and fine and uniformly distributed. At the copper plating time of 7 min, the deposition amount of copper on the surface of the carbon fiber increased significantly, and the surface was rough, thus indicating that the deposition amount of copper on the surface of the carbon fiber increased over time. In brief, the copper plating surface was smooth, and the quality was the optimal within 5 min of the plating time.
The SEM morphology of copper plating on carbon fiber cross-section was selected (figure 5) to further observe the copper plating of carbon fiber. The deposition rate v (μm/h) for electroless copper plating is determined by ASTM B733,where w 0 (mg) and w 1 (mg) are the weight before and after deposition. ρ (g cm −3 ) is the density of the coating,which was∼8.9 g/cm 3 for the general Cu coatings [24,25]. A (cm 2 ) is the surface area of sample and t (h) is the plating time.    It can be obtained from formulas (1) and (2): (3) is the formula for calculating the coating thickness, as shown in figure (5). When the plating time is 3 min, the calculated coating thickness is 0.2 μm.At the coating time oh 5 min, the coating thickness reached 0.5 μm, the carbon fiber coating thickness was moderate, and the surface was uniform. At the coating time after 7 min, the coating thickness was close to 1 μm, while the coating surface was rough and unevenly distributed.

Conclusion
(1)The result indicated that degumming effect of carbon fiber in acetone solution at 55°C was the optimal. After heating for 120 min, the weight of carbon fiber changed slightly, and the surface colloid was almost removed.
(2)When the 200 g/L NaOH solution, 65% concentrated nitric acid, and ammonium persulfate were selected as the coarsening agent, its coarsening effect in the 65% concentrated nitric acid was the optimal, with the most surface functional groups, followed by ammonium persulfate, and it was not significant in the 200 g/L NaOH solution.
(3)When Cu colloid, Ni colloid, and Cu-Ni mixed colloid served as the activators to plate copper on the carbon fiber surface, the Cu-Ni colloid exerted the optimal activation effect, and the carbon fiber surface was completely covered by copper layer. At the plating time of 5 min, the coating quality was the optimal, and the coating surface was uniform and bright.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).