Quantitative Image Analysis of Ni-P Coatings Deposited on Carbon Fibers
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In this work, polyacrylonitrile (PAN)-based carbon fibers coated with different thicknesses of Ni-P coatings were studied. The coatings were deposited by electroless metallization lasting from 3 to 22 min and consisted of approximately 3 wt.% phosphorous. Computer quantitative image analysis was used to characterize the surface features and thickness of the coatings as a function of the time of metallization. The results showed that quantitative image analysis is a useful technique for the measurement of the coating thickness and can be used as a tool for obtaining an innovative description of the Ni-P coating morphology. The morphology of the coatings and their thicknesses were investigated by scanning electron microscopy. The image analyses were performed using the proprietary software Micrometer, developed at the Faculty of Materials Science and Engineering, Warsaw University of Technology. The observations revealed that a specific feature of the coating topography is the hemispherical bulge of a diameter ranging from 0.1 to 10 μm. The thickness of the coatings increases linearly with the metallization time. The obtained results indicated that the methodology proposed in the present work can be successfully applied and possesses several advantages over the traditionally used weight measurements technique.
Keywordscarbon fibers electroless process image analysis Ni-P coatings
Nickel-phosphorus coatings are widely used due to their unique chemical and mechanical properties. Among the advantages, the most important are good corrosion and wear resistance and high hardness. For these reasons, Ni-P layers are used in micro-galvanic and optical applications and in the automotive industry as protective layers (Ref 1-3). The most popular techniques for obtaining these types of coatings are: electro- and electroless plating. Despite a few drawbacks (high amount of waste and the complex composition of the solution), the electroless technique exhibits several advantages. In particular, it allows uniform coating on non-conductive surfaces with complicated geometry (Ref 3, 4), especially on bundles of carbon fibers, which when coated with Ni-P layers are used in polymer matrix composites for electromagnetic shielding (Ref 5-9). Moreover, Ni-P layers on carbon fiber scan be used to fabricate reinforced metal matrix composites (Ref 10-12) because they increase the wettability between the fiber and the metal (Ref 13-16). The properties of Ni-P coatings on carbon fibers were described in previous research (Ref 17-19). However, their morphology and thickness were not addressed in a comprehensive way. In particular, the thickness of the coatings was estimated globally by measuring the weight of deposited Ni. The results of the morphology investigation presented in the presented paper can be correlated with the wettability tests of the coatings using liquid alloy droplets, where the coating morphology and roughness play crucial roles. This gives support to the efforts aimed at the quantitative characterization of the coating based on their images, the results of which are reported in this paper. A literature survey indicated that there has been no attempt to parameterize these features quantitatively or to demonstrate the dependence of the time of the metallization process on the function of the thickness of Ni-P coatings and the diameter of their hemispheres. The authors of this paper attempted to perform this task by computer quantitative image analysis.
The polyacrylonitrile (PAN)-based carbon fibers used in this work were Tenax HTA40, in bundles containing 3 K fibers with a diameter of 7 μm. The continuous fibers were cut into 11 cm pieces. The protective epoxy layer (sizing) deposited on the fibers by the manufacturer was removed by annealing in an oven for 1 h at 450 °C in an air atmosphere. The fiber surface preparation process for further metallization consisted of three stages: (1) sensitization, (2) activation, and (3) pre-reduction. The fibers were sensitized for 15 min in a solution consisting of SnCl2 and HCl. Afterwards, the fibers were cleaned in the distilled water for 2 min. To obtain a catalytic surface, fibers were activated in a solution containing PdCl2 and HCl. At the end of this process, the fibers were rinsed in distilled water. The pre-reduction step was performed at 70 °C for 5 min under ultrasonic vibrations in a solution consisting of 0.01 M NiSO4 and 0.2 M NaH2PO2. The metallization process was conducted at 70 °C in a thermostatic vessel (200 mL solution) under ultrasonic vibration. The duration of the processes were 3, 5.5, 11, and 22 min. The solution for Ni-P deposition was composed of NiSO4 (0.1 M), NaH2PO2 (0.2 M), glycine (0.21 M), and NaOH as a pH regulating additive (pH value was 8.5). Thiourea (0.2 mM) was added as the stabilizer, and cetyltrimethylammonium bromide (0.1 mM) was added as the surfactant.
To ensure that the measurements were based only on the cross sections of the coated fibers that were perpendicular to the surface of the sample, before investigation, the ratio of the maximal and minimal Ferret diameters of the fibers, (d max) and (d min), respectively, was analyzed. When the ratio deviated from unity by 5%, the fiber was rejected from further processing.
Results and Discussion
Additionally, the results achieved by the image analysis indicated that joining fibers into groups can disturb the proper coating thickness measurement. Theoretical calculations based on the quantity of deposited nickel are also improper because of the fiber agglomeration. Many fibers share one coating, and new layers of nickel phosphorous grow only on the surfaces not covered by other fibers.
Furthermore, for metallization time up to 11 min, the results agree with the estimates based on the weight measurements. The values were nearly the same or stayed within the range of the standard deviation (SD) values, as in the case of 11 min metallization. In contrast to the theoretical calculations (using Eq 2), the image analysis allows calculation of the SD values. Utilizing this parameter, it was possible to draw conclusions about the diversity of the coating thickness in the range of the fiber, which is important in optimization of the coating thickness for polymer and metal matrix composites.
The image analysis showed an increase in the coating thickness for 22 min metallization, but the thickness of the coatings was higher in comparison to the weight measurements of the nickel deposition, which can be explained by the non-uniform coating thickness distribution that was confirmed by the SD values. Fibers joined into group have thicker coatings outside of the bundles than inside. On the other hand, the weight measurements are relevant to the whole surface area of the fibers. Therefore, in practical applications of the coated fibers, i.e., for metal matrix composites (MMC), thick coatings or 1 or 1.6 µm are not advisable because of the formation of brittle phases when dissolving the coating in the matrix (Ref 21).
Considering this fact, for practical applications, the optimal time for the process is determined in a way that ensures a coating that is not too thick. Although the developed method can be applied for well-separated and distant fibers with thick coatings, in practice, due to technological reasons, carbon fibers are formed in tight bundles. This configuration limits their mobility inside the group of fibers. Moreover, a large thickness causes cracking of the coatings, even during the metallization process.
The histograms shown in Figure 9 indicate that the diameter distribution function for the lowest coating thickness (0.3 and 0.5 μm) is relatively narrow, with the most frequent values in the range 200-250 nm. The distribution function widens for sample with a 1.0-μm coating thickness, but the unbiased measure of dispersion, the CV, does not grow as rapidly.
Application of quantitative image analysis and advanced electron imaging techniques allows assessment of the diversity in the coating thickness between individual fibers in the bundle.
Estimates of the coating thickness based on weight measurements provide good results only for relatively thin coatings with thickness below 1 µm.
Profound changes in the morphology of the coatings occur at the thickness of 1.0 µm with the size of surface bulges becoming significantly diverse.
Finally, the results presented in this report provide tools for optimizing the thickness of the coatings obtained by electroless metallization. In particular, the thickness of the coating or metal matrix composites can be better controlled to prevent coating overgrowth. Moreover, because the Ni-P, Ni, and Cu reactive coatings in most studied cases dissolve when in contact with liquid aluminum alloys (Ref 16, 21, 22), the developed method can also help to control the formation of brittle phases during the infiltration process through the adjustment of the coating thickness and the mechanical properties of the aluminum alloys matrix composite.
This study was supported by the Polish Ministry of Science and Higher Education and by DFG in Germany as the Polish-German Bilateral Project “3D-textile reinforced Al-matrix composites (3D-CF/Al-MMC) for complex stressed components in automobile applications and mechanical engineering” No. 769/N-DFG/2010/0.
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