Particles Co-Deposition by Electroless Nickel

Scripta Materialia, Vol. 38, No. 9, pp. 1383–1389, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00

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PARTICLES CO-DEPOSITION BY ELECTROLESS NICKEL I. Apachitei, J. Duszczyk, L. Katgerman and P.J.B. Overkamp Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands (Received November 4, 1997) (Accepted in revised form January 23, 1998) Introduction Electroless, as well as electrochemical co-deposition of inert particles, represents a method to obtain metal matrix composite materials as thin foils (coatings) at low temperature ('90°C). Compared to electrodeposition, electroless can be applied on different substrates (conductive and nonconductive) and the obtained layer has a very homogeneous distribution regardless the substrate geometry [1]. An electroless nickel composite coating consists of small inert particles, such as: oxides, carbides, nitrides polymers etc., uniformly dispersed into a nickel-based matrix. During the redox reaction between Ni21 ions and sodium hypophosphite, the inert particles are physically entrapped in a growing Ni–P layer on the substrate surface. Each combination between a certain type of particle and the Ni–P matrix can lead to a new set of properties. Generally, in electroless nickel particles co-deposition, particle concentration in the deposit depends on factors like: bath chemistry (mainly type and amount of surfactant), particle characteristics (e.g. density, size distribution) and operating conditions (e.g. bath stirring, substrate position and movement) [2–5]. It is difficult to assess the influence of these or other factors on the process mechanism since this is not yet available. A recent review article on electrodeposited composite coatings [6] suggests that particle shape can also affect the adsorption of particles on the substrate surface (cathode), adsorption of ions on the particle surface, and suspension stability. In this paper, the results of a preliminary study on co-deposition of some hard particles (SiC, Al2O3, and B) within electroless Ni–P matrix are presented. The influence of particle characteristics, such as: shape and size distribution on the co-deposition process as well as coating morphology is discussed. Experimental Electroless Nickel Bath and Operating Conditions An electroless nickel bath for SiC composite coatings, with sodium hypophosphite as reducing agent was used. In order to obtain Ni–P composite coatings, several types of hard particles were added in the bath and kept in suspension by magnetical stirring. Particle characteristics and bath loadings are presented in Table 1. Particles were chemically cleaned using 36 –38% HCl, rinsed with demiwater and dried in the oven. Plating occurred in cylindrical glass vessels containing two liters of bath solution. All the samples were plated at 87°C and pH 4.6. 1383

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TABLE 1 Particle Characteristics and Bath Loadings

Particle type

Supplier

Density, (g z cm23)

Sic Al2O3-I Al2O3-S Saffil d-Al2O3 B

B.G.T., The Netherlands Lonza-Werke GmbH, Germany Showa Denko K.K., Japan I.C.I., Runcorn, U.K. H.C, Starck GmbH & Co. KG, Germany

3.22 3.97 3.97 3.97 2.34

Average size, (mm)

Maximum size, (mm)

Specific surface area, (m2 z g21)

Bath loading, (g z L21)

3.2 2.8 3.4 7.2*) 1

6.4 10.5 10.5 34*) 5.0

3.7 3.2 0.7 18.1 11.0

2.5 3.1 3.1 3.1 1.8

* Values indicate fiber diameter

Electroless Ni–P coatings were applied on an aluminium 6063-T6 alloy after a suitable pretreatment sequence including a deoxidizing step followed by a double immersion in a modified alloy zincate (MAZ) solution [7]. The substrates had a rectangular geometry of 100 3 100 3 3 mm3 (ratio surface area/bath volume was 1 dm2/L) and were vertically positioned in the bath. Analyses Particle surface morphology was examined by scanning electron microscopy (SEM) using a JSM6400F microscope. Particle size distribution was determined by a laser scattering Malvern 2600 D particle size analyzer (which gives a size distribution in the range 1 to 1800 mm). For this analysis, the particles were ultrasonated in 0.1% Na4P2O7z10H2O solution. Quantachrome Autosorb-6B equipment using nitrogen gas at 77 K was used to obtain the adsorption isotherms from which specific surface area of particles was calculated. The morphology of Ni–P and Ni–P–X (X 5 SiC, Al2O3, and B particles) coatings was investigated by optical microscopy. Particle concentration in Ni–P–X deposits was determined gravimetrically and expressed as volumetric percentages (density of Ni–P deposit was according to [8]). For this, the substrate was dissolved in 65% NaOH solution and Ni–P composite deposits were stripped in 65% HNO3 solution. Solutions containing the particles were filtered using GF 6 filter paper (0.3 mm), and dried in the oven at 105°C, for two hours. Results and Discussion Particles Characterization Shape and size are two fundamental aspects when characterizing powder particles. Surface morphology and size distribution (cumulative and frequency-wise) for the particles used in this study (as reinforcement in electroless Ni–P composite coatings) are presented in Figures 1, 2 and 3, respectively. As can be seen from Figure 1, (a and e), SiC and B particles are of irregular shape. Three different geometries of alumina particles were co-deposited: irregular (I), spherical (S) and fibers (Figure 1, b– d). Saffil d-alumina fibers are obtained by a solution-spinning process and are commercially available in three main product forms: Mat, Bulc and Milled. The Milled (RF 590) form was selected for this study, with an extra 1 h ball milling in a Turbola mixer using ceramic balls of 19 mm diameter. The balls-to-fibers weight ratio was 6:1. Fiber geometry is cylindrical with quite a large diameter distribution (avg. 7.2 mm) and a length (estimated by SEM measurements) less than 100 mm. On the other hand, SiC and Al2O3-S presented a relatively narrow size distribution (average size 3.2 and 3.4 mm,

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Figure 1. SEM micrographs of particles used as reinforcement in electroless Ni–P coatings: (a) SiC, (b) Al2O3-I, (c) Al2O3-S, (d) Saffil d-Al2O3, and (e) B.

respectively). Boron particles were found to be smallest (with an average size of 1 mm and 50% of submicron particles) and 25% of Al2O3-I particles were less than 1 mm. As can be observed from Table 1, Saffil d-alumina and boron particles have high specific surface area (18.1 and 11 m2zg21, respectively) as a result of fiber surface microporosity [9] and the very fine boron powders. In contrast, the Al2O3-S particles have a very low specific surface area due to their spherical morphology.

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Figure 2. Cumulative size distribution of the particles.

Coating Morphology Using the same type of electroless bath, the three types of hard particles—SiC, Al2O3, and B—were successfully co-deposited in the Ni–(8.22 wt.%)P matrix. To prevent bath decomposition during plating, the ratio particle/bath volume was kept constant for all types of hard materials. Surface morphology of the Ni–P and Ni–P–X coating is presented in Figure 4. From this figure, a uniform distribution of particles on coating surfaces can be observed. Also, a lamellar Ni–P grain (nodular) orientation with different grain sizes is distinguished on the Ni–P deposit (Figure 4, a). This is a consequence of the preferential growth of Ni–P grains on substrate defects [10 –12] which resulted from the metallurgical working process by hot rolling. The size and grain orientation are modified when the particles are incorporated throughout the deposit (Figure 3, b–f). Only in the Ni–P–Al2O3-I coating, the lamellar grain orientation is still present and this might be the result of a low particle concentration in the deposit (9.7 vol.%). Particle Concentration in the Deposits Particle concentrations found in the Ni–P–X deposits are presented in Table 2. For the same ratio particle/bath volume and operating conditions (pH, temperature, stirring rate, type and concentration of surfactant) particle concentration in the Ni–P matrix varied with particle type and shape. From the irregular particles, boron has the highest concentration in the Ni–P deposit. Compared to Al2O3-I and SiC, boron particles had the smallest size and also the lowest density enabling a more stable suspension which could result in higher particle concentration in the deposit. TABLE 2 Particle Concentration and Deposition Rate of Coatings Coating

Particle concentration, (vol. %)

Deposition rate, (mm z h21)

Ni-P Ni–P–SiC Ni–P–Al2O3-I Ni–P–Al2O3-S Ni–P–Saffil d-A12O3 Ni–P–B

— 19.6 9.7 28.6 10.7 27.0

15.0 14.0 12.5 13.5 12.0 11.5

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Figure 3. Differential size distribution of the particles: (a) SiC, (b) Al2O3-I, (c) Al2O3-S, (d) Saffil d-Al2O3, and (e) B.

Regarding the shape of particles (alumina: irregular, spherical and fibers) the deposits containing Al2O3-S showed the highest concentration. It seems that, the spherical geometry together with a narrow size distribution can be conditions for a better entrapping of these particles in the layer. However, to fully explain the relationship between different types of particles (hydrophilic, hydrophobic, different geometries) and deposit characteristics, factors which include bath chemistry, interaction particle-surfactant, agitation method and rate, substrate position and movement should be investigated.

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Figure 4. Surface morphology of as-deposited coatings: (a) Ni–P, (b) Ni–P–SiC, (c) Ni–P–Al2O3-I, (d) Ni–P–Al2O3-S, (e) Ni–P–Saffil d-Al2O3, and (f) Ni–P–B.

The values of the deposition rates of coatings are within the same range (Table 2). However, the deposition rates of composite coatings are smaller compared to the Ni–P deposit as a result of the more complex mechanism of the electroless co-deposition. Conclusions Different types of particles (SiC, Al2O3, and B) presenting different geometries and size were co-deposited by electroless nickel on an aluminium substrate. All the deposits presented a uniform

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distribution of the particles. However, for the same operating conditions, smaller particles and a narrower size distribution resulted in a higher concentration in the Ni–P matrix. Also, Al2O3-S particles showed a very good embedding in the Ni–P deposit. The deposition rates of Ni–P and Ni–P–X coatings were found to be almost in the same range. Acknowledgment Special thanks to B.G.T. Galvanische Technieken B.V., Eindhoven, The Netherlands for useful discussions and for providing bath proprietary solutions. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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