UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

122 IMMC 2016 | 18th International Metallurgy & Materials Congress

Preparation and Characterization of Electroless Ni-P Coated SiC Composite Particles

Ulaş Matik

Karabük University - Türkiye

Abstract The electroless deposition of Ni-P alloy on SiC particles (SiCp) with average size of 82 μm has been studied by following SnCl2 sensitization and PdCl2 activation processes. The bath for electroless deposition was prepared by using NiCl2·6H2O, NaH2PO2·H2O, HOC(COONa)(CH2COONa)2·2H2O and NH4Cl, and the deposition was carried out at a bath temperature of 82 °C with a pH value of 9. The phases and morphology of Ni-P coated SiCp were examined by XRD, SEM and EDX. The crystallization temperature of Ni-P coating were detected with a differential scanning calorimeter (DSC). XRD showed that the as-plated Ni-P deposit was amorphous. However, after heat treatment, the metallic deposits crystallized into Ni and Ni3P phases. The structure of Ni-P coated SiCp was assessed as a beneficial surface modification to improve interfacial bonding strength between the matrix phase and the dispersed phase for metal matrix composite manufacturing. 1. Introduction Metal matrix composites (MMCs) are the engineered materials produced by mixture of two or more unlike materials, at least one of which is basically a metal [1]. The interaction between the reinforcement and the matrix was recognized at an early stage as a critical factor in determining the properties of the resulting MMCs [2]. Properties of MMCs strongly depend on the interfacial phenomena between the metal matrix and ceramic reinforcement [3]. Stronger adhesion at the particle–matrix interface improves load transfer, increasing the yield strength and stiffness and delaying the onset of particle–matrix de-cohesion [4]. However, poor wettability is a major problem in composite systems [2,5]. There are some methods to overcome the problem of generally poor wetting of reinforcements by metals, such as reinforcement pretreatment, alloying modifications and reinforcement coating [6]. Among these coating methods, electroless nickel coating of the reinforcement, which is a simple, low-cost and an easy to use process, has been successfully applied to prevent undesired interfacial reactions and promote the

wettability through increasing the overall surface energy of the reinforcement [3]. Ni-P coatings can be classified into three categories with respect to the microstructural differences as a function of the phosphorus content; phosphorus supersaturated structure with a low phosphorous content (<7 at%), compounded nanocrystalline Ni and amorphous structure with a medium phosphorous content (8-19 at%), and fully amorphous structure with a high phosphorous content (>20 at%) [7,8]. Guo et al. [9] showed that in the as-deposited state, P in the solid solution had a minimal effect on wetting force and kinetics. Upon high temperature annealing, Ni3P precipitated out of the Ni matrix, resulting in degradation of solderability. SiCp have become one of the popular reinforcing phases for many metal matrix composites. They are hard and brittle ceramic particles with high strength, high modulus of elasticity, and high thermal and electrical resistance [10]. In this study, preparation and characterization electroless nickel (EN) coated SiC particles has been investigated. Another purpose of this study was to investigate the effect of heat treatment on structure of Ni-P coated SiCp. 2. Experimental procedure SiCp with an average size of 82 μm were used for experimental. The activation process had to be applied to the SiCp prior to the electroless deposition process because of their passive surface properties. For this aim, following process steps were carried out prior to deposition process; i) cleaning the SiCp with acetone, ii) sensitization in an aqueous solution containing 2.65

g/L SnCl2 and 10 ml/L HCl, 30 min iii) activation in an aqueous solution of 0.14 g/L

PdCl2 and 10 ml/L concentrated HCl, 30 min

After the activation processes, the particles were washed thoroughly by deionized water. Finally, electroless plating Ni-P was carried out on the particles.

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Composition of the electroless Ni-P plating bath and the operation conditions were given in Table 2. The plating solution was stirred using a heather magnetic stirrer. After the process of electroless plating, the particles were thoroughly rinsed with deionized water and dried.

Table 2. Composition of the electroless Ni-P plating bath

and the deposition parameters Chemical composition and operation conditions NiCl2·6H2O 35 g/L NaH2PO2·H2O 10 g/L HOC(COONa)(CH2COONa)2·2H2O 65 g/L NH4Cl 50 g/L NH4OH pH adjuster pH 9 Temperature 80 °C Stirring speed 400 rpm Time 30 min

The crystallization temperature of coatings were detected with a differential scanning calorimeter (Hitachi DSC7000X). The heating temperature ranged from 50 to 500 °C with 10 °C/min rate in N-flow atmosphere (30 ml/min). After the coating processes, according to the transformation temperature heat treatment was designed at 450 °C for 1 h to determine the change in structure. The structure, morphology and chemical composition analyses of uncoated and coated SiCp were performed using a field emission scanning electron microscope (SEM), (Carl Zeiss ULTRA PLUS FESEM equipped with EDX). X-ray diffraction (XRD) analyses of uncoated, coated and, coated and heat-treated SiCp at 450 °C with a Rigaku Ultra IV XRD applying Cu K radiation. Scanning was performed through 10°-90° (2 ), with a step size of 0.01° and counting time 4 s/step. 3. Results and discussion Fig. 1 shows the optical photographs of uncoated and Ni-P coated SiCp. It is observed that after the coating process, the color of SiCp has changed from green to gray.

Figure 1. The optical photographs of (a) uncoated and

(b) coated SiCp

Fig. 2 shows the DSC analysis of uncoated and coated Ni- SiCp in the temperature range from 50-500 °C at heating rate of 10 °C/min. The DSC trace of Ni-P coated SiCp exhibits the formation of an exothermic peak at

about 400°C. The exothermic peak is associated with the formation of Ni and Ni3P phases [7]. Also, DSC curves show that SiCp are thermodynamically stability up to 500 °C.

Figure 2. DSC traces of of (a) uncoated and (b) coated

SiCp at a heating rate of 10 ºC/min. The XRD patterns of uncoated, coated and, coated and heat-treated SiCp at 450 °C are shown in Fig. 3. There were only diffraction peaks of -SiC detected on raw particles. However, in the diffraction peaks of Ni-P coated SiCp, a sharp peak under 30° and a broad Ni peak around 45° can be clearly observed. It shows that the coating has an mixture of amorphous and microcrystalline structure. In the heat-treated sample, heating to 450 ºC, the nickel reflection became sharper and intensity increased. In addition, nickel phosphide (Ni3P) phases are present in deposits.

Figure 3. The XRD patterns of uncoated, coated and,

coated and heat-treated SiCp at 450 °C Fig. 4 shows the SEM images of SiCp before and after electroless deposition. It can be seen that the surface of Ni-P coated SiCp were not completely covered with Ni-P coating (Fig. 4, b). This is explaining why strong diffraction peaks of SiC and weak diffraction peaks of Ni

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

124 IMMC 2016 | 18th International Metallurgy & Materials Congress

both exist in its XRD patterns and EDX analysis (Fig. 5-b). However, it can be seen (Fig. 4 b-c) that the surface of coated particles were almost completely covered with large amount of Ni-P coating. The average thickness of coatings is approximately 0.8 μm for a deposition time of 30 min. EDX analysis indicated that the phosphorus content in the Ni-P coating is about 4.35 wt.% .

Figure 4. SEM images of (a) uncoated and (b-c) coated

SiCp

Figure 5. EDX analyses of (a) uncoated and (b) coated

SiCp 4. Conclusions In the present study, the structural and morphological properties of as-plated and heat-treated electroless Ni-P coating with low phosphorus content on SiCp were investigated. The following results were obtained: • The surface of SiCp was successfully activated by

SnCl2 sensitization and PdCl2 activation processes, • After the activation processes, SiCp were coated with

an alkaline electroless nickel plating solution, successfully.

• As-plated Ni-P coating has an amorphous structure, which crystallized at about 400°C.

• The final products of the crystallization of Ni-P coating were Ni and Ni3P.

• The structure of Ni-P coated SiCp was assessed as a beneficial surface modification to improve interfacial bonding strength between the matrix phase and the dispersed phase for metal matrix composite manufacturing.

• It can be used that the surface of Ni-P coated SiCp were not completely covered with Ni-P coating (Fig. 4, b).

• An ultrasonic bath can be used to prevent agglomeration of the particles during the activation and coating processes.

Acknowledgments This work was supported by Karabük University Scientific Research Foundation (Project number: KBÜ-BAP-14/2-KP-053) References

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