Preparation of Core-Shell Structured Cobalt Coated Tungsten Carbide Composite Powders by Intermittent Electrodeposition

doi:10.1016/j.jmst.2016.09.021

Abstract

Abstract:

Core-shell structured cobalt coated tungsten carbide (WC/Co) composite powders were prepared by intermittent electrodeposition. The influence of process parameters such as current density, single deposition pulse, pH value and surfactants on the formation of WC/Co was investigated and characterized by scanning electron microscopy (SEM), electrochemical station, acidometer and X-ray diffraction (XRD) techniques. The composite powders with 54% cobalt content were fabricated at a current density of 16 A dm^-2, with a load of 10 g dm^-3 WC powders and a stirring speed of 600 r min^-1 at an operation temperature of 40 ± 2 °C, and 90% current efficiency was obtained with a single deposition pulse of 1.5 min and single stirring pulse of 2 min during 12 min efficient electrodeposition time. The uniformly distributed WC/Co powders could be obtained in the cobalt electrolyte containing 300 mg dm^-3 PEG-2000. The spherical cobalt grains coated WC particles were prepared in the pH 4-5 electrolyte at the Co deposition rate of 0.58 g min^-1. A practical process for high efficient production of WC/Co powders by electrodeposition was developed in the present work.

Key words: Composite powder, Electrodeposition, Dispersion, Crystal structure

Zhong Huan,Ouyang Yuejun,Yu Gang,Hu Bonian,Yan Dalong. Preparation of Core-Shell Structured Cobalt Coated Tungsten Carbide Composite Powders by Intermittent Electrodeposition[J]. J. Mater. Sci. Technol., 2016, 32(11): 1171-1178.

Figures/Tables 16

Table 1. Pretreatment process of WC powders

Pretreatment	Solution composition	Operation conditions
Chemical degreasing	Na₃PO₄	30 g dm^-3
	Na₂CO₃	10 g dm^-3
	Na₂SiO₃	15 g dm^-3
	Temperature	80-90 °C
	Time	20 min
Activation	HNO₃	30 mm^-3dm^-3
	HF	30 mm^-3dm^-3
	NH₄F	3 g dm^-3
	Ultrasonic time	30 min

Table 2. Composition of the plating bath and plating process

Components	Parameters
CoCl₂ ⋅ 6H₂O	150 g dm^-3
H₃BO₃	30 g dm^-3
Surfactant	100-400 mg dm^-3
Temperature	40 °C
pH	2-6
Stirring speed	600 r min^-1
Efficient deposition time	12 min

Fig. 1. Schematic of current signal by intermittent electrodeposition.

Fig. 2. Surface morphologies of the WC particle: (a) untreated; (b) treated.

Fig. 3. (a) SEM images of WC/Co; (b) EDS spectrum of WC/Co; (c) XRD pattern of WC/Co.

Fig. 4. Mechanism of WC/Co powders by intermittent electrodeposition.

Fig. 5. Effects of different single deposition pulse on cobalt content in composite powders and coating ratio of WC powders.

Fig. 6. Surface morphologies of the composite powders at different current density: (a) 12 A dm-2; (b) 14 A dm-2; (c) 16 A dm-2; (d) 18 A dm-2, under the load of 10 g dm-3 WC powders in electrolyte, at operating temperature of 40 ± 2 °C and total deposition time for 12 min.

Fig. 7. Effects of current density on current efficiency and cobalt content in composite powders.

Fig. 8. Cathodic polarization curves of electrolytes with different surfactants on: (a) Nb electrode; (b) WC electrode.

Table 3. Deposition potential of the electrolyte solution with different surfactants

Deposition potential	E (V) ---------------------------------
Deposition potential	No surfactants	CTAB	PEG-2000	SOS
Nb electrode	-0.54	-0.56	-0.63	-0.68
WC electrode	-0.55	-0.56	-0.58	-0.61

Fig. 9. Effects of three types of surfactants on the absorbance of the WC particles in the electrolyte.

Fig. 10. Morphologies of powders electrodeposited with different surfactants: (a) PEG-2000; (b) SOS; (c) CTAB.

Fig. 11. Morphologies of composite powders at different pH values: (a) pH = 2; (b) pH = 4; (c) pH = 6.

Fig. 12. XRD patterns of cobalt coating on WC powders at different pH values.

Fig. 13. Effects of different pH on deposition rate.

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