CODEPOSITION OF CERIUM OXIDE WITH NICKEL AND COBALT: CORRELATION BETWEEN MICROSTRUCTURE AND MICROHARDNESS G. Ca ˆra ˆ c, L. Benea, C. Iticescu, T. Lampke, S. Steinha ¨ user and B. Wielage Composite layers were obtained by electrochemical deposition of cobalt or nickel from a plating bath with cerium oxide as dispersed particles. The changes in microstructure and microhardness of the composite were studied in correlation with the number of oxide particles incorporated. It was found that the presence of cerium oxide (CeO 2 ) particles has an influence on the codeposition process. The CeO 2 included particles lead to structural modifications of the metal matrix and have an effect on the mechanical as well as the surface properties of the composite. SE/S301 Drs Ca ˆra ˆc, Benea and Iticescu (geta.carac@ugal.ro) are in the University of Galati, Department of Chemistry, Domneasca Street 47, 800008-Galati, Romania. Drs Lampke, Steinha ¨user and Wielage are in the Chemnitz University of Technology, Institute of Composite Materials and Surface Technology, Erfenschlager Str. 73, D-09107 Chemnitz, Germany. Manuscript received 9 April 2004; accepted 4 May 2004. Keywords: Composite coatings, Nickel, Cobalt, Electrocodeposition, Cerium oxide, Microstructure, Microhardness # 2004 Institute of Materials, Minerals and Mining. Published by Maney on behalf of the Institute. INTRODUCTION Scientific and technical progress is always closely connected with the development and industrial application of new materials. Electrochemical manu- facture of composite coatings by codeposition is a relatively simple, inexpensive, versatile, easily con- trollable and reliable technique for obtaining material with the desired quality. Composite coatings can be defined as material obtained by electroplating of a metal or alloy with dispersed ceramic particles, such as oxides, carbides, nitrides, polymers and other insoluble phases. By codeposition of oxide particles during electro- plating, composite coatings with interesting proper- ties can be produced. The properties of such coatings are dependent on the nature of the particles incorporated, among other things. 1–4 For example, the inclusion of dispersed lanthanide oxides and their compounds could produce special properties includ- ing electrical, magneto-optical, catalytic and mechan- ical. The presence of microsized particles in a metal matrix can generally improve the mechanical and chemical properties to be beneficial in a wide range of applications. In the case of cobalt–ceria, these composite coatings provide higher corrosion resis- tance than does a pure cobalt matrix. 5 Nickel electroplating is one of the few surface finishing processes that can satisfy the requirements of decorative and functional applications. 6,7 It has been reported that nickel–ceria composites feature good wear resistance compared with nickel–zirconia. 8 In the present work, the codeposition of ceria with nickel and cobalt was carried out from a sulphate– chloride electrolyte. The mean diameter of the cerium oxide was 1 mm, maintained in suspension by mechanical stirring. Pure metal depositions were performed from the same electrolyte for comparison with composite coatings. The electrochemical bath was carefully prepared to a proper pH. All the coatings were deposited on steel substrate. The aim of this study was to correlate the microstructure and microhardness of different com- posite coatings deposited in nickel and cobalt matrices. Many investigations with varying working para- meters were necessary to find the optimal codeposi- tion conditions for obtaining composite layers with an appropriate profile of mechanical properties. 9,10 EXPERIMENTAL Composite layers in cobalt and nickel matrices were electrodeposited from electrolytes with the composi- tion described below and CeO 2 particles added in a plating bath. The composition of the cobalt bath was: 1 . 0 mol L –1 CoSO 4 ?7H 2 O, 0 . 2 mol L –1 CoCl 2 ?6H 2 O, 0 . 28 mol L –1 H 3 BO 3 and 0 . 4gL 21 sodium dodecyl- sulphate (CH 3 (CH 2 ) 11 OSO 3 Na). The pH of the electrolyte was 4 . 5–5 . 2, corrected with alkaline solution. The experiments were performed at room temperature. The nickel coatings were electrodeposited in a Watt’s electrolyte: 0 . 90 mol L –1 NiSO 4 ?6H 2 O, 0 . 20 mol L –1 NiCl 2 ?6H 2 O, 0 . 28 mol L –1 H 3 BO 3 and 0 . 4gL 21 CH 3 (CH 2 ) 11 OSO 3 Na. The pH value of the electrolyte was maintained at 4 . 2–4 . 5 by careful correction during the entire deposition processes. The experiments were performed at 50–55uC. CeO 2 as dispersed phase with a particles size of y1 mm and concentrations between 25 and 100 g L 21 (in the bath) was added. To keep the particles in suspension, the electrolyte was magneti- cally stirred at 700 rev min –1 . The codeposition experiments were performed in a large 500 ml DOI 10.1179/026708404X1134 Surface Engineering 2004 Vol. 20 No. 5 353