Effect of Current Density on the Microstructure and Adhesion of Ni-Co/Al 2 O 3 Composite Coatings
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Effect of Current Density on the Microstructure and Adhesion of Ni-Co/Al2O3 Composite Coatings Diana Alejo-Guerra1, Carlos A. León-Patiño1, Ena A. Aguilar-Reyes1, José E. Bedolla-Becerril1 1
Instituto de Investigación en Metalurgia y Materiales, Universidad Michoacana de San Nicolás
de Hidalgo, Av. Francisco J. Múgica S/N Ciudad Universitaria, Morelia, Michoacán, C.P. 58030, México. ABSTRACT Ni-Co/Al2O3 composites with alumina particles below 0.53 µm in size were electrochemically deposited on carbon steel AISI-O1 substrates. The influence of current density (2, 3, 4 and 5 mA/cm2) on the microstructure and adhesion behavior was investigated. It was found that the micro-hardness of the surface increased in 90% with respect to the plain substrate. SEM examination revealed an increased thickness from 3.7 to 10.7 µm at current density of 2 and 5 mA/cm2, respectively. The Co/Ni weight ratio in the coatings was determined by atomic absorption spectroscopy (AAS); the content of Co and Ni at 2 mA/cm2 were 42.4 %Co and 57.6 %Ni, while at 5 mA/cm2 were 44.5%Co and 55.5%Ni. The adhesion of the coatings was evaluated qualitatively according to the VDI 3198 norm; in all the cases, the adhesion was more than acceptable since no cracks or detachments were observed in the periphery of the marks. Details on the synthesis and properties of the Ni-Co/Al2O3 composite coatings are presented together with the excellent properties they show despite of their thin thickness. Keywords: Ni-Co/Al2O3 composite coatings, electrodeposition, current density, adhesion INTRODUCTION The Ni-Co alloys present excellent properties of hardness, wear and corrosion resistance. Recent studies show the use of Ni-Co coatings reinforced with hard oxides (Al2O3, TiO2, ZrO2) or carbides particles (SiC, WC) [1]. The use of electrodeposition to produce composite coatings results attractive because of its low cost and because it allows homogeneous coatings of constant thickness in the full surface of the parts irrespective of their geometry and size. Using electrochemical methods, all the particles are totally dispersed in the metallic matrix. One of the most used reinforcements is Al2O3 particles due to its high availability, chemical stability, resistance at high temperatures and high microhardness [2]. During the electrodeposition process, current density is the most important parameter to control since it affects the deposition rate, which in turn affects the composition of the metal matrix and the amount of oxide particles embedded in the matrix. Both variables are fundamental and determine the physical and mechanical properties, and the corrosion and wear behavior of the Ni-Co/Al2O3 coatings applied [3]. The present work shows the chemical and morphological characteristics of five electrodeposited Ni-Co/Al2O3 coatings when applying current densities from 2 to 5 mA/cm2. The effect of current density on microstructure, and adhesion behavior is studied.
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