Characterization of electroplated Ni/SiC and Ni/Al 2 O 3 composite coatings bearing nanoparticles
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Ultrafine SiC and Al2O3 particles with 30–50 nm sizes were used to codeposit with Ni in a sulfamate bath to form composite coatings. The microstructure and mechanical properties of the layers were investigated by x-ray diffractometry, scanning and transmission electron microscopy, high-resolution transmission electron microscopy, microindentation, and wear testing. The microstructural results revealed that 7 vol% of SiC or Al2O3 particles dispersed randomly in the Ni matrix. The addition of the ultrafine SiC or Al2O3 powder into the Ni matrix apparently reduced the size of Ni grains during the electroplating and inhibited the grain growth during heat treatment. The microhardness and wear resistance were improved by the addition of SiC and Al2O3 particles, especially for SiC/Ni samples after heat treatment at 400 °C for 24 h. The mechanisms of hardening and wearing of Ni-based electroplated layers are discussed.
I. INTRODUCTION
Composite coatings or electrolytic codepositions are prepared by dispersing insoluble ceramic, polymeric, or metallic powders in an electroplating solution during metal electroplating. The simultaneous deposition of the particulate inclusions and metal phase to form composite coating results in an improvement of physical and mechanical properties. Recently, a technical breakthrough was appreciated on the application of the codeposited composite with the hardened layers on the cylinder surface of a hot engine,1 or the surface of a cooling Cu mold for continuous casting of molten steel. The ceramic particles reported in the literature could be Al2O3, ZrO2, SiC, diamond, Cr2O3, TiO2, MoS2, and WC.2–14 Among these inclusions, SiC is one of the important ceramic materials for structural and electronic applications. SiC has high resistance to wear because of its large hardness, thermal shock, chemical corrosion resistance, and good stability at high temperature, which is mainly due to its covalent bonding character. Therefore, SiC has been widely used as an abrasive and the materials for hot ceramic engine and heating elements.15,16 Hard particles, in general, improve the hardness and tribological properties of the composite Ni layer by the dispersively strengthening mechanism. The hard
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J. Mater. Res., Vol. 18, No. 7, Jul 2003 Downloaded: 29 Jan 2015
inclusions can inhibit the movement of grain boundaries of Ni grains and retard the grain growth during thermal treatment. When the material suffers an external stress, the inclusions hinder the dislocation movement in metallic matrix, whereby plastic deformation becomes difficult.17 Therefore, the hardness of the composite layer increases, and the wear resistance can be improved. Besides, the wear resistance can also be improved by reducing the grain size of the metal matrix, which includes ceramic particles at grain boundaries.18 According to Orowan’s report, the mechan
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