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بخشی از مقاله انگلیسیعنوان انگلیسی:Electrodeposition of nickel/silicon carbide composite coatings on a rotating disc electrode~~en~~

Abstract

Composite coatings suitable for protection against wear were prepared by electrodeposition from a nickel Watts solution containing silicon carbide particles maintained in suspension. To obtain a better understanding of hydrodynamic effects on the codeposition process a rotating disc electrode, immersed in a vertical rising flow, was used. The local concentration of embedded SiC along the radius of the disc electrode was studied as a function of suspension concentration, rotation rate and the particle mean diameter. The effect of a rheoactive polymer was also examined. Although it is generally admitted that the particle incorporation rate is governed by a two-step adsorption process, the experimental results show that it is also dependent on the spatial distribution of the wall fluid flow. The normal component of the fluid velocity promotes particle impingement, whereas the parallel component tends to eject the loosely fixed particles. The competition between the forces which tend to maintain particles attached to the surface and the shear force which tends to remove them, depends on several parameters, in particular the surface chemistry and the size of the particles, the flow rate and the current density.

۱ Introduction

Composite coatings can be prepared by electrodeposition from an electrolytic solution containing a suspension of insoluble particles [1]. Interesting applications have been developed for purposes of wear resistance [2, 3], dry lubrication [4, 5], anticorrosion [6, 7] and dispersion hardening [8, 9]. Many systems were studied, including metals such as copper [10-13, 32], nickel [2, 3, 8, 9, 14], silver [5], cobalt [15, 16], lead [17] and gold [18, 35]. A great variety of particles was also used, including inert materials such as diamond [19], ceramic materials such as silicon carbide [2, 5, 20-23], titanium carbide [17], chromium carbide [6], alumina [8 to 11, 25], titania [9, 20], chromium oxide [16], lubricant materials such as PTFE [26], graphite [27] or molybdenium sulfide [4, 5] and also metallic materials such as chromium [28, 29]. Because of their excellent tribological properties nickel/silicon carbide coatings were first developed to protect the NSU-Wankel rotary engine [23] against wear and are used as substitutes for hard chromium coatings in four-stroke or twostroke aluminium engines [3, 31]. According to the literature, the rate of particle entrapment depends on many factors either related to the particles (size, density, composition, zeta potential [11, 25, 30, 39], conductivity [40]) or to the electrolytic solution (composition, pH, temperature, presence of additives [21, 30, 32]). Agitation of the bath appears to be an important factor but its effects are somewhat ambiguous. In most cases particle sizes are larger than 0.1 #m and therefore vigorous stirring of the electrolyte is necessary to obtain a homogeneous suspension. Various stirring techniques have been employed [1, 16]. In some industrial processes, several of these techniques are employed simultaneously [15, 26]. However, it has sometimes been observed that increase in fluid velocity results in a decrease in the incorporation rate of solid particles [30, 33].

۴ Discussion and conclusion

Use of a rotating disc electrode, has shown that both steps of the nickel/particle codeposition process is affected by hydrodynamics in spite of the fact that this factor is not considered in the Guglielmi model [42, 43, 47]. For example, the apparent adsorption coefficient is very sensitive to bath agitation and depends on it in a complex way (Table 2). Recently, Fransaer et al. [45, 46] derived a new codeposition model based on detailed analysis of particle trajectories by taking into account all particleelectrode interaction forces, as well as hydrodynamic forces. Particles are subjected to gravitational force, Fg, which, in our setup (Fig. 14(a)) was balanced by the vertical force, Fe, resulting from the liquid circula- tion by the external pumping circuit and to the forces exerted by the convective flow of the rotating disc electrode and, eventually, to attractive forces from the electrode surface if the particle approaches sufficiently close. For non-Brownian particles, the model shows that small particles tend to follow the fluid streamlines while the big particles tend to leave the streamlines when close to the wall. Particles which are attached to the surface are submitted to adhesion forces, Fadh, and frictional forces, Ffric, by the electrode surface and to a stagnation force, Fstagn, and a shear force, Fshea r, by the fluid flow (Fig. 14(b)).

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