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4.5 Coating on Metal Powders
In number of cases, the base metal powders may be coated by another chemical species. The purpose may be to produce a homogeneous mixing, e.g. W–Cu, a hard surface or deposition of soft low melting point metals on ceramics to impart better compressibility during compaction. One of the simplest method may be the mechanical method, say ball milling of WC–Co. However, the uniformity of coating in such case is questionable. Some of the more common methods are described below:
(i) Electroplating: In this method, a second dispersed phase of controlled particle size may be introduced into a plating solution. This is referred to as dispersion, inclusion, occlusion, composite or electrophoretic plating, deposition or coating. The requisites are:
– The particles must be insoluble (or only slightly soluble) in the solution;
– The particles must be compatible with the solution, i.e. not produce any detrimental effect;
– The particle must be dispersed either naturally (as colloid size particles) or mechanically (stirring, agitation) in order to contact physically the surface being coated.
Electrophoretic coating produces a coating on a conducting substrate from a dispersion of colloidal particles. The powder to be coated is immersed in an aqueous dispersion which dissociates into negatively charged colloidal particles and positive cations. An electric field is applied with the powder mass as anode; the colloidal particles are transported to the anode, where they are discharged and form a film. The coating is air dried and baked to remove the solvent medium. The coating is nonadherent and must be processed faster by compaction and sintering. The coating thicknesses can be varied by controlling voltage, electrode spacing, suspension concentration and time.