Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 369 of 707

elemental fluorine. The thermal stability of the CFx class of solid lubricants is high- er than PTFE, allowing the CFx composite to be postbaked for maximum hard- ness (1,100 VHN). The CFx composite exhibits high wear resistance coupled with a low coefficient of friction. The inclusion of these finely divided particles within an electroless matrix (15-25% by volume) involves the need to maintain uniform dispersion of the occluded material during metal deposition. Specialized equipment is required and part size, configuration, and deposit thickness are limited. Deposition rates will vary, depend- ing upon the type of electroless bath utilized. The surface morphology of the par- ticle used (i.e., type, size, and distribution in the matrix) will greatly influence the final codeposit properties and composition. The coefficient of friction and wear resistance of the composite are related to particle size and concentration in the electroless bath. Applications include food processing equipment, military components, molds for rubber and plastic components, fasteners, precision instrument parts, mat- ing components, drills, gauge blocks, tape recording heads, guides for comput- ers, and textile machine components. Due to the resultant matrix surface topography (when using diamonds or sil- icon carbide, for example), the final surface roughness must be considered. Special postplate surface finish operations must be employed to regain the required rms (microinch) finish. In severe abrasion applications involving high pressure foundry molding, it has been noted that the softer electroless nickel matrix wears first, exposing harder silicon carbide particles, which create poor drawability of the resin/binder from the mold. Polyalloys have been developed to produce deposits having three or four ele- ments with specific coating properties. These include applications where unique chem- ical and high temperature resistance or electrical, magnetic, or nonmagnetic properties are requirements. The use of nickel-cobalt-iron-phosphorus polyalloys produce magnetic (for memory) properties. Other polyalloys include nickel-iron-phosphorus, nickel-cobalt-phosphorus, nickel-phosphorus-boron, nickel-iron-boron, nickel-tung- sten-phosphorus, nickel-molybdenum-boron, nickel-tungsten-tin-phosphorus, and nickel-copper-phosphorus. The final selection is dependent upon the final applica- tion and the economics of achieving the results required. Electroless composites and polyalloys have made unique contributions to various engineering applications. Extensive field testing is ongoing to gain expe- rience for proper applications, inclusions and sizes, plus proper electroless bath operating parameters for these new forms of electroless plating. WASTE TREATMENT The electroless bath has limited life due to the formation of reaction byproducts. For example, in acid electroless nickel (hypophosphite-reduced) baths, the added accumulation or concentration of orthophosphite (HPO3 2— ) in the solution will eventually decrease the plating rate and deposit quality, requiring bath disposal. Also, the chelators and stabilizers make it difficult to reduce the electroless met- al content by alkaline precipitation. Regulations regarding effluent discharge vary globally and with respect to local POTW limits. In the United States, electroless metal legal discharge limits of 1 ppm or below are common for nickel and copper effluents. Conventional precipitation to form metal hydroxide or sulfide sludge through continuous or batch treatment involves a series of pH adjustment steps to con- 368

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