Metal Finishing Guide Book


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performance of other candidate coatings may be compared against it. Hardness Hardness measurements involve making an indentation on the surface (or cross section for thin coatings) of the deposit. The indenter has a specified geometry and is applied with a specified load. In the case of industrial nickel coatings, the most common hardness determination is the Vickers method of forcing a diamond point into the surface under a predetermined load (normally 100 g). This provides a measure of that surface to permanent deformation under load. The figure obtained is not necessarily related to the frictional properties of the material nor to its resistance to wear or abrasion. The measurement of microhardness of plated coatings is discussed in ASTM B 578. Internal Stress The magnitude of internal stress obtained in deposits is determined by plating onto one side of a thin strip of basis metal and measuring the force causing the strip to bend. One method used in commercial practice involves plating the exterior surface of a helically wound strip and measuring the resultant change of curvature. Another method is based on the flexure of a thin metal disc. See ASTM B 636 for the method of measuring internal stress with the spiral contractometer. Ductility Most of the tests that have been used for evaluating the ductility of plated coatings are qualitative in nature. Two bend tests are described in ASTM B 489 and B 490. Both of these procedures require a minimum amount of equipment. Another method for measuring the ductility of thick deposits is to determine the elongation of a specimen in a tensile testing machine. This method is limited to relatively thick foils of controlled geometry and thickness. A method specifically designed for plated thin foils has been used and is known as the hydraulic bulge test. A mechanical bulge test is also available. Adhesion In general, the adhesion between a nickel coating and the basis material should exceed the tensile strength of the weaker material. As a result, when a force is applied to a test specimen, which tends to pull the coating away from the basis metal, separation occurs within the weaker material rather than at the boundary between the basis metal and the nickel coating. A number of qualitative tests have been used that utilize various forces applied in a multitude of directions to the composite basis metal and coating, such as hammering, filing, grinding, and deforming. Quantitative tests have also been described in the literature. Achieving good adhesion requires a sound bond between the substrate and the coating. A sound metallurgical bond may be achieved on most materials by proper surface preparation prior to plating. The selection of grinding, polishing, pickling and conditioning treatments for a variety of basis metals varies from one material to another, and depends on the initial surface condition of the metal. The activating treatments that follow polishing and cleaning operations are listed in Table VIII for the most commonly plated basis metals. ASTM standards provide additional information. Nonconductive plastics and other materials can be plated by metallizing the material, using etching and catalyzing techniques (ASTM B 727). 306

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