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Residual macrostresses in electroless nickel (EN) coatings affect critical deposit properties such as hardness and adhesion. Straight thin aluminum, beryllium, and steel strips were plated in hot nickel hypophosphite baths varying in chemistry, pH, and usage. When the nickel-phosphorus (Ni-P) coating was stripped from one side the metal strip assumed either a concave (tensile) or a convex (compressive) arch because of internal stress in the EN coating. The degree of curvature was measured and the residual stress calculated. The total stress consists of an intrinsic component resulting from plating bath chemistry and usage and a thermal stress produced by the difference in thermal expansion coefficients between the EN coating and the substrate. Thus, on aluminum strong compressive stresses are induced by its larger shrinkage, which occurs during cooling from bath (90°C) to room temperature. Annealing decreases the compressive stress. With usage of the EN bath the coatings decrease in compressive stress and after three to five turnovers are tensile stressed and blister because of adhesive failure. On beryllium compressive stress is obtained if the EN coating contains >11% phosphorus. Annealing changes the stress to slightly tensile. On steel the initial low tensile stress increases linearly with bath usage and reaches 180 MPa (28 ksi) after six turnovers, which is moderated by annealing at 200°C. The internal stress of EN coatings depends on the chemistry and usage of the bath, the substrate, and the phosphorus content of the deposit. The plating of thin, rigid metal strips offers a simple, inexpensive method to determine the residual stress of coatings on any metal substrate. The results obtained are in good agreement with contractometer stress measurements of EN coatings.
Electroless nickel properties, nickel phosphorus alloys, internal stress measurements, autocatalytic nickel coatings, stress of metallic coatings, electroless plating, method of stress measurement
Consultant, Park Ridge, IL