Determination of Modulus of Elasticity, Nanohardness and Residual Stresses in Brush-plated Gold and Silver Coatings on Copper Substrate
In the current study the investigated brush-plated gold coatings are generally used for repairing the commutators of generators and sliding contacts made in most cases of copper. Considering the cost, gold is sometimes replaced by silver in manufacturing electronic components.
Nickel-hardened gold and silver coatings were brush plated from a commercial SIFCO Dalic Solution (Gold Hard Alloy), Code SPS 5370, and Silver Hard Heavy Build, Code SPS 3080, on unclosed thin-walled copper ring substrates.
The magnitudes of the modulus of elasticity and of nanohardness of the coatings were obtained by instrumented indentation using the MTS Nano Indenter XR® and the Micromaterials Nano Test system pendulum-type nanohardness tester. Residual stresses in the coatings were calculated from the curvature changes of the substrates and they represented tensile stresses. Relaxation of residual stresses was observed. An equation for approximation of the change of residual stresses was applied assuming that the dependence of residual stresses on relaxation time is linear-fractional. The values of residual stresses in the gold and silver coatings decreased considerably, during the first weeks in particular. The equation for approximation of the change of residual stresses allows to predict the finishing residual stresses in the coating for the exploitation period within the limits of measurement uncertainty.The surface morphology and microstructure of the coatings was studied by means of scanning electron microscopy (SEM).
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