Nanostructured gradient Co-Sn electrodeposits as alternative to Sn connector contacts

• Electrodeposition of nanostructured Co-Sn gradient coatings
• Effect of Sn content on microstructure and mechanical properties of Co-Sn sub-layers
• Tribological properties of nanostructured Co-Sn gradient coatings
• Co-Sn gradients have superior wear resistance when compared to pure Sn coatings

Gluconate plating baths were successfully used for the synthesis of homogenous, crack-free nanostructured gradient Co-Sn coatings. The grain size of electrodeposited Co-Sn sub-layers was found by X-ray Diffraction to range from 10 up to 143 nm, whereas the increase of Sn content into the Co-Sn sub-layers led to an increase of average grain size. The average hardness of the gradient coating was found to be significantly higher than that of the pure Sn electrocoating, due to the presence of intermetallic phases. In addition, the frictional properties of the Co-Sn gradient coatings strongly depend on the thickness of the pure Sn top layer, as the thin top layer can shear when sliding against the counterface material. On the other hand, thick Sn layers as in the case of pure Sn electrodeposits result in a higher coefficient of friction of the tribosystem due to adhesive wear phenomena, brought about by the formation of debris at the sliding contact. Furthermore, nanostructured gradient Co-Sn coatings are found to have significantly higher wear resistance when compared to pure Sn coatings. The main wear mechanisms observed were adhesive and abrasive wear of the Sn top layers and subsequent deformation and cracking of the intermetallic based Co-Sn sub-layers.