Residual stress induced wetting variation on electric brush-plated Cu film

Nanocrystalline Cu film with a mirror surface finishing is prepared by the electric brush-plating technique. The as-prepared Cu film exhibits a superhydrophilic behavior with an apparent water contact angle smaller than 10 degrees. A subsequent increase in the water contact angle and a final wetting transition from inherent hydrophilicity with water contact angle smaller than 90 degrees to apparent hydrophobicity with water contact angle larger than 90 degrees are observed when the Cu film is subjected to natural aging. Analysis based on the measurement of hardness with nanoindentation and the theory of the bond-order-length-strength correlation reveals that this wetting variation on the Cu film is attributed to the relaxation of residual stress generated during brush-plating deposition and a surface hydrophobization role associated with the broken bond polarization induced by surface nanostructure.