Processing and properties of electrodeposited layered surface coatings

Abstract

Hard chromium, produced by conventional direct current (DC) electrodeposition, cannot be deposited to thicknesses greater than about 5μm because of the buildup of processing stresses which cause channel cracks in the coating. Much thicker chromium coatings may be produced by depositing a layered structure using alternate DC plating and periodic current reversal (PR). Such layering produces a through thickness stepped gradient in residual stresses. Most importantly, a bending moment develops in the coating whenever the substrate is compliant. For thin, compliant substrates, the coating cracks and spalls off early on. For thick, non-compliant coatings, much thicker coatings can be formed. Fracture resistance must be considered in relation to both specimen and loading geometries. Since the inherent bending moment causes a maximum tensile stress at the coating surface, the loading geometry is almost always analogous to bending, and fracture resistance is provided through deviation of the channel crack by weak interfaces, resulting in ‘terrace cracking’.