The continued development of multilayered and compositionally modulated electrodeposits

ABSTRACT Traditionally, electroplating has involved the continuous deposition of a single layer of metal at constant current. However, electrodeposition of alternate layers can offer benefits such as reduced wear, improved corrosion resistance and higher tensile strength. The alternate layers can involve different morphology or thickness of metal, different metals or the alloy composition of layers with and without included particles. In the case of a single bath, electrocrystallisation is continuous but layers can be tailored to have different chemical composition, phase composition, morphology and microstructure. The composition of layers can also be systematically modified in a gradient fashion. The thickness of each metal layer can vary from >20 μm down to ≈1 nm; in the case of nanometre thick layers, up to 500 layers of 1 nm thick individual layers might be involved. Compact multilayer deposition from a single bath is often achieved by applying a potential waveform in the laboratory or pulsed current in industry. While multilayer electrodeposition is going through a phase of rediscovery, growth and diversification, the field can be traced back to a patent involving Cu–Ni multilayers, in 1905. Progress in multi-layered electrodeposition has made use of contemporary trends in electroplating research, including self-assembled layers, nanowire arrays and the use of deep eutectic solvents for electrolytes. The developing uses of multilayer deposits are seen to span industries as diverse as wear and corrosion resistant coatings, tool bits and heavy engineering. Speciality uses include electronic, optical and magnetic materials as well as catalytic electrode surfaces for electrochemical technology. Recommendations are made for topics which deserve further R & D.