Properties of electroless and electroplated Ni-P and its application in microgalvanics

摘要

Composition, microstructure, surface morphology, mechanical properties and electrochemical behaviour of electroless (el) and electroplated (ep) Ni-P deposits are studied using XPS, SEM-EDX, AFM, nanoindentation measurements, cyclic voltammetry and capacitance measurements. Ni-P layers were compared with ep Ni films and bulk Ni. Ni-P layers prepared by both techniques contain 12-14 wt% phosphorus, present in oxidation states of P~0 and P~(3-). El and ep Ni-P deposits are amorphous and are characterised by a relatively low average surface roughness (2 and 4 nm, respectively). The ep layers possess a rhythmic-lamellar microstructure indicating a periodic change of electrodeposition conditions. The el Ni-P layers do not show such laminated structure but exhibit small surface pores, which are absent in the ep layers. Comparable values for the hardness and the reduced elasticity modulus of el and ep coatings are determined from the nanoindentation data. The observed small differences indicate that the mechanical properties of Ni-P deposits depend not only on the phosphorus content but also on the deposit microstructure. Microelectrochemical measurements with the so-called droplet cell show that the electrochemical behaviour of both el and ep Ni-P coatings is practically identical and does not depend on the location on the sample surface. Evolution of O_2 and H_2 on Ni-P are similar to pure Ni (ep and bulk), but the corrosion resistance in acid solution is much better. The very similar properties and electrochemical behaviour of el and ep Ni-P deposits suggest that both materials are suitable for various applications in microsystem technology. For different substrates and microstructures of different size and geometry, deposition conditions have still to be optimised.