Interface between electroless amorphous Ni-Cu-P coatings and Al substrate

The initial phase arising at the interface with aluminum substrate during electroless deposition of amorphous high phosphorus (16-19 at.%) Ni-Cu-P alloy with alloyed copper 1.7-3.4 at.% is studied. Stripped deposit foils are inspected by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy coupled with energy dispersive X-ray analysis (TEM-EDX). Depending on the plating conditions, various particles, some of them crystalline, are revealed at the foils sides formerly adhering to the substrate. EDX analysis of the crystallites shows diverse copper contents of about 34-64 at.%, without any phosphorus. Electron diffraction analysis indicates a lattice structure very close to that of Cu. The presence of a phase without phosphorus and enriched in copper at the interface with the Al substrate is also confirmed by Auger electron spectroscopy. This initially arising crystalline phase might exert a limited effect on the main properties of bulk Ni-Cu-P deposits: the composition uniformity through the thickness, amorphous structure and paramagnetic behavior. A modification of the plating process is recommended, which is decreasing the probability of inhomogeneities formation.