CORROSION AND XPS INVESTIGATIONS OF NITROGEN-IMPLANTED ALUMINUM

Low energy ion implantation of nitrogen into oxide free aluminium surf aces was used to obtain stoichiometric and pore-free AlN layers of abo ut 8-10 nm thickness. The electrochemical stability of these layers wa s investigated by anodic cyclic voltammetry, pulse measurements and X- ray Photoelectron Spectroscopy (XPS). For comparison AIN layers prepar ed by Physical Vapor Deposition (PVD, thickness 2 mu m) were examined as well. While AlN is hydrolysed at pH 14 as predicted by thermodynami cs, electrochemical stability up to 1.6 V was observed in acetate buff er solution (pH 5.9). Surface analysis reveals that at potentials belo w 2 V, AlN is covered by an oxide layer of 2 nm. Molecular N-2 dissolv ed at the AlN/Al2O3 phase boundary was detected as a product of nitrid e decomposition. At higher anodic potentials, the oxide layer thicknes s increases whereas the AlN layer diminishes. Oxidation takes place in a two-step mechanism. Beyond a potential of 1.6 V, the electric field is sufficiently high to initiate the oxidation of N3- via tunnelling. Although migration of mobile ions (Al3+, N3-, O2-) starts. the layer composition remains nearly unchanged. At potentials above 4 V, high-fi eld oxide growth starts until the nitride is completely decomposed. In contrast, the properties of PVD layers on steel substrates are determ ined mainly by the porosity of the coating. Due to substrate propertie s, iron dissolution and transpassive oxygen evolution were observed. ( C) 1997 Elsevier Science Ltd.