Preparation and surface characterization of zincated aluminium memory-disc substrates

Abstract

The surface compositions of CW66 aluminium alloy memory-disc substrates following commercial alkaline and acidic cleaning treatments and a commercial zincating treatment were investigated using the techniques of X-ray photoelectron spectroscopy, low-energy ionscattering spectroscopy, secondary ion mass spectrometry, and scanning electron microscopy. The commercial treatments were performed on the disc substrates following diamond turning, or diamond turning followed by alumina-slurry polishing. Results indicated that alumina-slurry polishing of CW66 memory-disc substrates produces a slightly thicker and more hydrated aluminium oxide film compared to diamond-turned only discs. Commercial alkaline (Alprep 204) and acidic (Alprep 230) cleaners do not significantly affect the aluminium oxide composition nor thickness on CW66 alloy. These cleaning treatments also do not appear to significantly affect the amount of surface carbon contamination, nor the amounts of trace surface contaminants, such as sulphur and chlorine, that are present on the metal. Nitric acid stripping treatments performed between the first and second zincating treatments do not completely remove all of the first zincate film. A small amount of zinc remains on the disc surface. After removal of the first zincate film with nitric acid, SEM results showed that the first zincate treatment significantly roughened the aluminium substrate compared to the cleaned discs. The zincate films prepared in this study were discontinuous and consisted primarily of a Zn(OH)₂/ZnO mixture on the surface and zinc metal in the bulk of the film. A variety of trace contaminants (i.e. sodium, silicon, sulphur, chlorine, potassium, calcium and iron) were also present in the zincate films. Some of these contaminants (i.e., sodium, chlorine and iron) most likely resulted from minor components in the zincating solutions. The others most likely resulted from inadvertent sample handling or from the adsorption of airborne contaminants prior to the analyses.