Depth-resolved analysis of Ni–P plated aluminium hard disks by radiofrequency glow discharge optical emission spectroscopy (rf-GD-OES)

Abstract

A parametric evaluation of the discharge conditions which affect the quality of radiofrequency glow discharge optical emission spectroscopy (rf-GD-OES) depth profiles of Ni–P plated aluminium hard disks is described. This technique has been shown previously to provide good depth resolution for metallic and oxide layers, also being able to examine contaminants at the layer interfaces. A range of discharge parameters is investigated relative to their contributions to analyte responses and resultant depth profile characteristics. The optimal discharge operating parameters for this application were determined to be an rf power of 40 W and an argon discharge gas pressure of 6 Torr. The depth-resolved analysis of the Ni–P layer plated on the Al–Mg alloy substrate reflects the microscopic non-uniformity of the surface of the Al alloy base material. Diamond stylus profilometry is employed to extract crater profiles and sputtering rates obtained over a range of discharge conditions of rf power and argon discharge gas pressure. SEM, TEM and EDX images of specimen cross sections are used to examine the physical structure of the layer interfaces on a microscopic scale. For each of the aluminium hard disks investigated in this study, the interface breadths (Δz) obtained at the Ni–P/Al–Mg interface is ultimately limited by the roughness of the aluminium disk substrate to which the overlayer is applied. Optimized discharge conditions are employed to profile hard disks extracted from two commercial drives of different technological generations. Distinct compositional differences are observed that correlate with improvements in the base technology over the years 1992–1998.