ION-BOMBARDMENT ETCHING TECHNIQUES AS APPLIED TO POWDER METALLURGY MICROSTRUCTURES

Abstract

The application of low vacuum ion-bombardment etching techniques to porous systems offers a suitable surface treatment prior to the examination by both optical and electron microscopy. Two glow-discharge chambers are described and their efficiency of operation investigated. Reproducible results were possible only when a stable glow discharge could be maintained. Some aspects of general apparatus design with respect to discharge stability are illustrated. The variation of etching efficiency with discharge voltage and current density, background pressure, and time of bombardment were studied. Generally background pressures of < 1·33 N/m2 (10-2 torr) were required to avoid redeposition of etched material. At these pressures axial magnetic fields were employed to increase the discharge current density. Results illustrating the variation of etching profile with cathode geometry, specimen edge effects, and chamber geometry are given. These indicate that the etching profile is radically altered by distortions of the electrical field in the neighbourhood of the specimen. This leads to non-uniform etching profiles, which renders the technique unsuitable for delineating microstructures to be examined by automated image analysis. Based on these results, a modified discharge apparatus has been developed which produced uniform etching profiles, suitable for optical microscopy, electron microscopy, and automated image analysis of powder metallurgy microstructures.