Electron microscopy of sharp edges and corners by ion-assisted PVD.

The thesis examines ion-assisted physical-vapour deposition (PVD) of thin coatings on non-flat three-dimensional samples, concentrating on the case of free-standing edges and comers. Changes in the electric field in the vicinity of sharp edges lead to local changes in the ion bombardment (ion flux and angle of incidence) which can significantly affect the ion-surface interaction and thus the properties and the performance of the coatings growing in the edge region. This work presents a detailed electron microscopy study of the edge-related changes in the coating properties and develops a physical model to explain and quantify the effects. The problem is studied on a system typical for industrial coating of cutting tools used in dry high speed cutting: TiAlN-type coatings (TiAlN/VN and TiAlCrYN) deposited on wedge-shaped samples by closed-field unbalanced magnetron sputtering (UBM), using high-flux, low-energy Ar+ ion irradiation (J[i]/J[me]~4, E[i] = 75-150 eV). The morphology and composition of the coatings in the edge region, as a function of the edge geometry (angle and radius of curvature) and the deposition conditions (substrate bias), is studied using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM+EDX). The internal structure of the coatings growing on sharp edges is examined by transmission electron microscopy (TEM). A detailed theoretical analysis of the effects, based on the simulations of the plasma sheath around the samples and the resulting ion bombardment distribution, is presented. A direct relationship between the experimentally observed magnitude and spatial extent of the changes in the edge region and the simulated characteristics of the plasma sheath around the edges is shown.