The role of ion-induced substrate damage in thin film adhesion strength

Variable-energy positrons are combined with Auger electron spectroscopy, scanning electron microscopy, and scratch test critical load measurements to study interfacial properties in thin film adhesion. This work complements an earlier investigation of the adhesion strength enhancement produced by ion bombardment of the substrate surface before deposition. In this study, we have investigated SiO2 films deposited by radio-frequency (RF) sputtering onto stainless steel substrates. Extended ion bombardment etching leads to three related effects: the scratch test critical load is increased significantly, Auger electron spectroscopy shows a greater penetration of the film material into the substrate; and the interfacial positron annihilation signal is dominated by large, open-volume defects. These results are interpreted as confirmation that ion bombardment leads to the formation of microvoids in the interface layer and, consequently, to an increased adhesion strength by allowing mechanical interlocking between the coating and the substrate.