Residual stress in ion-assisted coatings

Abstract

Ion beam irradiation of growing coatings offers the possibility of close control of the flux and energy of the bombarding ions which leads to control of the structure and properties of the deposited coating. For physically vapour-deposited thin films it is important to understand the microstructure—property relationships since these ultimately dictate the performance of the coating in any application. The residual stress in ion-assisted coatings can be either tensile or compressive depending on the conditions of ion bombardment and this is related to the structure of the deposited coatings. Tensile stresses are associated with open coating structures where ion bombardment can lead to some compaction of the growing film whereas compressive stresses are associated with fully dense coatings and are generated as a response to the damage created by the ion bombardment. At a critical momentum transfer from the ion beam there is a change from the open structure to the dense structure and compressive stresses are developed in the coating; thus the levels of residual stress can be used to monitor coating structure. In this paper the development of residual stress as a function of ion beam parameters will be discussed for a number of metal and ceramic coating systems. The models for stress generation in ion-assisted coating are critically reviewed in the light of the experimental observations.