Adhesion of advanced overlay coatings: mechanisms and quantitative assessment

Abstract

Functional properties and adhesion determine the performance of overlay coatings. The adhesive strength transmits the surface loads into the component and takes up the stresses that arise because of the differing properties of coating and substrate. Therefore, it is important to understand the mechanisms involved and to develop testing methods to measure the adhesive strength. Generally, three basic mechanisms of adhesion can be distinguished: (1) mechanical interlocking, (2) physical bonding, (3) chemical bonding. Depending on the coating/substrate system and deposition process, one mechanism usually plays a dominant role, although the others are present as well. In this way, a three-dimensional interfacial zone is formed, the width ranging from one atomic layer to several millimetres, again depending on materials, processes and parameters. Measurements of adhesive strength are often problematic, particularly with thin coatings and at high adhesive strength levels. For this reason numerous assessment tests have been developed, e.d. pull-off test, shear test, scratch test, indentation test, etc. An interesting solution to the problem is the fracture mechanics approach, where crack propagation in the interface is assessed. The paper discusses adhesion and adhesion testing with several examples of advanced coating systems.