Fracture initiation mechanisms of thin hard coatings during the impact test

The impact test can be successfully applied for the characterization of thin and thick coatings fatigue and creep properties, respectively. In the present paper experimental and analytical investigations concerning thin films fracture initiation during this test are introduced. The damage initiation of thin hard coatings deposited on smooth surfaces is mainly induced by fatigue mechanisms and characterized by the removal of film segments in the contact area between the ball indenter and the specimen in highly superficially loaded contact regions during the impact test. Herein the currently used coating stress determination methodologies, when applied, lead in most cases to sufficiently accurate results. Superficial PVD layers deposited on a basic coating do not affect the occurring stress field practically and thus no differences concerning the fracture initiation in the basic coating are encountered. When the surface roughness increases, localized stress concentrations throughout the contact area occur, resulting in film failures in randomly distributed spots, at predictable high impact loads. In cases of relatively soft PVD films or at elevated substrate roughness, the abrasive wear contribution to the coating damage initiation has to be considered. In such cases, as will be explained, the first film fracture appears in a region, close to the middle of the contact radius.