Acoustic Emission Generated during Scratch Test of Various Thin Films

摘要

Thin films are used as a functional coating of components. These coatings should improve or modify the surface properties of the component materials in operation, or they serve as protective tribological films with sufficient load bearing capacity, wear resistance and with good sliding properties, alternatively. The limiting factor of their application is the adhesion to substrate. Scratch test is the laboratory method used to investigate their adhesion and mechanical toughness. This method emulates a mechanism of the abrasive wear in a way. Irreversible deformations (either plastic or brittle fracture), which arise during the scratch test, are also a source of an acoustic emission (AE). By recording the increasing depth of penetration together with the increasing load applied to the tip of the moving stylus in the nanotester and alongside by synchronous recording of the generated digitized AE signal, one can obtain data files, which contain a course of the whole experiment including AE. Using appropriate software one can process and analyze the AE data from different points of view and with various settings in the post-processing at any time (i.e. independently on time, place and environment of the experiment). For example hits corresponding with inception of micro cracks in the investigated sample can be calculated from particular events. Start of the AE recording is synchronized with the start of the actual load and penetration of the stylus tip (during the scratch test), which enables very accurate matching of acoustic events with the progress of the stylus loading. It is a sort of an "in-situ" experimental method of the scratch test. The method is demonstrated on various thin film-substrate systems. It can be mainly used for investigation of brittle films. It can reveal cohesion rupture either on the surface or inside of the material or adhesion ruptures of the film to the substrate. The method is also useful for investigation of transparent films in cases where the microscopic scanning is failing. Results of testing experiments are presented and compared.