The concept of an advanced impact tester supported by evaluation software for the fatigue strength characterization of hard layered media

The coating impact test, in combination with its finite elements method (FEM) simulation, is successfully used to characterize the fatigue performance of thin hard monolayer coatings, as well as of multilayer ones. The test is based on successive impacts of a cemented carbide ball onto a plane-coated specimen, which induce hard contact loads and strain superficially the layered compound. The fatigue failure mode of each specimen is classified by means of SEM observations, EDX microanalyses and profilometry. FEM simulating models of the impact tests are used to determine the critical stress components, which introduce coatings fatigue failure. Critical values for stress components, responsible for distinctive fatigue failure modes of the coating substrate compounds are obtained and the fatigue Limits of various coatings are illustrated in general applicable Smith and Woehler diagrams. To further improve this method, an advanced impact tester supported by appropriate software facilities, able to evaluate the fatigue strength of hard coatings, was developed. In this enhanced tester the contact loads as well as the number of impacts can be readily varied so that the fatigue failure for coatings with different technological specifications and material properties can be obtained. Moreover, the continuous data acquisition as well as the real time monitoring and evaluation of the test bench is enabled. The test results are recorded in diagrams containing the impact load versus the number of successive impacts that a coating substrate compound can withstand. Thus, through appropriate computer software, the fatigue strength of thin hard coatings can be automatically determined and expressed in form of Smith and Woehler diagrams. The code is supported by an extended data base, implemented into an analytical procedure based on pre-conducted FEM calculations, covering a wide range of coating substrate compounds, considering also a variety of technological specifications and material properties. (C) 1999 Elsevier Science S.A. All rights reserved.