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 (FE M) simulation, is successfully used to characterize the fatigue performance of thin hard monolayer coatings, as well as of multilayer ones. The test i s based on successive impacts of a cemented carbide ball onto a plane-coate d specimen, which induce hard contact loads and strain superficially the la yered compound. The fatigue failure mode of each specimen is classified by means of SEM observations, EDX microanalyses and profilometry. FEM simulati ng models of the impact tests are used to determine the critical stress com ponents, which introduce coatings fatigue failure. Critical values for stre ss components, responsible for distinctive fatigue failure modes of the coa ting substrate compounds are obtained and the fatigue Limits of various coa tings are illustrated in general applicable Smith and Woehler diagrams. To further improve this method, an advanced impact tester supported by appropr iate software facilities, able to evaluate the fatigue strength of hard coa tings, 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 properti es can be obtained. Moreover, the continuous data acquisition as well as th e real time monitoring and evaluation of the test bench is enabled. The tes t results are recorded in diagrams containing the impact load versus the nu mber 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 Smit h and Woehler diagrams. The code is supported by an extended data base, imp lemented into an analytical procedure based on pre-conducted FEM calculatio ns, 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.