Acoustic emission monitoring of single cracking events and associated damage mechanism analysis in indentation and scratch testing

A detailed study on optimisation of single cracking events in indentation a nd scratch testing of brittle coatings is presented. These results are illu strated for nitrogen-doped stainless-steel coatings deposited by reactive m agnetron sputtering. The best results are obtained with a resonant piezo tr ansducer on the indenter shaft and high-speed electronics allowing for the detection of single cracks. Waveforms are seen to contain only information on the crack energy and the instant of nucleation. The latter is exploited for precise synchronisation of all test-specific parameters with the real i mage of the cracking failure. With this approach, it is possible to carry o ut an advanced failure mechanism analysis of the test-induced surface damag e. This is practised for indentation, where cracking is seen to occur in th e elastically strained region outside the real contact area (size of the pl astic indent). The shape of the plastic indent is seen to be unmodified ela stic recovery on unloading. Tn scratch testing, a clear correlation between high-energy AE pulses and cracking failure is achieved, and the first atte mpts to analyse the prevailing failure mechanisms and experimental 'dead ti mes' for separation of consecutive events are presented. (C) 1999 Elsevier Science S.A. All rights reserved.