Wavelet Processing of Acoustic Emission Signals from Scratch Tests on Nitride Coatings

Wavelet Transform was applied to Acoustic Emission (AE) signals from Scratch Tests (ST) on stainless steel samples with Cr and Ti nitride surface coatings obtained by PVD. The implementetion of the signal processing method was aimed at characterizing the coating failure stages and evaluating the quality of and the differences between both type of coatings. In previous work we had identified different zones along scratches both with Scanning Electron Microscopy and the directly measured AE parameters. The zones corresponded to transversal microfractures on the coating and/or deboding microfractures between the substrate (steel) and the coating. AE permitted an earlier detection, but with the traditional AE inspection the identification of the different zones was not sufficiently clear. This fact showed the necessity of more sophisticated signal processing. The WT, adequate to analize the essentialy non staionary AE signals, was applied in this paper and the different stages could be precisely delimited by a small number of adequate parameters. One of the best was the fMWP , frequency at which the WT calculated mean power of signals takes its highest value, considering its evolution along the ST.