Characterization of plasma electrolytic oxidationby the acoustic emission technique

摘要

Plasma Electrolytic Oxidation (PEO) is attractive for a number of industrial applications likernhexavalent chromium replacement in anti-corrosion-protection or tribological propertiesrnimprovement of metals. This electrolytic process differs from "conventional" anodising by usingrnvoltages above the dielectric breakdown potential of the anodic oxide yielded. This leads to thernlocal formation of a plasma, as indicated by the presence of sparks on the metal surfacernaccompanied by gas evolution.rnThe need for a technique suitable to study the characteristics of these discharge phenomena inrnthis process led us to consider the application of the Acoustic Emission (AE) technique for PEOrnmonitoring. Indeed, a part of the energy released during PEO is converted into mechanical energyrnand yields elastic waves propagating through the sample, which are detectable by standard AErntechniques.rnThe various phenomena which take place simultaneously at the surface of the electrode duringrnPEO of AM60 were monitored by acoustic emission and separated using the signal statisticalrntreatment by neural network. Before the breakdown voltage, a conventional anodising takesrnplace, and the following phenomena were observed: oxygen evolution, growth of the oxide filmrnporosity and oxide film growth. At the beginning, a compact barrier film is form and yielding byrna high acoustic activity, following by a step of porous film growth. After the breakdown voltage,rnthe number, duration and energy of the microdischarges were evaluate. The duration ofrnmicrodischarges varied in the range of milliseconds; the duration and energy increased duringrnanodising. The acoustic energy of oxygen evolution and film relaxation increases as a function ofrntime, which is consistent with the increase of the microdischarges acoustic energy.