Fracture behaviour of plasma electrolytic oxide coatings on an aluminium substrate using acoustic emission

Due to their low density and high specific strength, aluminium alloys provide a great range of application, but they are limited regarding corrosion and wear resistance. Plasma electrolytic oxidation (PEO) is an innovative process to produce oxide ceramic surfaces on aluminium to protect against corrosion and wear. However, due to their brittle nature and an unfavourable residual stress ratio (substrate/coating), PEO coatings have a limited fatigue resistance. The characterisation of the failure behaviour using conventional material testing methods is mostly elaborate and time-consuming. Furthermore, microstructure analysis methods using metallographically prepared specimen sections only provide local two-dimensional information about failure. In contrast, acoustic emission (AE) analysis is an integral method for the examination of the cracking behaviour and fracture mechanisms of materials. However, its application for PEO coatings is a novel approach. In this work, the suitability of the AE analysis for these coatings is evaluated by the characterisation of the fracture and failure behaviour under three-point bending loading. Since the fatigue test is a complex loading case, the focus is first on the static test. Signals were acquired during mechanical testing using an AE sensor and processed by Fourier transform. Two characteristic frequency bands between 150 kHz and 250 kHz as well as 350 kHz and 450 kHz were observed in the generated frequency spectra. From the correlation with the microscopic images, the low-frequency band was assigned to vertical surface cracks, whereas the higher-frequency band was primarily attributed to horizontal interface cracks.