In-situ SEM analysis of brittle plasma electrolytic oxidation coating bonded to plastic aluminum substrate: Microstructure and fracture behaviors

The interfacial fracture behaviors between brittle ceramic coating and ductile metal substrate system are vital under uniaxial tensile loading for engineering applications. In this work, the fracture process of crack initiation and propagation between plasma electrolytic oxidation (PEO) ceramic coating and aluminum substrate were analyzed by in-situ SEM method. The effects of ‘over-growth’ regions existing at the ceramic coating/substrate interface on the fracture behaviors of coated samples were investigated comparatively. The tensile test results of bare and PEO coated aluminum alloy show that PEO treatment decreases tensile strength and elongation of the aluminum substrate, while enhances the elastic modulus slightly. At the same elongation, the thicker ceramic coating, the more peeling area of the coating. The real-time crack initiation and propagation behaviors recorded by in-situ SEM indicate that cracks initiate at the bottom of ‘over-growth’ regions between the coating/substrate interface, which implies the cracks initiation is sensitive to the defects caused by ‘over-growth’ regions. The thicker PEO coating is easier to induce the earlier crack initiation owing to the lager stress concentration. The cracks are prone to propagate into aluminum alloy rather than the coating/substrate interface indicating that the ceramic coating has superior adhesion with aluminum alloy substrate.