Effect of Plasma Electrolytic Oxidation coating on the specific strength of open-cell aluminium foams

Plasma Electrolytic Oxidation (PEO) coating treatment is applied to open celled aluminium foams with different structures, aiming to enhance the mechanical performance of the composite material. The mechanical properties of the coated foams produced are assessed experimentally, both in tension and compression. From experimental results, yield stress is found to increase initially with increasing PEO coating thickness, though this trend is not maintained with thicker coatings. This is caused by the transformation of greater quantities of metal to ceramic with thicker coatings (leading to more flaw-sensitive behaviour), and higher defect density in the surface layer (reducing the strength of the coating material). The specific strength of the samples (the yield strength per unit weight for a fixed volume) is shown to initially increase with coating thickness, although, due to the diminishing mechanical benefit and constant weight increase, the effect of substantial coatings is less beneficial. The optimum coating thickness appears to be in the region of 13 μm for the low porosity replicated foams tested in compression, and a value between 18 and 50 μm for the high porosity investment cast foams tested in tension.