Effect of Power Duty Cycle on Plasma Electrolytic Oxidation of A356-Nb2O5 Metal Matrix Composites

Dense outer layer with inter-oxide composite intermediate layer coatings was formed on Al-356 matrix with Nb2O5 reinforcement particles composite via soft-sparking power duty cycle plasma electrolytic oxidation (PEO) in alkaline silicate electrolyte. The x-ray diffraction and transmission electron microscopy results proved the formation of γ-alumina layer containing Nb-Al mixed oxides. The band and discrete pores at and near the metal/coating interface were reduced by applying bipolar-soft power cycle duty discharges with higher negative half cycles. The applied mixed oxide coating showed high corrosion resistance in aggressive and corrosive mediums specially during the long-time exposure tests. This behavior was associated with the triple layered PEO coating, containing very few open channels from metal substrate to the outer layers. The high surface hardness, small surface roughness, and low surface frictions ensure high wear and abrasion resistance of the coatings.