One of the remarkable features of plasma electrolyte coatings is the presence of micro pores and cracks on the coating surface. Plasma electrolytic oxide coatings are generally recognized for high hardness, wear resistance, and corrosion resistance.  However, the coating properties are highly dependent on the substrate used, as well as on the composition of the electrolyte and the electrical regime used (see Equipment used section, above).
Even on aluminium, the coating properties can vary strongly according to the exact alloy composition.  For instance, the hardest coatings can be achieved on 2XXX series aluminium alloys, where the highest proportion of crystalline phase corundum (α-AlO) is formed, resulting in hardnesses of ~2000 HV, whereas coatings on the 5XXX series have less of this important constituent and are hence softer.  Extensive work is being pursued by Prof. T. W. Clyne at the University of Cambridge to investigate the fundamental electrical and plasma physical processes  involved in this process, having previously elucidated some of the micromechanical (& pore architectural), mechanical and thermal characteristics of PEO coatings.