From Nanoparticles to Microparticles in Plasma Electrolytic Oxidation: Transport, Incorporation, and Coating Performance

Plasma electrolytic oxidation (PEO) can incorporate suspended particles from the electrolyte into growing ceramic-like oxide coatings, but particle size strongly affects their transport, capture, transformation, and final distribution. This review compares nanoparticle and microparticle additions, with particular emphasis on suspension stability, electrophoretic and convective transport, access to discharge channels, interaction with transient oxide melts, and distribution across the coating thickness. Well-dispersed nanoparticles generally remain suspended more readily, access finer pores, and undergo more extensive interfacial reaction or dissolution, whereas microparticles are more strongly affected by sedimentation and are more often retained as discrete or partially fused inclusions in outer coating regions. These differences influence coating morphology, porosity, roughness, phase evolution, hardness, wear and corrosion resistance, photocatalytic activity, and biofunctionality. Primary particle size alone, however, is insufficient to predict coating behavior because agglomeration, surface charge, particle morphology, electrolyte chemistry, waveform, and reactor hydrodynamics can alter the effective size-dependent response. Meaningful comparisons of particle-assisted PEO coatings should therefore account for the effective in-bath particle size and suspension state.