This work describes an investigation into the mechanisms responsible for the incorporation of particles and fibres suspended in the electrolyte during plasma electrolytic oxidation, and provides a comparison of the processing and discharge characteristics of aluminium, titanium and magnesium.

Well-established methods such as synchronised small area and high speed video monitoring, which have previously been successfully applied to the study individual discharge characteristics of aluminium, are extended to magnesium and titanium substrates. It was found that while magnesium behaves quite similarly to aluminium, titanium discharges are by contrast much less energetic, due to much lower discharge currents and durations. These observations are attributed to differences in the properties of each metal oxide.

Particle incorporation into coatings on Al and Ti substrates was studied using three types of oxide powder; 30 $\mu$m alumina, 7 $\mu$m magnesia and 100 nm alumina. It has been established that, where such reactions are chemically favoured, phase changes can occur that must have involved the particulate reaching very high temperatures. From this and other evidence, it is concluded that the main incorporation mechanism involved is that of (fine) particulate being swept into the pores associated with active discharge sites, while they are being refilled with electrolyte immediately after collapse of the plasma. They are then likely to become entrapped, and in many cases to be strongly heated as the plasma is created during the next discharge cycle.

Alumina (Saffil®) fibres were successfully incorporated into PEO coatings on Ti and Al substrates and it is concluded that incorporation is due to the the trapping (and subsequent partial or full melting) of fibres in and around discharge pores. Fibre incorporation is observed to be more homogeneous, on Ti substrates, in cases where the (fibre-containing) electrolyte is jetted across the face of the sample.