First of all, it is possible to produce particle-reinforced coatings with addition of relatively large-sized Si3N4 particles (0.1–0.8 μm and 1–5 μm). Particles have been found distributed uniformly throughout the cross section of PPEO(M) and PPEO(L). Taking the microstructure and EDS analysis of PEO coatings into consideration, the uniform distribution of particles is caused by two reasons. On the one hand, particles could flow towards the inner coating through discharge aurora kinase inhibitor even reaching the inner layer. On the other hand, Si3N4 particles will be pasted on melted material on the surface of the coating and finally the growing coating is embedding them. Pores on the surface of PEO coating have been filled by particles, as can be seen in Fig. 2d anf f. Even though the surface could be optically greatly improved by the particle addition, there is no significant impact on enhancing cross-sectional quality of PEO coatings.
It can also be concluded that mainly an inert incorporation of Si3N4 particles (0.1–0.8 μm and 1–5 μm) occurs without reaction with other components of the coating or electrolyte forming new compounds and phases during PEO processing. In other words, the plasma temperature or at least the transferred energy into the coating produced by the discharges during PEO processing from electrolytes with Si3N4 particles (0.1–0.8 μm and 1–5 μm) addition is not high enough to reach the melting temperature of Si3N4 (1900 °C). The inert particles may be considered as obstacles for coating growth reducing the effective area for formation of conversion products which are finally converted by the discharges into the coating. Furthermore the coating has to grow around the particles thus large-sized particles are bigger obstacles resulting in the observed thinner coatings with increasing particle size. For the electrolyte with the smallest particles (0.02 μm) addition, a normal PEO coating could not be achieved. Coating formation was observed and the breakdown potential was exceeded but a final voltage of 450 V was never reached. The plan view of those specimens revealed severe cracking, with crack opening up to 10 μm and down to the substrate. If this cracking occurs during the processing continuously one can understand that the final voltage cannot be reached because always new substrate is exposed. It might be possible that the nano-sized particles (which are generally known to have lower melting temperatures [17,18]) are being reactive incorporated forming a new and brittle coating phase that cannot withstand the internal stresses generated by the PEO processing. However, this assumption needs further studies to be proven.
In terms of improving corrosion resistance of PEO coating, incorporating Si3N4 particles into electrolyte seems to be not effective. Although Si3N4 particles are stable and distributed uniformly throughout the cross section, they do not form a dense and thick film. Thus they cannot protect the substrate better than the PPEO coating only. In general, the quality and thickness of the cross section, especially the inner layer, play more important role in the corrosion resistance in contrast with the surface of PEO coating.
Magnesium and its alloys have hexagonal close-packed (HCP) crystal structure and activation of several variants of both slip systems and twin systems is required for an arbitrary shape change. Dislocation slip in HCP crystals includes both basal and non-basal systems. For Mg and its alloys, the critical resolved shear stress (CRSS) of basal slip is less affected by strain rate and temperature than non-basal slip [1,2], especially for pure magnesium . Deformation in c-axis tension may occur by tensile twinning and in c-axis compression by double twinning e.g. Refs. [3,4]. The critical stress to activate tensile twinning is lower than that for compression twinning e.g. Refs. [5,6]. Experimental work has shown that plastic deformation of Mg alloys can be slip-dominant or twinning-dominant depending on loading, texture and microstructure e.g. Ref. . The effects of strain rate and temperature on yield and flow stress, as well as the structural performance under crash conditions, are strongly influenced by the controlling deformation mechanisms and therefore by loading mode, texture and microstructure.