Different-sized porosity and thermal conductivity of oxide layers formed by plasma-electrolytic oxidation on the AK12D silumin

Oxide layers formed by plasma-electrolytic oxidation (PEO) are characterized by a sufficiently high porosity, which influences almost the whole complex of service characteristics. However, the known data on the integral porosity of PEO-produced layers are rather contradictory, and the nature of the pore size distribution in these layers remains understudied. As a result of processing the images of the layer cross-section produced in a wide magnification range (scanning electron microscopy – SEM, image-based analysis), the authors obtained the pore size distribution in the range of 10 nm to 10 µm, which is sufficiently well described by a lognormal distribution function (pore geometry was approximated by a spherical shape). Such distribution pattern indicates the nature of pore formation, which can be related to the thermally activated process of gas emission from a liquid melt, the volume and average temperature of which, in their turn, depend on the micro-arc discharge energy. The paper presents the results of identifying the oxide layer phase composition and crystallites sizes by the X-ray crystallography method. Comparing the results of X-ray spectral microanalysis and X-ray crystallography, the amorphous component phase composition was evaluated. Using the stationary method and the method of pulsed laser heating, the authors determined the thermal conductivity of the initial oxide layer and the layer after the removal of its highly-porous outer part. The porosity values obtained experimentally and calculated based on the analysis of SEM-images and the results of determining the phase composition, including amorphous phases, allowed evaluating the oxide layer thermal conductivity with the help of four known analytical models. The results of calculating the thermal conductivity using the Loeb model showed good convergence with the experimental results obtained in this work. The modeling demonstrated that the size of crystallites influence the oxide layer thermal conductivity much less than the porosity and amorphous phase.