Frontier Materials & Technologies (Dec 2022)
Different-sized porosity and thermal conductivity of oxide layers formed by plasma-electrolytic oxidation on the AlSi12Mg silumin
Abstract
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 pore size distribution in these layers remains understudied. Pore size distribution in the range of 10 nm to 10 µm (pore geometry was approximated by a spherical shape) was obtained by using analysis of scanning electron microscopy (SEM) images in a wide range of magnifications. Lognormal distribution function fits the shape of pore size distribution sufficiently well. Such distribution 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 turn, depend on the micro-arc discharge energy. The results of the oxide layer phase composition and crystallites sizes by the X-ray crystallography were described in the present paper. The amorphous component phase composition was estimated by the comparing of the of X-ray spectral microanalysis and X-ray crystallography methods. The thermal conductivity of the intact oxide layer and the polished layer (after the removal of its highly-porous outer part) was evaluated by using of the steady-state method and the laser flash method. The porosity values 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 use of four known analytical models. The results of calculating the thermal conductivity using the Loeb model demonstrate the good convergence with the experimental results obtained in this paper. Modeling results the size of crystallites effect on the oxide layer thermal conductivity significantly less than the porosity and amorphous phase.
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