Investigation of micro-arc oxidation coatings using sub-THz ellipsometry

Abstract

The paper is devoted to the investigation of the structure and properties of coatings obtained by micro-arc oxidation (MAO). For the first time, the possibility of using sub-terahertz ellipsometry with an operating frequency f = 0.14 THz for non-destructive non-contact determination of the integral values of the thickness, structure, and composition of such coatings is studied. The frequency was chosen since the corresponding wavelength ( λ = 2.14 mm) significantly exceeds the characteristic size of the roughness of the coatings, which makes it possible to use the geometrical optics approximation in the analysis of experimental results. When constructing models of the structure of coatings and determining their parameters, additional independent methods were used, such as optical microscopy, XRD, SEM, terahertz reflectometry, and the theoretical modeling of the electric field distribution during the microplasma oxidation process. The A1 alloy 1100 was used as the material for research. It is shown that ellipsometry is very sensitive to processes on the surface during MAO even at the earliest stages of oxide layer formation. Based on the data of the ellipsometry experiment, several planar models of the surface structure were constructed, and their parameters were determined at different stages of oxidation. The constructed models and their parameters (thickness, phase composition, and optical constants) are in good agreement with the results of independent measurements and do not contradict the physical concepts of the processes occurring during MAO. For the first time, it was proposed to use piecewise models for interpreting ellipsometric measurements of film structures non-uniform over the surface. The proposed model adequately describes the structure of MAO coatings at all stages of the technological process. It makes it possible to use ellipsometry for non-destructive non-contact express control of the state of these coatings during production without resorting to additional more complex analytical methods (XRD, SEM, etc). When analyzing ellipsometric data, estimates for the optical constants of mullite (Al 5 SiO 10− x ) in the terahertz range were obtained for the first time. It is also shown that the main results of the work can be used in the analysis of MAO coatings on other metals. • THz ellipsometry is found to be an efficient non-destructive method for MAO coatings characterization. • Information about the coating parameters was obtained, consistent with the data of structural and other independent studies. • Estimates for the optical constants of mullite in the sub-THz region are obtained. • A new model is proposed for the analysis of ellipsometric measurements of coatings non-uniform over the surface.