Features of Ceramic Coating Formation by a Method of Microspark Oxidation in an Electrolyte Based on Boric Acid
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Vol. 61, No. 3, September, 2020
FEATURES OF CERAMIC COATING FORMATION BY A METHOD OF MICROSPARK OXIDATION IN AN ELECTROLYTE BASED ON BORIC ACID M. A. Markov,1,4 Yu. A. Kuznetsov,2 A. V. Krasikov,1 A. A, Slobodov,3 A. D. Bykova,1 and S. N. Perevislov1 Translated from Novye Ogneupory, No. 5, pp. 50 – 55, May, 2020.
Original article submitted February 11, 2020. Results of theoretical and practical studies of the formation of functional ceramic coatings by a method of microspark oxidation in an electrolyte based on boric acid are presented. Good chemical stability of an oxide film in the borate electrolyte is noted, It is shown that treatment of an aluminum surface for 2.5 h at a current density of 5 A/dm2 leads to formation of a ceramic coating with porosity of not more than 3% and thickness of about 50 mm impregnated with corundum. The main factors affecting borate electrolyte durability in the process of aluminum microspark oxidation are determined. Keywords: microspark oxidation (MAO), oxide coating, borate electrolyte, current density, electrolyte durability.
The electrolyte is a key factor governing properties of the MSO coating formed. There is most extensive use in industry of MSO for aluminum alloys combined with a silicate-alkaline electrolyte of the alkali – Na2SO4 type. Within this composition alkali is an activator that exhibits an agitation capacity that makes it forma strengthened layer within an oxidized surface of relatively nominal size of a component and to improve coating adhesion. Water glass acts as a passivator and by being absorbed creates a film on metal with high resistance, due to which during anodic polarization spark discharge development becomes possible. In silicate-alkali electrolyte at the surface of aluminum there is formation of a coating complex with respect to phase composition including high-temperature modifications of Al2O3 and mullite 3Al2O3·2SiO2. A coating may be considered as a composite within which corundum is a strengthening phase, although at the oxide layer surface there is formation of a film of amorphous SiO2, treated as a loose production layer requiring grinding [4]. The microhardness of these coatings on wrought aluminum alloys reaches 16 – 25 GPa [5]. As a passivator in MSO electrolytes it is possible to use salts of some acids, for example borates, citrates, tartrates, and sometimes succinates and acetates [6]. Borate electrolyte is most preferable in view of good stability (towards oxida-
A fundamentally new step on the path of creating coatings with good physicomechanical properties on aluminum alloys is transfer from traditional anodizing to preparation of oxide ceramic coatings under conditions of a spark discharge by a microspark oxidation (MSO) method [1 – 3]. The essence of MSO includes the fact that on a component of valve metal or alloy located in an electrolytic bath an anodic current is fed means of a special supply source leading to an increase in thickness of a natural oxide film existing upon it. In this case there is a marked increase in voltage
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