Carbon in oxide layers formed under electric discharge conditions

The presence of carbon on the surface and in deep layers of oxide coatings produced by plasmaelectrolytic oxidation is studied. Graphite is found in the coatings formed on titanium under the conditions of cathodic polarization in an organic electrolyte. Carbon in the amount of 16–40 at % is present on the surface of coatings produced on aluminum at the anodic polarization in aqueous electrolytes. Compounds with C-O (oxidized carbon), C-C or C-H (aliphatic carbon), and C-M (carbide carbon) bonds are determined. The carbon content decreases to a few atomic percents upon etching the surface with a high-energy argon beam and removing a nearly 3-nm layer. Aliphatic and metal-oxide carbon is present at this depth. The carbon content on either the surface or in deep layers of the coatings is the same, regardless of the stage of the coating formation, namely, prespark, spark, or microarc anodizing.