Phase Composition and Wear Resistance of Coatings Formed on the VT6 Titanium Alloy by Plasma Electrolytic Oxidation

Abstract

The growth kinetics of the coating during the plasma electrolytic oxidation (PEO) of the VT6 (Ti–6Al–4V) alloy with a specified density of 10 A/dm2 in an alkaline aqueous solution containing sodium aluminate (NaAlO2) in an amount of 40 g/L is investigated. The wear resistance of coatings of different thicknesses (30 and 80 μm) formed on the VT6 alloy is studied by the “pin-on-disc” test using a “High-temperature tribometer” automated friction machine and WYKO NT1100B optical profilometer. The dependences of the phase composition of coatings on the PEO duration and wear resistance of coatings on this composition are established. The following growth mechanisms of the coating thickness, which explain the growth kinetic features, are proposed: (i) the migration and diffusion of metal cations to the external phase interface in segments adjoining the microdischarges; (ii) the thermochemical transformation of deposited ions or polyanions, in particular, sodium tetrahydroxoaluminate; and (iii) high-temperature oxidation of the metallic base of the bottom of through coating pores, in which the plasma anodic microdischarges were implemented. The considered equivalent circuit of proceeding the anodic component of the alternate current during the PEO of the titanium alloy allows one to understand the causes of a substantial initial decrease in the coating growth rate during the PEO of the VT6 alloy without lowering the anodic voltage. The feature of this circuit is the presence of rheostats, because the flowing resistance of alternate current components depends largely on the PEO duration. It is shown that the presence of the high-temperature modification (α‑Al2O3) in the coating based on spinel TiAl2O5 makes it possible to increase the wear resistance of the VT6 alloy almost sixfold if the coating thickness is ~80 μm.