PHASE COMPOSITION AND WEAR RESISTANCE OF COATINGS FORMED ON VT6 (TI–6AL–4V) TITANIUM ALLOY BY PLASMA ELECTROLYTIC OXIDATION

Abstract

The study covers the growth kinetics of the coating formed on VT6 titanium alloy by plasma electrolytic oxidation (PEO) at a specified current density of 10 А/dm2 in the alkali aqueous solution containing 40 g/l of NaAlO2. Coatings with varied thicknesses (30, 80 μm) formed on VT6 titanium alloy was tested for wear resistance by the «pin-on-disc» test using the Hightemperature tribometer and WYKO NT1100В optical surface profiler. Relationships between phase coating composition and PEO process duration, as well as wear resistance are determined. The mechanisms of coating thickness growth that explain its kinetic features are suggested. The mechanisms are as follows: 1) migration and diffusion of metal cations towards the outer phase boundary on sections adjacent to microdischarges; 2) thermochemical transformation of deposited ions or polyanions, in particular, tetrahydroxyaluminate; 3) high-temperature oxidation of the metal substrate at the bottom of coating pores where plasma anode microdischarges occurred. The considered equivalent scheme of the anodic component of the alternating current at titanium alloy PEO allows us to understand the causes of a significant decrease in the initial coating growth rate at VT6 alloy PEO without anodic voltage reduction. The peculiarity of this scheme is the presence of rheostats since the flow resistance of alternate current components depends largely on the PEO process time. It is shown that the presence of a high-temperature modification (α-Al2O3) in the TiAl2O5 spinel coating makes it possible to increase VT6 alloy wear resistance by almost 6 times when the coating thickness is ~80 μm.