Comparison of tribological properties of VT6 titanium alloy surface after oxidation and nitriding

Abstract

Titanium alloys offer advantages over steel due to their low specific density and high specific strength. This makes them indispensable in products for the aerospace, marine, and damping industries, among others. However, their poor frictional properties significantly limit their applications. This study proposes a way to minimize this disadvantage of titanium alloys by modifying their surface layers using an advanced thermochemical treatment (TCT) in a gas environment with controlled oxygen and nitrogen contents. Surface layers of VT6 titanium alloy were modified at 750°C with limited oxygen access and a controlled nitrogen flow. This TCT mode limited the thickness of TiO2 or TiN uniform layers formed on the surface of the alloy to 1–2 µm. Immediately beneath them, layers of solid-solution-hardened material with a gradient of oxygen or nitrogen content formed. Their microhardness gradually decreased, reaching the level of untreated titanium at a depth of 29–30 µm. To confirm the effectiveness of TCT, a friction test method was used under conditions of oscillatory sliding of steel and ceramic balls (as counterbodies) on a hardened surface. Wear profiles revealed small indentations from spherical counterbodies into the surface-hardened layers (from 3 to 1 µm under dry friction and boundary lubrication conditions, respectively). Hard particles of titanium dioxide or nitride altered the wear track profile on the surface of the TCT samples from relatively uniform to textured due to the selective removal of small chips. Despite the higher microhardness, the nitrided layer turned out to be 40-50% worse than the oxidized layer in terms of wear resistance, and this did not depend on the friction conditions (counterbody material, friction conditions, or the type of lubricant used). This was explained by the charging of the surface layers of the samples with superhard TiN fragments that entered the friction zone. Under dry friction, the friction coefficient between the steel ball and the surface- hardened VT6 alloy remained stable, but increased when using a ceramic ball. Under boundary lubrication, the friction coefficient remained constant by removing wear debris from the friction zone. Overall, the tests showed that controlled surface oxidation of the VT6 titanium alloy provides superior wear resistance compared to nitriding.