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

Abstract

Due to their low specific weight and strength, titanium alloy products offer advantages over steel in aircraft and shipbuilding, space technology, damping systems, etc. However, their poor frictional properties significantly limit their application. This article proposes a method to eliminate this drawback of titanium alloys by modifying their surface layers using an improved technology of chemical-thermal treatment (CHT) in a gas environment with controlled oxygen and nitrogen content, as well as a subsequent justification for improving their performance under oscillating friction conditions. VT6 titanium alloy specimens were held at 750°C with limited oxygen access and controlled nitrogen supply. This prevented the formation of uniform layers of titanium dioxide or nitride, which penetrated only to a depth of 1–2 µm. Layers of solid solution-hardened material with a gradient in oxygen or nitrogen content formed directly beneath these layers. The microhardness of the layers reached the level of untreated titanium at depths of up to 29–30 µm. The wear track profiles on top of the surface-hardened layers revealed minor (from 3 μm without lubrication to 1 μm with boundary lubrication) depressions from counterfaces (steel or ceramic balls). During friction, hard titanium dioxide or nitride particles removed fine chips from the surface of the treated specimens and changed the wear track profile from relatively smooth to irregular. Despite the higher microhardness of the nitride layer, its wear resistance was 40-50% worse than that of the oxidized layer, and this was independent of friction conditions (counterface material, conditions, or lubricant type). This is explained by the introduction of super hard TiN particles into the contact zone on the surface of both the hardened layer and the counter body. Without lubrication, the friction coefficient f between the steel ball and both hardened layers remained unchanged during the tests, whereas with the ceramic ball it gradually increased due to the increasing role of the ceramic counter body itself in the friction process. During friction in the boundary lubrication mode, the friction coefficient remained constant due to the washing out of solid wear products from the friction zone.