Effect of Pre-Oxidation on Microstructure and Wear Resistance of Titanium Alloy by Low Temperature Plasma Oxynitriding

Titanium alloys are used in the aerospace industries, chemical industries, biomedicine, marine ships and other fields because of their high strength-to-weight ratio, good corrosion resistance, and biocompatibility. However, titanium alloys have low hardness and poor wear resistance, which limit their applications, especially under sliding contact. Plasma nitriding (PN) is an effective method for improving titanium alloy's tribological properties. PN can produce a composite layer composed of TiN and Ti2N to improve the friction and wear properties of titanium alloy. However, the high temperature in the nitriding treatment results in a brittle "alpha-stabilized layer" (a continuous layer of alpha phase titanium enriched with interstitial nitrogen atoms) and unfavorable phase transformations in the substrate that can impair the fracture toughness, ductility, and fatigue properties. In this study, a low-temperature plasma-composite treatment, consisting of plasma oxidizing and oxynitriding, has been developed to improve the hardness and wear resistance of Ti-6Al-4V alloy. The effect of the preoxidation process on the surface microstructure, phase composition, and wear performance of titanium alloy was studied. The microstructure and phase composition of the plasma-composite layer were observed using SEM, TEM, XRD, and other methods. The results showed that the composite layer of titanium alloy treated using plasma-composite-treatment is composed of compound and diffusion layers, and the phase is TiO2 (rutile) and TiN0.26. Microhardness and wear-resistance properties of the composite layer were characterized using microhardness tester, nanoindentation, and reciprocating-friction tester. The plasma-composite treatment can increase infiltration depth of the diffusion layer, surface hardness, elastic modulus, and wear resistance of the titanium alloy more than the traditional nitriding process.