Tribological Properties of Micro-arc Oxidized TC4 Titanium Alloy under the Action of Oil-soluble Organic Molybdenum Additives

Titanium and its alloy have been introduced into automobile manufacturing due to their excellent mechanical properties and relatively low density. However, titanium alloy has poor tribological performance even with efficient lubrication. Thus, surface engineering is widely used to enhance the wear protection properties of titanium alloys. In this paper, high hardness oxide films were constructed on the surface of TC4 titanium alloy by means of micro-arc oxidation (MAO) technology in electrolyte contacting silicate and phosphate. The structure of the MAO coatings was analyzed by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. XRD patterns of the MAO coatings suggest a layer of titanium oxide in the form of rutile and anatase on the TC4 substrate after the MAO process. MAO-treated TC4 substrate was characterized by the typical morphology, i.e., a uniform distribution of pores of different sizes on the surface. The MAO coatings possessed higher hardness than the TC4 substrate. The tribological experiments were conducted to test the tribological properties the MAO treated TC4 under lubrication of PAO base oil containing molybdenum dialkyldithiocarbamate (MoDTC) additive, a commonly used friction modifier. It was found that the addition of MoDTC to the base oil did not significantly change the friction performance of untreated TC4, suggesting that MoDTC was not effective in reducing the friction coefficient for the bare TC4. However, this was not the case for the TC4 samples after the MAO treatment. Specifically, the friction coefficient of the MAO-coated TC4 sample lubricated with the base oil was stable at 0.11 during the entire sliding period. After the addition of 2% MoDTC to the base oil, the friction coefficient was further reduced to ~0.05 after a short running-in period, suggesting best matching effort for the surface MAO coatings with MoDTC additive. XPS analysis results implied that a large amount of MoS₂ as well as MoO₃ and MoS₂ was found on the wear track, and a small amount of metallic molybdenum was even observed, which were not found in the steel/steel system. This observation suggested that a complete decomposition reaction of MoDTC occurred in the MAO-treated TC4 samples. In other words, the surface MAO coating may exhibit a tribocatalytic effect, resulting in the complete decomposition of MoDTC to MoS₂ during the sliding process and leading to reductions in the friction coefficient and wear rate. Thus, the present study demonstrated that that micro-arc oxidization was an efficient method to enhance the wear resistance of TC4 significantly due to the formation of a hard oxide film. MAO oxide film further facilitated the formation of an effective lubricating film containing MoS₂ to reduce friction and wear in the presence of base oil containing MoDTC. Our findings are expected to promote the wider use of titanium alloys in energy-efficient engine systems.