Effect Of Carbon Nanotubes On Tribological Properties Of Micro-arc Oxidation Ceramic Film On TC4 Titanium Alloy

With the continuous increase of car ownership and the increasing attention to environmental pollution in various countries,the emission standards for automobile exhaust are becoming more and more stringent.Automobile lightweight design has become one of the important ways for major automobile manufacturers to solve the problem.Titanium alloy is becoming a high-quality replacement material for automotive parts because of its high specific strength,light weight,corrosion resistance and other excellent properties.The key parts of the engine made of titanium alloy can reduce the inertia mass,decrease the friction force and improve the fuel efficiency of the engine.On the other hand,the size and the mass of the engine and the whole vehicle can be decreased with increased the rotational speed and output power of the engine.Therefore,today,when the automobile is developing in the direction of lightweight,environmental protection and energy saving,titanium alloy is undoubtedly the most potential key parts material of automobile engine.However,titanium alloys have high adhesive wear and abrasive wear,and show poor tribological properties even under lubrication conditions,which limits the direct application of titanium alloys.In this paper,a hard wear-resistant TiO₂ ceramic layer was constructed on the surface of TC4 titanium alloy by micro-arc oxidation technology,and carbon nanotube(CNTs)additive was used to further improve its corrosion resistance and tribological properties.The effect of MoDTC additive on the tribological properties of MAO film was studied.The results show that:(1)The effect of CNTs concentration in electrolyte on the surface morphology and mechanical properties of micro-arc oxidation film on TC4 titanium alloy was investigated.When the concentration of CNTs is 0.15 g/L,the surface micro-arc oxidation film has the lowest porosity,the smallest roughness and the highest surface hardness due to the incorporation of CNTs.The friction coefficient and wear rate are reduced by about 40%and 50%compared with the samples without CNTs.However,with the further increase of the concentration of CNTs to 0.2 g/L in the electrolyte,the porosity and roughness of the film increased,while the wear resistance decreased.(2)The effect of the concentration of CNTs in electrolyte on the corrosion resistance and wear resistance of micro-arc oxidation film on the surface of TC4 titanium alloy in3.5%NaCl solution was studied.The micro-arc oxidation film obtained in the electrolyte containing CNTs concentration of 0.15 g/L has the best corrosion performance,and its self-corrosion potential has a positive shift of 500mV compared with the TC4 substrate with the positive shift of about 100 mV and the reduced self-corrosion current phase by55%.The open-circuit potential is positive by 300 mV,and the wear rate is reduced by24%.In the seawater immersion test for 60 days,no pitting corrosion was found in the samples of micro-arc oxidation,which showed excellent corrosion resistance.(3)The antifriction and wear resistance of common lubricating oil additive MoDTC on micro-arc oxidation film on the surface of TC4 titanium alloy were investigated.Compared with the untreated TC4 samples,under the lubrication of PAO base oil containing 2%MoDTC,the friction coefficient decreased by 87.4%and the wear rate decreased by 99.8%.This is mainly due to the fact that the TiO₂ oxide film formed on the surface of TC4 titanium alloy after arc oxidation treatment improves the surface hardness and wear resistance,promotes the tribochemical reaction of MoDTC additives under boundary lubrication conditions,and forms a lubricating layer containing MoS₂ on the surface of the contact zone,which greatly reduces friction and wear.