Friction Properties of Microarc Oxidation Coating on Titanium Alloy Modified by Ni Nanoparticles
Titanium alloys have the advantages of light weight,high specific strength,and good corrosion resistance,but the disadvantages of low hardness and poor abrasion resistance limit their applications.Ceramic-like oxide coatings metallurgically bonded with the matrix can be prepared on titanium alloys by microarc oxidation(MAO),improving the hardness and friction resistance of the titanium alloys.However,these oxide coatings are usually composed mainly of TiO2,and there are many discharge holes and bulges,so the density,hardness,and abrasion resistance of MAO coatings are not good enough.The MAO coatings were modified by adding Ni nanoparticles in the MAO electrolyte of TC4 alloy.On the one hand,the densification of the coating could be increased;on the other hand,the excellent toughness of Ni could enhance the hardness and brittleness of the MAO coatings.Finally,the abrasion resistance of the MAO coatings could be improved.The effects of the amount of Ni nanoparticles in the electrolyte on the surface microstructure,cross-sectional morphology,phase composition,and elemental composition of the MAO coatings were analyzed by scanning electron microscope(SEM),energy dispersive spectrometer(EDS),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS).A Vickers hardness tester and a friction and wear testing machine were used to detect the hardness and friction performance of the MAO coatings prepared in different electrolytes.The morphology and volume loss of the MAO coatings after the friction test were studied by 3D topography instrument.The results showed that the thickness and compactness of the MAO coatings increased with the increase of the content of Ni nanoparticles in the electrolyte.The oxide coatings prepared in different electrolytes mainly comprised rutile TiO2 and anatase TiO2.The Ni nanoparticles added in the electrolyte participated in the MAO coating formation process and existed in the coatings in the form of NiO.When the amount of Ni nanoparticles was 2 g/L,the surface micropores of the MAO coatings were evenly distributed,and the pores and cracks were few.The MAO treatment increased the hardness of TC4 alloy.The hardness of the MAO coatings prepared in the base electrolyte for 45 min was about 420HV.The MAO coatings obtained in the electrolyte containing 2 g/L nickel nanoparticles for 45 min had the highest hardness(about 452HV).The friction coefficient of the base MAO coatings changed significantly during the friction experiment,while the addition of Ni nanoparticles could stabilize the friction coefficient of the MAO coatings.The base MAO coatings was worn through in the friction test,and the addition of Ni nanoparticles reduced the wear volume of the oxide coating.The MAO coating formed in the electrolyte with 2 g/L Ni nanoparticles was the best,its friction coefficient as stable at 0.75,and the wear volume was reduced by 67%compared with the base MAO coating.Adding an appropriate amount of Ni nanoparticles in the electrolyte and a reasonable MAO time can reduce the micropores and microcracks of MAO coatings,change their phase composition,and further improve the hardness and abrasion resistance of MAO coatings.
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