Preparation And Properties Of Composite Coating Formed Via Micro-arc Oxidation On Pure Ti With Doping CNTs/B₄C

Titanium has excellent properties such as high strength to weigh ratio,high temperature resistance and good biocompatibility.Unfortunately,pure Ti has low surface hardness and poor wear resistance,and its corrosion resistance needs to be further improved under harsh conditions.Therefore,coating can be prepared on the surface by micro-arc oxidation to improve its wear and corrosion resistance.Boron carbide(B₄C)has low density,high strength,good high temperature stability and chemical stability.Carbon nanotubes(CNTs)have excellent mechanical,electrical and thermal properties.Adding B₄C and CNTs as additives to micro-arc oxidation ceramic coating on pure titanium can further improve the microstructure,wear resistance and corrosion resistance of the coatings,which has broad application prospects.In this paper,pure titanium was used as matrix.Firstly,in aluminate system,the parameters of micro-arc oxidation,such as pickling time,electrical parameters(current density,pulse frequency,duty cycle)and oxidation time,were optimized.Under the optimum technological parameters,micro-arc oxidation ceramic coatings in different electrolyte systems were prepared.In aluminate system,TiO₂-B₄C composite ceramic coatings and TiO₂-CNTs composite ceramic coatings were prepared.The effects of electrolyte composition,the doping content of B₄C particle,the doping mode and content of CNTs particle on the properties of the coatings were studied by SEM and XRD characterization,microhardness,adhesion,wear resistance and corrosion resistance test.The result shows that the coatings in different electrolyte systems had honeycomb porous structure.The penetration of micro-pore on the surface of P-coating was obvious,and the micro-pore on the surface of Si-coating was interlaced layer by layer.Both P-coating and Al-coating consisted of rutile TiO₂ and anatase TiO₂,while Si-ccoating consisted only of anatase TiO₂.P-coating had the largest microhardness(375.3 HV)and the strongest adhesion(21.3 N).The order of wear resistance of the coatings in different electrolyte systems was P-coating>P/Al-coating>P/Si-coating>Al-coating>Si-coating.P-coating had the longest wear-resistant time(64.39 min)and the narrowest wear-scar width(555.20?m),showing the best wear resistance.The order of corrosion resistance of the coatings in different electrolyte systems was:Si-coating>P-coating>P/Si-coating>P/Al-coating>Al-coating.Si-coating had the largest self-corrosion potential(-191.22 m V),the largest polarization resistance(4.14×10⁴?·cm²),and the smallest corrosion current density(4.56×10^-6 A·cm²),showing the best corrsion resistance.For TiO₂-B₄C composite ceramic coatings,with the increase of B₄C particle doping content,the surface of TiO₂-B₄C coating gradually smoothed.When B₄C particle doping content was too high,cracks and other defects appeared.The surface of TiO₂-0.9B₄C coating was the densest and the micropore size was the most uniform.TiO₂-B₄C coating was composed of rutile TiO₂,anatase TiO₂ and B₄C.With the increase of B₄C particle doping content,the microhardness and adhesion of TiO₂-B₄C coatings firstly increased and then decreased,the wear resistance and corrosion resistance of TiO₂-B₄C coatings firstly enhanced and then weakened.TiO₂-0.9B₄C coating showed the best comprehensive performance.The TiO₂-0.9B₄C coating had the largest microhardness(268.8 HV)and the strongest adhesion(22.6 N).It had the longest wear-resistant time(19.24 min)and the narrowest wear-scar width(384.53?m),showing the best wear resistance.Its wear mechanism was abrasive wear and fatigue wear.It had the largest self-corrosion potential(-213.38 m V),the largest polarization resistance(5.47×10⁴?·cm²),and the smallest corrosion current density(2.37×10^-6 A·cm²),showing the best corrsion resistance.According to the two time constants of the Bode diagram,the coatings were composed of a dense inner layer and a loose outer layer.For TiO₂-CNTs composite ceramic coatings,after acidification,the length-diameter ratio of CNTs particles decreased,the superhydrophobicity of CNTs particles became hydrophilic,and oxygen-containing functional groups were introduced into the CNTs particles,which increased the solubility and dispersion of CNTs particles in solvents.The addition of CNTs particles made the coating smoother and denser.The TiO₂-CNTs coating was composed of rutile TiO₂,anatase TiO₂ and CNTs.The TiO₂-A-0.05CNTs coating and the TiO₂-A-0.1CNTs coating showed better comprehensive performance.The TiO₂-A-0.1CNTs coating had the largest microhardness(236.6 HV).The TiO₂-A-0.05CNTs coating had low porosity,small pore size and small size fluctuation.It was not completely damaged after 20 minutes friction test.And the surface debris was the least and the size was uniform,showing the best wear resistance.The TiO₂-A-0.05CNTs coating had the largest polarization resistance(7.47×10⁴?·cm²)and the smallest corrosion current density(6.78×10^-7A·cm²),showing the best corrosion resistance.Due to its lamellar structure,the anti-shearing force of the TiO₂-A-0.05CNTs coating and the TiO₂-A-0.1CNTs coating was reduced.So the adhesion was slightly smaller than that of the TiO₂-AC-0.1CNTs coating.