Design, Fabrication And Tribological Behavior Of MAO/DLC Composite Coatings On Titanium

Titanium and its alloys are widely used in aviation,warships and other fields,but their low surface hardness and poor tribological properties limit their application in the field of mechanical transmission.In order to improve the tribological properties of titanium alloys,direct current(DC)/alternating current(AC)micro-arc oxidation(MAO)coatings and MAO/DLC composite coatings were designed and prepared on the surface of titanium by the methods of AC/DC micro-arc oxidation and physical vapor deposition(PVD).The effects of electrical parameters of AC MAO on the growth behavior of the coatings were investigated,and the microstructure and tribological properties of AC and DC MAO/DLC composite coatings were also studied in this paper.The results show that there are many micro-pores on the surface,and the inner layer is dense,the outer layer is loose for both AC and DC MAO coatings.In the electrolyte system with NaAlO2+NaOH as the basic composition,the thickness,the surface roughness and micropore size of AC MAO coating increase with the increasing of pulse voltage and duty cycle.The surface roughness and micropore size of AC MAO coating increase with the rising of pulse frequency,while the coating thickness increases first and then decreases.The microstructure of AC/DC MAO coatings were improved by depositing DLC on their surfaces.The results show that the surface micropore size of AC/DC MAO coatings can be decreased and the surface roughness and porosity of the coatings can be reduced by depositing DLC.Finally,a denser and smoother MAO/DLC composite coating was obtained.However,the effect of DLC deposition is different for the MAO coating with different surface morphology.When the surface of MAO coating is flat and the micropore size is small,it can be found that a better coating is formed on the surface of MAO coating.But on the surface of MAO coating with higher surface roughness and larger micropore size,the DLC presents obvious discontinuous distribution characteristics.The tribological properties of AC MAO coatings with different surface morphologies was quite different.The results show that the higher the surface roughness is and the larger the micropore size is,the higher the friction coefficient and wear rate of MAO coating are.The tribological properties of AC MAO coating are significantly improved by depositing DLC,but the effect depends on the surface morphology of MAO coating.When the surface roughness and micropore size of MAO coating are small,the stable friction coefficient can be reduced from 0.85 to 0.24,and the wear rate can be reduced by 71%.In contrast,the friction coefficient can be reduced from 0.77~0.97 to 0.67,and the wear rate can only be reduced by 9%.The microstructure and tribological properties of DC MAO/DLC composite coating were also studied.The results show that the surface roughness and porosity of DC MAO coating are significantly reduced by depositing DLC.Under the load of 1 N,the stable friction coefficient of DC MAO/DLC composite coating is about 0.15,and the wear rate is approximately zero.Under the load of 10 N,the friction coefficient of DC MAO/DLC composite coating is about 0.18,and the wear life of DC MAO/DLC composite coating is obviously higher than that of DLC coating.