Study On The Mchanism Of Cathodic Plasma Electrolytic Deposition On Ti6Al4V

The more extensive application of Ti6A14V in engineering and biomedical field is limited for its poor wear resistance and bioactivity.As one subclass of the newly developed plasma electrolytic deposition technology(PEDT),Cathodic plasma electrolytic deposition(CPED)provides a new promising way to modify the surface properties of Titanium and its alloy.With comparison to anodic plasma electrolytic deposition(APED),which mainly referring to plasma electrolytic oxidation(PEO),the other subclass of PEDT,CPED has many advantages.It could deposit multicomponent functional coatings on nearly any kind of metals with the aids of proper design of the electrolyte component.To enclose the mechanisms of the ceramic coating formation and the changes of surface properties,the temperature field on the surface of the cathodic sample in CPED is the key knowledge.However,the temperature of the cathodic surface was hard to measure by general test methods because of the discharge on the surface and local melting of the surface.So it is necessary and significant to calculate the temperature on surface in CPED.In this work,the physical model,the heat transfer equation and its boundary conditions of sample-plasma envelop-electrolyte in direct current source powerd CPED(DC-CPED)was established.The temperature of the cathodic surface,which was the same with that of the interface between the sample and the plasma envelop in the eatablished four models were calculated.The method for the measurement of the surface temperature was designed according to the simulation results of the internal distribution of the temperature in the cathodic sample.It was found that when the vaporization heat of the electrolyte and the heat absorbed by the supplementary electrolyte were introduced into the calculation of the heat flux density,the calculation results were consistent with the experimental data on the promise that the thermal conductivity of envelope was the function of temperature and the electrical conductivity of envelope was constant.As for the pulsed powered CPED(P-CPED),Two models were established to discuss the different conditions of heat conduction on the sample.The calculated results fit the experiment data better,when the heat exchange between the sample and the vapor envelop be ignored during the power-off stage.When the pulse voltage was fixed,the temperature of the sample rose with the increase of the duty cycle.The frequency had little effect on the temperature of the cathodic sample.The simulation results of the temperature field in the single micro-discharged channel showed the extreme high temperature 18000℃ and 8000℃ is located at the center of micro-discharged channel and the radial location 3μm to the center,respectively.It elucidated the formation mechanism of the coating and the mcro-discharged channel.The possible formation of amorphous and nano-crystalline in the coating also could be inferred.Ti(C,N)(-TiO2)coating was prepared by DC-CPED and P-CPED,the optimum parameters of the lateral one were applied into the fabrication of HA-Ti(C,N)-Ti02 coating.Their formation mechanism and the evolution of their structure and properties with the change of parameters were discussed on the base of the calculated results of cathodic temperature.Afer being treated with 220V,the thickness of the Ti(C,N)increased with the growth of the duration time of DC-CPED.Accompanying the process of CPED,the micro-discharged channels turned more,bigger,and conjected to each other,then formed the depression zone and the island-shaped buldges or spheroids on the surface of the coating.All samples showed better wear resistance and corrosion resistance after the DC-CPED,among which,the one treated with 50min has the best properties with one order of magnitute scale lower wear volume losss and corrosion current density than those of substrate alloy respectively.The temperature of cathodic kept growing with the increase of the applied voltage.Keeping the time as 20min,when the voltage grew to 260V,the temperature reached 1025℃,the phase of anatase formed in Ti(C,N)coating.The existence of amorphous and nano-crystalline Ti(C0.3N0.7)in the coating of this sample was verified by the HRTEM analysis.Among the samples treated by DC-CPED,the best wear resistance and corrosion resistance were obtained after 20min duration when the applied voltage was 260V.After been treated by P-CPED in 20min,Ti(C,N)-TiO2 coatings formed on all the samples,which were also characteriazed as porous,depressions and buldges.Nanocrystalline were confirmed to consist the bulges.The rising temperature in P-CPED increased the thickness of Ti(C,N)-TiO2 coating and its wear resistance.However with the increase of temperature,the crossion resistance of the coating acsendent first and then drop.The better properties of samples treated by P-CPED,were got at 1142℃ and 1212℃,corresponding to the voltage of 300V and 320V respectively,which was chosen for the synthesis of HA-Ti(C,N)-TiO2 coating.The addition of calcium nitrate and sodium dihydrogen phosphate in formamide electrolyte contributed to the HA formation in coatings.The formed HA distributed homogenously on surfaces and partially filled in the pores.They displayed as clusters,which were congregated by nano-rods at 1142℃ and by nano-plate at 1212℃ individually.The friction coefficient of HA-Ti(C,N)-TiO2 coatings were slightly higher than that of the substrate,but lower than that of HA free coatings.The wear resistance and corrosion resistance were better than those of Ti(C,N)-TiO2 coatings.The best properties could be obtained for HA-Ti(C,N)-TiO2 coatings at 1142℃ after 30min P-CPED.