| In the present paper, plasma electrolytic oxidation of Ti-6Al-4V alloy was carriedout under constant voltage or current regimes in electrolyte system of Na2SiO3and(NaPO3)6. The waveforms of the electric pulses were recorded during the PEOprocesses, surface and cross sectional morphologies and phase compositions of thecoatings were investigated in detail to reveal the coating growth mechanisims.Electrochemical tests, dry sliding wear and nanoindentation tests were also performedto identify the corrosion performance, wear resistance and hardness of the coatings.Finally, the difference in the mechanisms of PEO of dissimilar valve metals wasinvestigated by comparing the growth efficiency and morphology of PEO coatings ofTi and Zr alloy. The following conclutions were drawn from this study:(1) Unlike other valve metals, Ti-6Al-4V alloy was more suitable to be treatedwith lower duty cycles. Reduce in the duty cycle can increase the peak value ofcurrent pulse, which was necessary to maintain the PEO process of Ti alloy as thecoating thickened.(2) In the first10minute of PEO treatment, the film grows rapidly with anaverage rate of3.3μm/min under constant voltage of600V, then slowed significantlyto less than0.2μm/min by30minute, after which the rate remained approximatelyconstant. The film of Ti alloy contains rutile, anatase and large amounts of amorphousmaterial, which may be related to the presence of considerable amount of silicon andphosphorus species. Rutile is enriched by extended times of treatment. The filmproduced by constant current regime is thicker than that by constant voltage regime.The thickness of the coating with constant current regime is~79μm compared to~44μm of the coating with constant voltage regime after60min of PEO.(3) Sliding wear test proved that either coating with constant voltage or constantcurrent regimes have improved the wear performance of Ti alloy, and a long treatmenttime produced improved durability. The improved wear resistance could be ascribed tothe high ratio of hardness to elastic modulus (H/E) of PEO film which have highhardness and low elastic modulus. Nanoindentation measurements proved that thehardness of the out layer can reach~6.1GPa, while in the inner layer, the hardness ofthe coating was reduced to~3.8GPa, which is close to~3.5GPa of the substrate. Theelastic modulus of the coating varies from~48.5GPa in the inner layer to64.6GPa in the out layer, which are much less than the substrate (88.8GPa). The H/E ratio isincreased from inner layer to out layer.(4) Electrochemical tests showed that the passive current densities of the coatedsamples were lower about1-2orders than that of the Ti substrate, which means bettercorrosion resistance of the coatings. However, the corrosion resistances for coatingsformed using relatively short times of treatment are better than that of long timestreatment, which was indicated by the high passive current density values for longertreatment times. The long times treatment was probably attributed to the increasedporosity and other defect of the coatings.(5) The kinetics of coating growth and morphologies of PEO coatings ofTi-6Al-4V alloy is quite different compared to that of Zr alloy under the sameelectrolytic system. The coating growth rate of Ti alloy was quicker than that of Zralloy at the beginning but it was slowed down significantly and falled behind that ofthe Zr alloy as the coating thickened. Thus, the coating growth efficiency is muchlower for the Ti-6Al-4V alloy than that of Zr alloy. The lower efficiency of PEO of Timay ascribe to its tendency to liberation of gas instead of sparking.The morphology analysis shows that the surface of PEO film of Ti-6Al-4Vreveals pores, cracks and coral-like features, however, the PEO film of Zr alloyreveals pores, cracks but a lot of pancake-like features which were very dissimilar tothat of Ti. The different features were ascribed to different types of discharge duringPEO. |
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