Dependence of growth features of microarc oxidation coatings of titanium alloy on control modes of alternate pulse

Abstract

In order to optimize the production regime for effective coatings of required thickness on Ti6Al4V surface, a comparative study was made on the kinetics of ceramic coatings growth, and was associated with the evolution of microstructure and phase composition during microarc oxidation (MAO) under constant voltage and constant current density regime, respectively, using an Ac pulse power supply in Na 2SiO 3–KOH–(NaPO 3) 6 solution. The thickness of the resulting coatings was measured using a coating thickness gauge based on eddy principle. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were employed to characterize the microstructure and phase composition of the coatings. The results show that as treatment time increases, the coating grows in an exponential function accompanied by a gradually decaying current density for constant voltage control mode ( U=500 V), For constant density control mode ( J=60 mA/cm 2), voltage increases rapidly up to a relatively stable value and the coating grows linearly; compared with the constant voltage mode, the coating grows faster. Phase composition and microstructure evolution during MAO process are almost independent of control mode. With increasing treatment time, the predominant phase composition varies from anatase to rutile, which indicates that phase transformation of anatase into rutile occurs in the duration of oxidizing process. Meanwhile, the size of micropores existing on the coating surface increases and thus the surface becomes much coarser.