Role of polymorph microstructure of Ti6Al4V alloy on PEO coating formation in phosphate electrolyte

In this work, the formation behavior of PEO coatings on polymorph Ti6Al4V alloy with two different lattice structures (hexagonal close packed α-Ti and cubic body centered β-Ti phases) was studied, using two galvanostatically controlled processes. The phase formation as a function of treatment time was studied ex-situ locally on the two Ti phases by interrupting the treatment after certain time intervals and in-situ continuously in an integral manner by recording diffraction patterns from the changing surface during the PEO process. The initial discharges were preferred to start on β-Ti phases localized at the grain boundaries and then extends to the surrounding α-Ti grains. The presence of α- and β-Ti phases was also responsible for the different local coating surface morphology. A highly sintered morphology and larger-sized micro-pores developed firstly on the layer on β-Ti phases, with higher concentration of vanadium. These findings highlight the importance of microstructure of Ti6Al4V alloy on the formation of PEO coatings. Under the selected conditions of PEO processing, the higher current density promotes the growing of the coating and reduces the surface roughness of the resultant coating. The ex-situ characterization of the coatings after certain periods shows that the final coatings mainly consist of anatase, rutile and an amorphous phase containing phosphorous. The formation of rutile, transformed from anatase, is influenced by the effective temperature of the discharges, thus the ratio of anatase to rutile is decreasing with treatment time. The in-situ characterization of phase evolution showed the formation of an amorphous phase in parallel with an expanded α-Ti phase, which is a result of oxygen incorporation into solid solution into the titanium lattice. Interestingly, the main formation of crystalline phases occur only when the HV is turned off and the coating obviously cools down.