Microstructure and tribological properties of microarc oxidation coatings formed on Ti6Al4V alloy in Na₂SiO₃ system aqueous solution

An AC power supply was used to perform microarc oxidation in Na₂SiO₃-(NaPO₃)₆ - Na₃MoO₄ aqueous solution so as to prepare a ceramic oxide coating on the surface of Ti6Al4V alloy. The microstructure and the morphology of the coating were analyzed by means of X-ray diffraction and scanning electron microscopy, while the tribological properties of the coating in sliding against GCr15 steel were investigated on a pinon-disc friction and wear tester. A nanoindenter was employed to measure the hardness of the dense layer of the microarc oxidation coating, while the morphology of the worn coating surface and the elemental composition of the worn coating surface and wear debris were observed and determined with a scanning electron microscope and an energy dispersive spectrometer, respectively. The results show that the coating about 20 μm thick are dense and homogeneous, mainly composed of anatase and rutile titanium oxide. The dense layer of the coating has a maximum hardness about 13.5 GPa. The microarc oxidation coating in sliding against the steel registers a friction coefficient as low as 0.18-0.20 at an applied load of 0.5 N and the sliding cycle below 2000, which is much smaller than that of the Ti6Al4V alloy against the same steel counterpart. The softer GCr15 steel is liable to transfer onto the coating surface during the sliding, while the microarc oxidation ceramic coating is characterized by slight abrasive wear and adhesion wear.