Formmation Mechanism And Tribological Behavior Of Microarc Oxidation Coatings On Ti6Al4V Alloy

In order to obtain a good antifrictional coating on surface of Ti6Al4V alloy, Si-P-(Al or Mo), P-F-Al and Al-C coatings were prepared in the corresponding electrolyte system by microarc oxidation (MAO) method, and the optimal MAO parameters were obtained. Microstructure of the coatings was characterized using XRD, SEM, EPMA, XPS and TEM techniques. The formation mechanism of MAO coating was investigated. The tribological properties of the coatings were examined in sliding and fretting contact conditions.Experimental results show that MAO coating is compact in the inner layer, puff in the outer layer, with many micropores (pore size 2~5μm) on the surface. The growth rate and microstructure of the coatings are mainly dependent on discharging energy of a single pulse in the same electrolyte. Compared with constant current density mode, oxidation with constant voltage mode gives the faster growth of the coating, but with a much puffer surface. Applying a constant current density combined with a stepped adjustment of electrical parameters, the growth rate of the coating increases, and more important is that in this case the coating is more compact, with almost no cracks on the surface. Considering the good surface morphology and high growth rate, the optimal MAO parameters were selectedThe microstructure of coatings depends mainly on the type of electrolyte. Si-P-(Al or Mo) coatings are mainly composed of nano-structured (<50nm) rutile and anatase TiO2, with a small amount of SiO2 and other amorphous compounds. The P-F-Al coating consists of nano-scaled (<60nm) rutile TiO2 and AlPO4. The Al-C coating is mainly composed of lath-shaped Al2TiO5 (100 nm wide and several hundred nm in length), with a small amount of Al2O3 and Ti2O in the inner layer.The element P is predominantly concentrated in the inner region of the coating, indicating that the transport of PO43- ion follows a short-circuit path of the discharge channel through the coating to reach the inner region of the coating in vicinity of the coating / substrate interface. Thus new coating products form within the inner layer close to and at the coating / substrate interface. The melting products formed in the discharge channels are rapidly cooled and solidified to deposit on the inner wall of the channels. However, the temperature...