Structure and properties of Al₂O₃ coatings formed on NiTi alloy by cathodic plasma electrolytic deposition

Al₂O₃ coatings were prepared on NiTi alloy by cathodic plasma electrolytic deposition (CPED) in an aluminum nitrate solution. Scanning electron microscopy (SEM) shows that the coating exhibits a typical porous surface, rough micro structure and excellent adhesive interface between the coating and the substrate. Energy dispersive X-ray spectrometer (EDS) shows that when the treatment time is 45min, this ceramic coating is composed of Al, O, and C with the atomic concentration of 43.37%, 48.52% and 8.11%, respectively, and the concentration of Ni is lower than the minimum instrument detection standard. X-ray diffraction (XRD) indicates that the coating consists of α-Al₂O₃ and γ-Al₂O₃ crystal phase. Direct pull-off test shows that the coating on NiTi substrate has a mean coating-substrate bonding strength of 15±2MPa. Cross-sectional hardness measurement illustrates that the average microhardness of the coating is less than the values of this parameter for the substrate. The results of potentiodynamic polarization test indicate that the corrosion resistance of the coated sample is increased by 3 times compared with that of the uncoated sample on the basis of corrosion current density.