Microstructural evolution and biological properties of PEO coating on SLM-prepared NiTi alloy

Selective laser melting (SLM)-prepared NiTi alloy is considered to have great application potential in the fabrication of complex-structured metal implant materials. However, the SLM-NiTi alloy still suffers from some disadvantages, such as poor bio-corrosion resistance, bio-compatibility, and health hazards caused by the release of nickel ions. In order to improve the surface biological properties of SLM-NiTi alloys, bio-active coatings rich in Ca and P elements were prepared on SLM-NiTi alloys by PEO process in this work. Confocal laser scanning microscopy, SEM, EDX, XRD, and XPS were utilized to feature the micro-structure, thickness, element information, and composition of SLM/PEO coatings at different oxidation times. The adhesion strength was determined using scratch testing. A potential polarization test and a simulated body fluid (SBF) immersion test were applied to assess the corrosion resistance and detect the biological activity of SLM/PEO coatings at varied oxidation periods, respectively. The results showed that the growth of SLM/PEO coatings was hindered due to the overflow of Ni elements at the spark discharge stage (stage II). However, with the PEO process, the inhibition effect of Ni element was gradually eliminated. The pore size, thickness and the TiO2 rutile phase content of the SLM/PEO coatings gradually increased over time. However, the coating density, roughness and adhesion strength increased first and then decreased, and the SLM/PEO coatings were maximized at 15 min. The main composition of SLM/PEO coatings was TiO2, CaO, CaHPO4 and Ca3(PO4)2, and hydroxyapatite (HA) was gradually formed in the coating with time. The SLM/PEO-15 min coating with high thicknesses and rutile phase content, as well as optimum density provided optimum corrosion resistance (1.615 × 10−6 A cm−2). Meanwhile, the SLM/PEO-15 min coating possessed the best surface roughness and higher active element content, providing an excellent nucleation point and growth environment for the deposition of apatite. And the Ni element on the SLM-NiTi alloy substrate surface (oxidation time from 0 to 20 min) was significantly reduced from 12.49 % to 0.52 % after immersing in SBF solution for 24 h. Therefore, the PEO process can effectively improve the corrosion resistance and biological activity of SLM-NiTi alloy and inhibit the release of Ni elements.