Electroless Ni–P alloys on nanoporous ATO surface of Ti substrate

Followed by well-known AAO templates, anodic titanium oxide (ATO) surface with nanoporous structure has gained extensive attraction as implants due to its excellent surface properties. So, the objective of this work was to explore a novel approach for electroless Ni–P film on nanoporous ATO surface, with an aim to obtain a strong bonding interface between Ti substrate and its surface films. Impressively, with anodizing DC potential extending from 100 to 300 V, a two-step anodizing process was created to achieve nanoporous ATO surface with a suitable diameter size that acted as bonding pinholes for amorphous Ni–P growth. Structural design of electroless Ni–P film onto ATO surface was innovatively addressed for increasing interfacial bonding strength between Ti substrate and its surface film. Regarding the ATO surface with nondestructive characteristics, several pretreatments were processed with anodizing, sensitizing, activating, following by electroless Ni–P film. Among them, Pd-reducing crystal sites, which were capable of effectively adsorbing on ATO surface, were emerged as preferential locations for catalytic growth of Ni–P film to make pinning effects toward ATO nanopores. Results indicated that the well-organized ATO nanoporous arrays were successfully processed onto the surface of Ti substrate through the mixed sulfuric–phosphoric acids electrolyte at DC 210 V. When nanosized Ni–P grains were planted inside nanoporous arrays of ATO surface, it was expected as growing sites to refine the subsequent Ni–P growth and thus resulting in higher compactness of electroless Ni–P film on ATO surface. Further studies of the laser-assisted MAG welding on the as-received ATO surface within electroless Ni–P film were conducted, thereby achieving a superior metallurgical bonding for Ti/Ni–P interface. More important, this was the first attempt to design nanoporous arrays for Ni–P growth on ATO surface to increase its weldability of Ti alloys, also supporting a new guideline for multi-functional films on porous surface of the as-anodized Ti, Mg, and Al alloys.