Surface Properties and In Vitro Bioactivity of Fluorapatite/TiO2 Coatings Deposited on Ti Substrates by Nd:YAG Laser Cladding
Fluorapatite (FA)/TiO2 composite coatings were deposited on Ti–6Al–4V substrates with an Nd:YAG laser cladding process. Two TiO2 powder phases, namely anatase (A) and rutile (R), were used. After cladding, the FA/TiO2(R) specimen had a rougher surface morphology than that of FA/TiO2(A). Both coatings had a cellular-like main microstructure near the interface of the coating (CL) and transition layers (TL). However, a fine metallurgical bonding state was found that existed between CL and TL of the FA + TiO2(R) specimen. The X-ray diffraction (XRD) analysis results show that the coatings of both specimens were composed principally of FA, CaTiO3, and Al2O3phases. With the high-energy–density laser cladding process, a portion of the FA in the original coating material remained, while all of the TiO2 powder was decomposed and reacted with the Ca in the FA (Ca-rich phase) to produce CaTiO3. Upon immersion of the clad specimens in simulated body fluid, apatite grew more rapidly on the FA/TiO2(R) coating than on the FA/TiO2(A) coating. The Ca/P ratio of the FA/TiO2(R) specimen approached the ideal bioactivity value after just 2 days of immersion. In contrast, that of the FA/TiO2(A) specimen did not reach the ideal value until 7 days of immersion. Furthermore, a peak corresponding to hydroxycarbonated apatite (HCA) appeared in the XRD patterns of both specimens. For the FA/TiO2(R) specimen, this HCA peak appears after a shorter immersion time. The FA/TiO2(R) specimen with a rougher surface morphology had better in vitro bioactivity than that of FA/TiO2(A).