Influence of tantalum pentoxide secondary phase on surface features and mechanical properties of hydroxyapatite coating on NiTi alloy produced by electrophoretic deposition

In the current research, the electrophoretic deposition (EPD) process has been applied to create the natural hydroxyapatite (nHA)-tantalum pentoxide (Ta2O5) coating on the NiTi substrate with the various contents of Ta2O5 of 0, 10, 15, and 20 wt.%. The subsequent sintering step has been conducted in the two temperatures of 850 and 800 degrees C to investigate the coating composition. The microstructure and roughness of sintered samples at 800 degrees C were examined using the field emission scanning electron microscope (FESEM) and optical profilometer respectively. The positive effect of Ta2O5 presence and its content augmentation in the nHA matrix on the corrosion resistance of NiTi was approved through determining the concentration of released Ni ions from the coated substrate in phosphate buffered saline (PBS). Then the effect of Ta2O5 secondary phase content on the mechanical performances of the nHA layer on the NiTi was interpreted by micro-indentation responses of samples. According to the achievements, the hardness of nHA coated NiTi was increased about 4, 12, and 19 times by the addition of 10, 15, and 20 wt.% of Ta2O5 respectively. Moreover, the elastic modulus of nHA was raised favorably by Ta2O5 enhancement as reached to 32.85 GPa in nHA-20 wt.% Ta2O5 coating, the closest value to that of human bones. At last the bonding behavior of coatings was assessed by considering the critical contact pressure parameter magnitude measured from nano-scratch data.