Hydroxyapatite coated orthodontic shape memory Ni-Ti alloys and biocompatibility

In this thesis, the biocompatibility of Ni-Ti (Nitinol) alloy coated with hydroxyapatite (HA) was investigated. Many thin film coating methods have been developed in order to be able to do thin film coating with ceramic materials. In this study, RF Sputtering and Sol gel methods (dipping and spin coating) were used. It is aimed to increase the biocompatibility of the Ni-Ti alloy with the HA coatings. In the coatings performed with the RF Sputtering method the thicknesses were determined with respect to the coating time and 3, 6 and 9 hours coatings were done for three different samples. Four different coatings at different thicknesses were done with Sol gel dipping method. The coating thickness was proportional to the number of dipping and after each coating process, to dry the samples a heat treatment for 1 hour at 100 oC were done. Subsequently, the samples were sintered for 1 hour at 500 oC. In the spin coating methods, the coating thicknesses were determined according to the number of revolutions per second and two different rotation speeds, which are 300 and 1350 rpm, were used. The coated samples were heat treated at 100 oC for 1 hour and then at 500 oC for 1 hour. The HA solution used for the coating was prepared using calcium nitrate tetrahydrate (Ca(NO3)2.4H2O) and ammonium dihydrogen phosphate (NH4H2PO4) precursors. The optimum conditions (martensite-austenite phase transformation temperature) where the Ni-Ti alloy exhibits shape memory effect were determined by DSC measurements.The morphological properties of the coating surfaces were investigated by SEM-EDX analysis, and the crystallographic structure of HA was determined by XRD and FTIR analysis. In the dip coating method, it has been observed that the cracks form on the surface as the coating thickness increases and it will have a negative effect on biocompatibility because of the release of harmful Ni ions to the body from these cracks. However, these surface deformations were not observed in the coatings done by spin coating method, and more homogeneous coatings were obtained compared to the dipping method.