Electrophoretic deposition and characterization of thin hydroxyapatite coatings formed on the surface of NiTi shape memory alloy

The NiTi surface was modified so as to improve its biocompatibility by forming multifunctional protective layers of TiO2 and hydroxyapatite (HAp). A special focus was placed on obtaining relatively thin (similar to 2.4 mu m), crack-free and homogenous HAp coatings using the electrophoretic deposition method (EPD). A well-dispersed and stable colloidal suspension with the predominant particles size of ca 100 nm was used for the deposition process. The coatings were obtained at the following deposition parameters: 40 V /120 s and after vacuum-sintering at 800 degrees C by 2 h providing to an increase of the surface roughness. It was revealed that the composite coatings had adhesion features of (15.1 +/- 1.4) MPa and crack resistance to deformation of 2.48%. Moreover, no layer spalling was found, even at the maximum deformation value epsilon=3.53%. The newly manufactured coatings were also observed to have no impact on the martensitic transformation responsible for shape memory effect. Additionally, a two-step martensitic transformation below room temperature was observed.