Martensitic transformation in TiNi alloy after surface modification done by hydroxyapatite layer deposition

A hydroxyapatite (HAp) layer was deposited on the surface of an NiTi alloy using a spark plasma technique. The effects of spraying conditions on the martensitic transformation were investigated in three heat-affected zones: the zone under the handle, the transition zone and in a representative zone with HAp coverage. In the representative area, a reduction in the amount of non-transformable phases was observed. A part of the sublayer showed a nanocrystalline or completely amorphous structure. This sublayer was approximately 30 mu m thick. Structural changes contributed to the reduction of the temperature of the reverse martensitic transition down to 25 degrees C. Moreover, modification of the surface revealed a positive influence on the transformation course by narrowing its temperature range and increasing its enthalpy.