Near surface martensitic transformation and recrystallization in a Ti-24Nb-4Zr-7.9Sn alloy substrate after application of a HA coating by plasma spraying

Plasma sprayed hydroxyapatite (HA) coatings on titanium alloy substrate have been used extensively due to their excellent biocompatibility and osteoconductivity. However, the influence of the high temperature process on substrates can not to be ignored, especially for metastable β type titanium alloy substrates. In this paper, martensitic transformation and recrystallization occurred in near surface of the low-modulus Ti-24Nb-4Zr-7.9Sn alloy substrate after application of a HA coating. Various analyses revealed that the martensitic transformation and recrystallization were mainly related to the high temperature and cooling process. The different microstructures can be attributed to inhomogeneous temperature and cooling rate distributions in the substrate. Corresponding to the microstructure, the Young's modulus and micro-hardness of the near surface of the alloy showed a similar graded distribution along the direction perpendicular to the coating/substrate interface. In addition, the martensitic transformation and recrystallization caused remarkable changes of the entire alloy.