The Study Of The Structure And Properties Of Pulsed Laser Deposited Hydroxyapatite Thin Films On TC4 Alloy

Hydroxyapatite (HA) films coated on Ti6A14V (TC4) alloy can combine the high strength and toughness of the metallic substrates and the bioactivity of HA. In this work, HA films were prepared on TC4 alloy by pulsed laser deposition. Influence of the parameters of PLD, including pressure, laser output power, deposition time, depositing atmosphere and substrate temperature on the microstructure was studied. And effect of heat treatment on the microstructure, bonding strength and biological activity was investigated. The mechanism in which the crystallinity of the laser nitriding substrate was improved was primitively discussed.The surface of HA films was compact and consisted of many particles without any cracks when they were deposited under 45Pa Ar atmosphere. The results of XRD and FTIR showed the film was amorphous. The Ca/P ratio of the film was higher than theoretical value. During deposition, the atmospheric pressure was higher, the particle numbers were more and the Ca/P ratio was lower. The droplets and the surface roughness were increased when the laser output power was higher or the deposition duration was longer.After heat treatment, the amorphous film transformed into the crystalline state, and the Ca/P ratio of the surface became lower, which was closer to the theoretical value of HA, what was more, the film was more hydrophilic. But due to the mismatch in the thermal expansion coefficients between the substrate and the film, cracks could be discovered when the temperature of heat treatment was higher (700℃), and then the bonding strength was lower. When the films were immersed 5 days into Hank's solution, there were some pores at the droplet places at the surface of the amorphous film due to the dissolution. There were many small reprecipiated apatite grains on the big droplets which were at the surface of the annealed films, and the Ca/P ratio decreased. The result of FTIR showed there were no changes about the kinds of groups.There were fewer droplets when the films deposited by pulsed laser under 45Pa H2O atmosphere comparing with 45Pa Ar atmosphere, because H2O could participate the reaction and the H2O pressure could decreased. The influence of different substrate temperatures showed no obvious changes, but it could affect the crystallinity. The temperature was higher, the crystallinity was also higher. The TiN dendrite with three-dimensional network prepared by laser nitriding was beneficial to the nucleation for HA particles, the film on which had a higher crystallinity, and the Ca/P ratio was more closer to the correct value.