Preparation And Biological Performance Of HA Coatings Fabricated By Laser Rapid Forming On Cp-Ti

Pure titanium and titanium alloys possess excellent mechanical properties and biocompatibility, but no bioactivity. As the implant material, they cannot induce the chemical integration with the host bone tissue, and their fixation only relys on the mechnical interlocking. The Ca-P ceramic possess excellent bioactivity, therefore they can induce the steady chemical integration with the host bone tissue. However, the Ca-P ceramics is not strong to bare the physical load. Coatings technology has the ability to take advantage of both Ti and Ca-P ceramic. Until now, many methods have been applied in preparation of Ca-P coatings on titanium and titanium alloys, but there are some problem need to resolve, such as low bonding strength and poor-bioactive phase. Laser cladding series technology can produce a sound metallurgical bonding between Ca-P coatings and metallic substrate, however the bioactive phase is less and poor stability. In this study, the Laser Rapid Forming （LRF） technology was applied to fabricate the Ca-P coatings on the pure titanium with mixed powder of CaHPO₄·2H₂O and CaCO₃ as raw material. The effect of technological parameter, such as the ratio of Ca to P in raw material, the laser power, the scanning velocity and the heat-treat condition, on the phase composition of coatings were studied. Under the optimized process, the HA coatings on the pure titanium was obtained. The mechanical properties, the physical and chemical properties and the biological properties of the HA coatings were evaluated. At the end, a new method was formed to fabricate the HA coatings on pure titanium and the experimental basis was built for the further research and application of this fabricating methods.Methods:1. The coatings were fabricated by the different parameters and the phase compositions of coatings were analyzed by the means of XRD and EDS. The effects of the ratio of Ca to P in raw material, the laser power, the scanning velocity and the heat-treatment condition on the phase composition of the coatings were revealed.2. Under the optimized parameter, the HA coatings on pure titanium were fabricated and the microstructure, the distribution of elements, the phase composition and the mechanical properties were characterized by the means of XRD, EDS, SEM and mechanical tests.3. The static immersing test was performed to analyze the behaviors of the HA coatings in simulated body fluid （SBF）. At the same time, the pure titanium was tested as control group, which were subjected to sand-blast and acid pickling beforehand.4. The osteoblasts were cultured on the surface of the HA coatings. The adherence and proliferation of the osteoblasts were investigated using MTT assay and the spreading was observed by the means of SEM. In addition, the titanium coated with the HA by LRF were implanted into the femur of the experimental rabbits. After the healing period of 4, 8 and 12 weeks, the interfaces between the host bone tissue and the surface of implants were observed and evaluated by histomorphometric methods. In the above tests, the surface-treated titanium was tested as control group.Results:1. The ratio of Ca to P in the mixed powder of CaHPO₄·2H₂O and CaCO₃ can affect remarkably the phase composition of the coatings fabricated by LRF. When the ratio was 2.0, the phase composition of the fabricated coatings was made up of HA,α-TCP and TTCP, which all possess bioactivity.2. In the lower range of laser power, from 450 w to 550 w, the less HA in the fabricated coatings was decomposed and the content of bioactive ingredient was high. On the other hand, the effect of the scanning velocity on the phase composition of coating was not remarkable.3. The optimized parameters were obtained as follow: the ratio of Ca/P was 2.0, the laser power was 450 w, the scanning velocity was 200 mm/min, the overlap rate was 35 %, the speed of powder feeding was 2.0 g/min, and the diameter of laser was 3.0 mm. The coatings fabricated by these parameters contained HA,α-TCP and TTCP. The surface of the coating was uneven and contained many micro pores. The metallurgical bonding between the coatings and the Titanium substrate was clearly observed. From the substrate to the surface of the coatings, there were three layers which were Titanium substrate penetrated by P （Ti₃P）, CaTiO₃ layer （transitional layer） and ceramic layer. The hardness in the Ti3P layer was the highest at 1263.0±35.3 HV_0.1, the hardness in ceramic layer was the lowest at 482.2±20.3HV0.1.4. Theα-TCP and TTCP in the coatings fabricated by the optimized LRF parameters can be transformed to HA under the condition of heating to 800℃and keeping for 6 h. After heat-treatment, the bonding strength between the coatings and the substrate was 24.50±0.72 MPa.5. After immersing in SBF, the coatings on the Titanium released some Ca and P ion and induced HA particles to deposit on the coatings surface.6. The coatings on the pure titanium improved the adherence, the proliferation and the ALP activity of osteoblast at the early stage. The osteoblast spread well on the coatings surface.7. The titanium implant with the coatings improved the regeneration and the reconstruction of the host bone tissue. At the same time, the coatings combined with the new bone tightly.Coclusion:Under the optimized process, LRF combined with heat-treatment, the HA coatings on pure titanium can be obtained. The coatings bond to the substrate well, and possess the excellent bioactivity which can improve the adherence and the proliferation of osteoblast, improve the regeneration and the reconstruction of the host bone tissue and can combine with the new bone tightly.