| HA is an excellent biomaterial known for its’ good bioactivity and the similarity of ingredients to human bone tissue. It is widespread used in the research of titanium surface modification, because HA can build a stable chemical bo nd with bone tissue.For the purpose of improving the bioactivity of titanium medical implants, the morphology and composition of the surfaces were modified. In this study, Ca HPO4.H2O/CaCO3 powders were used to produce HA composite layer in situ synthesis, fabricated by laser cladding with presetting method. Well-formed HA coating, with tight metallurgical bond between the coating and substrate, was obtained by optimizing the process parameters, just like the ratio of powder, laser power and scanning speed. The optimized process parameters were as followed: Ca/P=2:1,laser power 350 W, scanning speed 0.001m/s. The HA layer was translucent milk white, continuous uniform distributing on the titanium surface without any obvious drawback.The microstructures and formed phases of the composite layer are studied by XRD, SEM, and EDX. Results indicate that the main phases of HA layer are massive or mesh pattern HA, a little Ca Ti O3 and Ti P. From the HA layer to titanium matrix, the transition of microstructures are as followed: HA+Ca Ti O3→Ti P→CaTi O3→TC4+Ti3P→TC4. Since there are several transition layer between the HA layer and titanium matrix, it can remit the stress concentration phenomenon effectively and the metallurgical bonding was fine.During the processing of laser cladding, HA was produced in situ synthesis. Elements Ti would diffuse into the layer and form a little CaTi O3;Elements Ti would spread into the matrix and form a bit Ti P. The interdiffusion would form a metallurgical bond between the HA layer and titanium matrix, but it was really sensitive to the process parameters. Lager heat increase will lead to the result of most elements P diffusing into the matrix and forming Ti P, leaving CaTi O3 and Ca O in the layer, which makes it difficult to creating HA.The microhardness of the composite coating was tested. The result shows that the hardness is very high around the bonding surface between the layer and matrix. It maybe caused by the differences of linear expansion coefficient between layer and matrix during the rapid cooling. However, the hardness decreased slowly from the bonding surface into the matrix due to the phase Ti P formed, which relief the cracking tendency effectively. Wettability was tested by the contact angle tester with SBF, the result of which shows that the contact angle of layer is clearly smaller than that of matrix. So after the laser claydding HA layer, the wettability of titanium was improved obviously.After soaking in SBF for 1,2,3 weeks, the sediments on the surface of HA layer changed in 3 stages: granulations→loosened net structure→gathering net structure→sphere structure on large bulks. The soaking experiment shows that HA layer has the ability to introduce Ca2+ and HPO4- to deposit on its’ surface. And this ability offer a basic condition for osteoblast to produce bone tissue on it. The samples of HA coating were implanted in leg bone tissue of healthy male SD rat for 30 days. It was found that HA composite layer has can induce the osteoblast grow and develop compact bone matrix tissue on it. And the osteoblast cells are in vigorous life state. Therefore, the HA layer created in our experiment has got the good bioactivity. |
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