Optimization Of Preparation Technology And Characterization Of Hydroxyapatite Coatings On Carbon/Carbon Composites

Carbon/Carbon (C/C) composites when used as biomaterials have excellent biocompatibility, with their elastic modulus close to that of human bone and thus can effectively reduce the bone absorption induced by stress shielding. However, C/C composites are bio-inert; the surface of C/C composites without any treatment is hydrophobic. The dissociative carbon can flow with body fluids and thus cause "black skin effect" due to the carbon deposition within the skin. Hydroxyapatite (HA) has excellent biocompatibility and bioactivity. Its components and crystal structure are similar to the mineral phase of bone. So HA has been widely used in clinic application. However, the bulk HA material has obvious brittleness and other limited mechanical properties. For example, fatigue damage can easily occur under physiological environment. So HA can not be used as implant materials for load-bearing applications. In this paper, HA and C/C composite materials were studied, moreover, they were combined together through plasma spraying method and thus an artificial bone substitute material with good properties can be expected to be fabricated.The main research works include: Firstly, HA powders were prepared with chemical deposition method integrated with hydrothermal synthesis method. The effect of reaction temperature and concentration on the crystallinity and morphology of the prepared HA powders were also studied. In order to avoid the formation of calcium-deficient apatite, the effect of Ca/P ratio of raw materials on the prepared HA powders was studied. Ultrasonic waves was introduced during the process of preparation of HA powders through chemical deposition method. Moreover, the effect of ultrasonic waves on the crystallinity and morphology of HA was studied. Secondly, sandblasting treatment was performed on the surface of C/C composite materials by using a small sandblasting equipment used for dental application. C/Ccomposites with different surface morphology and roughness were prepared through different sandblasting technology, and the optimum sandblasting technology was determined based on the shear strength between the HA coating and C/C substrates. HA coatings on C/C composites were fabricated by using a 3710 type of atmospheric plasma spray equipment made in America; the phase compositions and microstructures of the HA coatings were also studied. Finally, related animal experiment and the manufacture of lumbar fusion cage are developed. The results show that HA powders were synthesized by chemical precipitation method with Ca (NO₃) 2·4H₂O and (NH₄) ₂HPO₄ as raw materials. There are no impurity diffraction peak are detected by XRD in HA phase prepared under different water-bath temperature, which indicates that the higher purity of HA powders are produced. The crystallinity of the HA powders without heat treatment and the length/diameter ratio of HA crystal grain increase with the increase of water-bath temperature. Therefore, higher reaction temperature can promote the growth of HA crystal. And the maximum compressive strength of HA is in the range of 50℃～60℃ of the water-bath temperature.No impurity phases are detected by XRD in HA prepared under different reaction concentrations, so the higher purity of HA is also produced. The crystallinity of HA powders prepared under the low concentration is much higher because the low concentration can promote the growth of HA crystal.During the preparation of HA powders, the high purity of HA powders can be obtained through increasing the Ca/P ratio of raw materials properly.The crystallinity of HA powders prepared by ultrasonic waves and mechanical stirring is much higher than that of HA powders prepared only by mechanical stirring before and after heat treatment. Moreover, the crystal grains of HA prepared by using double stirring are uniform, and spherical HA crystal is gradually obtained. Furthermore, the HA powders were perfectly dispersed. After heat treatment, the sizes of crystal grains of HA are also uniform.The samples were sandblasted by 1500 mesh of SiC powders with the sandblasting air pressure of 0.2 MPa. Four kinds of sandblasted surface were prepared as follows: (1) rough sandblasting of the face surface; (2) rough sandblasting of the side surface; (3) fine sandblasting of the face surface; and (4) carbon fiber puncture visible on the face surface. The shear strength between the HA coatings and the C/C composites were reduced as follows: (1)>(4)>(2)>(3). As above, it can be seen that the shearing strength of the fourth surface morphology is higher, and its rough degree is lower. So, the fourth surface has an more ideal sandblasting morphology.The HA coatings on C/C composites were carried out by using a Plexus 3710 type of plasma spray equipment made in America. The main spraying parameters included 30 KW of spray power, 70mm of spray distance and 200mm/s of gun displacement speed, respectively. The particle sizes of HA powders are chosen to be 38～75μm.XRD results showed that the quantity of constituents phase contents of HA coating on C/C substrate were as follows: HA, α-TCP (tricalcium phosphate) and β-TCP. Moreover, the crystallinity of HA coating is lower, and thus the degradation speed might be higher when it is implanted in organism.The surface morphologies of HA coatings were affected by that of C/C substrates. The molten HA particles impacted to the substrate, and their refrigerated morphologies were cake-like, flat-like or irregular shape. Due to the high difference of thermal expansion coefficient between the HA and the C/C composites, there are a lot of microcracks appeared in the HA coatings. The higher magnification SEM imagines indicated that the HA particles are composed of nanosized-microcrystals.Mechanical bond was the main mode of bond between HA coating and the C/C substrates. HA particles come into the interspace between the fibers, and formed a high efficient bond under the effect of plasma spraying. The damage of the fiber bundle caused by the sandblasting treatment were favorable to the interaction of the molten HA particles and the fibers, and thus improved bond strength of the interface between the HA coatings and the C/C substrates.The results of cell toxicity testing in vitro indicated that the cell toxicity of C/C and C/C+HA were accorded with the national standard and criterion for third class of medical appliance.C/C+HA composites were implanted in the bone tissue of New Zealand rabbits. The histological observation results after 90 days of implantation showed that the materials were favourable for health, no inflammatory reactions were found and the coatings combined to the substance firmly. The osteoblast had grown on the interface between the coatings and the bone. All of these results indicated that as a new bone-repairing material, C/C+HA composite has more preferable feasibility in clinical application.The shape and size of lumbar fusion cage was designed, and the specimens were fabricated. Besides, the HA coatings was sprayed on C/C substrate, which is prepared for the further animal experiments and clinical applications.