| The composites of titanium alloy with bioceramics coating, mainly containing hydroxyapatite (HA), have been developed for the hard tissue implant for their superior bioactivity and mechanical properties. Because these composites are thought to be the most promising substitute of hard tissues, many efforts are made to develop new cladding techniques, improve their properties and so on. Based on those facts that HA can be synthesized during sintering the proper mixture of some Ca-P-containing salts, and laser beam is widely used in coating techniques, a new method was proposed to synthesize and clad the HA-based bioceramic coatings on titanium alloy with laser beam. Additionally, a few amount of Y2C>3 were added into the coating.The surface of titanium alloy (TC4) was remelted with laser beam beforehand, then mixed powders of 80%CaHPO4'2H2O and 20%CaCO3 with 1% Y2O3 in addition were preplaced on it. During laser-treated once more, HA was surely synthesized and a RE-containing bioceramic coating was cladded on the substrate at the same time. The phases in this coating are: Ca10 (P04), (OH) ^ 0 -Ca3(PO4)^ a -Ca3(PO4)2> Ca2P207 ^ Ca2Y2O4 and CaY4O7. The optimal laser processing parameters recommended in this paper are: power density=13~15W/mm2, scanning rate=630mm/min. With fine grains and porous top, the morphologies of the coating are similar to that of natural bone. The compositions, microstructures and mechanical properties graded vary along the depth direction. And the mechanical properties of this bioceramic-TC4 composite are excellent. Compared with natural hard tissues, its strength and hardness are much higher, and it exhibits some ductility and toughness as well. Furthermore, the biomechanical compatibility of the implant made of the composite can be improved for the coating possesses an elastic modulus between TC4 and natural bones.As illustrated by biocomparability experiment in vitro, the coating has not toxic effects on muscles, red cells and fibroblasts. And acute toxic reactions were not observed in experimental mice having been injected the coating material in large dose. So its excellent compatibility with blood and tissue was well proved. Implanting experiments were also performed in dogs. It was found that both the substrate and coating are biocomparable in vivo. The substrate TC4 is bioinert, so it will wholly packed with connective tissues after a certain period in vivo. Whereas, the coating is bioactive, it doesn't affect the activity of osteoblasts and osteoclasts, and it isbiodegradable induced by cells. 60 days later in vivo, new bones were found to form on the coating, which means it is osteoconduct.Yttrium is helpful to improve phase compositions, microstructures and properties. During laser heating, Y2O3 didn't melt as discovered by numerical simulation of the laser temperature field. So they are apt to sink in the laser pool for their larger density, and Y mainly segregate in interface area as a result, which was ascertained by chemical composition analysis. These yttrium can restrain the substrate elements from moving to coating, and decrease the dilute of coating, which is beneficial to the forming of HA and other calcium phosphate bioceramics. Moreover, DSC analysis confirmed that Y2O3 could catalyze the forming reaction of HA, i.e., decreasing its forming temperature, and that Y could increase the stability of HA and P -TCP at elevated temperature. Those Y retaining in coating make the grains finer so increase its mechanical properties. The Y-containing coating has an interface bonding strength of 42.76MPa with substrate, more than three times as high as that of Y-free coating. Additionally, yttrium can also improve corrosion-resistance of the coating in acid, alkali and salt mediums.
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