| Calcium phosphate ceramics have become of prime importance for biological applications in the field of hard tissue repair and reconstruction, and their mechanical properties, solution and degradation behaviors, etc. are still immediate areas of research focus. In this work, hydroxyapatite (HA) and amorphous calcium phosphate (ACP) ceramic powder are synthesized by chemical coprecipitation process, and the ACP/HA powder mixture are prepared via mechanically mixing and suspension mixing method, respectively. Moreover, the sintering behaviors of the powder mixture are studied during the ordinary pressureless sintering, microwave sintering and hot-pressing sintering processes. In addition, physical, mechanical and dissolution properties of the sintered bodies are investigated from the view of the crystallization of ACP.The Ca/P ratio, pH value, reaction time are the major factors influencing the nature of the product for the chemical precipitation of ACP. The results show that ACP is formed when the Ca/P ratio of the reacting solution is1.50,1.67and2.0and the ACP would transformed into beta tricalcium phosphate (β-TCP) after the heat treatment at900℃.The adjusting of pH value during the reaction process has not affected the formation of ACP if the pH values of corresponding reacting solution were adjusted to11before the reaction. Additionally, the reaction time kept within30min is advantageous in obtaining of ACP powder.ACP/HA powder mixture with ACP content of25,50and75wt.%are prepared by mechanically mixing and suspension mixing methods. The ACP powder have morphologies of hollow sphere with diameters of20～50μm in both of mechanically and suspension mixing ACP/HA powder mixture, and the HA powder are composed of granular grains with diameters of100nm in the former mixture while rod-like grains with length of20～40nm in the latter. The specific surface area and total pore volume of suspension mixing ACP/HA powder mixture is larger than that of mechanically mixing one with the same composition due to the difference of HA morphology. Moreover, the specific surface areas and total pore volume are increased as the fraction of ACP increased in the powder mixture prepared via the same method.Biphasic calcium phosphate (BCP) ceramics consisting of β-TCP and HA are obtained after the mechanically and suspension mixing ACP/HA powder mixture are sintered via ordinary pressureless sintering, microwave sintering and hot pressing sintering. In addition, the weight ratios of (3-TCP in the BCP ceramics are roughly equal to the weight ratios of ACP in the ACP/HA powder mixture, indicating that the phase content of BCP ceramics can be tailored via the control of ACP content in the powder mixture.Dense BCP ceramics with limited grain size can be generated when the pressureless sintering is carried out at1150℃for2h. The grain size of β-TCP ceramics made from ACP powder and HA ceramics made from HA powder are5μm and1μm, respectively, while BCP ceramics made from the ACP/HA powder mixture with the ACP mass fraction ranging from25to75%consist of two kinds of grain sizes. The larger grains are about5～10μm, the smaller are1～2μm and they distribute inhomogeneously in the pressureless sintered BCP ceramics when the ACP/HA powder mixture are synthesized through mechanically mixing method. Moreover, the amount and size of pore are increased as the ACP content increases. On the contrary, the larger grains are only2～3μm, the smaller are still about1μm and they distribute homogeneously in the BCP ceramics made by suspension mixing ACP/HA powder mixture. The amount and size of pore are decreased as the ACP content increases.Presintering affects the phase content and microstructure of BCP ceramics when microwave sintering is carried out on the ACP/HA bodies. It increase the content of β-TCP phase in the BCP ceramics made by suspension mixing ACP/HA powder and leads to plenty of pores in all of BCP ceramics. The grain sizes are smaller than1μm.Grains with size of5μm and with straight boundaries are found in the hot pressing sintered BCP ceramics made by mechanically mixing ACP/HA powder (ωACP=25～75wt.%). Conversely, grain sizes are not larger than2.5μm if the ACP/HA powder mixture are prepared via suspension mixing method. In addition, there is nearly on pore in all of hot pressing sintered BCP ceramics.In general, the mechanical properties including hardness, fracture toughness and compressive strength of sintered bodies made of suspension mixing powder are all enhanced from those of mechanically mixing powder under the same sintering conditions. Moreover, the mechanical properties of sintered bodies increase as the mass fraction of ACP increased from0to75%in the original suspension mixing powder while the mechanically mixing powder follow the inverse regularity. The hardness and fracture toughness of pressureless sintered bodies made from suspension mixing powder with75wt.%ACP are5GPa and1.27MPa-m1/2.In vitro dissolution experiments of sintered ACP/HA bodies is carried out in Tris-HCl buffer solution, the results show that the weight loss of soaked powder samples (315～400nm) increases as the content of ACP in ACP/HA powder mixture increases. Additionally, the weight loss of powder sample made of suspension mixing method is less than that of mechanically mixing under the same condition after soaked for14days, and the dissolution is homogeneously and continuously. For the bulk samples made of mechanically mixing composite powder, weight loss also increases with the ACP content in the original powder increases after28days immersing. Meanwhile, large sizes of dissolved pores are formed on the surface and distribute inhomogeneously. However, weight loss, for the bulk samples made of suspension mixing composite powder, doesn’t follow the above rule. The dissolved pores on their surface are small and distribute homogeneously, which is mainly influenced by the phase content, grain size and pore volume. |
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