ИСТИНА

Ceramics based on calcium phosphates, in particular hydroxyapatite (HAp, Ca10(PO4)6(OH)2), is well known as a prospective material for biomedical applications since it is the base of the mineral part of a skeleton. There are several ways to improve on the densification of powder compacts and consequently increase mechanical properties of bioceramics. One of them consists in the use of a sintering additives and utilization of the liquid-phase sintering. Various types of additives specific to HAp have been considered earlier [1]. Biocompatible chlorides are noteworthy compounds for bioceramics applications due to low melting point and bioresorbable phases formation as a result of melt-solid reaction. Melt and solid interaction plays very important role in the densification and sintering process. Chemical reactions during thermal treatment, wetting of solid phase by melt, surface tension of solid/liquid interfaces are the most important factors for liquid phase sintering. In the present work we consider hydroxyapatite bioceramics sintering and microstructure formation through the prism of chlorides/Hap chemical reaction and influence of additives on HAp surface tension. The powders of HAp were obtained via conventional wet precipitation technique, that is, by dropwise adding of ammonium hydrophosphate solution (C=0.3 M) to the solution containing calcium nitrate (C=0.5 M) keeping the following conditions of the synthesis: 10Ca(NO3)2 + 6(NH4)2HPO4 + 8NH4OH = Ca10(PO4)6(OH)2 + 20NH4NO3 + 6H2O T=60 °C, pH=9. Surface tension of HAp investigated using thermal grooving of Hap in air and chlorides vapor atmosphere. The width and depth of the thermal grooves were measured by atomic force microscopy (AFM). For ceramics preparation and microstructure investigations a surplus of chlorides (NaCl, KCl and CaCl2) in amount of 5 wt.% to a powder charge, was added at the stage of disaggregating. The Hap surface tension has the greatest effect of NaCl (about 330 and 183 mJ/m2 for pure Hap and Hap in NaCl vapor environment respectively at 830 °C). NaCl and KCl additives lead to whiskers formation after high temperature treatment that inhibits the ceramics sintering. At the same time CaCl2 addition results in uniform microstructure of ceramics through the active reaction with Hap.