Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) ceramics is a well-known prospective material for skeleton restoration. Sintering of HAp powder is still a challenging task, and liquid-phase sintering seems to be a reasonable solution in improving microstructure and strength of the phosphate ceramics. Chemical reactions during heating, wettability of HAp by a melt, solid/liquid interfacial energies are the most important factors affecting liquid phase sintering. Various types of additives specific to HAp sintering have been considered earlier [1], among them, biocompatible chlorides are of particular interest [2]. The scope of this work is the impact of chemical reaction between Hap and chlorides as well as surface tension in the presence of the chloride melts on sintering of the ceramics.
The powders of HAp were obtained via conventional wet precipitation technique at T=60 °C and pH=9 from Ca(NO3)2 and (NH4)2HPO4 solutions. A surplus of chlorides (NaCl, KCl or CaCl2) in amount of 5 wt.% to a powder charge, was added at the stage of ball milling prior to making powder compacts. Surface tension of HAp in chloride environment was estimated in form of γgb/γs ratio, where γs is specific surface energy and γgb is the energy of a grain boundary terminated by surface of a sample. The γgb/γs ratio was extracted from the experiments on thermal grooving of the terminated boundary in HAp ceramics in air and chloride (NaCl, KCl and CaCl2) vapor atmosphere. Geometrical characteristics of the thermal grooves were measured by atomic force microscopy (AFM), and the groove profiles were treated in terms of Mullins’ theory of thermal grooving [3].
It was demonstrated that among other halides NaCl has the greatest effect on HAp surface tension (γgb/γs= 0,35 ± 0,05 and γgb/γs= 0,93 ± 0,09 for pure HAp and HAp in NaCl vapor environment at 830 °C, respectively). Interfacial tension at liquid halide/HAp interface was estimated under the assumption of perfect wetting of HAp ceramics with chloride melt. NaCl and KCl additives lead to formation of whiskers that inhibit ceramics sintering, whereas the addition of CaCl2 results in uniform microstructure of ceramics due to its chemical reaction with HAp.
This work was partially supported by RFBR grants №№09-03-01078, 10-03-00866, 11-03-12179-ofi-m-2011.