Possibilities for strengthening hydroxyapatite ceramics

Some physics during heating and sintering of powder pressings of hydroxyapa tite (HA) under conventional (usual) conditions have been studied. It is re vealed that heating and firing of the pressings of a middle-dispersity powd er are accompanied by release of gases. The gas release hinders and can sto p the shrinkage (sintering). The microhardness is low and has a complicated distribution on the surface and in the volume of ceramics. A slight degass ing (drying) of the pressing before sintering heightens the density and imp roves the microhardness characteristics of the ceramics. The shrinkage stop effect is eliminated in pressings of a fine powder. On the basis of the re sults, a technique and some methods for quality improvement of ceramics wer e proposed and approbated. They consisted of the manufacture of samples of a mixture of two powders with different dispersity, use of a press techniqu e with two male dies, thermal treatment of pressing before sintering, and t he choice of moderate sintering conditions. The resulting ceramics had a de nsity close to the theoretical, almost homogeneous microhardness distributi on in the sample and much higher values of microhardness and compressive st rength. (C) 1999 John Wiley & Sons, Inc.