Sintering peculiarities for hydroxyapatite with different degrees of crystallinity.

It has been shown that reduction of sintering temperature of hydroxyapatite is connected to the use of a powder, the particles of which consist of crystalline and amorphous-like constituents. Shrinkage of the pressings made of the powder starts at the middle temperatures (600-700 degrees C) and is realized by a mechanism of a mutual sliding of the particles. If the firing temperature of the pressings increases, the second stage, realized by a diffusion mechanism (over approximately 900 degrees C), begins. A shrinkage curve on the sliding stage has a stepped character. It is caused by a stopping effect on the shrinkage of the pressure of a gas, which is exuded as a result of crystallization of the amorphous-like constituent, and partly accumulates in closed cavities and pores. The contribution of the two mechanisms to the shrinkage process is determined by the degree of crystallinity of the powder (i.e. the ratio of the crystalline and the amorphous-like constituents). If a powder has a higher degree of crystallinity, higher temperatures result, and the stepped character of the shrinkage curve is smoothed. The sliding stage is practically lacking for pressings of crystalline powder, and a noticeable shrinkage starts at high temperatures (over approximately 900 degrees C) as diffusive processes develop. Copyright 2000 John Wiley & Sons, Inc.