AN IMPROVEMENT IN PROCESSING OF HYDROXYAPATITE CERAMICS

Hydroxyapatite ceramics have been fabricated via two different process ing routes, a conventional processing route and an emulsion-refined ro ute. The conventional precipitation processing of powder precursors fo r hydroxyapatite ceramics results in the formation of hard particle ag glomerates, which degrade both the compaction and densification behavi our of the resultant powder compacts. An emulsion-refinement step has been shown to be effective in ''softening'' particle agglomerates pres ent in the conventionally processed powder precursor. As a result, the emulsion-refined powder compact exhibits both a higher green density and a higher sintered density than the un-refined powder compact, on s intering at temperatures above 800 degrees C. The effect of powder agg lomeration on densification during both the initial and later stage of sintering is discussed. The attainable sintered density of the conven tionally processed material was found to be limited by the presence of hard powder agglomerates, which were not effectively eliminated by th e application of a pressing pressure of 200 MPa. These hard powder agg lomerates, which form highly densified regions in the sintered ceramic body, commenced densification at around 400 degrees C which is more t han 100 degrees C lower than the densification onset temperature for t he emulsion-refined powder compact, when heated at a rate of 5 degrees C min(-1). The inter-agglomerate voids, manifested by the differentia l sintering, resulted in the formation of large, crack-like pores, whi ch act as the strength-limiting microstructural defects in the convent ionally processed hydroxyapatite. A fracture strength of 170 +/- 12.3 MPa was measured for the emulsion-refined material compared to 70 +/- 15.4 MPa for the conventionally processed material, when both were sin tered at 1100 degrees C for 2 h.