Sintering of the ultrahigh pressure densified hydroxyapatite monolithic xerogels
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Sintering of the ultrahigh pressure densified hydroxyapatite monolithic xerogels J´an Majling, Peter Zn´aik, Angela Palov´a, and Stefan Svet´ık Department of Ceramics, Slovak Technical University, 812 37 Bratislava, Slovak Republic
Stefan Koval´ık Department of Materials Science and Technologies, Slovak Technical University, 812 37 Bratislava, Slovak Republic
Dinesh K. Agrawal and Rustum Roy Intercollege Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 16 February 1996; accepted 28 June 1996)
Dense and translucent ceramics were prepared by sintering of cylindrical preforms of hydroxyapatite extruded from xerogels. Extruded specimens were dried as monoliths and then consolidated by applying cold isostatic pressure, ranging from 500 to 1500 MPa. Upon heating the samples began to densify at 610 ±C, and the densification/sintering was completed at 870 ±C as was evidenced by the dilatometry plot indicating no further shrinkage. The sintered specimens thus formed were translucent in appearance. Further heating of the samples up to 1200 ±C resulted in their “bloating” or creation of pores in the originally dense matrix. Pore creation within the structure is reproducible, it proceeds from the surface to the interior of the sample, and its spreading can be thermally controlled. Pore evolution within the single phase dense polycrystalline material is not related to the frequently occurring phenomenon of microcracking in ceramics during cooling. I. INTRODUCTION
Hydroxyapatite (HAp) ceramics draw special interest for their biocompatibility. Dense apatite ceramics are in commercial use for bone replacement in the human body. Fabrication procedures for dense hydroxyapatite ceramics are quite diversified. Most of the researchers have used methods involving the formation of suitable gel-like precipitates1,2 and then drying the obtained powders. The resulting xerogels are then calcined,3,4 milled, compacted,5,6 and finally subjected to sintering.7,8 Aggregation of powders during drying and calcination stages (dry route) causes nonuniformity in packing of particles in green compacts, and thus prevents full densification during sintering. Colloidal or sol/gel processes are therefore considered in order to prevent the aggregation of powders. A defect-free fine microstructure in a ceramic is synonymous with the optimum mechanical properties of polycrystalline materials and is usually manifested by the translucency or transparency of the respective materials. The preparation of a translucent1 HAp ceramics was reported in which solution route assisted in the partial consolidation of colloidal particles. The transparent HAp was obtained after hot pressing.9,10 In a recent report, however, it was also prepared by the pressureless sintering,11 using a noncalcination route and hydrothermally synthesized powder (an aggregate free
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http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 1, Jan 1997
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