CALCIUM-STRONTIUM PHOSPHATE BIOCERAMICS

Ceramics were made for potential use as prosthetic bone and tooth materials from powders of calcium hydroxyapatite (CaHA) and strontium hydroxyapatite (SrHA) prepared by aqueous precipitation. The powders were characterized as to the size and shape of the crystallites, the density and the Ca:P ratio. An attempt was made to convert these powders into their fluorine analogs, calcium fluorapatite (CaFA) and strontium fluorapatite (SrFA) by ion-exchange reactions. The powders were treated with NH(,4)F solutions using both a constant pH and a cyclic variation of pH techniques at room and elevated temperatures. The best fluorapatite was obtained by the cyclic pH method at elevated temperatures. A high temperature phase study was conducted on CaHA and SrHA between 600(DEGREES)-1400(DEGREES)C using both air-quenching and slow-cooling. CaHA was found to gradually dehydroxylate up to 1335(DEGREES)C to form a hydroxy-oxyapatite solid solution. At 1335(DEGREES)C transformation of the apatite to (alpha)-Ca(,3)(PO(,4))(,2) and Ca(,4)O(PO(,4))(,2) occurred. Pure oxyapatite was formed by lowering the temperature to just below this transformation and then air-quenching. Similarly, SrHA dehydroxylated to form a hydroxy-oxyapatite solid solution when heating to 1400(DEGREES)C. Bioceramics of CaHA and SrHA were then fabricated by extrusion of the powders into rods and then sintering at selected intervals from 900(DEGREES) to 1400(DEGREES)C for 2 hrs. The best ceramic bodies having high density and small grain size were obtained by firing at 1150(DEGREES)C. The moduli of rupture were 65 MN/m('2) and 54 MN/m('2) for the CaHA and SrHA ceramics, respectively, while the elasticity moduli were 50 GN/m('2) and 46 GN/m('2), respectively. Surface ion-exchange was attempted to improve the strength of the CaHA ceramic by producing a surface layer in compressive stress. Reactions were carried-out using salts of Sr(NO(,3))(,2), CaCl(,2) and SrCl(,2) in both the molten and solid state. The best result was obtained with SrCl(,2) in the solid state at 650(DEGREES)-700(DEGREES)C where Cl-ions diffused 50 (mu)m into the ceramic surface. The modulus of rupture of this SrCl(,2)-treated ceramic was 79 MN/m('2) which corresponded to a 22% increase in strength.