Hardness and fracture toughness of dense calcium-phosphate-based materials

Hardness, H, and fracture toughness, KIc, have been determined as well as fracture energy and embrittlement index, H/KIc for six calcium-phosphate ceramics differing in phase composition. These materials were produced from initial calcium-phosphate precipitates with Ca-P molar ratios ranging between 1.50 and 1.73, synthesized by wet methods. After uniaxial or isostatic moulding and thermal treatment at 1250 degrees C, the obtained dense sinters constituted mono-, bi- or triphase ceramic materials containing hydroxyapatite (HAp), beta-TCP, alpha-TCP and CaO. When comparing the investigated materials, the best parameters, i.e. relatively high hardness accompanied by high KIc, were observed in the case of a HAp-TCP composite, containing approximately 15 wt% HAp. It has been stated that free CaO occurring on the surface of the HAp samples obtained from powders with Ca-P ratios exceeding 1.67, transforms partially to CaCO3 due to contact with the surrounding atmosphere. The well shaped calcite crystals existing on the surfaces of such sinters significantly reduce hardness and increase fracture energy of the material when comparing both with the monophase HAp and the biphase HAp-TCP ceramics.