Synthesis, sintering and mechanical characteristics of nonstoichiometric apatite ceramics

摘要

The effect of Ca/P atomic ratio on the thermal behavior and mechanical characteristics of HAP based ceramics was investigated. Ca-deficient HAP powders with Ca/P ratio ranging from 1.6667 to 1.6315 were prepared by a precipitation method. The synthesized powders were single phased, of apatitic structure : Ca_(10-x)(PO_4)_(6-x)(HPO_4)_x(OH)_(2-x). During calcination, the specific surface area of powders started to decrease at 400 deg C and the surface reduction rate increases with the decrease of Ca/P ratio. The decomposition of the initial apatitic phase in a biphasic compound (stoichiometric HAP and TCP Ca_3(PO_4)_2) occurred from 700 deg C. The sintering, studied from dilatometry using powders calcined at a temperature below dissociation, showed that a decrease of Ca/P ratio of the initial phase decreased the densification rate during the first stage of sintering. This indicated that HPO_4~(2-) groups influenced the diffusion mechanisms. Nearly fully dense biphasic (HAP+TCP) materials were produced by hot pressing. The highest mechanical properties were obtained for Ca-deficient HAP ceramics having an atomic ratio Ca/P close to 1.65 ( sigma _f approx approx 150 MPa; K_(IC) approx approx 1 MPa.m~(1/2). Conversely, stoichiometric HAP had the lowest fracture strength and toughness.