Microwave sintering improves the mechanical properties of biphasic calcium phosphates from hydroxyapatite microspheres produced from hydrothermal processing

Starting from two microspherical agglomerated HAP powders, porous biphasic HAP/TCP bioceramics were obtained by microwave sintering. During the sintering the HAP powders turned into biphasic mixtures, whereby HAP was the dominant crystalline phase in the case of the sample with the higher Ca/P ratio (HAP1) while α-TCP was the dominant crystalline phase in the sample with lower Ca/P ratio (HAP2). The porous microstructures of the obtained bioceramics were characterized by spherical intra-agglomerate pores and shapeless inter-agglomerate pores. The fracture toughness of the HAP1 and HAP2 samples microwave sintered at 1200 °C for 15 min were 1.25 MPa m1/2. The phase composition of the obtained bioceramics only had a minor effect on the indentation fracture toughness compared to a unique microstructure consisting of spherical intra-agglomerate pores with strong bonds between the spherical agglomerates. Cold isostatic pressing at 400 MPa before microwave sintering led to an increase in the fracture toughness of the biphasic HAP/TCP bioceramics to 1.35 MPa m1/2.