The effect of lanthanum addition on the microstructure and mechanical properties of Mg-modified hydroxyapatite ceramics

In the present paper, the effect of La2O3 (0.25, 0.5 and 1 wt.%) addition on the phase stability, porosity, density, hardness, fracture toughness, compressive strength and brittleness index of hydroxyapatite modified with 1 wt.% of MgO was investigated. Hydroxyapatite (HA) without additives sintered at 1300°C has mixture of dominant hydroxyapatite phase with beta-tricalcium phosphate (α-TCP), alpha-tricalcium phosphate (β-TCP) and calcium oxide (CaO) phases. The microstructure is characterized with transgranular microcracks and oversized grains. Although the density and hardness of the pure HA increased with increasing temperature, a steady decrease in fracture toughness (from 0.96 to 0.71MPa•m1/2) and compressive strength (from 130.2 to 65.6MPa) was observed. For the HA modified with 1 wt.% of MgO the highest compressive strength (183.2MPa) and fracture toughness (1.47MPa•m1/2) were obtained at 1200°C and at this temperature the brittleness index was 3.24 μm-1/2. Increase in the sintering temperature led to the increase of the brittleness index of the pure HA,MgO modified HA and La2O3-MgO modified HA samples. The addition of 1 wt.%La2O3 to 1 wt.% MgO-HA contributed to the increase in the compressive strength of about 10%(from 183.2 to 202.0MPa), fracture toughness of about 69% (from 1.37 to 2.32MPa•m1/2) and also decrease of the brittleness index from 3.24 to 2.18 μm-1/2. The best performance after sintering at 1300°C was obtained for the MgO-HA sample with 0.25wt.% La2O3. As a result of this study, a new candidate material for biomedical application with superior mechanical properties and the phases that do not cause adverse reactions in the human body could be 1 wt.% MgO-HA modified with 1 wt.% La2O3 and sintered at 1200°C.