Mechanical properties and in vitro bioactivity of silicon nitride ceramics with SiO₂ , CaO, and MgO additions

Silicon nitride ceramics with SiO₂ , CaO, and MgO as sintering aids were investigated in view of biomedical applications. In the current study, samples with four different compositions were pressureless sintered at 1750^°C for 1 h under a nitrogen atmosphere. The samples were evaluated concerning densification, microstructure, mechanical properties, and in vitro bioactivity. Microstructures with elongated β-Si₃ N₄ grains dispersed in an intergranular phase and with densities from 78.77 to 97.14% of the theoretical density were obtained. Higher contents of SiO₂ resulted in the best densification and mechanical properties. Besides, replacements of CaO by MgO in the initial compositions affected Young's modulus and in vitro bioactivity. Considering the samples with relative density higher than 94.14%, those with lower values of Young's modulus had lower SiO₂ /MgO ratios. After immersion in SBF (Simulated Body Fluid), the samples with high porosity and/or partial replacements of CaO by MgO had their surfaces coated with a layer rich in calcium and phosphorus, morphologically similar to hydroxyapatite. Hence, producing silicon nitride ceramics with the potential to be used as orthopedic implants must consider ideal amounts of additives. In this article, the best combination of mechanical properties and mineralization capability was reached by the composition with low content of MgO, and high content of SiO₂ and CaO.