Effect of the presence of SiO₂, CaO and MgO on silion nitride bioceramics, evaluating the microstructure, biological behavior and mechanical properties

Silicon nitride ceramics have aroused interest for applications in the medical and dental field due to their relatively high fracture toughness, wear resistance, and low friction coefficient as well as adequate osseointegration. In addition, it is known that the presence of magnesium stimulates new bone growth, enhancing cell proliferation and differentiation, and the collagen formation. Silica assists in the densification of material during sintering, due to its low melting temperature, and positively interferes with bone growth. Calcium enhance osteoblastic activities, stimulating the formation of bone tissue. This Project aims to analyze or affect the presence of SiO₂, CaO and MgO sintering additives in silicon nitride ceramics, to evaluate a microstructure, biological behavior and mechanical properties, in order to improve the response of these materials in vivo. Different compositions were prepared and formed by uniaxial, cold isostatic pressing, and conventional sintering (reaching 1750°C). The sintered materials were evaluated for density, microstructure and biological behavior in vitro. The microstructural evaluation showed that the studied compositions resulted in a microstructure containing elongated grains of β-Si₃N₄ dispersed in secondary amorphous phase. However, the density values varies from 79 to 97% of the theoretical density, according to the composition, with the compositions with higher values of SiO₂ being the ones with higher values of density. After immersing the samples in SBF (Simulated Body Fluid), the presence of deposits rich in calcium and phosphorus was observed with a typical hydroxyapatite morphology, and the compositions with higher values of MgO and CaO showed better results. The Vickers hardness values of the compositions ranged from 9 to approximately 12 GPa, with the compositions with higher values of SiO₂ showing higher values. The fracture toughness values ranged from 4 to approximately 7 MPa.m1/2, with better results in compositions with higher values of SiO₂, while the values of Youngs modulus ranged from 132 to approximately 269 GPa (the compositions with higher values of MgO and CaO showed higher values of this property), and the flexural strength values varied between 180 MPa and approximately 620 MPa, with the compositions with higher values of SiO₂ presenting higher values of flexural strength. All samples showed bioactivity and non-cytotoxicity in the ALP activity tests, and the cells achieved differentiation in the extracellular matrix mineralization test, with the best results being obtained by the compositions with higher levels of MgO and CaO. Thus, the materials studied seem to have great potential for applications in implants in both orthopedic and dental areas. (author)