The formation of aluminum titanate was investigated by isothermal treatments of samples obtained from equimolar mixtures of alumina and titania, containing small amounts of silica and magnesia. Results of differential thermal analysis and Rietveld refinements of data collected by X-ray powder diffraction (XRPD) showed that additions of silica in amounts used in this work did not influence the formation of aluminum titanate. However, the presence of magnesia favored the formation of aluminum titanate in two steps, first one by incorporating Mg²⁺ into Al₂TiO₅ lattice during its initial formation, and the second one by accelerating the Al₂TiO₅ formation, contributing to large quantities of this phase. MgO doped samples have also developed a more suitable microstructure for stabilizing of Al₂TiO₅, what make them promising for applications such as thermal barriers, internal combustion engines and support material for catalyst.

The bioinert surface of silicon nitride ceramics led us to investigate the additions of SiO₂, CaO and Al₂O₃ in order to aid the liquid phase sintering and improve the mechanical properties and biological behavior of the final materials. The sintered materials reached ca. 97% of theoretical density and total α→β-Si₃N₄ transformation. The samples had relatively high values of fracture toughness while their elastic modulus values were lower than those of conventional silicon nitride ceramics. Apatite deposits were observed on the surfaces analyzed after SBF (simulated body fluid) immersions, suggesting their bioactivity. Osteoblasts proliferation and calcified matrix were also detected as response to cells/materials contact. This combination of properties suggests that all studied compositions are promising for applications in biomedical devices. Moreover, compositions with alumina additions and higher silica content had better in vitro biological behavior, densification and mechanical properties, suggesting greater potential to be used in bone substitute devices.

Titanium, silicon and calcium ions have demonstrated positive effects in bone healing. Therefore, this paper investigates the densification, mechanical properties and in vitro biological behaviour of TiO2, together with SiO2 and CaO, on silicon nitride ceramics to be used in clinical applications. The results revealed that the sintered samples reached high values of relative density and fracture toughness, non-cytotoxicity as well as good ability to promote cell proliferation and form a layer of carbonated hydroxyapatite on their surfaces, after immersion in simulated body fluid. The sample with the highest amount of TiO2 reached the highest value of relative density and the best results of cell proliferation and fracture toughness, indicating the great potential to be explored by in vivo experiments for later application as intervertebral devices.

In this paper, the oxidation behavior of silicon nitride with different contents of TiN was evaluated at 1400 ^oC for 64 hours in air. The oxidized samples were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. Weight gain measurements have shown that the oxidation followed a multiple-law model with linear, parabolic, and logarithmic contributions. The samples presented high weight gain at the beginning of the process followed by the formation of an amorphous silica surface layer containing Y₂Ti₂O₇ and rutile crystals. Cracks and holes were detected on the oxide layer. The oxidation resistance of the composites was strongly influenced by the initial content of TiN.