Thermal stability of in situ formed Si3N4–Si2N2O–TiN composites

To investigate the potential of in situ formed Si3N4–Si2N2O–TiN composites, mixtures of Si3N4 with TiO2 and TiN powders were hot-pressed. Seven different compositions were successfully densified at 1650 °C resulting in various Si3N4-, Si2N2O- and TiN-volume fractions. The effect of elevated temperatures (up to 1400 °C) on the microstructure of these composites was explored, using internal friction measurements with the impulse excitation technique (IET) and high temperature X-ray diffraction (HTXRD). The bulk microstructure of the tested composites is stable up to 1400 °C, both with respect to the crystalline phase composition, as well as the intergranular glass phases. Sample surfaces are affected due to the reducing atmosphere in which IET and HTXRD tests are performed (N2 or vacuum). This involves the silicon oxynitride phase, which is not stable at temperatures above 1300 °C in the presence of a substantial amount of amorphous intergranular phase. Based on the results of indentation toughness and hardness tests, a high-Si2N2O/high-TiN/β-Si3N4 is selected as most promising for further development.