Ceramics-Silikáty 62, (4) 382 - 388 (2018)

Dense Si₃N₄-based composites with O'-SiAlON bonding phase were prepared by hot pressing of the mixture of Si₃N₄ matrix and polysiloxane (samples SA) and polysilazane (samples SB) based sintering aids. In both samples α-Si₃N₄ and β-Si₃N₄ were identified as major phases and O'-SiAlON with a chemical formula Si_1.84Al_0.16O_1.16N_1.84 as a minor phase. The hydrothermal corrosion tests were performed in subcritical conditions at 250 °C for 100 hours. The weight loss was rather low, 0.96 wt.% and 1.32 wt.% for samples SA and SB, respectively. The corrosion rate of samples SA and SB was 206 mg.m^-2.h^-1 and 233 mg.m^-2.h^-1. The higher corrosion rate of sample SB prepared with polysilazane-based sintering aid is most probably due to the hydrothermal dissolution of Si₃N₄ grains producing ammonia, as the pH value of eluate of this sample was higher, 8.5 compared to pH = 8.0 determined for sample SA. In sample SA a continuous SiO₂-based passivation layer was formed, while in sample SB the passivation layer was discontinuous and the corrosion medium could easily attack the Si₃N₄ grains. As the corrosion solution was more basic in sample SB, the SiO₂ passivation layer did not form so effectively, or dissolved faster than in sample SA. The higher corrosion resistance of sample SA was partly improved also by using Y₂O₃ sintering additive, which formed a rather stable Y₂SiO₅ phase.