Effect of water vapor on the mechanical behaviors of hot isostatically pressed silicon nitride containing Y2O3

The effect of water vapor on the mechanical behavior of Si₃N₄ ceramics was studied. Strength measurements by flexural dynamic fatigue tests were made at temperatures from 1038°C to 1350°C and at actuator speeds of 8.4 × 10^–2 and 8.4 × 10^–5 mm/s (200 and 0.2 MPa/s). Step stress rupture tests were also performed at 1288°C and 1150°C. Water vapor had a beneficial effect on the flexural strength due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrated that the beneficial effects of water vapor on the strength increases as temperature increases and/or loading rate decreases. Time-to-failure was always longer in wet air during step stress rupture testing. Creep crack growth by formation and coalescence of cavities ahead of the crack tip generated from the oxidation pits or subsurface pores were the primary mechanism for slow crack growth for NT 164 Si₃N₄.