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 degrees C to 1350 degrees C and at actuator speed s of 8.4 x 10(-2) and 8.4 x 10(-5) mm/s (similar to 200 and similar to 0.2 MPa/s). Step stress rupture tests were also performed at 1288 degrees C and 1150 degrees C. Water vapor had a beneficial effect on the flexural streng th due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrated that the beneficial effects of water vapor on the strength in creases as temperature increases and/or loading rate decreases. Time-to-fai lure was always longer in wet air during step stress rupture testing. Creep crack growth by formation and coalescence of cavities ahead of the crack t ip generated from the oxidation pits or subsurface pores were the primary m echanism for slow crack growth for NT 164 Si3N4. (C) 2000 Kluwer Academic P ublishers.