Effects of Moisture on Grain-Boundary Strength, Fracture, and Fatigue Properties of Alumina

The role of moisture in affecting both intrinsic and extrinsic aspects of the fracture and fatigue-crack growth resistance of a polycrystalline alumina (99.5% pure, 25 μm grain size) has been examined in both moist and dry environments at ambient temperature. The intrinsic (crack-tip) toughness, deduced from measured crack-opening profiles, is found to be less than for a single crystal and is 30% lower (∼0.6 MPa·m^1/2) in moist air versus in dry N₂, implying that the grain-boundary theoretical strength is higher in a dry environment. Despite this, in dry atmospheres, the R-curves (which derive from crack deflection and grain bridging) initially rose more steeply and nominal fatigue-crack growth thresholds for short crack sizes (20-60 μm) were more than 1.3 MPa·m^1/2 higher. Owing to this quicker crack bridging development, strengths for natural flaws could be more than doubled in dry atmospheres, a difference that well exceeds the effect solely due to the intrinsic toughness change. After ∼2 mm of crack growth, however, the R-curve and steady-state fatigue behavior appeared similar in both environments, although altering the atmosphere for such fatigue cracks in situ induced large, abrupt changes in transient growth rates. The environment influences the nature of the bridging zones, with uncracked-ligament bridges playing a larger role in dry atmospheres, while frictional bridges are predominant in moist air. Evidently, to achieve optimal toughness in bridging ceramics, the window for the requisite grain-boundary strength may be small; whereas weak boundaries are required to induce the necessary intergranular fracture, if too weak, shallower R-curves, less strengthening, and poorer fatigue resistance all follow.