Slow crack-growth behavior of alumina ceramics

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Abstract
The fracture behavior of high-purity alumina ceramics with grain sizes ranging from 2 to 13 μm is studied by means of the double torsion method. Crack-propagation tests conducted in air, water, and silicon oil, for crack velocities from 10−7 to 10−2 m/s, show that slow crack growth is due to stress corrosion by water molecules. An increase of the grain size leads to enhanced crack resistance, which is indicated by a shift of the V–KI (crack velocity versus applied stress intensity factor) plot toward high values of KI. Moreover, the slope of the curve is apparently higher for coarse grain alumina. However, if the R-curve effect is substracted from the experimental results, a unique V–KItip (crack velocity versus stress intensity factor at the crack tip) law is obtained for all alumina ceramics, independently of the grain size. This means that the crack-growth mechanism (stress corrosion by water molecules) is the same and that the apparent change of the V–KI law with grain size is a direct effect of crack bridging.

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