Slow crack-growth behavior of alumina ceramics

摘要

The fracture behavior of high-purity alumina ceramics with grain sizes ranging from 2 to 13 pm is studied by means of the double torsion method. Crack-propagation tests conducted in air, water, and silicon oil, for crack velocities from l0~-7 to l0~-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 VK (crack velocity versus applied stress intensity factor) plot toward high values of K. 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-K_itip. (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 VK law with grain size is a direct effect of crack bridging.