Effect of Water on Toughness of MgO Crystals

It was possible to drive plastically blunted cleavage cracks through double cantilever beam specimens of impure MgO monocrystals in a series of small propagations. The forces required to propagate the cracks often defined a fairly constant apparent surface energy and provided an environment‐sensitive measure of toughness. Apparent surface energies between 1700 and 3100 ergs/cm 2 were obtained on MgO crystals tested in air. Values of apparent surface energy obtained from crystals tested in a low water content environment agreed well with values obtained on the same specimens in air, suggesting that water vapor in laboratory air does not significantly affect the toughness of MgO. Specimens tested while immersed in water, however, were approximately 30% tougher than those tested in air and exhibited a faceted fracture surface in contrast to the relatively smooth fracture surface produced in air.