The modification of the Gilman cleavage technique described recently by Westwood and Hitch has been used to determine the surface energy γ0 of {100} MgO at 298°K. The value 1150±80 ergs/cm2 is in good agreement with previous experimental estimates of γ0, but somewhat lower than theoretical estimates. Metallographic studies were made of the plastic relaxation that occurs in the vicinity of the crack tip when cleavage cracks are repropagated, and a correlation between the extent of plastic relaxation, crack length, and the measured cleavage surface energy was observed.

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The effects of certain liquid environments on the penetration of a carbide spade drill and on the mobility of near-surface edge dislocations in freshly cleaved MgO and CaF, monocrystals were determined at room temperature. The environments were water, toluene, DMF, DMSO, DMF-DMSO solutions, n-alkanes ranging from heptane to hexadecane, and aqueous solutions of AlCl,. Drilling efficiency and near-surface dislocation mobility are directly related, and "softening" environments facilitate crack initiation in MgO. The n-alkanes are extremely surface-active with respect to MgO and CaF,. It is proposed that adsorbed surface-active species control the drilling behavior of MgO and CaFz (and perhaps of other notch-sensitive nonmetallic solids (minerals) also) as a consequence of their influence on the flow and flow-dependent fracture properties of the near-surface regions of these solids. Possible effects of adsorption-induced reductions in surface free energy are of minor importance.

A study has been made of the influence of adsorbed species on the room-temperature mobility of near-surface dislocations introduced into freshly cleaved {111} CaF2 surfaces by a diamond indenter. It was found that, in general, adsorbed surface-active ions or molecules decrease dislocation mobility, i.e., increase microhardness. Studies of the relaxation behavior of near-surface dislocations in CaF2 exposed to solutions of DMF in DMSO revealed that these environments also can influence dislocation mobility in the absence of stresses imposed by a loaded indenter. Moreover, for any particular DMSO-DMF environment, a correlation exists between the extent of dislocation motion induced by a loaded indenter and the amount of relaxation which occurs after the indenter is removed. From these and other observations it is concluded that indenter-lubrication effects do not play a fundamental role in the occurrence of Rebinder phenomena in CaF2. The results of this work are consistent, however, with the view that Rebinder effects in ionic solids are associated with chemisorption-induced changes in the state of ionization of near-surface point defects and dislocations. Such changes introduce variations in their mutual interactions which are then manifested as environmentally induced changes in the mobility of near-surface dislocations, and hence in microhardness.

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