Mechanism for Environmental Control of Drilling in MgO and CaF2 Monocrystals

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 CaF2 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 AlCl3. 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 CaF2. It is proposed that adsorbed surface‐active species control the drilling behavior of MgO and CaF2 (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.