The effects of surface condition on the mechanical properties of lithium fluoride crystals

A comparative study has been made of the mechanical behaviour of as-cleaved, chemically polished and coated lithium fluoride crystals. Surface coatings were produced by Vacuum deposition and by chemical reaction. Thin (≯ 10 μ) reacted-coatings increased the yield stress some 50% and were responsible for catastrophic stress drops observed at the critical resolved shear stress. Coatings also decreased the initial rate of work-hardening and the stress and strain to fracture. Failure occurred by the formation of Strch-type cracks. An explanation for these and other results is offered based on considerations of the distribution and number of operable dislocation sources, and the piling up of dislocations at intersecting slip bands and beneath surface films. Metallographic evidence is presented for the latter phenomena. Experimental observations are more consistent with a barrier-mechanism of work-hardening than with a defect-mechanism.