Occurrence and Mechanism of Rebinder Effects in CaF2

Abstract

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.