Rebinder effects in MgO

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Abstract
A study has been made of the effects of adsorbed complex ions and organic molecules of various dipole moments on the room temperature mobility of near-surface dislocation half-loops introduced by a diamond indenter into freshly cleaved MgO surfaces. In general, adsorbed complexes of high charge (positive or negative), or molecules of high dipole moment, significantly enhance dislocation mobility, leading to the time dependent reductions in hardness termed ‘anomalous indentation-creep’. In certain environments, however, for example in aqueous 10−3 N AgNO3, dislocation mobility is reduced. It is considered that both the latter effect, termed ‘adsorption-locking’, and anomalous indentation-creep effects belong to the class of phenomena known as Rebinder effects. Observations of a photomechanical effect in MgO, and of inhibition of enhanced mobility also are described. A possible explanation for Rebinder effects in ionic crystals is suggested. This is that chemisorption-induced band bending in the surface regions of the crystal alters both the electronic core structure (charge) of near-surface dislocations, and the state of ionization of point defects such as vacancies and impurities. As a consequence, the nature of the interactions between moving dislocations and such lattice defects is changed, affecting dislocation mobility and, hence, microhardness. The existence of Rebinder effects in metals is briefly discussed.