SURFACE FORCES AT CRACK INTERFACES IN MICA IN THE PRESENCE OF CAPILLARY CONDENSATION

A fracture mechanics model for brittle cracks with capillary condensation is developed. The model is based on an integration of intersurface forces, comprising solid-liquid-solid forces within the capillary and solid-vapour-solid forces without. The work of adhesion thus contains terms from these two regions of the adhesion zone, plus a term from the surface tension of the meniscus. Three interface types are considered: virgin, corresponding to first propagation of the crack through the bulk solid; healed-matched, corresponding to repropagation after crack retraction of a partial cleavage; and healed-misoriented, where the cleavage halves are first fully separated and rotated before closure. Notwithstanding the possibility that a layer of environmental molecules may become trapped on interfacial closure, thereby reducing the intrinsic cohesion, we predict the work of separation for healed-matched interfaces to have the same dependence on partial pressure of interactive environmental species as for virgin interfaces, provided lattice coherence is maintained between the matching surfaces. A different dependence is predicted for healed-misoriented surfaces, because of the loss of such coherence, in which case the greater part of the adhesion comes from the liquid surface tension. Results of equilibrium cleavage experiments on mica in water vapour confirm these predictions. The strongly increasing work of adhesion observed for virgin and healed-matched surfaces with diminishing relative humidity is indicative of a long-range, Coulombic component of bonding in the pristine mica structure. Implications of the results in the general description of brittle fracture, and of contact adhesion, are discussed. © 1990.