FRACTURE AND CONTACT ADHESION ENERGIES OF MICA MICA, SILICA SILICA, AND MICA SILICA INTERFACES IN DRY AND MOIST ATMOSPHERES

A study is made of the factors that contribute to the energy of mica-mica, silica-silica, and mica-silica interfaces in the presence of moist atmospheres. Energies are measured using brittle fracture and contact adhesion techniques. Both "virgin" and "healed" interfaces are investigated, with special attention on the latter. The fracture and adhesion data overlap, reflecting a common underlying separation process by "sharp-crack propagation." The study identifies several contributors to the interface adhesion energies. At virgin mica-mica and silica-silica interfaces the energy is determined by primary ionic-covalent attraction, and by the screening of this attraction by condensed moisture from the atmosphere. At healed interfaces the energy depends on both environmental interaction and "lattice" coherence. At retracted cracks in mica-mica most of the virgin ionic attraction is retained. On misorienting separated cleavage halves prior to recontact the interaction energy drops substantially: in "dry" atmospheres (relative humidity < 5%) a portion of the Coulombic interaction persists in the form of "macroscopic domains" of electrostatic charge, attributable to long-range order in the cation sublattice; in "wet" atmospheres (relative humidity > 50%) capillary forces dominate. The dissimilar mica-silica system exhibits the same dominance by capillary forces in wet atmospheres. However, in dry atmospheres the adhesion energy becomes inordinately high, from spontaneous transfer of electronic charge from one surface to the other. The implications of these observations concerning mechanical properties of brittle ceramics are discussed.