DISLOCATION NUCLEATION AT METAL CERAMIC INTERFACES

The ductile vs brittle behaviour of metal-ceramic interfaces is discussed within an atomistic framework, in which the mechanical response of an interfacial crack is assumed to be ultimately controlled by the competition between atomic decohesion and dislocation nucleation ahead of the crack tip. As in later versions of the Rice-Thomson model, this competition may be evaluated in terms of the parameters G(cleave), the energy release rate for cleavage of the metal-ceramic interface, and G(disl), the energy release rate associated with the emission of a single dislocation within the metal. The various models of dislocation nucleation are discussed, with emphasis on an approach which makes use of Peierls-like stress vs displacement relations on a slip plane ahead of a crack tip. A recent analytical result by Rice shows that for a mode II or III shear crack, with a slip plane parallel to the crack plane, a dislocation is emitted when G = gamma(us) (G is the energy release rate corresponding to the "screened" crack tip stress field and gamma(us). is the "unstable stacking" energy associated with the sliding of atomic planes past one another). This treatment permits the existence of an extended dislocation core, which eliminates the need for the core cutoff radii required by the Rice-Thomson model of emission. Results are presented here for the nucleation of dislocations under more realistic assumptions for metal-ceramic cracks, namely, the emission on inclined slip planes within a mixed-mode crack-tip field. The specific case of a copper crystal bonded on a {221} face to sapphire is analyzed, and the results are used to interpret the recent experimental observations of Beltz and Wang [Acta metall. mater. 40, 1675 (1992)] on directional toughness along this type of interface.