Toughening and crack tip shielding in brittle materials by residually stressed thin films

Abstract

The apparent fracture toughness of brittle components can be greatly enhanced via surface-deposited thin films which are under residual, possibly nonuniform compression since the residual compression counteracts the effect of the applied loads at the tip of a surface crack. Examples are epitaxial thin films on crystalline components, thin films in a state of compression due to the film deposition process, or thin films of ion-exchange in glass. We review the micromechanics of the strengthening process by considering a surface microcrack under the combined action of the residual stress in the film (which shields the crack tip and may cause partial crack closure) and of the externally applied loads (thermal or mechanical). We examine crack tip shielding as the interaction of microscopic dilatant spots with the crack tip. This model predicts rising crack growth resistance curves for limited amounts of crack growth. We review and analyze experimental results on ion exchange in thin films of glass in small-, medium-, and large-size glass components for high average power solid state lasers. These measurements show that the apparent fracture strength increases by as much as five to six times.