Creep fracture in ceramics containing small amounts of a liquid phase

Abstract

Even small quantities of a fluid phase, say a few volume percent, can have a dominant influence on fracture behavior of polycrystalline ceramics. It is possible to conceive of several reasons why: 1. (i) the liquid film in two grain junctions can promote grain boundary sliding, 2. (ii) since the liquid is often constrained in pockets at triple grain junctions, it can become stressed in hydrostatic tension, and as a result cavitate. 3. (iii) grain boundaries can become separated by the lateral growth of penny shaped bubbles in the fluid film when a tensile stress is applied across the interface, and 4. (iv) the fluid may lead to accelerated crack growth along the interface if the meniscus along the crack front becomes unstable and breaks up into a finger-like morphology. This paper is an attempt to synthesize the results from published literature on these separate topics and apply them to fracture in polycrystalline ceramics, with an emphasis on quantifying properties and phenomena of engineering interest such as modulus of rupture, creep crack-growth and superplastic flow. Also some other relevant issues such as multiaxial or mixed mode fracture, fracture from small flaws vs large flaws, and the question of localized microcrack damage near crack tips vs more general cavitation damage, are discussed.