Stephen J. Bennison

flaw tolerance in ceramics with rising crack resistance characteristics

The stabilizing influence of increasing toughness with crack size associated with a cumulative closure-stress process (R-curve, orT-curve) on the strength properties of brittle ceramic materials is analysed. Three strength-controlling flaw types are examined in quantitative detail: microcracks with closure-stress history through both the initial formation and the extension in subsequent strength testing; microcracks with closure stresses active only during the subsequent extension; spherical pores. Using a polycrystalline alumina with pronouncedT-curve behaviour as a case study, it is demonstrated that the strength is insensitive to a greater or lesser extent on the initial size of the flaw, i.e. the material exhibits the quality of “flaw tolerance”. This insensitivity is particularly striking for the flaws with full closure-stress history, with virtually total independence on initial size up to some 100μm; for the flaws with only post-evolutionary exposure to the closure elements the effect is less dramatic, but the strength characteristics are nevertheless significantly more insensitive to initial flaw size than their counterparts for materials with single-value toughnesses. The implications of these results to engineering design methodologies, as expressed in conventionalR-curve constructions, and to processing strategies for tailoring materials with optimal crack resistance properties, are discussed.

Fabrication of flaw-tolerant aluminum-titanate-reinforced alumina

Abstract The fabrication and the flaw tolerance behavior of particulate aluminum-titanate-reinforced alumina composites have been studied. High-density (∼99% theoretical) composites with controlled microstructures are readily produced via a conventional ceramics processing scheme using starting powders of α-alumina and β-aluminum titanate. Indentation-strength measurements demonstrate that these composites are highly flaw tolerant. Direct observations of crack evolution from Vickers indentations during loading reveal a strong crack-stabilization with prefailure extensions of a millimeter or more. This stabilization gives rise to the flaw tolerance properties and results from pronounced crack-resistance (R-curve) behavior. Grain-localized crack bridging is active in these materials and is believed to be a contributor to the R-curve properties.

Microstructure, Toughness Curves and Mechanical Properties of Alumina Ceramics

The microstructural variables that determine the toughness (T-curve) characteristics of alumina and other structural ceramics are considered. Alumina ceramics gain their toughness from shielding by grain-interlock bridging at the interface behind the crack tip. A general fracture mechanics formalism for describing the bridging is outlined in terms of desirable microstructural elements, such as weak internal boundaries, high internal stress, coarse microstructure. The T-curve imparts the quality of flaw tolerance to the strength properties. We examine this quality, under both inert and interactive environmental conditions, monotonic and cyclic loading, using indentation flaws. In situ observations of bridging sites during loading in the scanning electron microscope provide insight into the bridge degradation micromechanisms. Finally, short-crack properties, spontaneous microcracking and wear degradation, are examined in light of the bridging model. It is concluded that design with ceramics may require certain tradeoffs, long vs short cracks, high strength vs flaw tolerance, etc. The key to optimal performance in ceramics rests with microstructural processing for specific properties.