Font Size: 

In this presentation, the primary mechanisms associated with the growth of fatigue cracks under cyclic loading in monolithic ceramics (e.g., Al2O3, Si3N4, SiC) are examined at both low and elevated temperatures, with emphasis on the role of the grainboundary films and phases. It is shown that the process of fatigue-crack growth in ceramics involves a mutual competition between intrinsic damage mechanisms ahead of the crack tip, which cause the extension of the crack and are essentially identical to crack-advance mechanisms under static loading, and extrinsic shielding mechanisms behind the crack tip, which impede crack advance primarily through the process of crack bridging. All these mechanisms are intimately controlled by the nature of the grain-boundary phase. At ambient temperatures, where intergranular cracking ahead of the tip is generally balanced by interlocking grain bridging behind it, the presence of the ubiquitous grain-boundary glassy phase can promote such bridging and is thus a vital factor in the development of crackgrowth resistance. However, at elevated temperatures where creep damage ahead of the tip is generally balanced by viscous-phase bridging behind it, the presence of the glassy phase conversely can be a detriment to crack-growth resistance by providing the primary site for softening, creep cavitation and oxidation damage. This compromise between low temperature toughness and high temperature creep resistance can be alleviated in silicon carbide ceramics, however, by crystallization of the grain-boundary amorphous phase at
elevated temperatures. Using the example of an in situ toughened SiC (ABC-SiC), it is shown that the attainment of good low and high temperature fatigue and fracture properties can be achieved through such a process of in situ crystallization. The primary mechanisms of shielding and damage in SiC associated resulting from this microstructural transformation are described.