Effect of microstructure and slow crack growth on lifetime prediction of monolithic silicon carbide

The lifetime of silicon carbide (SiC) based materials is strongly dependent on the presence of pre-existing flaws or cracks and their extension under an applied load during their service life. The purpose of this work is to determine the long term strength of SiC based materials with different grain morphologies and grain boundary chemistry. Solid state (SS) sintering of SiC with carbon and boron and liquid phase (LP) sintering of SiC using alumina and yttria as additives were used to produce fine and coarse grained materials to clarify the role of chemistry and grain morphology respectively. Fracture toughness, strength and slow crack growth (SCG) data were used to determine lifetime diagrams to accurately evaluate the long term strength behaviour for natural and artificial defects. The LP-SiC materials have more susceptibility to SCG compared to SS-SiC. However, the LP-SiC with coarse grains has a higher toughness and can be used at higher stresses after a large defect has been accidentally introduced. This indicates that the effect of the slow crack growth as a result of introducing oxides on the grain boundaries is not sufficient to alter the ranking between materials in terms of their deterministic allowable stress after such a damage event. On the other hand, the allowable stress in terms of the natural defect population revealed different results for using a low probability of failure (5%) and a much higher probability of failure (63.2%): the ranking of the materials alters when the stress level at which it is to be used changes.