Effects of crack-crystallite interaction on the fracture behavior of a cordierite glass-ceramic
Crack-microstructure interactions occurring during the flaw introduction process were studied in a model brittle composite, a cordierite glass-ceramic. Microstructural effects associated with the repropagation of the introduced flaws under the imposition of a mechanical load were also examined.
Two general types of crystallized microstructures were investigated for samples heat-treated from the original glass: a fine structure composed of a uniform precipitation of very small (< 0.1 µm) crystallites, and a coarser structure characterized by crystallites, and a coarser structure characterized by crystallites ~ 1-2 µm in diameter dispersed within a much finer-grained (< 0.1 µm) crystalline matrix. Surface damage was simulated by the Vicker's microhardness technique, with indentations being made over a wide load range to duplicate varying degrees of severity in the contact events.
Direct measurement of the indentation flaws was made by calibrated scanning electron microscopy. Fracture toughness values were determined by direct calculation from the indentation parameters. The repropagation of the indentation flaws was investigated by strength tests performed in biaxial flexure.
The results indicated that flaw introduction, as well as strength, fracture toughness, and the magnitude of strength loss sustained from surface damage, were all significantly affected by crack interactions with the crystallites in the glass-ceramic samples. The crack-crystallite interactions were extensive in the coarse microstructure samples. Crack pinning by the dispersed phase crystallites occurred at flaw sizes approximately equal to the mean free path distance between the dispersions, while at larger flaw sizes, crack deflection around the dispersed crystallites took place. Crack-microstructure interactions were absent in these same samples at flaw sizes less than the mean free path distance, and were not observed at all in the original glass or in samples heat-treated to yield only the fine microstructure.
In the coarse microstructure samples, the size of flaws introduced by surface contact was found to be limited by the crack pinning interaction, thus confirming the basic concept of the dispersion-strengthening model for brittle composites. A substantial toughening effect in these same samples was realized from the crack deflection. Fracture toughness for the coarse microstructure samples exhibited a crack size-dependency, with toughness values corresponding to that of the matrix measured at small flaw sizes, and to that of the composite, at larger flaw sizes. The phenomenon was not present in either the original glass or in the fine microstructure samples.
The crack-crystallite interactions occurring in the coarse microstructure samples greatly improved mechanical performance. The combination of decreased flaw size from crack pinning and increased fracture toughness from crack deflection resulted in strength values which were superior to those of the original glass. The crack size dependent fracture toughness enhanced the ability of the coarse microstructure samples to avoid potential strength losses following surface contact.