Crack interactions and thermal shock properties of ceramic laminates

Ceramic materials tend to fail catastrophically. Recent researches have been shown that thin compressive layers in the ceramics can stop crack propagation and prevent catastrophic failure to produce a threshold strength, namely, a strength below which the probability of failure is zero. The current work describes the multiple crack interactions and thermal shock properties of the ceramic laminates that have a threshold strength.; The threshold strength has been previously measured with a single crack which extends within a thicker layer. The behavior of multiple cracks in ceramic laminates within the laminate has been investigated, to observe the mechanism of crack interaction and measure their influence on the threshold strength. It was found that when the cracks in adjacent thick layers were offset by a distance less than twice the thickness of the thick layer, the cracks would interact and decrease the threshold strength. The number of interacting cracks, their orientation, and location can also have an effect on the threshold strength.; In a second series of experiments, thermal shock testing was performed with both monolith and laminate ceramics using a water quenching method following by a strength measurement. Laminate ceramics showed better thermal shock resistance than monolith ceramics by increasing the critical temperature differences defined by sudden drop of the strength retained after the thermal shock. The ceramic laminates showed characteristic crack patterns: edge cracks and lateral cracks. This was due to the residual stress in the laminate ceramics.; To further improve the thermal shock resistances of laminate ceramic, an external compressive coating was proposed and applied with dip-coating technique. Different processing problems were solved and the quality of the coating was controlled by understanding the rheological properties of the slurry and their effect producing an external coating. It was shown that the compressive external coating not only produced strengthening, but also produced improved thermal shock resistance: the critical quench temperature differences for all specimens were increased due to the external compressive coating.