Laminar ceramics that exhibit a threshold strength

The current work describes a new method for fabricating reliable, damage-tolerant brittle materials. This is accomplished by incorporating thin layers of material under residual compression on the surface and throughout the bulk of another material. The residual compression developed within these thin layers, e.g. via differential thermal contraction of the laminae during cooling from processing, acts to arrest the propagation of otherwise catastrophic cracks. A specific level of applied stress, defined as the threshold strength, is required to cause the cracks to ‘puncture’ through the compressive layers and produce catastrophic failure. Therefore, since failure at stresses below the threshold strength is not possible, this new class of composite represents a potentially powerful new method for ensuring the reliability of ceramic-based components.; A model developed to describe the threshold strength, based on the stress intensity shielding effect of the compressive layers, shows that the threshold strength is expected to be dependent upon the properties of the compressive layers, i.e. their thickness, separation distance, and magnitude of residual compression. Experimental data show that the predictions made by the model agree well with the observed behavior of laminates in which the thickness of the compressive layers and/or magnitude of residual compression is small. The data further show, however, that when the thickness of the compressive layers and/or the magnitude of residual compression is large, the model ceases to accurately describe the observed behavior. This discrepancy is attributed to crack bifurcation, which causes the cracking within the compressive layers to follow paths inconsistent with the assumptions upon which the model is based.