Processing and properties of ceramic composites with three-dimensional architectures of thin compressive layers

Threshold strength behavior and flaw insensitivity have recently been observed in laminar ceramic composites containing thick stress-free layers and thin layers in residual compression (due to thermal expansion mismatch). The threshold strength behavior is due to a crack tip shielding effect in which the stress intensity factor cracks initiating in the thick layers and propagating through the thin layers is reduced by the compressive stresses. Consequently, a well-defined level of stress (the threshold strength) is required to cause the cracks to propagate entirely through the compressive layers to cause failure of the specimens regardless of the initial flaw size. The threshold strength behavior is only observed in one loading orientation relative to the layers.; The current work describes the processing procedures and fracture behavior of ceramic composites with three-dimensional architectures of thin compressive layers which are expected to exhibit threshold strength behavior in any tensile loading orientation and possess higher threshold strengths than laminates of similar architectural dimensions. Processing of the composites consists of producing spherical agglomerates, coating these agglomerates with a thin layer of material which upon heat treatment develops a residual compressive stress, and consolidating the coated agglomerates into bars suitable for mechanical testing. Finite element modeling of the residual stresses in the composite architecture is used to explain fracture behavior of the composites which is shown to depend on the architectural variables. Fractography of failed specimens is instrumental in elucidating the processing flaws within the architectures, one of the most pervasive of which is inter-agglomerate voids due to incomplete consolidation; additional processing steps are developed to remove these flaws and produce flaw-free composites suitable for a study of a flaw-insensitivity through the introduction of controlled flaws. Mechanical testing of composites produced utilizing these additional processing steps are shown to have higher average failure strengths; however, subsequent fractography reveals that inter-agglomerate voids continue to act as failure origins rendering any study of flaw insensitivity unfeasible.