July 2005 Ceramic Composites with Three-Dimensional Architectures 1883 1 aKU (c) Fig 4. SEM micrographs of representative fracture surfaces for composite architectures considered in this study.(a)thin( 30 um)25 vol% mullite /75 vol% alumina layers, (b)thick(60 um)25 vol% mullite/75 vol% alumina layers, (c)thin( 30 um)55 vol% mullite/45 vol% alumina layers, and (d)thick(-60 um)55 vol% mullite/45 vol% alumina layers the first paper of this series; consequently and for reasons de- plete consolidation of the agglomerates described in Part I of cribed below, cracks running down the center of the compress ive layers intersecting the surface of the specimens(edge cracks) Figure 7 summarizes the results of an extensive EDX explo- were observed for the composites containing both thick and thin ation of mating fracture surfaces of the thick 55 vol% mullite compressive layer composite just beneath the tensile surfac Table II lists the average strength for the monoliths and com- posite specimens as well as the range of strengths obtained for each architecture. As shown the strengths of the monoliths were larger than those of the composites, but their values exhibited larger scatter in values. The strengths of the composites decrease ith increasing layer thickness and larger apparent compressive composite architectures considered. The fracture surfaces in- crease in roughness with both increasing layer t hickness and in- m asing mullite content Figures 5 and 6 show surfaces within the composites that encompass a crack-like void for thick compressive laye taining 25 and 55 vol% mullite, respectively. These surfaces could be identified as voids because the surfaces are dentical to those of an external surface of a dense poly ne body, where grooves are present wherever a grain boundary ntercepts an external surface. They are easily d from a fracture surface due to the rounded ap 2 m EDX analysis of the non-bonded regions on matin surfaces confirmed the presence of Si, namely, the Inous of mullite. on both surfaces indicating that the void bonded lie entirely within the compressive layer and result from incom- 60 uregion on the fracture surface of a composite with thick 25 vol% mullite/75 vol% alumina laythe first paper of this series;5 consequently and for reasons de￾scribed below, cracks running down the center of the compress￾ive layers intersecting the surface of the specimens (edge cracks) were observed for the composites containing both thick and thin layers formulated with 55 vol% mullite. Table II lists the average strength for the monoliths and com￾posite specimens as well as the range of strengths obtained for each architecture. As shown, the strengths of the monoliths were larger than those of the composites, but their values exhibited a larger scatter in values. The strengths of the composites decrease with increasing layer thickness and larger apparent compressive stress (increasing mullite content). The range of strengths ob￾served for the composites was relatively small (o710 MPa). Figure 4 shows representative fracture surfaces of the four composite architectures considered. The fracture surfaces in￾crease in roughness with both increasing layer thickness and in￾creasing mullite content. Figures 5 and 6 show surfaces within the composites that encompass a crack-like void for thick compressive layers containing 25 and 55 vol% mullite, respectively. These surfaces could be identified as voids because the surfaces are identical to those of an external surface of a dense polycrystal￾line body, where grooves are present wherever a grain boundary intercepts an external surface. They are easily distinguished from a fracture surface due to the rounded appearance of the grains. EDX analysis of the non-bonded regions on mating fracture surfaces confirmed the presence of Si, namely, the presence of mullite, on both surfaces, indicating that the voids lie entirely within the compressive layer and result from incom￾plete consolidation of the agglomerates described in Part I of this series. Figure 7 summarizes the results of an extensive EDX explo￾ration of mating fracture surfaces of the thick 55 vol% mullite compressive layer composite just beneath the tensile surface. Fig. 4. SEM micrographs of representative fracture surfaces for composite architectures considered in this study. (a) thin (B30 mm) 25 vol% mullite /75 vol% alumina layers, (b) thick (B60 mm) 25 vol% mullite/75 vol% alumina layers, (c) thin (B30 mm) 55 vol% mullite/45 vol% alumina layers, and (d) thick (B60 mm) 55 vol% mullite/45 vol% alumina layers. Fig. 5. SEM micrographs at various magnifications showing a non￾bonded region on the fracture surface of a composite with thick (B60 mm) 25 vol% mullite/75 vol% alumina layers. July 2005 Ceramic Composites with Three-Dimensional Architectures 1883