Mullite (3Al2O3·2SiO2)–Aluminum Phosphate (AlPO4), Oxide, Fibrous Monolithic Composites

Abstract

Mullite–AlPO4 fibrous monolithic composites were fabricated by a co‐extrusion technique using ethylene vinyl acetate (EVA) as a binder. Processing routes such as mixing formulation, extrusion sequence, binder removal cycle, pressing, and sintering procedures are described. An effort to make tougher composites was conducted by modifying the microstructures of the composites. Different kinds of monolithic composites were fabricated by changing the number of filaments, and the composition and thickness of interphase layers, and their microstructural and mechanical properties were characterized. To make the interphase more porous and to facilitate debonding and fiber pullout in the composite, graphite was added as a fugitive “space filler” into the interphase material and then removed. A fibrous monolithic composite with a sintered interphase thickness of 5–10 μm and an interphase composition of 50 vol% graphite and 50 vol% AlPO4 had a three‐point bend strength and a work of fracture of 129 ± 2 MPa and 0.86 ± 0.05 kJ/m2, respectively. This corresponded to 42% of the strength but 162% of the work of fracture when compared with the values for a single‐phase mullite. Two‐layer, mixed 50% two‐layer:50% three‐layer, and three‐layer fibrous monoliths were fabricated and their microstructural and mechanical properties were studied. The difference in the sintering behaviors of the two‐layer and three‐layer composites is described.