This study focuses on the interfacial properties of a family of porous matrix oxide composites with uncoated fibers. Measurements of debond energy and sliding stress are made using a modified version of the established fiber push-in test. Modifications include the following: (i) use of a sphero-conical indenter (not a sharp-tipped one) to produce only elastic deformation of the fibers, and (ii) analysis of the loop width (instead of absolute displacements) to ascertain interface properties. The method obviates the need for indentation tests on reference (non-sliding) fibers. It also mitigates the problems associated with the elastic deformation of the surrounding matrix. The measured debond toughnesses (about 0.05 J/m 2 ) are about two orders of magnitude lower than the fiber toughness. This ensures that debonding will occur when a matrix crack impinges on a fiber. Additionally, the sliding stresses are in the same range as those reported for C-coated Nicalon fibers in glass-ceramic matrices (about 5 MPa). The latter results are qualitatively consistent with the observed damage tolerance in these two seemingly disparate systems, as manifested in the degree of fiber pullout as well as the notch sensitivity of tensile strength.

Fabrication of oxide fiber composites is accompanied by the development of drying cracks in the matrix following slurry infiltration. The cracks are a result of the inherent shrinkage in particle compacts during drying coupled with the mechanical constraints imposed on the matrix by the fibers. The effects are most pronounced in systems with three‐dimensional fiber architectures. A mitigation strategy based on the addition of coarse matrix particles to the fine particulates has been devised and demonstrated. Among the various implementation strategies explored, the most effective involves combining the two particle types (coarse and fine) into a single slurry and coinfiltrating the slurry through sequential vibration‐ and vacuum‐assisted processes. Regardless of the infiltration route, the SiC particles have no apparent detrimental effect on the fiber bundle properties. Additionally, they increase the through‐thickness thermal diffusivity by 50%–100%.

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Long‐term durability of ceramic composites requires the retention of weak interfaces to promote crack deflection and subsequent sliding at the fiber–matrix boundary. The present article addresses the efficacy of fugitive carbon coatings in achieving this goal in all‐oxide systems. A solution precursor route with a low‐temperature pyrolysis treatment has been devised and implemented for coating oxide fibers within a woven cloth. The coating is oxidized following completion of matrix processing to produce an interfacial gap. Upon comparing the mechanical properties of two continuous fiber ceramic composites—with and without a fugitive coating—the gap is shown to reduce the interfacial sliding resistance and significantly enhance the fracture resistance of the composite. It also enables an extension in the useful life of the composite when subjected to long‐term (1000 h) exposure at high temperature (1200°C). The results also suggest that modifications in the coating thickness may provide additional enhancement in composite durability.

A method for applying monazite coatings onto oxide fibers after matrix infiltration and sintering is devised and demonstrated. It uses an intermediate fugitive coating, followed by impregnation and pyrolysis of a monazite precursor solution. Composites with monazite coatings produced in this manner as well as those with only a fugitive coating exhibit superior mechanical performance to composites without any coating, as manifested in higher notched strength and fracture energy as well as significantly greater amounts of fiber pullout.