Woven Hi-Nicalon TM -reinforced melt-infiltrated SiC-matrix composites were tested under tensile stress-rupture conditions in air at intermediate temperatures. A comprehensive examination of the damage state and the fiber properties at failure was performed. Modal acoustic emission analysis was used to monitor damage during the experiment. Extensive microscopy of the composite fracture surfaces and the individual fiber fracture surfaces was used to determine the mechanisms leading to ultimate failure. The rupture properties of these composites were significantly worse than expected compared with the fiber properties under similar conditions. This was due to the oxidation of the BN interphase. Oxidation occurred through the matrix cracks that intersected the surface or edge of a tensile bar. These oxidation reactions resulted in strong bonding of the fibers to one another at regions of near fiber-to-fiber contact. It was found that two regimes for rupture exist for this material: a high-stress regime where rupture occurs at a fast rate and a low-stress regime where rupture occurs at a slower rate. For the high-stress regime, the matrix damage state consisted of through-thickness cracks. The average fracture strength of fibers that were pulled out (the final fibers to break before ultimate failure) was controlled by the slow-crack-growth rupture criterion in the literature for individual Hi-Nicalon fibers. For the low-stress regime, the matrix damage state consisted of microcracks which grew during the rupture test. The average fracture strength of fibers that were pulled out in this regime was the same as the average fracture strength of individual fibers pulled out in as-produced composites tested at room temperature.

Boron nitride nanotubes (BNNT) of significant lengths were synthesized by reaction of boron with nitrogen. Barium calcium aluminosilicate glass composites reinforced with∼4 wt% of BNNT were fabricated by hot pressing. Ambient‐temperature flexure strength and fracture toughness of the glass‐BNNT composites were determined. The strength and fracture toughness of the composite were higher by as much as 90% and 35%, respectively, than those of the unreinforced glass. Microscopic examination of the composite fracture surfaces showed pullout of the BNNT. The preliminary results on the processing and improvement in mechanical properties of BNNT‐reinforced glass matrix composites are being reported here for the first time.

The successful application of SiC/SiC ceramic matrix composites as high-temperature structural materials depends strongly on maximizing the fracture or rupture life of the load-bearing fiber and matrix constituents.

The most recent and innovative silicon carbide (SiC) fiber ceramic matrix composites, used for lightweight high-heat engine parts in aerospace applications, are woven, layered, and then surrounded by a SiC ceramic matrix composite (CMC). To further improve both the mechanical properties and thermal and oxidative resistance abilities of this material, SiC nanotubes and nanowires (SiCNT/NWs) are grown on the surface of the SiC fiber via carbon nanotube conversion. This conversion utilizes the shape memory synthesis (SMS) method, starting with carbon nanotube (CNT) growth on the SiC fiber surface, to capitalize on the ease of dense surface morphology optimization and the ability to effectively engineer the CNT-SiC fiber interface to create a secure nanotube-fiber attachment. Then, by converting the CNTs to SiCNT/NWs, the relative morphology, advantageous mechanical properties, and secure connection of the initial CNT-SiC fiber architecture are retained, with the addition of high temperature and oxidation resistance. The resultant SiCNT/NW-SiC fiber can be used inside the SiC ceramic matrix composite for a high-heat turbo engine part with longer fatigue life and higher temperature resistance. The differing sides of the woven SiCNT/NWs act as the "hook and loop" mechanism of Velcro but in much smaller scale.