Degradation of a SiC‐SiC composite in water vapor environments

Abstract

The article examines degradation of a SiC‐based fiber composite containing Tyranno ZMI fibers in water vapor at elevated temperatures (800°C and 1100°C). Degradation is characterized through mechanical tests under cyclic and quasi‐static tensile loading in the near‐threshold regime, at stresses at or slightly above the matrix cracking limit. These tests are augmented by examinations of fracture surfaces and polished cross‐sections, measurements of fracture mirror radii, and measurements of interfacial debond toughness and sliding resistance. Degradation involves highly localized consumption of fibers through reactions of water vapor with the fibers and the BN coatings in regions adjacent to the few matrix cracks present at low stresses; the global hysteresis response and the average interfacial properties are minimally affected. Boria formed by oxidation of BN appears to play a fluxing role; it combines with silica on the fibers to form a non‐protective molten glass. Inhomogeneous fiber consumption leads to stress concentrations in the fibers and hence reduced fiber strength. Spatial variations in the degradation process occur at two length scales: at the macroscopic scale, because of cracking of the external CVI SiC overcoat and subsequent water ingress through the cracks, and at the tow‐scale, because of cracking of the CVI SiC around the tows. Parsing the kinetic processes over the two length scales remains a significant challenge.

Optimizing the fiber push‐out method to evaluate interfacial failure in SiC/BN/SiC ceramic matrix composites

“…Variations in inter-tow mechanical properties as opposed to intra-tow variations were investigated by Zok et al 42 who found no direct correlation between testing fibers close (1 mm) or from a distance (10 mm) to the fracture plane, as well as no evidence of fibers locking (from interphase debris or other fibers) during testing. However, there exists a difference in coating thickness (for both BN and SiC) deposited by CVI proportional to the distance of the tow to the outskirts of the CMC-thus potentially influencing the interfacial measurements.…”

Cracking during pyrolysis of preceramic polymers within glass microtubes

“…For instance, in orthogonal weaves, prismatic matrix pockets with dimensions exceeding 1 mm are common. 34,35 One specific strategy for creating more uniform pore distributions involves use of SiC particles with dimensions comparable to the fiber diameter to fill the larger spaces between the tows and between the layers. The utility of this approach has been demonstrated in oxide composites.…”

A synergistic model of stress and oxidation induced damage and failure in silicon carbide‐based ceramic matrix composites

“…Finally, by considering appropriate chemical kinetics, the above model can be modified to investigate the deleterious effect of water vapor on SiC/SiC composites 46 and to model the harmful effect of moisture in oxide/oxide CMCs 47 . The above model can also be extended to study the effect of environmental barrier coatings on SiC/SiC CMCs 48 …”