The influence of fiber type and method of composite fabrication on the thermal shock behavior of 2-D fiber-reinforced ceramic composites is studied. Thermal shock tests are performed using a water quench technique, and thermal shock damage is characterized by both destructive and nondestructive techniques. It is shown that the composites possessed superior resistance to thermal shock damage than the monolithic ceramics. Catastrophic failure due to severe thermal stresses is prevented in composites and a significant portion of their original strength is retained at a quench temperature difference up to 1000°C. These results along with an analysis of the thermal shock damage mechanism based on the destructive and nondestructive tests is described.

The burner rig thermal fatigue properties of SiC continuous fiber/Si3N4 ceramic composites were examined under impinged jet fuel flame, a constant applied tensile stress and thermal cycling in the temperature range 500-1350 °C. The SCS-9 SiC fiber/Si3N4 composites failed within the flame impinged zone, whereas the SCS-6 fiber/Si3N4 composites failed outside the flame impinged zone due to the high thermal stresses resulting from high-temperature gradients. Analytical transmission electron microscopy was used to investigate the microstructure and chemistry of the fiber, matrix and fiber/matrix interface in the failed SCS-9 SiC fiber/Si3N4 composites. The partial degradation of columnar structure of the fiber was interpreted as the dominant mechanism of burner rig thermal fatigue failure of SCS-9 SiC fiber/Si3N4 composites.

In many fiber reinforced composite systems, protective coatings on the fibers are required in order to protect the fiber from reaction with the matrix, to modify the mechanical properties of the fiber-matrix interface, or to protect the fiber during processing steps such as chemical vapor infiltration. We have developed techniques for coating single crystal sapphire fibers with thin AIN layers, using CVD from an organometallic precursor in a cold wall reactor. Side by side tests of coated and uncoated fibers introduced into a MoSi2 matrix, either by hot isostatic pressing or by reactive vapor infiltration (RVI), indicate that the coating inhibits both fibermatrix interdiffusion and volatilization of the fibers by HC1 produced in the RVI process.

β-SiC/TiC composites were synthesized using the process of self propagating-high temperature combustion synthesis (SHS). The heat released during the exothermic reaction between Si and C powders (with a δH of -14kcal/mole) and that Ti and C powders (with a δH of -44.1 kcal/mole) is sufficient to cause the melting of the powders into which the carbon diffuses and from which β-SiC and TiC precipitate out of the supersaturated solution. The composite was characterized using X-ray diffraction techniques, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. An attempt was made to understand the mechanism of formation of the composite. We have proposed a mechanism to understand the formation of the SHS compounds based on dissolution, diffusion and precipitation from the supersaturated solution. There is no evidence for the presence of TiSi2 and an attempt was made to explain this observation based on free energy considerations. Diamond film was then grown on the pellet by hot filament CVD technique using methane and hydrogen gas as the reactants. The deposition was conducted for a period of four hours. A continuous film of diamond was found to grow on β-SiC/TiC composite using this technique. The diamond film was characterized by using Raman spectroscopy and SEM. The diamond film showed both (001) and (11) facets with average grain size of 5 μm.

The elevated temperature rheology of continuous SiC (Nicalon®) calcium aluminosilicate glass-ceramic matrix composites is evaluated in uniaxial compression creep experiments (-σ1 = 20-to-40 MPa; T = 1300–1320°C). The steady state strain rate is demonstrated.to be highly sensitive to the orientation of the reinforcement relative to the maximum compressional stress, with highest bulk specimen strain rates noted for conditions in vWhich the sliding between the fiber and the matrix is optimized as a kinetic flow response (i.e., a fiber orientation of approximately 40-50° from σ1). One further discovers that the temperature sensitivity (i.e., activation energy) of flow increases as the amount of interface flow/sliding increases. The experimental results suggest that the high-temperature, low-stress interface response in this composite system is related to the ductile flow of the “Planar” SiO2 reaction-layer interphase that exists (in addition to the well-recognized planar carbon interphase) in these materials. The results of these simple experiments are used to calibrate a microscale-to-macroscale rheologic model in which the fibermatrix interface is described by a viscous constitutive relationship.