Toughening mechanisms in weak matrix ceramic composites

Tough behavior of ceramic matrix composites is closely related to the weak interface between fiber and matrix. The weak interfaces in composites have been provided by reducing the chemical bonding and the residual thermal stresses between constituents. A new way to introduce the weak interface by induced porosity or damaged zones, so called porosity toughening or damage toughening respectively, was proposed in this study to provide the rationale for guiding development of oxide/oxide composites. The existence of porosity toughening was examined by qualitative theoretical consideration and experiments. Porous oxide/oxide composites were fabricated using alumino-silicate fibers (Nextel 440) and magnesium alumino-silicate matrix (1:1 mixture of kaolinite and talc), and sintered at 1100 °C and 1200 °C. Both type of composites failed non-catastrophically in bending and tensile tests. The porous matrix composites showed the low ultimate strength of 12.7±3.4 MPa, but showed high work of fracture of 630±206 J/m2 for the specimens fired at 1100° C when tested in tension. Damaged zones or porous phases were introduced into the interface region of model composites and effects determined using single fiber pullout specimen or a single rod pullout specimen. Single fiber pullout specimens with a damaged zone (by the oxidation of SiC fiber in situ) produced higher frictional stress than debonding strength. Frictional stresses increased up to 6.4 ± 3.4 MPa as pullout of the damaged fiber proceeded. The porous phase was produced by coating alumina rods with zirconia or alumino-silicates fired at various temperatures. Distinct changes in pullout behavior were observed in data for the specimens with an alumino-silicate coating fired at 1450° C. Magnitudes of frictional stress and debonding strength of the interface were both less than 2 MPa. Both coatings produced a gradual change of the pullout load during the pullout tests --Abstract, page iii