| Fabrication of composite materials by laminate processing provides an opportunity for control of whisker orientation and/or whisker placement; however, it is a relatively new technique for making whisker-reinforced ceramic matrix composites. This thesis examines three major aspects, namely, (1) laminate processing, (2) composite structure, and (3) mechanical properties of oriented SiC whisker-reinforced mullite composites.;In the first part, the experimental processes for fabrication of dense, homogeneous, oriented SiC whiskers reinforced mullite composites with up to 50 vol% reinforcement were systematically developed. The key parameters and critical conditions of tape-casting formulation, lamination and hot pressing of these materials were identified. In particular, the fundamental factors and physical background of tape-casting based laminate processing, such as fluid dynamics of whisker orientation and formation of anisotropic composite microstructure, were established. The ultimate whisker orientation was a function of whisker aspect ratio, whisker starting position, casting parameters and slurry rheology. Whisker orientation increased as the shear stress of tape-casting was increased.;In the second part, two levels in the composite structure, i.e., whisker orientation in the cast tapes and structural anisotropy of hot-pressed composites, were characterized. Based on the relationship between the crystal structure and growth habit of SiC whiskers, XRD was used to quantitatively monitor the change of whisker orientation and whisker orientation distribution in the matrix. Based on the semiconducting nature of SiC whiskers, electrical resistivity and dielectric permittivity measurements could display the composite structural anisotropy and whisker percolation behavior.;The final part of the thesis covers the measurements and related modeling of the mechanical properties of these composites, including the elastic modulus, coefficient of thermal expansion, flexural strength and fracture toughness. Young's modulus of the composites was increased by addition of whiskers, and the behavior was fit well by the Halpin-Tsai equations. The coefficient of thermal expansion was decreased by incorporating SiC whiskers, and the composite behavior was in good agreement with the Schapery predictions. The maximum flexural strength and fracture toughness were 940 MPa and 6.85 MPa... |
