Characterization of ceramic composites and of their constituants: precursors, fibers and interphases

Examples from recent works are chosen to illustrate the importance of TEM in the study of high performance ceramics: a study on SiC whiskers and Si 3N4 crystals has evidenced that both are coated with a poorly organized thin film. The film was found to be aromatic turbostratic carbon for the whiskers, and non stoechiometric silicon oxynitride for Si3N 4. The study of sintered Si3N4 + SiC composites (MgO as sintering aid) has revealed crystallized triple points which are not simple magnesium silicates. On the contrary, triple points are amorphous within Si3N4 + SiC + Al2O3 + Y2O3 composites. In another work, the thermal degradation mechanisms of a Nicalon fiber have been evidenced. The comparison with a bulk ceramic derived from the same precursor (but uncured) has shown that the oxygen release induces the free carbon degradation in the fiber. On the contrary, free carbon remains unaltered in the bulk (oxygen-free) ceramic. Consequences are that the drastic SiC crystal growth, which occurs by coalescence due to the grain boundaries degradation, is favoured in the fiber and delayed for the uncured ceramic. A last example on 2D SiC/SiC or SiC/C/SiC composite has allowed to correlate the nature and the texture of the interphases found at the fiber-matrix interfaces with some mechanical properties such as resilience.