Development and characterization by HRTEM of hot-pressed Si3N4-SiC(w) composites

The activation of toughening mechanisms in Si3N4-SiC(w) composites depends essentially on interfacial characteristics. They are affected by the chemical compatibility and the difference of the thermal expansion coefficients between the SiC whiskers and the Si3N4 matrix. In this work, Si3N4-SiC(w) composites were produced by hot-pressing, using AlN/Y2O3 as additives. The SiC whiskers-Si3N4 matrix interface has been investigated by high-resolution electron transmission microscopy (HRTEM). A thin amorphous phase has been detected at the whisker/matrix interface and at grain boundaries, triple junctions of matrix grains. This intergranular phase causes weak bonding between the whiskers and the matrix, enhancing crack deflection toughening mechanisms. As a result, and due to the high aspect ratio of the SiC whiskers, a significant improvement of the fracture toughness is achieved, when compared to Si3N4 monolithic ceramics.