Study of SiC fiber-matrix interaction in ultra-high temperature ceramics by transmission electron microscopy.

Recent activities on UHTC have been focused on the improvement of the fracture toughness, which is still one of the major drawbacks affecting this class of materials. The first campaigns carried out on ZrB2 containing Hi-Nicalon SiC chopped fibers pointed out that the standard fracture toughness of 3.5 MPam1/2 can be raised up to a maximum of 6.2 MPam1/2 upon meticulous choice of the sintering additive and sintering cycle, which notably impact on the fibers integrity. However, in order to trigger effective toughening mechanisms, such as crack deflection, bridging and pullout, the interface fiber/matrix has to be carefully controlled. To this aim, different thermal/chemical treatments were performed on the Hi-Nicalon fibers to limit the reaction with the matrix and enable the fiber pullout. In addition another type of fibers was experimented, Tyranno, which indeed showed higher stability at high temperature and lower interaction extent with the matrix. Microstructural and preliminary mechanical properties are presented and correlated with the modified fiber/matrix interface. The effect of the type of fiber on the microstructure evolution and the thermo-mechanical properties is also discussed.