UHTCMCs: short vs continuous fibers

ISTEC ongoing research is focused on fabrication and characterization of UHTCs-based ceramics containing short randomized or continuous SiC and C fibers/preforms (UHTCMCs). In this talk, processing and characterization are first shown for short SiC or C fibers incorporation, considering different kinds of commercial products and various compositions. Usually, the densification of these composites is achieved through hot pressing, but a major issue is that the fibers may change their chemistry and properties if the sintering temperature exceeds 1800°C for SiC fibers and 1900°C for C fibers. Therefore, the use of sintering aids is advisable for maintaining the densification temperature below these limit values. At the same time, the formation of a fiber/matrix interphase is one of the most critical issues and depends on many factors: the densification temperature first of all, but also processing procedure, type of sintering aid and type of fiber. On the basis of fundamental studies on boride/fiber interaction, the best systems are selected for fabrication of continuous SiC or C fibers - ZrB2 composites. In this case we used a hybrid processing method that couples a vacuum bag infiltration technique for impregnation of 0-90°C unidirectional fiber laminates to conventional hot pressing. The use of a different processing technique and a diverse fiber architecture enable us to analyze these composites from another point of view, finding interesting analogies and differences compared to short fibers. Emblematic cases, e.g. ZrB2 matrices containing 50% of randomized short fibers and 50% of continuous fibers are hence illustrated to compare densification, microstructure, fiber/matrix interphase, fracture behavior and properties.