Synthesis, microstructure and mechanical properties of lamellar YB2C2 – based ultra-high temperature ceramic composites

A significant improvement of the mechanical performance was observed following the introduction of rare earth oxides in ZrB2-based ultra-high temperature ceramic matrix composites (UHTCMCs), resulting in the formation of ternary boro-carbides of general formula REB2C2, belonging to a new class of layered compounds akin to MAX phases. These layered phases possess high melting points and could be responsible for the toughening of UHTCMCs, but their formation, properties and role were never fully investigated. In this study we focused on the potential routes for the synthesis of YB2C2 phases at temperatures typical of UHTC sintering, starting from Y2O3, carbon and four different boron sources (B, B2O3, BN, B4C) and their microstructure was analysed by SEM, XRD and TEM. The mixture with B4C led to the highest selectivity towards the formation of YB2C2 and was selected to fabricate a long carbon fibre reinforced YB2C2 ceramic composite, which was mechanically tested displaying a flexural strength of 380 MPa. Finally, the chemical stability in air of these materials was assessed.