Y5Si3C and Y3Si2C2: Theoretically predicted MAX phase like damage tolerant ceramics and promising interphase materials for SiCf/SiC composites

Researching for interphase materials that can protect SiC fibers from oxygen and water vapor attacks has become one of the most important issues for the applications of SiCf/SiC composites in high-temperature combustion environment. However, such kinds of interphase materials are not available yet. Herein, we report theoretically predicted properties of two promising interphase materials Y5Si3C and Y3Si2C2. Although crystallizing in different structures, they share the common features of layered structure, anisotropic chemical bonding, anisotropic electrical and mechanical properties, and low shear deformation resistance. The bulk moduli for Y5Si3C and Y3Si2C2 are 78 and 93 GPa, respectively; while their shear moduli are 52 and 50 GPa, respectively. The maximum to minimum Young's modulus ratios are 1.44 for Y5Si3C and 3.27 for Y3Si2C2. Based on the low shear deformation resistance and low Pugh's ratios (G/B = 0.666 for Y5Si3C and 0.537 for Y3Si2C2; G: shear modulus; B: bulk modulus), they are predicted as damage tolerant and soft ceramics with predicted Vickers hardness of 9.6 and 6.9 GPa, respectively. The cleavage plane and possible slip systems are (0000l) and (0001)[11 (2) over bar0] and (10 (1) over bar0)[0001] for Y5Si3C, and those for Y3Si2C2 are {h00} and (010)[101]. Since the oxidation products are water-vapor resistant Y2Si2O7, Y2SiO5 and/or Y2O3 upon oxidation, and the volume expansions are ca 140% and ca 26% for Y5Si3C and Y3Si2C2, they are expected to seal the interfacial cracks in SiCf/SiC composites. The unique combination of easy cleavage, low shear deformation resistance, volume expansions upon oxidation, and the resistance of the oxidation products to water vapor attack warrant them promising as interphase materials of SiCf/SiC composites for water-vapor laden environment applications. (C) 2018 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.