Intergranular Oxynitride to Regulate Solution–Reprecipitation Process in Gas–Pressure-Sintered SiC Ceramics with AlN–Y₂O₃ Additives

Core−rim structures are found as a general microstructural feature in SiC ceramics gas–pressure sintered with AlN−Y₂O₃ additives in different ratio, combined with a range of α/β-SiC seeding to control the phase transformation. Using analytical electron microscopy, the authors investigate the re-distribution of additives to detect uniform level of AlN solution into SiC rims, independent to the seeding ratio, and also not proportional to additives. The behavior of AlN solution reveals the presence of sintering melt based on nitro-carbide to start the reprecipitation of SiC rims, while the cores are remnants from starting powders to control the phase transformation as well as the grain growth behavior. Among various intergranular phases, Y₁₀Al_2-zSi_3+zO_18+zN_4-z is observed as common hence the secondary phase, while its Al level changes in an opposite trend to AlN solution in SiC rims. This oxynitride phase precipitates from the Y-rich oxynitride melt, which extends and regulates the solution−reprecipitation process and develops into the Y−Si−O−(C) melt to create a series of residual intergranular oxides. Post-annealing can fully devitrify the viscous intergranular glass to reach complete crystallization of ceramic body, which leads to improved creep resistance at high temperature for this SiC ceramic system.