Effects of Zr and chopped C fiber on microstructure and mechanical properties of SiBCN ceramics

To further improve the thermal shock resistance and ablation resistance of SiBCN system ceramics targeted much harsh environment, both ultra-high temperature ceramic phases and carbon fibers are expected to be incorporated simultaneously as the reinforcements. However, tough problems like difficulty in densification as well as degradation of C fibers (Cf) due to interfacial reaction usually cannot be avoided. Thus, in this study, Zr as well as chopped Cf with BN coating were introduced into SiBCN ceramics by mechanical alloying and after hot-pressing sintering to prepare C/SiBCNZr composites. The structure and phase component development have been characterized in detail by XRD, SEM, TEM and XPS etc., and mechanical properties, fracture behavior and toughening mechanisms were also investigated. Zr-B and Zr-O bonds are detected besides Si-C, N-B and N-B-C bonds when pre-alloyed ZrB2 amorphous powder was incorporated into SiBCN system by further mechanical-alloying treatment. After hot-pressing, the ultra-high temperature phases of ZrB2 and ZrN were formed as expected besides the original matrix phases of BN(C) and SiC. Excellent mechanical properties can be obtained for SiBCNZr ceramic. The BN coating on carbon fibers successfully avoids the severe fiber degradation and makes it possible for moderating strong interface bonding, which ensuring the debonding, crack deflection, fiber pulling-out and bridging during the fracture. As a result, the C/SiBCNZr composites retain good mechanical properties and fractures in a pseudoplastic fracture manner, showing better potential applications in harsh environments. Optimization of the BN coating on Cf and Zr incorporation by pre-mechanical alloying can be a good route to achieve Cf/SiBCNZr composites with a better balance between thermal shock resistance, ablation resistance and mechanical properties.