Synergistic strengthening and toughening of graphite composites with a biomimetic bilayer ceramic cytoderm

The development of graphite composites for extreme environments is persistently hindered by inefficient reinforcement, primarily due to poor spatial control of ceramic-reinforcement phases and weak interfacial bonding. Herein, biomimetic cellular-structured bilayer Mo_4.8Si₃C_0.6-SiC reinforced graphite microbead (MCMB@MSC-SiC) composites were fabricated via reactive spark plasma sintering (RSPS) of MoSi₂ and MCMB. The synergistic strengthening and toughening were governed by sintering temperatures and ceramic components. The sample sintered at 1600 °C achieved near-full densification (99.5% relative density) with a continuous bilayer ceramic cytoderm comprising an outer MSC layer and an inner SiC transition layer. This structure formed via C diffusion-dominated reactions (MoSi₂→Mo₅Si₃ + Si_free → MSC + SiC), with SiC growth driven by C diffusion at the MCMB/SiC interface. By coupling a three-dimensional continuous ceramic cytoderm with well-bonded phase interfaces, the composites yielded exceptional flexural strength (295 MPa) and fracture toughness (3.8 MPa m^1/2) at optimized reinforcement content (21 vol% MSC and 26 vol% SiC). Key toughening mechanisms include interfacial crack deflection, graphite nanoflake bridging/pull-out, microcracking, bilayer-synergistic crack bridging/deflection, and suppressed debonding due to residual compressive stresses at the MCMB/SiC and MSC/SiC interfaces. This work establishes a paradigm of biomimetic bilayer design for developing high-performance graphite composites tailored for extreme environments.