Low‐Temperature Construction of Self‑Strengthening, Reusable, Ultralight Mullite Fiber Composites with Fish‑Scale‑Like Adaptive Boron‑Rich Layers

ABSTRACT Reusable thermal protection materials are prone to structural damage accumulation and performance degradation under cyclic thermomechanical loading. Overcoming the conventional “cycling equals damage” paradigm is a critical challenge for long‐term service. Here, we exploit low‐temperature ceramization of a boron‐rich precursor at 600°C to in situ construct a fish‐scale‐like adaptive ceramic reinforcement layer (ACRL) on mullite fiber surfaces and junctions. The composite exhibits remarkable thermomechanical‐cycling‐induced strengthening (TMCIS) during cyclic compression at 420°C, with the average modulus nearly tripling after 100 cycles. In situ SEM reveals that thermomechanical coupling drives local displacement of adjacent fiber segments and node reconstruction; the ACRL of viscous softening acts as a dynamic “high‐temperature solder” that bridges fiber contacts and reconfigures the load‐transfer network, enabling a service‐induced strengthening mechanism. After 50 cycles (totaling 25 h) at 420°C, the mass loss per cycle is only 0.07%. Furthermore, the material simultaneously achieves a low density (0.29–0.35 g/cm 3 ), high compressive strength (1.60 MPa), and low thermal conductivity (0.062 W/(m·K)), realizing a synergy of lightweight, load‐bearing, and thermal insulation. This work converts cyclic loading from a damage source into a driving force for structural strengthening, establishing a new adaptive paradigm for long‐life reusable thermal protection materials.