Investigation on the multi-scale mechanical strengthening mechanism of SiC-coated 3D high thermal conductivity C/C composites

Single-layer SiC coatings were prepared on high thermal conductivity C/C (HTC-C/C) composites using two distinct methods: pack cementation and chemical vapor deposition. Three-point bending tests reveal surprisingly enhanced mechanical strength of the composites after coating fabrication. The enhancement mechanism is elucidated based on real-time acoustic emission monitoring, interface stress and strength analysis, and porosity and phase characterization. Results show that SiC phase infiltration during coating fabrication fills the composites’ pores. Due to thermal expansion coefficient mismatch, the carbon fiber/pyrolytic carbon (CF/PyC) interface stress state changes: PyC tensile stress decreases, the carbon fiber compressive stress increases, and the interface stress gradient is reduced. And the shear strength at the CF/PyC interface was enhanced from 15.69 ± 4.26 MPa to 52.59 ± 5.37 and 29.07 ± 4.88 MPa, which were increased by 235.18%and 85.28%, respectively, effectively mitigating fiber debonding and enabling efficient interface-to-fiber force transfer under external loading. Thus, the HTC-C/C composites shifted from fiber fracture-dominated single energy dissipation to fiber-PyC-interface multi-scale synergistic energy dissipation, ultimately enhancing mechanical properties. The highest flexural strength of the SiC-coated HTC-C/C composites can reach 298.42 ± 7.59 MPa. This work provides theoretical guidance for the preparation of surface coatings of HTC-C/C composites in the future.