Abstract: Ultra-high-temperature ceramic modified carbon/carbon composites (UHTC-modified C/C) are rapidly developing for extreme environments such as hypersonic thermal structures; however, the dominant mechanisms controlling their mechanical reliability remain inconclusive. In this study, a representative C/C-ZrC-SiC system was investigated. A decoupled comparison strategy was employed to separate and quantify the modulus effect and the interfacial effect. Digital image correlation (DIC) measurements were conducted during uniaxial compression and three-point bending, and a multiscale finite element model incorporating explicit interfaces was established. The results reveal that the interface layers formed by ceramic modification exhibit significant differences in thickness and chemical reaction degree. The C/C-ZrC-SiC composite possesses a heterogeneous interface of moderate strength, thereby achieving an superior strength and more stable crack propagation, its stability against tensile-shear coupling increased by 169.6 % compared to C/C-SiC. Under three-point bending, the stability against tensile-shear coupling improved by 14.7 % and 39.3 % compared to C/C-ZrC and C/C-SiC, respectively. The failure mode shifted from shear-dominated instability induced by excessively strong interfaces or interlayer cracking caused by overly weak interfaces toward more stable crack propagation and energy dissipation pathways. Inter-bundle scale simulations further indicate that, compared to the strong interfacial state, the moderate-strength interface reduces the peak matrix stress by approximately 56.6 %. Overall, the final failure of UHTC-modified C/C composites is governed by a critical synergy between interfacial strength and modulus ratio. Based on this, a generalizable design guideline is proposed to guide the targeted design and reliability improvement of UHTC-modified C/C composites for extreme service conditions.

Key words: UHTC-modified C/C composites, Decoupled analysis, Interfacial, DIC, Mechanical response