Chemo-mechanical coupling model for ablation behavior of C/C-HfC composites at 2100 °C

Hafnium carbide (HfC) coatings are critical for protecting C/C composites in high-temperature and oxygen-rich environments. During service, ablation-induced chemical reactions and the associated volumetric expansion generate complex internal stress fields. Ultimately, these stresses drive the cracking and spallation of the coating. This work proposes a coupled chemo-mechanical ablation model that incorporates the feedback loop between stress-dependent reaction kinetics and chemical expansion strain. The model is implemented via a user-defined element (UEL) subroutine. It is employed to investigate the critical role of surface roughness in the coevolution of composition and internal stress within HfC coatings. Its predictive accuracy is validated against experimental post-ablation residual stresses and oxygen distribution profiles. The results identify the peak-to-valley transition regions as critical failure sites, where prolonged ablation (up to 40 s) elevates shear stress to 375 MPa. Furthermore, increasing surface roughness (Ra) from 4 to 12 μm accelerates the degradation of oxygen barrier properties and amplifies the maximum tensile and shear stresses by over 48%. These findings provide fundamental insights into the failure mechanisms of HfC-based thermal protection systems.