Microscopic Phase-Field Modeling with Accurate Interface Thickness Representation: Applied to Ceramic Matrix Composites.

Ceramic matrix composites (CMCs) are promising candidates for high-temperature structural applications. However, their fracture toughness remains low due to strong chemical bonding between fibers and the matrix. To improve toughness, engineered interfaces such as pyrolytic carbon (PyC) and hexagonal boron nitride (h-BN) are commonly introduced. These interfaces promote crack deflection and fiber bridging, leading to improved damage tolerance and pseudo-ductile behavior. To investigate the influence of interface thickness on mechanical performance and to identify optimal thickness ranges, we propose a microscopic phase-field model that accurately resolves interface thickness and material contrast. This model overcomes the limitations of conventional smeared interface approaches, particularly in systems with variable interface thickness and closely packed fibers. We apply the model to simulate the fracture behavior of unidirectional SiC fiber reinforced SiC matrix (SiCf/SiCm) composites with PyC and h-BN interfaces of varying thickness. The numerical results show strong agreement with experimental findings from the literature and reveal optimal interface thicknesses that maximize toughening effects. These results demonstrate the model's predictive capabilities and its potential as a tool for interface design in brittle composite systems.