Thermoelastic mechanism of matrix cracking behavior of cross-ply melt-infiltrated SiC ceramic matrix composites with varied free silicon

The free silicon content in silicon melt infiltrated ceramic matrix composites (CMC) causes the GPa-range residual microstress in silicon and SiC phases and the MPa-range average residual matrix stress in the Si-SiC matrix. These residual stresses are naturally linked to the matrix cracking stress. The complex effect of free silicon on the matrix cracking behavior in SiC fiber reinforced cross-ply laminate CMCs has been investigated by experiment and thermoelastic analysis. The CMC with the lowest silicon exhibits the highest onset cracking stress. A proposed thermoelastic model, which includes silicon crystallization expansion, reveals that the Si-SiC matrix fracture energy in the low-silicon CMC is nearly doubled due to suppressed elastic potential energy associated with the GPa-range microstresses. This improved fracture energy overturns the effect of matrix residual stress and becomes the dominant factor in determining cracking stress. These results offer an energy-based guideline for improving onset cracking stress.