The fracture behavior of silicon carbide-graphite composites

The fracture behavior of CVD (chemical vapor deposition) SiC-graphite (coating-substrate) composites was investigated. The CVD SiC was deposited at various temperatures (1200-1300{dollar}spcirc{dollar}C) and times (2-4 h). All samples were in a sandwich configuration (SiC/graphite/SiC) and were characterized with respect to phase, geometry (coating thickness), Young's modulus, residual stress, and strength. The average Young's moduli of polished coatings were measured in situ using strain gage and dynamic resonance techniques and determined to be 439 and 414 GPa, respectively. These moduli were in good agreement with literature values and were consistent with the observed preferred orientation in the {dollar}beta{dollar}-SiC (cubic) coatings. The as-deposited residual strains in the composites were measured through the use of a strain gage/polishing technique. The average residual stresses in the coatings ranged from {dollar}-{dollar}80 to {dollar}-{dollar}400 MPa (compressive) and were highly dependent on coating thickness.; The fracture behavior of the in situ as-deposited coatings was examined in four-point flexure. When the coating thickness exceeded a "critical thickness" of {dollar}sim{dollar}50 {dollar}mu{dollar}m, catastrophic failure of the sample was observed. However, when the coating thickness was less than {dollar}sim{dollar}50 {dollar}mu{dollar}m, crack arrest/multiple cracking was observed in the tensile coating surface despite the fact that the graphite has a lower toughness than CVD SiC. Observation of the crack arrest/multiple cracking phenomenon in the composites was accomplished using a fluorescent dye penetrant. SEM observations suggested that the cracks arrested in the graphite.; The conditions for the arrest of the initial crack were modelled using an approximate stress intensity factor approach and were found to depend on the coating strength, substrate toughness, relative Young's moduli, and residual stresses. Subsequently, experimental data were applied to the model to test its validity and to predict a critical coating thickness. The proposed model provided a good representation of the observed fracture behavior of these composites and predicts a critical coating thickness within 20% of the observed values.