UC Santa Barbara

The mechanical response of ceramic matrix composites depends critically on slip along the matrix-fiber interface, which is usually achieved with thin coatings on the fibers. Environmental attack of such coatings (enabled by ingress of reactants through matrix cracks) often leads to significant degradation, through removal of the coating via volatilization and oxidation of exposed SiC surfaces. The extent of the volatilization region extending from the matrix crack plane (i.e. recession length) is strongly coupled to the formation of oxide, which ultimately fills open gaps and arrests further reactions. This dissertation presents models to quantify these effects over a broad range of environmental conditions, coating thickness and matrix crack opening. Analytical solutions are presented for the time to close recession gaps via oxidation, and the associated terminal recession lengths obtained near free surfaces. A broad parameter study illustrates that recession behaviors are controlled by a competition between volatilization and oxidation rates. As such, the extent of recession is highly sensitive to water vapor and temperature, providing an explanation for disparate observations of recession under seemingly similar conditions. The extent of recession in the interior of composites is also illustrated, using a straightforward reaction-transport model. Recession lengths decay rapidly away from the free surface, with the extent of recession penetration scaling with maximum recession at the free surface.

Models are also presented to quantify the impact of BN coating recession on SiC/SiC ceramic matrix composite strength as a function of environmental conditions and crack spacing. Under the assumption of uniform coating recession, an analytical expression for BN recession length is combined with composite fragmentation models to calculate retained strength. The results are characterized by two terminal conditions: (i) SiC oxidation seals gaps left by coating recession before the coating is entirely volatilized and the retained strength remains high (oxidation dominant); (ii) BN recession completely removes the fiber coating before fiber oxidation can seal the gaps and the retained strength hits a minimum (recession dominant). The composite strength of the oxidation dominant condition is described by the single fiber composite (SFC) while the composite strength of the recession dominant condition is described by the dry fiber bundle. The crack spacing sets the coating recession required to reach the minimum composite strength (i.e. that of a dry fiber bundle). Composites with large crack spacings exhibit a high residual strength (governed by the SFC model), while composites with small crack spacings always reach the dry fiber bundle limit prior to gap closure. Trends of retained strength with temperature and water vapor are complex, yet are generally inversely related to the coating recession length. Hot, wet environments (1000 C, 1 atm water vapor) lead to the oxidation dominant regime with minimal coating recession and high residual strength while cold, dry environments (700 C, 0.01 atm water vapor) lead to the recession dominant regime and low residual strengths.