Low-Temperature Oxidation Embrittlement of SiC (Nicalon™)/CAS Ceramic Matrix Composites

The influence of extended duration (up to 1000 h), low temperature oxidation heat-treatments (375°–600°C) has been assessed using a model ceramic matrix composite system with a graphitic fiber/matrix interphase. For this study a Nicalon^™ fiber reinforced CaO–Al₂O₃–SiO₂ matrix composite was selected (Nicalon^™/CAS), which possesses a thin (∼20–40 nm) carbon-based interphase. Oxidation exposure has been conducted under both unloaded and static fatigue-loaded conditions. For unstressed oxidation exposure, degradation of the carbon-based interphase is apparent at temperatures as low as 375°C, after 1000 h exposure, resulting in a transition to a nominally brittle failure mode (i.e., negligible fiber pull-out). The degree of mechanical property degradation increases with increasing temperature, such that strength degradation, and a transition to nominally brittle failure, is apparent after just 10 h at 600°C. Static fatigue loading between 450° and 600°C demonstrated generally similar trends, with reduced lifetimes being observed with increasing temperature. Based upon the unloaded oxidation experiments, combined with previously obtained intermediate and high-temperature oxidation stability studies, a simple environmental embrittlement failure mechanism map is presented for Nicalon^™/CAS. The implications of this study for advanced composite designs with multiple thin carbon-based interphase layers are also discussed.