Abstract
The influence of extended duration (up to 1,000 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 (CAS/Nicalon), 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 1,000 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 temperatures, 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 600C 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 CAS/Nicalon. The implications of this study for advanced composite designs with multiple thin carbon-based interphase layers are also discussed.