Oxidation of silicon carbide fibers in ceramic composites

The oxidation behavior of Nicalon SiC fibers, having three different chemistries, has been investigated. The active and passive oxidation conditions at 1473 K for the SiC fibers has been established. A parabolic oxidation rate (K{dollar}sb{lcub}rm p{rcub}{dollar}) for the Nicalon SiC fibers at 1473 K under a P{dollar}sb{lcub}rm O2{rcub}{dollar} =.1 MPa was determined. The measured K{dollar}sb{lcub}rm p{rcub}{dollar} is in excellent agreement with data published for fine grained SiC materials. In addition, a model is developed to determine the oxide scale growth kinetics on a curved fiber surface. Nicalon SiC fibers were subjected to composite fabrication conditions at 1473 K. These conditions included vacuum exposures in which the P{dollar}sb{lcub}rm CO{rcub}{dollar} was extremely low and vacuum hot pressing conditions where the P{dollar}sb{lcub}rm CO{rcub}{dollar} is on the order of the vacuum (1mPa). These conditions are very reducing and allow for active oxidation of the SiC fibers to occur.; Changes in the oxidation behavior of Nicalon SiC fiber due to composite fabrication have also been determined. Several Nicalon SiC fiber reinforced lithium aluminosilicate glass-ceramic composites were fabricated. The chemical composition of the glass-ceramic matrix was varied to change the fiber/matrix interface. These different interfaces caused the composite systems to have different oxidation behavior at elevated temperatures. The Nicalon SiC fibers were extracted from the glass-ceramic matrices and analyzed. It was found that the fiber surface chemistry and microstructure are altered during composite fabrication. Furthermore, the presence of matrix elements in the fiber significantly modifies the formation of a protective oxide scale on the fiber surface during oxidation. The changes in the oxidation behavior of the Nicalon fibers that underwent composite fabrication are compared to the properties of the as-received fibers.