Rare Earth Silicate Forming Matrices For Sic Based Ceramic Matrix Composites

Silicon carbide (SiC) ceramic matrix composites (CMC) have excellent high temperature durability. Application of CMCs in a combustion environment requires the use of an Environmental Barrier Coating (EBC) to prevent water vapor accelerated volatilization.  The EBC can crack or spall off due to stress caused by mismatched coefficients of thermal expansion with the underlying CMC.  By doping the SiC matrix with rare earth compounds we can make the CMC more robust because the matrix will oxidize to form a rare earth silicate scale on the surface protecting from further oxidation.  Samples were made from compressed SiC, Y2O3, YB2, LaB6, and La2O3 powders and densified using polymer infiltration and pyrolysis (PIP).  Dense samples were oxidized at 1400°C for 24 hours to determine oxidation behavior. X-ray diffraction and scanning electron microscopy were used to evaluate samples before and after oxidation. Prior to oxidation, a uniform distribution of rare earth compound particles is observed in the SiC matrix. Oxidized specimens display a layered structure. The material is unchanged at its center. A region depleted in rare earth compounds is observed below the surface followed by a layer enriched zone in rare earth elements.  The outer surface is an oxide scale containing silica and rare earth silicates.  It is expected that the silica will volatilize in high temperature water vapor and the rare earth silicate will remain to protect the CMC.