Synthesis and characterization of high-temperature resistant yttrium-doped lanthanum monazite coatings on SiC fibers

To improve the long-term oxidation resistance of SiC fibers, a novel yttrium-doped lanthanum monazite (Y-LaPO4) coating was deposited onto the surface of SiC fibers (Y-LaPO4/SiCf) by a simple hydrothermal method. The static oxidation behavior of Y-LaPO4/SiCf, SiC fibers coated with lanthanum monazite (LaPO4/SiCf), and SiC fibers without coatings (SiCf) was studied at 1350 degrees C for 50 h. The results indicated that yttrium diffused into the oxide scale during oxidation process. The thicknesses of the oxide scales for the oxidized fibers in Y-LaPO4/SiCf and LaPO4/SiCf were about 4.56 and 11.31 mu m. SiC fibers without coatings were completely oxidized to silica. Furthermore, the oxidation kinetic curves of SiCf, LaPO4/SiCf, and Y-LaPO4/SiCf exhibited a parabolic trend during oxidation for 15, 20, and 50 h. Calculated the oxidation rate constants of SiCf, LaPO4/SiCf, and Y-LaPO4/ SiCf were found to be 1.37, 1.08, and 0.40 mu m2/h, respectively. The system of Y-LaPO4 coatings not only retains LaPO4 as a framework of the interface, but also incorporates yttrium to improve the oxidation resistance of fibers. Therefore, doping yttrium in lanthanum monazite can enhance the adherence between coatings and oxide scales and improve the oxidation resistance of SiC fibers.