Carbothermal Synthesis of Al-O-N Coatings Increasing Strength of SiC Fibers

Non-bridging Al-O-N coatings have been synthesized on the surface of Tyranno ZMI SiC fibers by a low-cost carbothermal nitridation method. First, a nanoporous carbide-derived carbon (CDC) layer is produced on the surface of SiC fiber by the extraction of Si with chlorine; the CDC layer on the fiber is then infiltrated by AlCl₃ solution, and finally nitrided in ammonia at atmospheric pressure to produce the coating. The intermediate carbon layer acts as a template for the coating, facilitates the formation of aluminum oxynitride, and helps to build a strong bonding between the fiber and coating. Optimization of the process parameters led to a more than 65% improvement in the tensile strength (up to ∼5.1 GPa) and a three-time increase in the Weibull modulus for the fiber with 200 nm coating compared to the as-received fibers. The coated fiber exceeds the strength of all other small-diameter SiC fibers reported in the literature. Al-O-N coating may also provide oxidation protection for the fibers in high-temperature applications.