Effectiveness of Monazite Coatings in Oxide/Oxide Composites after Long‐Term Exposure at High Temperature

The effectiveness of monazite (LaPO 4 ) in providing an oxidation‐resistant weak fiber/matrix interface was evaluated in a fiber roving/thin coating/ceramic‐matrix composite with >20% fiber volume fraction. Nextel™ 610/monazite/alumina composites were fabricated and tensile tested after isothermal exposures of up to 1000 h. Some strength loss was seen after short‐term exposures (1100°–1200°C/5–250 h); however, no further loss was observed after 1000 h at 1200°C. Conversely, control samples containing uncoated fiber displayed >70% strength losses after only 5 h at 1200°C. Fiber pullout was seen in monazite‐containing samples even after 1000 h at 1200°C. Debonding was predominantly in the coating or at either the fiber/coating or coating/matrix interface. Push‐out testing confirmed the weakness of the monazite coating interface.