Effectiveness of monazite coatings in Oxide/Oxide composites after long-term exposure at high temperature

The effectiveness of monazite (LaPO4) 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(TM) 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 (1100degrees-1200degreesC/5-250 h); however, no further loss was observed after 1000 h at 1200degreesC. Conversely, control samples containing uncoated fiber displayed >70% strength losses after only 5 h at 1200degreesC. Fiber pullout was seen in monazite-containing samples even after 1000 h at 1200degreesC. 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.