MONAZITE FIBER COATING FOR STRENGTH AND TOUGHNESS IMPROVEMENT OF GEOPOLYMER MATRIX COMPOSITES.

Geopolymers, or inorganic polymers, are an interesting family of materials that are able to be processed near room temperature, but have continuous use temperatures well above those of organic polymers. By themselves, geopolymers exhibit relatively poor mechanical properties, limited by their brittle nature. Incorporation of continuous ceramic fibers has been demonstrated to significantly improve the strength and stiffness. Still, the performance of these composites is limited by toughness. In ceramic matrix composites (CMC), damage tolerance is commonly developed by application of a weak fiber coating. This use of an interphase material has been shown to produce stronger and tougher CMCs, yet identifying effective interphase materials that are stable at high temperatures remains a challenge. In this study, geopolymer matrix composites reinforced with monazite (LaPO₄) coated, continuous Nextel 720 fibers were evaluated in 3- point flexure to better understand the potential for toughness improvement. Results indicate that the use of monazite fiber coating is favorable. The oxide coating resulted in >20 % increase in flexural strength over samples without coating. Analysis of stress-strain curves indicates improved toughness in samples containing monazite. Fracture surfaces of the monazite coated fiber reinforced samples appear to reveal a more tortuous crack path than samples with uncoated fibers. [ABSTRACT FROM AUTHOR]