Use of interphase in geopolymer matrix composites for improved toughness

Geopolymers are a lightweight material with excellent fire and thermal resistance. When combined with continuous fiber reinforcement, geopolymers demonstrate potential to replace heavier, structural components, especially those that are exposed to elevated temperature regimes (> 400 degrees C) in air. Oxidation resistance is essential to continuous operations in these environments. While oxide fibers can drastically improve the mechanical properties of geopolymers, their incorporation has been met with challenges. These geopolymer matrix composites (GMC) lack the necessary toughness to find much use as a structural component. In this study, concepts and materials used to reduce interfacial strength in ceramic matrix composites for the purpose of improving toughness are employed in GMCs. To investigate the potential to improve toughness, composites containing an alumina-based fiber and geopolymer were prepared with three different interfacial conditions: heat cleaned, carbon coated, and monazite coated. From the fabricated composites, specimens for four-point flexural testing and single fiber pushout were prepared. Additionally, composite samples were heat treated to 650 degrees C in oxidizing and non-oxidizing environments. To varying degrees, composites that utilized a fiber coating demonstrated improvements in toughness and relative damage tolerance as compared to those without any modification to the interface. In some cases, the improvement was over 160% as compared to those without a coating. Pushout testing showed specimens with coated fiber to have weaker interfacial conditions. The combined results support the use of tailored interface concepts in GMCs for improving toughness.