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Mechanical Engineering Seminars - Alumino-Silicate (Geopolymer) Matrix-Ceramic Fiber Composites for High Temperature Structural Application

John Bridge
IPhD Program
University of Maine

Abstract

Geopolymer matrix-ceramic fiber composites are attractive for high-temperature structural applications since they are lightweight, have been reportedly tested successfully to over 1000°C, and are processed at near-room temperatures. The geopolymer matrix consists of alumino-silicate “sialate” cross-linked units that are processed like a polymer but exhibit ceramic-like properties. The purpose of the current work is to better understand the mechanisms for toughening these composites by studying the interfacial failure characteristics of the fiber and matrix under various heat-treatment conditions.

Geopolymer matrix samples for this initial study were processed using a manufacturer-provided “MEYEB” resin system coupled with a Nextel, alumina-based, ceramic plain weave fabric. Efforts were made to increase geopolymer composite strength and toughness by attempting to reduce the fiber-matrix bonding strength. Fabric plies were pyrolized at 500°C to create a carbon-coating and were processed with the geopolymer resin to create composite test samples. Another set of Nextel fabric plies were processed to remove all coatings on the fiber (i.e.; sizing) and made into test samples. All fabricated composite samples were subjected to elevated temperatures up to 900°C for varying periods of time and subsequently tested under short-beam shear flexure conditions.

The results clearly show that conditioning of the fibers to reduce the interfacial strength markedly improves strength (and toughness) at temperatures below 650°C. Strength values for coated samples increased by 20% and fracture surface microscopy showed fiber pull-out suggesting enhanced toughness versus a more brittle response for the uncoated fiber samples. Plans for future research include testing unidirectional composite samples at elevated usage temperatures, examining other reinforcement fibers, such as silicon carbide, and applying/testing different fiber coatings.

 


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