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Interfacial modifications in fiber reinforced geopolymer matrix composites for improved toughness




Jackson, Patrick R., author
Radford, Donald, advisor
Heyliger, Paul, committee member
Kota, Arun, committee member
Ma, Kaka, committee member

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Geopolymers have emerged in the recent decades as a potential matrix material for advanced composites. Geopolymers, or more generically inorganic polymers, extend the use temperature range over more common organic polymers, while retaining relatively low processing temperatures. In fact, some of the techniques used to process geopolymers are very similar to those developed for thermosetting polymers. This allows for processing of near net shape components without many of the complexities that would be associated with conventional ceramics or metals manufacturing technologies. To-date, fiber reinforced geopolymers have seen limited use, primarily in areas that emphasize high temperature resistance and good manufacturability over structural performance. The use of all-oxide geopolymer matrix composites (GMC) for high temperature structural applications remains uncertain due to limited toughness. Attempts to improve toughness in these materials through the use of an interphase material, such as those associated with ceramic matrix composites (CMC), have yielded mixed results. In some cases, an increase in toughness was observed, but at the expense of modulus and sometimes strength. The result was a composite that was less tough, or no tougher, than the composite with the untailored interface condition. Additionally, methods that would indicate differences between the tailored and untailored interface have not been employed leaving uncertainty as to what is providing improved toughness. This research examines the ability of weak interface concepts, often employed in ceramic matrix composites and created using fiber coatings, as means of producing greater toughness in GMCs exposed to elevated temperatures. This was accomplished through examination of composite mechanical properties and interfacial conditions. Geopolymer matrix composites reinforced with the 3M Nextel series of ceramic fibers were fabricated and exposed to elevated temperatures. From the fabricated composites, samples were prepared for testing in flexure, tension, short beam shear, and single fiber push-out. Microscopy techniques were employed to analyze fracture surfaces and results of push-out testing of composites. Both a model coating, carbon and thermally stable oxide coating known as monazite were applied to the surface of fibers and compared against a baseline condition to support the changes observed. The results of the research indicate the importance of ensuring adequate cure time of the geopolymer matrix, which enhances it properties. In GMCs using carbon coated fibers to achieve a weak interfacial condition, low mechanical properties of inadequately cured matrix produced composites with limited shear resistance and limited ability to transfer stresses to fibers. A moderate increase in the mechanical properties of the matrix via extended cure time from 1 to 5 hours resulted in a roughly 50% increase in modulus and 150% increase in strength for GMCs containing the interphase material. In all cases the use of fiber coatings resulted in a reduction of interfacial strength. This was revealed by fiber push-out testing, which constitutes the first known use of this technique in GMCs to directly analyze the strength of the bond between the fiber and the matrix. Analysis of the interfaces in specimens further revealed that simple reductions in bond strength are not sufficient for producing better toughness and mechanical properties in GMCs, but that there is a delicate interplay between the interface properties and improved mechanical behavior. Increased toughness was observed in the specimens containing the carbon coated and monazite coated fiber surfaces except in the instance where coating was degraded by the oxidizing environment. GMCs containing the monazite coated fiber demonstrated the greatest improvement in toughness. The improvement in toughness was the result of increased damage tolerance and also a roughly ~32 to 44% increase in strength as compared to GMCs without coated fiber surfaces. Both limited and elongated elevated temperature exposure did not limit greater toughness from being achieved in monazite coated fiber GMCs as compared to those composites without coatings. In general, the use of fiber coatings did improve the toughness of GMCs as result of weaker interfacial conditions and it was demonstrated that careful tailoring of the interfacial strength can result in retention of mechanical properties.


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fiber reinforced


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