Multifunctional hybrid composites are proposed as novel solutions to meet the demands in various industrial applications ranging from aerospace to biomedicine. The combination of carbon fibers and/or fabric, metal foil and carbon nanotubes are utilized to develop such composites. This study focuses on processing of and fracture toughness characterization of the carbon fiber reinforced polymer matrix composites (PMC) and the CNT modified interface between PMC and a metal foil. The laminate fabrication process using H-VARTM, and the mode I interlaminar fracture toughness via double cantilever beam (DCB) tests at both room temperature and high temperature are conducted. The cross-sections and fracture surfaces of the panels are characterized using optical and scanning electron microscopes to verify the existence of CNTs at the interface before and after fracture tests. The experimental results reveal that CNT’s improve bonding at the hybrid interfaces. Computational models are developed to assist the interpretation of experimental results and further investigate damage modes. In this work, analytical solutions to compute the total strain energy release rate as well as mode I and mode II strain energy release rates of asymmetric configurations layups are utilized. Finite element models are developed in which the virtual crack closure technique is adopted to calculate strain energy release rates and investigate the degree and effect of mode-mixity. Results from analytical solutions agree well with each other and with results obtained from finite element models.

Maturity of high-temperature polymer-reinforced composites defer to conventionally expensive and intensive methods in both material and manufacturing aspects. Even traditional carbon-carbon, aerogel, and ceramic approaches are highly limited by difficult manufacturing techniques and are subject to sensitive handling throughout their processing and lifetime. Despite their utility in extreme environments, the high costs of existing high-temperature composites find limited practical applicability under high-performance applications. The development of continuous fiber-reinforced pyrolysis-derived carbon-matrix composites aim to circumvent the issues surrounding the manufacturing and handling of conventional high-temperature composites. Polymer matrix composites (PMCs) have a number of attractive properties including light weight, high stiffness-to-weight and strength-to-weight ratios, ease of installation on the field, potential lower system-level cost, high overall durability and less susceptibility to environmental deterioration than conventional materials. However, since PMCs contain the polymer matrix, their applications are limited to lower temperatures. In this study, a pyrolysis approach was used to convert the matrix material of phenolic resin into carbon-matrix to improve the mechanical and thermal properties of the composites. Composite material consisting of basalt fiber and phenolic resin was pyrolyzed to produce basalt-carbon composites through a novel method in which the pyrolysis promoted in-situ carbon nanotube growth to form “fuzzy fibers”. The carbon phenolic composites were pyrolyzed to produce carbon-carbon composites. Several types of composites are examined and compared, including conventional phenolic and carbon-matrix composites. Through Raman spectroscopy and scanning electron microscopy, the composition of materials are verified before testing. Investigation into the improvements from in-situ carbon growth was conducted with an open-flame oxyacetylene test (ASTM-E285), to establish high-temperature thermal behavior, in addition to mechanical testing by three-point bending (ASTM-D790), to evaluate the mechanical and thermal properties of the pyrolyzed composites.

<p>For many applications within the defense, aerospace, and electricity-producing industries, available material choices for high-performance devices that fulfill necessary requirements are limited. Choosing a metallic material or a ceramic material may be optimal for only some of the required properties. For instance, choosing a metal may optimize ductility but compromise oxidation resistance, yield strength, or creep resistance. Of potential interest, ceramic-metal (cermet) composites can address several fundamental concerns such as high temperature mechanical toughness and stiffness and oxidation/corrosion resistance. However, cost-effective, scalable manufacturing of complex-shaped, high-temperature cermets can be challenging.</p> <p>A cermet of interest is niobium and yttrium oxide, Y2O3. Both materials exhibit high melting points with similar coefficients of thermal expansion. Basic thermodynamic calculations suggest that these materials are chemically compatible, and that Y2O3/Nb cermets may be generated by reactive melt infiltration using the patented Displacive Compensation of Porosity (DCP) process. With the DCP process, a liquid fills a porous perform, and a displacement reaction occurs to produce products of larger solid volume. This reaction yields the cermet of interest, formed in a reduced-stress condition, while maintaining a generally near net shape and high relative density.</p> <p>In order to get to the point of designing cermet components for various applications, a focus of this work has been to create a Y2O3/Nb composite by hot pressing powders at high temperatures at the predicted stoichiometric ratios, and then characterizing the thermal and mechanical properties. The reduction reaction between liquid yttrium and solid niobium (IV) oxide (NbO2) was then characterized to evaluate kinetic mechanisms affecting the reaction rate which is necessary for future DCP-based cermet component manufacturing.</p> <p>Lastly, the mechanical behavior of this cermet was modeled and compared to another cermet processed using liquid metal infiltration using a temperature-dependent elasto-visco-plastic self-consistent model. The effects of cooling from processing temperatures, as well as thermally cycling of these cermets, were quantified. As high temperature experiments can be time intensive with high costs, it is advantageous to have a computationally efficient, desktop design tool to quantify the impacts of changing processing and use conditions on material performance.</p>

Ultra-high temperature (i.e., greater than 2500°C) engineering applications present continued materials challenges. Refractory metal/ceramic composites have great potential to satisfy the demands of extreme environments (e.g., the environments found in solid rocket motors upon ignition), though general scalable processing techniques to fabricate complex shaped parts are lacking. The work embodied in this dissertation advances scientific knowledge in the development of processing techniques to form complex, near net-shape, near net-dimension, near fully-dense refractory metal/ceramic composites with controlled phase contents and microstructure. Three research thrusts are detailed in this document. First, the utilization of rapid prototyping techniques, such as computer numerical controlled machining and three dimensional printing, for the fabrication of porous tungsten carbide preforms and their application with the Displacive Compensation of Porosity process is demonstrated. Second, carbon substrates and preforms have been reactively converted to porous tungsten/tungsten carbide replicas via a novel gas-solid displacement reaction. Lastly, non-oxide ceramic solid solutions have been internally reduced to create intragranular metal/ceramic micro/nanocomposites. All three techniques combined have the potential to produce nanostructured refractory metal/ceramic composite materials with tailorable microstructure for ultra-high temperature applications.