In-situ mechanical properties of reaction bonded silicon carbide/silicon and multi-walled carbon nanotubes reinforced polymer composites at the nanoscale

Date
2019
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Publisher
University of Delaware
Abstract
Mechanical properties of nanoscale specimens are of considerable interests to scientists for various kinds of applications due to their unique performances at the small scale and the in-situ mechanical testing at nanoscale offers additional opportunities for mechanistic understanding of material structural responses to the exertion of a force. For the preparation of nano-specimens, however, there are some drawbacks associated with the generally adopted photolithography nanofabrication techniques. They may not only significantly influence the final experimental results due to residual stress introduced during deposition, etching, or handling for the specimen preparation, but also limit the selectivity of testing materials as the suitable materials for lithography preparation are limited. A dual-beam system, focused ion beam and field emission scanning electron microscope (FIB/FE-SEM), on the other hand, provides powerful fabrication and characterization capabilities for the in-situ study of material behaviors in order to explore the tensile properties of materials at the nanoscale and the deformation behavior of individual phases and structural features such as interfaces. In this study, a novel method for the fabrication of small specimens with desired shape and thickness of a few hundred nanometers was developed utilizing the FIB/FE-SEM and a technique of in-situ tensile test inside the FIB/FE-SEM was established. Two types of advanced composites, reaction bonded SiC/Si (RBSC) and multi-walled carbon nanotubes (MWCNTs) reinforced polymers, were chosen for the in-situ study to understand their mechanical properties at small scale, the behavior of composite interfaces and components, and the adaptability of the testing method using the same platform for different materials and structures. ☐ RBSC composites have been widely used for personal protection components, semiconductor platforms and high-energy laser mirrors due to their excellent strength-to-weight ratio, adequate electronic properties, outstanding thermal transport capabilities and stability in extreme environments. While there is considerable knowledge established about their macroscopic mechanical properties and performances, the mechanistic behaviors at the nanoscale, especially of the SiC/Si interface when the composite experiences loading have yet to be fully understood. In this work, the mechanical performance of RBSC was investigated using the in-situ testing method. Nanoscale SiC/Si interface and -SiC tensile specimens of 20 µm x 4 µm x 200-300 nm were prepared from bulk composites. The tensile properties and fractography of the SiC/Si interface and monolithic -SiC prepared directly from the RBSC composites were evaluated. The maximum SiC/Si interfacial strength and the maximum -SiC tensile strength were measured to be 7.2 GPa and 22.9 GPa, respectively, suggesting that the SiC/Si interface was a weak structural link in the RBSC system. Additionally, the in-situ testing revealed a phenomenon of specimen size dependency of the tensile strength of monolithic -SiC due to possible reduction of defects of critical size in smaller specimens. High resolution transmission electron microscopy (HRTEM) confirmed that the SiC/Si interface included an amorphous Si layer likely from the lattice mismatch of SiC and Si phases and the liquid phase infiltration bonding reactions. The fracture appeared to be a cohesive failure occurring in the interfacial amorphous Si (a-Si). A Weibull modulus of 4.1 for the -SiC test obtained from data analysis suggested a non-uniform defect distribution in the nanoscale specimen compared to that of the bulk specimen. ☐ Multi-walled carbon nanotube (MWCNT) has become an ideal filler to enhance the performance of polymers including thermal, electrical and mechanical properties in automobile and aviation industries. In structural applications, however, the experimental ultimate tensile strength and Young’s modulus are significantly lower than those of theoretical predictions. The discrepancy is generally attributed to various type of defects in CNTs and the integrity of CNTs/polymer interface. To minimize bulk defect effects and evaluate CNTs/polymer properties and dynamics, a small size specimen is most desirable. In this work, the same FIB/FE-SEM nano-specimen preparation and the in-situ testing methods used for RBSC composite investigation were employed to investigate the mechanical behavior of MWCNT reinforced polymer-based composites. The average tensile strength was found to be 388.7 MPa, about 4 times higher than that of the bulk composites. The nanoscale specimen failed in a brittle manner, suggesting a volume-size reduction of the amorphous phase in nano-specimen polymer matrix required for the plastic deformation. Real-time testing observation and record revealed a general failure sequence as follows: crack formation, crack growth, MWCNTs bridging between fractured PEEK surfaces, and MWCNTs pullout. ☐ To better design and fabricate advanced MWCNT reinforced composites and understand the essential individual MWCNT tensile strength and behavior, it is desirable to test the mechanical properties of individual MWCNTs which are incorporated in a polymer matrix of interest. In this work, AFM probes with known cantilever spring constants were properly mounted on the piezo stage of the in-situ testing platform. The fracture strengths of individual MWCNTs were evaluated for a composite of 27.9 wt.% MWCNT-reinforced nitrile butadiene rubber (NBR). In parallel, the mechanical properties of the composites at macroscale were characterized and a maximum fracture strength was found to be 50 GPa for MWCNT (27.9 wt.%)/NBR, which is 28 and 2.5 times higher than that of pure NBR and carbon black reinforced NBR, respectively. This improvement was found to be attributable to the cellulation effect from the formation of a three-dimensional MWCNT network in the composite with 8.8 wt.% or higher MWCNT in addition to high quality and greater length (250 µm, compared with 20 m spheres of carbon black) of as-fabricated MWCNTs and an effective MWCNTs/NBR interface. Indeed, using the AFM probe in-situ testing platform, the maximum individual MWCNT fracture strength was measured to be 88 GPa indicating a high preservation of the MWCNT integrity and an efficient load transfer at the MWCNTs/NBR interface. Fractography study revealed both of full breakage and sword-in-sheath failure mechanisms for MWCNTs, as also observed with in-situ composite fracture testing and recorded in video clips. The sword-in-sheath failure corresponded to weaker tensile strength possibly due to randomly distributed defects or non-hexagon carbon ring structure in the MWCNTs. ☐ In summary, a general method of nanoscale tensile specimen preparation utilizing FIB/FE-SEM and a homebuilt universal in-situ tensile testing platform were developed for the mechanical characterization of SiC, SiC/Si interface, MWCNTs/polymers and individual MWCNTs. The SiC/Si interfacial strength and MWCNTs/PEEK nano-mechanics were characterized with nanoscale specimens for the first time. The tensile strengths of both -SiC and MWCNT/PEEK composites exhibited similar phenomenon of size dependency, with a trend of strength increase when the specimen dimension reduces to the nanoscale. The fracture during tensile testing appeared to start at the surface of the specimens for both of the monolithic -SiC and the MWCNTs/PEEK composite. The SiC/Si interface failed cohesively in the a-Si region of the interface, while the sword-in-sheath failure mechanism was found to dominate the MWCNTs/NBR fracture.
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