Structure-Mechanical Property Relationships in Porous SiC Derived from Wood

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Biomorphic silicon carbide (bioSiC) is a novel porous ceramic material derived directly from wood precursors. This material is fabricated by pyrolysis of a natural wood precursor in an inert atmosphere leaving an amorphous carbon scaffold. The amorphous carbon is infiltrated with molten Si in vacuum at elevated temperature, which reacts with the scaffold to form SiC. Finally, any residual Si is removed using an acid solution producing a porous SiC material with a microstructure that is analogous to the wood precursor. In order to understand mechanical behavior and identify this material for potential applications, fundamental structure-mechanical property relationships in bioSiC were established. This was accomplished by analysis of bioSiC from five different wood precursors, which covered a range of pore volumes, pore sizes, and pore size distributions. The structure and phase composition of these materials were characterized and coupled to mechanical behavior conducted by mechanical testing and finite-element analysis. In addition to volume of porosity and orientation, mechanical properties were found to be a function of phase composition and structure. All bioSiC materials were found to have unreacted carbon that had a deleterious effect on compressive strength and elastic modulus (E) but no significant effect on fracture toughness (K<sub>IC</sub>). A procedure was developed to quantify the fraction of unreacted carbon in bioSiC, and results showed that materials with a higher fraction of small pores contained higher amounts of this phase. Additionally, misalignment of tubular pores was found to lower compressive strength and E in axial compression, and random pore arrangement and misorientation of tubular pores with elliptical cross-sections were found to lower compressive strength and E in transverse compression. Analysis also revealed preferred crack paths in planes parallel to the axial direction. Cracks likely initiate from nodes and curved areas in tubular pores of bioSiC, which act as stress concentrators. Additionally, wood features such as rays and growth rings that manifest themselves as density gradients in bioSiC served as crack deflectors.

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  • 05/14/2018
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