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1. Effect of pyrolysis parameters on mechanical properties of polymer-derived ceramics

   https://doi.org/10.1142/s2047684122500154  

   Ma, Chi ; Zhao, Huan ; Li, Yan ( March 2023 , International Journal of Computational Materials Science and Engineering)

   Polymer-derived ceramics (PDCs) which are fabricated through pyrolysis of preceramic polymers have attracted increasing attention due to their versatility in structure architecture design and property tailoring. Shaping at the polymer state using 3D printing allows the final ceramic products to exhibit arbitrary shapes and complex architectures that are otherwise impossible to achieve through traditional processing routes. The polymer-to-ceramic phase transition also provides additional space for mechanical property tailoring. A multiscale computational model is developed to explore the phase transition mechanisms and their correlations with processing parameters and failure response. Calculations in this work concern PMHS/DVB preceramic polymers. Molecular dynamics (MD) simulations are carried out first to track the atomic structure evolution at different temperatures. Continuum-scale ceramic phase formation is calculated on the basis of the competition between gas generation and gas diffusion. The effect of processing parameters on mechanical properties of pyrolyzed PMHS/DVB is systematically studied. Conclusions from this study can provide direct guidance for fabricating PDC composites with tailored mechanical properties. 

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2. Energy dissipation in functionally two-dimensional phase transforming cellular materials

   https://doi.org/10.1038/s41598-019-48581-8  

   Zhang, Yunlan ; Restrepo, David ; Velay-Lizancos, Mirian ; Mankame, Nilesh D. ; Zavattieri, Pablo D. ( August 2019 , Scientific Reports)

   Phase Transforming Cellular Materials (PXCMs) are periodic cellular materials whose unit cells exhibit multiple stable or meta-stable configurations. Transitions between the various (meta-) stable configurations at the unit cell level enable these materials to exhibit reusable solid state energy dissipation. This energy dissipation arises from the storage and non-equilibrium release of strain energy accompanying the limit point traversals underlying these transitions. The material deformation is fully recoverable, and thus the material can be reused to absorb and dissipate energy multiple times. In this work, we present two designs for functionally two-dimensional PXCMs: theS-typewith four axes of reflectional symmetry based on a square motif and, theT-typewith six axes of symmetry based on a triangular motif. We employ experiments and simulations to understand the various mechanisms that are triggered under multiaxial loading conditions. Our numerical and experimental results indicate that these materials exhibit similar solid state energy dissipation for loads applied along the various axes of reflectional symmetry of the material. The specific energy dissipation capacity of theT-typeis slightly greater and less sensitive to the loading direction than theS-typeunder the most of loading directions. However, both types of material are shown to be very effective in dissipating energy.

    

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3. Phase content prediction in polymer‐derived ceramics with metal additives

   https://doi.org/10.1111/jace.17769  

   Yang, Ni ; Lu, Kathy ( March 2021 , Journal of the American Ceramic Society)

   In this study, silicon oxycarbide (SiOC) is selected as the base polymer to derive a SiOC ceramic (PDC) matrix, and four transition metals M (M = Ni, Mo, Co, and Zr) are individually introduced into the SiOC base to form various SiOC/M systems. SiOC‐Ni, SiOC‐MoC_x, and SiOC‐CoSi_xare obtained by pyrolysis at 1100°C, whereas SiOC‐ZrO_xforms upon pyrolysis at 1400°C. The selected SiOC/M systems encompass four different types of phase separation pathways—pure metal, metal carbide, metal silicide, and metal oxide (SiC‐SiO₂‐C‐Ni, SiC‐SiO₂‐C‐MoC_x, SiC‐SiO₂‐C‐CoSi_x, and SiC‐SiO₂‐C‐ZrO_x). The driving force for crystallization has been analyzed using a Gibbs free energy minimization method and phase fractions of these different PDC systems are calculated based on the lever rule. This work also reveals the energetics related to the quaternary systems and provides guidance to synthesizing metal‐containing PDCs with desired phase contents. In addition, we have examined the broad applicability of the phase content prediction method for a variety of other SiOC/M systems.

    

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4. Accelerating dynamic exchange and self-healing using mechanical forces in crosslinked polymers

   https://doi.org/10.1039/C9MH01938C  

   De Alwis Watuthanthrige, Nethmi ; Ahammed, Ballal ; Dolan, Madison T. ; Fang, Qinghua ; Wu, Jian ; Sparks, Jessica L. ; Zanjani, Mehdi B. ; Konkolewicz, Dominik ; Ye, Zhijiang ( June 2020 , Materials Horizons)

   null (Ed.)

   Dynamically crosslinked polymers and their composites have tremendous potential in the development of the next round of advanced materials for aerospace, sensing, and tribological applications. These materials have self-healing properties, or the ability to recover from scratches and cuts. Applied forces can have a significant impact on the mechanical properties of non-dynamic systems. However, the impacts of forces on the self-healing ability of dynamically bonded systems are still poorly understood. Here, we used a combined computational and experimental approach to study the impact of mechanical forces on the self-healing of a model dynamic covalent crosslinked polymer system. Surprisingly, the mechanical history of the materials has a distinct impact on the observed recovery of the mechanical properties after the material is damaged. Higher compressive forces and sustained forces lead to greater self-healing, indicating that mechanical forces can promote dynamic chemistry. The atomistic details provided in molecular dynamics simulations are used to understand the mechanism with both non-covalent and dynamic covalent linkage responses to the external loading. Finite element analysis is performed to bridge the gap between experiments and simulations and to further explore the underlying mechanisms. The self-healing behavior of the crosslinked polymers is explained using reaction rate theory, with the applied force proposed to lower the energy barrier to bond exchange. Overall, our study provides fundamental understanding of how and why the self-healing of cross-linked polymers is affected by a compressive force and the force application time. 

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5. Diamond-reinforced cutting tools using laser-based additive manufacturing

   https://doi.org/doi.org/10.1016/j.addma.2020.101602  

   Traxel, Kellen ; Bandyopadhyay, Amit ( January 2021 , Additive manufacturing)

   null (Ed.)

   Production-volume and cost requirements currently limit machine tool manufacturers’ ability to produce application-specific tooling with traditional methods, motivating the development of innovative manufacturing technologies. To this end, we detail a manufacturing framework for the design and production of application-specific cutting tools based on industry standard tungsten carbide-cobalt (WC-Co)-based “carbide” cutting materials using additive manufacturing (AM). Herein, novel diamond-reinforced carbide structures were designed and manufactured via AM and subsequently tested in comparison to current commercial products that are traditionally-processed. The resulting diamond-reinforced composites were free from large scale cracking and maintained microstructures with multiple reinforcing phases. Diamond incorporation had a remarkable effect on the processing, microstructure, and machining performance of the WC-Co based material in comparison to a commercial carbide cutting tool of similar composition as well as the base WC-Co matrix. Detailed microstructure and phase analysis, as well as machining experiments, demonstrate the ability to exploit laser-based directed energy deposition (DED)-based AM to create multifunctional cutting tools that can be designed to meet ever-increasing manufacturing demands across many industries. 

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