Silicon carbide (SiC) is a widely used high-temperature structural material due to its high thermal conductivity, chemical inertness, thermal stability, and low thermal expansion coefficient. The traditional shaping or forming methods of ceramics such as pressing, injection molding, and casting require high processing temperatures and highly challenging to make complex geometries due to their high hardness and melting points. Besides, the sintered SiC structures contain additives that can cause severe degradation, specifically at higher temperatures. Additive manufacturing (AM) technologies surpassed these limitations without mold needs. Various additive technologies are currently available for ceramics printing based on feedstock's nature, such as powder or slurry-based. Among those, printing and densifying a polymer-derived ceramics green body has become a popular technique for producing Si-based ceramic structures. The process entails printing green bodies and several post-printing steps, including binder removal and densification. The ceramic parts' properties and their final dimensions depend on ceramic particles' densification and porosity during the post-printing steps. Conventionally the SiC ceramics are densified by heating the green bodies in an inert atmosphere.&nbsp; Due to the highly covalent character of the Si-C bonds in SiC, high sintering temperatures greater than 1500°C are required to achieve enough neck growth between particles, limiting the practical applications of SiC ceramics. Hence one of the critical issues in manufacturing SiC ceramics is high processing temperatures.&nbsp;In contrast, the polymer-derived ceramics require low temperatures (1100-1300°C) for densification. However, they undergo about 30% volumetric shrinkage. Thus, process science is necessary to ensure high quality and dimensionally predictable parts for additive manufacturing of SiC ceramics. This study hypothesizes that creating dense oxide shells over the SiC particles and sintering/diffusion bonding can yield high strength SiC/oxide composites. The overall aim of this thesis work is to make high-strength skin/core damage-tolerant ceramic structures.However, there are several challenges to be overcome to create a complex-shaped and defect-free composite. The first problem is growing a silica layer beyond 200nm without damage onset.&nbsp; Secondly, the process-induced residual stress states are complicated because of green body shrinkage due to polymer burnout and oxide layer expansion. Lastly, the formulation of a printable photocurable resin filled with SiC particles is a challenge. To create the necessary science-based framework to produce high-quality 3D printed ceramic composites, we constructed a high-resolution numerical model that simulates the densification and oxidation steps. Using the model, we determined optimal process windows for creating high-performance oxide ceramics.&nbsp; This work helps to additively manufacture complex-shaped SiC/silica composite structures with higher oxidation resistance and better strength for a given porosity. These composites find applications in heat exchangers and gas filters.&nbsp;

Additively Manufactured High-Performance Silicon Carbide Composites

Paper Proceedings: https://asmedigitalcollection.asme.org/MSEC/MSEC2021/volume/85062

2021 International Manufacturing Science and Engineering Conference (MSEC2021)

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