Thermal Processing of Injection-Molded Silicon Carbide Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/jq085p89h

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  • Silicon carbide is an important and versatile nonoxide ceramic. Powder injection molding (PIM) is a method of high-speed fabrication of complex near-net shapes of SiC and other powders. Green micro-machining (GMM) is used to extend the shaping capability of green ceramics and powder metallurgy to smaller feature sizes. Debinding--removal of organic additives--is the rate-limiting step in PIM. Sintering aids enable densification of sintered powders, especially in the absence of applied pressure during sintering. Thermal Processing of Injection-Molded Silicon Carbide presents a study of the effects of GMM, debinding, sintering aids and sintering on two size distributions of PIM α-SiC with 5% each of Y₂O₃ and AlN as sintering aids. The use of 10% 20-nm particles, i.e., a bimodal size distribution, to increase the packing density of the green bodies was found to have a small effect on the rate of debinding, the liquid-phase sintering (LPS) precipitates, the microstructural development and the mechanical properties of SiC compared to the conventional monomodal size distribution, where D₅₀ = 0.7 μm. The nanoparticles and debinding methods did have a strong effect on the feasibility of GMM on SiC. The nanoparticles, debinding methods and GMM in combination significantly affected the sinterability of SiC. The rates and effects of solvent debinding and thermal debinding were measured and compared by various kinetic models. The catalytic effect of the bimodal SiC, if any, was small compared to PIM SiC with monomodal particles. The activation energy for thermal debinding was similar to that of solvent debinding. Too rapid of debinding by either method was detrimental to sintering in the form of fracture in the green body by residual stress. The debinding mechanism shifted from surface dissolution to bulk diffusion as the solvent debinding progressed. Changes in thermal debinding mechanisms were also noted as a function of heating rate. Thermal debinding was problematic in PIM bars with a large characteristic diffusion path length ψ, which led to fractures during sintering. Weak particle bonding and uncontrolled grain growth were observed in some cases after thermal debinding, and attributed to dissolution of aluminum in SiC, excess oxidation of the SiC and premature decomposition of polypropylene. Solvent debinding was less stressful, but not without fractures in some instances due to the swelling of the wax as it dissolved. Monomodal SiC was much more amenable to GMM than bimodal, whether solvent or thermally debound. The GMM swarf adhered to the monomodal more than to bimodal, even after the wax holding the swarf to the substrate was dissolved. The bimodal SiC had about one percentage point better densification than the monomodal. The grain size, precipitate content and Knoop hardness were about the same for monomodal and bimodal, whether solvent or thermally debound, with or without GMM, except in the case of thermally dewaxed bimodal SiC.
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