MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TEXTURED SILICON CARBIDE FORMED VIA DIRECT INK WRITING AND TEMPLATED GRAIN GROWTH
Silicon carbide (SiC) is a ceramic material of interest for many applications due to its mechanical properties, oxidation resistance, and high thermal conductivity. However, one limitation of SiC is its low fracture toughness. There is evidence that SiC with crystallographic texture and an anisotropic microstructure of aligned plate-shaped grains has improved fracture toughness without sacrificing strength. Previous techniques to create these materials have made use of either pressure during densification or a strong magnetic field, but these processes limit possible geometries that can be created. In this dissertation, the additive manufacturing technique direct ink writing (DIW) and pressureless templated grain growth (TGG) are proposed as a route to complex-shaped textured SiC.
DIW is a colloidal processing technique where ceramic suspensions are extruded through a nozzle along a path, building up a part layer-by-layer. High aspect ratio particles can be aligned via the forces in the print nozzle. In this work, single crystal SiC platelet seed particles were added to a SiC suspension and aligned with DIW. After densification, samples were annealed above the sintering temperature. During annealing, TGG occurs where the platelet seed particles grow at the expense of the finer matrix particles, and this results in crystallographic texture.
First, work on the development of the DIW process for the creation of textured SiC is shown. Aqueous SiC suspensions were developed with a high solids loading (> 50 vol%) and low polymer content (< 5 vol%) to maximize the density achieved during sintering, which is ideal for TGG. Four rheological parameters (viscosity of the suspension at 5 s-1, zero shear viscosity, oscillatory yield stress, and equilibrium storage modulus) were related to the amount of viscosity modifying polymer (polyvinylpyrrolidone) and observed quality of prints. The best prints were made from suspensions that had a viscosity of 30-35 Pa s, ZSV of 5000-7000 Pa s, and yield stress 100-150 Pa. The best suspension for printing was identified to be 53 vol% solids with 0.2 vol% PVP due to its high particle loading and ability to create consistent prints. The addition of 5 vol% platelet particles to the suspension did not impact the rheology or printability significantly.
Next, textured SiC ceramics over 95% theoretical density were created via pressureless sintering and annealing. Samples were fabricated with and without 5 vol% platelet seeds, and with and without annealing at 2050 ºC and 2150 ºC. The effects of DIW, seed particles, and annealing temperature on the microstructure and crystallographic texture are presented. Annealing lead to the development of large, high aspect ratio plate-shaped grains among a matrix of many finer, low aspect ratio grains. Higher annealing temperatures and addition of platelet seeds both increased the size of the large grains. Samples were found to be textured regardless of having platelet seeds. Via x-ray diffraction and electron backscatter diffraction, unseeded SiC was found to have texture where the crystallographic direction  had a preferred orientation perpendicular to the normal direction. This occurred for both DIW and cast SiC, so the texture development must have occurred during sintering, though the mechanism is unknown. For seeded SiC, platelet seeds aligned in DIW successfully seeded the grain growth to develop crystallographic texture. The texture was mainly influenced by the alignment of platelet seed particles via shear stresses in the print nozzle, causing a one-dimensional texture where  is perpendicular to the printing direction. However, it was found that the texture was not the expected one-dimensional, concentric alignment of platelet particles in DIW, so the shear stresses in the nozzle are not solely responsible for the texture developed.
Finally, the mechanical behavior of these materials was explored via 4-point flexural strength testing and Weibull analysis. The effect of texture, print orientation, and printing defects on the mechanical and fracture behavior of these materials is discussed. Mechanical tests were conducted both parallel and perpendicular to the printing direction. DIW samples were found to have a variety of defects after densification, including visible print lines, air bubbles, and porosity. Unseeded SiC annealed at 2050 ºC tested parallel to the print direction was found to have the best combination of mechanical properties among all annealed SiC, with evidence of toughening on the fracture surface, flexural strength 405 ± 16 MPa, and Weibull modulus of 15.4. Seeded SiC annealed at 2050 ºC had a high degree of transgranular fracture among large plate-shaped grains, but still had a flexural strength 339 ± 41 MPa. However, improved alignment of grains in future work may increase the incidence of intergranular fracture. At both annealing temperatures, textured SiC created with aligned platelet seed particles was found to have comparable mechanical strength to those fabricated without seed particles despite having a coarser microstructure, suggesting texture may influence the mechanical properties.
Purdue University Graduate School Frederick N Andrews Fellowship
- Doctor of Philosophy
- Materials Engineering
- West Lafayette