Processing of Preceramic Polymers for Direct-Ink Writing

Preceramic polymers are organosilicon polymers that, when pyrolyzed to above 1000°C, convert from a polymer to an amorphous ceramic. These polymers have been used for fiber spinning, polymer infiltration, and casting of materials but have recently gained interest for use as the feedstock material for additive manufacturing techniques. This work explores preceramic polymers being used for direct-ink writing (an additive manufacturing method) and many of the issues that occur with the polymers during curing and pyrolysis.

The first chapter of this dissertation provides a review of preceramic polymers, while the second and third chapters focus on the development of inks made of preceramic polymers. The second chapter uses a polysilazane polymer mixed with up to 43.3 volume percent hexagonal boron nitride as the rheological modifier to enable printing. The pyrolyzed parts are tested with 3-point flexure and microhardness indentation to observe failure behavior. The third chapter uses a polycarbosilane polymer with zirconium diboride and silicon carbide fibers as constituents for printable inks. These polycarbosilane-based inks exhibit much more porosity and crack development during curing and pyrolysis than the inks in the second chapter. Defects are characterized with micro-computed tomography and scanning electron microscopy. From the measured defects, new suggestions for decreasing porosity and crack development are discussed.

Building from the observations in the third chapter, the fourth chapter focuses on how the size of printed material influences the development of defects and overall strength. Two new inks, similar to those in chapter three, are used with the addition that one of the formulations utilizes fumed alumina as an added viscosity modifier. The final study investigates printed rods of varying diameters (0.45 to 1.7 millimeter) to observe the effects of off-gassing during curing on the development of porosity. Failure strength is measured with 3-point flexure and Weibull statistics are used to understand how specimen size and ink formulation affect final specimen strength.

Overall, this dissertation shows that preceramic polymers are a viable option as a feedstock material for direct ink-writing and begins to quantify the degree to which part size and filler selection affect overall porosity development after curing and pyrolysis.