"Preventing Carbon Fiber Degradation in Cf/ZrB2-SiC UHTCMCs Fabricated via Direct Ink Writing" 

David Calvo, MSE MS Candidate 

Advisors: Prof. Rodney Trice, Prof. Jeffrey P. Youngblood

ABSTRACT

As an ultra-high temperature ceramic (UHTC) with a melting temperature over 3200°C, zirconium diboride (ZrB2) is a candidate matrix material for use in extreme environments, particularly those experienced by high-speed aerospace vehicles. A crucial limitation of UHTCs is that they are prone to catastrophic brittle failure. By adding fibers to a UHTC matrix, the resulting UHTC matrix composites (UHTCMCs) can provide enhanced damage tolerance and thermal shock resistance. However, a major challenge in UHTCMC manufacturing is the degradation of carbon fiber reinforcing elements during processing. Fiber/matrix interactions compromise fiber integrity and form strong bonds at the interface, counteracting the benefits of fiber reinforcement.

In this work, the phenomenon of carbon fiber degradation in Cf/ZrB2-SiC is analyzed and our strategy to prevent deleterious fiber/matrix reactions is presented. First, parameters for aqueous ZrB2-20 vol.% SiC inks were optimized, comprised of ceramic powders, a polymeric binder and dispersing agent, and 0% or 10 vol.% pitch-based carbon fiber. Ink behavior and suitability for printing via the additive manufacturing method of direct ink writing (DIW) was studied with rheological measurements. Both pressureless sintering (1950°C, 2150°C) and hot-pressing (1750°C) densification methods were tested. Pressureless sintering, regardless of temperature, was ineffective at fully densifying Cf/ZrB2-SiC and caused severe fiber degradation. Hot-pressing at 1750°C, however, yielded parts with densities >99.50% of theoretical and suppressed fiber degradation. Specimen mechanical properties, including flexural strengths and elastic moduli, were collected via four-point bend testing and the impulse excitation technique, respectively. While the inclusion of 10% Cf resulted in reduced strengths and elastic moduli relative to monolithic ZrB2-SiC prints, fiber-reinforced prints displayed significantly greater Weibull moduli, suggesting failure occurred in a more consistent, predicable manner. SEM was used to survey fiber condition after densification and image fracture surfaces, which revealed fiber pullout and provided insight into the strength of fiber/matrix bonds. Lastly, to assess the viability of the ZrB2-SiC and Cf/ZrB2-SiC prints for service at extreme temperatures, oxy-acetylene torch testing was performed.