Masters Theses

Date of Award

8-2024

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Uday Vaidya

Committee Members

Uday Vaidya, Dustin Gilmer, Brett Compton

Abstract

Carbon-fiber-reinforced ceramic matrix composites (CMCs) are frequently used in applications where high thermo-mechanical loads are induced along with weight limitations. The first step of producing the fibrous preform allows the near-net shape of the final part to be formed with fibers placed in the desired orientation, typically relying on traditional polymer matrix composite (PMC) manufacturing methods. Recent work has demonstrated a new method to produce discontinuous fiber preforms through various forms of additive manufacturing including fused filament fabrication (FFF). This work uses Polyetheretherketone with carbon fibers (CF PEEK) for additive manufacturing of the carbon-rich polymer precursor. Critically, thermoplastic precursor materials suffer severe instability during pyrolysis as they move from melt to degradation. This work evaluates oxidative stabilization of the additively manufactured preform to preserve form at the macrostructural and microstructural scales. It is shown that thermo-oxidative stabilization near the melt temperature is an oxygen-dependent process that forms carbonyl groups to cross-link polymer chains and that shrinkage is anisotropic, measuring 17.89% in Y, 20.10% in Z, and 1.18% along the fibers in X. It also shown that, while thermo-oxidative stabilization can produce a viable preform, increased crosslinking does reduce permeability to evolved gases, leading to foaming and failure in pyrolysis.

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