PhD Oral Exam - Mohammad Azami, Electrical and Computer Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

NASA’s Artemis program targets sustained lunar presence, making in-space manufacturing (ISM) and in-situ resource utilization (ISRU) essential. Materials extrusion (MEX), notably fused filament fabrication (FFF), offers a low-mass, energy-efficient route for ISM. This thesis investigates FFF of space-grade PEEK composites filled with lunar regolith simulant (LRS), supported by modeling and a powder bed fusion (PBF) benchmark.

Using a FFF system, we compounded and printed 70/30 wt% PEEK/LRS (PEEK/LRS30) to study the feasibility of using LRS as a filler, with neat PEEK as baseline. LRS increased melt viscosity and porosity, yielding a ∼27% strength loss attributable to porosity. Fractography showed embrittlement and reduced elongation, and microstructural analysis confirmed uniform LRS dispersion with visible pores. LRS also improved interlayer bonding and reduced warping.

PEEK and LMS-1D LRS were melt-compounded (0–50 wt%), FFF-printed, and annealed at 300 °C. Characterization (density, thermal, tensile, microstructural/elemental) showed all filaments above 96% dense. As-printed porosity rose from below 1% (neat PEEK) to 7.5% at 50 wt% LRS. Regolith increased crystallinity (17.4% → 20.5%) and elastic modulus (6–41%), while reducing delamination/warping and improving dimensional accuracy and yield. Tensile strength fell from 107 to 90 MPa through 40 wt% LRS, then to ∼70 MPa at 50 wt%. Annealing improved density and stiffness up to 30 wt% LRS (diminishing thereafter). Process refinement cut defect size/frequency, raising PEEK/LRS30 tensile strength from 67.1 to 94.8 MPa. At 50 wt% strength and ductility declined more sharply, consistent with micrography-observed defect growth.

Finite element analysis (FEA) of defect-free printed composites matches the measured stiffness up to ∼40 wt% LRS. The divergence at 50 wt% aligns with higher porosity and weak inter-bead bonding. A defect-aware model that incorporates large crack-like discontinuities at layer boundaries derived from micrographs predicts a marked reduction in modulus and, together with mechanistic reasoning, explains the observed gap.

A PBF feasibility study is conducted on regolith simulant and a 20 wt% regolith–Invar 36 composite. FFF is simpler and lower in energy/cost, while PBF enables alternative densification and resilience at higher temperatures with greater on-site use, but poor flowability and weak laser–powder coupling in regolith feeds yield porous parts with frequent large defects. The results include lessons learned on the processability window for key parameters and print conditions.

This study strengthens the technological basis for AM in lunar conditions and accelerates ISM adoption.