Directed Energy Deposition of Additively Grown Carbon Fibers from Various Hydrocarbon Precursors

Directed Energy Deposition (DED) by Laser Chemical Vapor Deposition (LCVD) is an additive processing technique in which freestanding fibers can be grown under dynamic conditions that drive the system far from equilibrium conditions. In this study the relationships between deposited carbon fiber microstructure, mechanical properties, and processing conditions were compared using two different hydrocarbon precursor gases (methane (CH₄) vs. ethylene (C₂H₄)). Each set of fibers were grown at 2, 4 and 6 bar at rates determined by the power used to initiate the growth. Depending on the process conditions, the fibers exhibit drastic changes in morphology and mechanical behavior. Whether a core shell or nodular fiber morphology nucleated and grew was largely found to be dependent upon the combination of pressure and growth speed. These transitions were easily initiated in our processing space for ethylene as compared to methane. The methane source resulted in approximately 1.5x lower growth speeds for equivalent pressures to ethylene even when it was at higher power settings. The methane formed fibers also exhibited a near doubling of their diameter as compared to the ethylene source. In all cases, the tensile strength at failure were low (< 300 MPa); however, a Weibull analysis reveals a high modulus suggesting fewer or more homogenously distributed flaws. The graphitic bond structure for the fibers has been characterized and linked to the processing outcomes of these fiber morphologies and strengths.