Isovolumetric synthesis of chromium carbide for selective laser reaction sintering (SLRS)

Abstract

A novel synthesis approach for the in-situ formation of isovolumetric non-oxide ceramics was demonstrated for application with selective laser sintering additive manufacturing (SLS-AM) methods. By optimizing the ratio of a metal/metal-oxide precursors system, the conversion of a 86/14 molar Cr/Cr2O3 composite precursor film to Cr3C2 by gas-solid reaction with CH4 was investigated as a model reaction synthesis process. The composite precursor mixture was ball-milled to nano/micro-sizes to enhance gas-phase mass transport for conversion. The feedstock was deposited as a 250 μm thick film representative of a single SLS layer using a high-volume low-pressure spray deposition method. The effects of H2 and trace amounts of oxidizing gas impurities on the reaction thermodynamics of the conversion process and microstructural evolution were investigated. The gas-solid reactions were carried out at 950 °C with an isothermal dwell time of 0.1 h under atmospheres of Ar/H2/CH4 and Ar/CH4. The results suggest that expeditious and thorough conversion of the precursor to Cr3C2 was achieved (greater ≥90 wt% in Ar/CH4 and ≥95 wt% in Ar/H2/CH4). Phase characterization of sample cross-sections using SEM image contrast methods were used to assess apparent volumetric occupancy of the as-deposited Cr/Cr2O3 precursor film and the Cr3C2 product ceramic. Results indicate approximate fill fractions of 81.3% (SD = 1.78) and 78.9% (SD = 1.52) respectively, suggesting near isovolumetric conversion. Ultimately, the results in this model system demonstrate isovolumetric reaction syntheses techniques broadly applicable for in-situ non-oxide ceramic production via selective laser sintering-AM.