Selective laser sintering in reactive atmospheres: Towards in-situ synthesis of net-shaped carbide and nitride ceramics

Abstract

Selective laser reaction sintering techniques (SLRS) were investigated to produce near net-shape non-oxide ceramic materials that were previously incompatible with powder-bed fusion additive manufacturing (AM) methodologies. Specifically, reaction-bonded layers of carbide and nitride materials (Cr3C2 and CrN) were fabricated using SLRS from a model system of Cr and Cr2O3 precursor materials. Varied particle sizes (<5–45 µm) were simultaneously chemically-converted and reaction bonded into layers during selective laser processing in 100 vol% CH4 or NH3 gas. For each precursor material, factors such as particle size, chemistry, optical properties, and microstructure were investigated using quantitative phase characterization and microstructural imaging to understand important aspects of carbide and nitride phase formation within an AM context. SLRS processing of metal or metal oxide precursor systems produced yields up to 100 wt% total for carbide formation and 100 wt% total for nitrides during reactive processing. However, when either metal or metal oxide precursor was processed by itself, the simultaneous synthesis and reaction bonding approach resulted in volumetric changes during in-situ gas-solid conversion that adversely affected layer microstructure. To circumvent the internal stresses that accompanied volumetric changes induced from conversion of the metal (which expands during conversion) and the metal oxide (which contracts), we demonstrate an alternative method for the production of near-net shape carbide and nitride materials for AM. By combining optimized ratios of metal/metal oxide precursors for processing in CH4 or NH3 reactant gas, the novel precursor formulations produced undistorted, crack-free refractory ceramic layers with sub-millimeter spatial resolution and viable layer thickness for powder bed fusion-AM. The results not only demonstrate the formation of near net-shape carbides and nitrides using AM-compatible techniques but suggest that transition metal carbide and nitrides may be compatible with AM processing even though they cannot be processed using more traditional laser-assisted AM approaches.