Feasibility of rapid debinding and sintering of additively manufactured ceramics

In recent years, additive manufacturing (AM) of ceramics has significantly advanced in terms of the range of equipment available, printing resolution and productivity. Among all the available AM technologies developed for ceramics, material-extrusion (MEX) based techniques outperform the others in several aspects and are widely used for rapid prototyping of ceramics. This was made possible due to the availability of low-cost open-source printers and the simplicity of the overall process. Unlike, vat photopolymerization, where differences in the refractive index between the ceramic particles and the resin increase the scattering or in SLS, where the absorption of the laser power is dependent on the optical properties of the starting powder, MEX-based AM techniques operate independently of the optical properties of the ceramic powder. However, the bottleneck of the complete fabrication process is denoted by the slow debinding process (up to 1 °C/min) and the subsequent sintering process required to obtain a dense ceramic component, making it an energy-intensive process. Thus, there is a real need to develop feedstocks that are compatible with rapid thermal treatments and to explore energy-efficient rapid sintering approaches to accomplish this goal. In this aspect, this PhD thesis addresses two research fields that have been only marginally communicated with each other to date: non-conventional sintering and additive manufacturing. The main goal of this thesis was to validate the feasibility of coupling rapid sintering approaches with material-extrusion (MEX)-based techniques, including direct ink writing (DIW) and fused filament fabrication (FFF). To validate this, both commercial 3 mol.% yttria stabilized zirconia (3YSZ) and laboratory-made filaments of barium titanate (BaTiO3) and alumina (Al2O3) were used. Additionally, inks with suitable viscoelastic properties for DIW were developed in-house using commercial BaTiO3 and Al2O3 powders. The printed samples with optimized processing parameters could be debinded and sintered to almost full densities without any defects in a single-step process using different rapid sintering techniques such as ultra-fast high-temperature sintering (UHS), fast firing (FF), and pressureless-spark plasma sintering (P-SPS) lasting around few seconds to minutes. The printed samples behave very differently when processed using different processing conditions. For instance, the FFF samples only work when chemical debinding is performed prior to the thermal debinding process. Additionally, the sintering process is strongly affected by the sintering atmosphere. For DIW, the samples printed with BaTiO3 could resist such rapid heating approaches but the undesired phase transformation associated when sintered in argon, makes such a process unsuitable for these compositions. Especially for UHS and P-SPS, the presence of an inert atmosphere is necessary to prevent the decomposition of the graphite felt or the die. This thesis therefore provides a first of kind of work reporting ultra-rapid processing (debinding and sintering) of additively manufactured ceramics, thereby reducing the overall processing time by 99%. The rapid debinding/sintering approach developed in this thesis could be potentially transferred to other optimized ceramic feedstocks as well.