Additive Manufacturing of Technical Ceramics: Laser Sintering of Alumina and Silicon Carbide

Technical ceramics, like alumina (Al2O3) and silicon carbide (SiC), display a number of material properties which make them interesting candidates for various engineering applications. They are typically produced through a powder metallurgy (PM) process, consisting of (1) feedstock preparation, (2) shaping, (3) debinding and (4) densification, with possible additional machining operations. Shaping, the second step in the PM process, is conventionally done using techniques such as uniaxial pressing, injection moulding, extrusion or slip casting. However, additive manufacturing (AM), a rapidly emerging production technique for polymers and metals, is a promising alternative shaping process for technical ceramics. AM has the potential to shape prototypes or small series of components with intricate shapes at low lead times and consequently to be complementary to conventional ceramic shaping techniques while at the same time pushing the use of ceramics towards a range of new applications. The objective of this dissertation is to produce fully dense technical ceramics using a laser powder-bed fusion AM process as shaping technique. To that end, three different strategies were investigated. The first strategy is powder-based indirect laser sintering and focuses on silicon carbide. The feedstock material was prepared by using a thermally induced phase separation (TIPS) technique to produce SiC-polypropylene (PP) microspheres. These microspheres were further processed through indirect laser sintering, melting the polypropylene binder in order to shape SiC-PP green parts. The PP was subsequently debinded in an oven and the resulting porous SiC components were densified by liquid silicon infiltration in a vacuum furnace, yielding fully dense ceramics. However, the produced components contained only 35% of SiC. Furthermore, debinding lead to cracking and shrinkage of the components, resulting in a loss of geometrical accuracy and therefore a reduced net-shaping capability. The second strategy that was investigated is direct laser sintering of ceramic powders without the addition of a sacrificial polymer binder. This strategy focuses on carbide ceramics, specifically silicon carbide and boron carbide (B4C). The feedstock material was prepared by mechanically mixing SiC or B4C powder with silicon (Si) powder in a 60:40 volume ratio. The resulting powder blends were then laser sintered. The Si in the powder mix melted and re-solidified, binding the carbide particles together and yielding porous Si-SiC or Si-B4C components. These were post-processed by infiltration with a phenolic resin and subsequent pyrolysis of this resin to produce free carbon in the laser sintered component. Finally, liquid silicon infiltration was used as a densification technique, leading to fully dense Si-SiC or Si-B4C ceramics. Additionally, during densification, the liquid Si reacted with the free carbon introduced during post-processing to form a beta-SiC phase. The resulting ceramics had a low residual Si content, promising mechanical properties and compared favourably against a commercially available Si-SiC material. Furthermore, the net-shaping capability proved to be very high, since no shrinkage occurred during post-processing thanks to the absence of a sacrificial polymer binder during laser sintering. Finally, slurry-based indirect laser sintering was used as a third strategy, focusing on alumina ceramics. Feedstock material was prepared by means of a two-step milling and mixing process. First, Al2O3 powder was de-agglomerated with water and a dispersant, then the resulting blend was mixed with a poly-vinyl alcohol (PVA) binder. The resulting slurry was optimized using viscosity measurements. A conventional laser sintering machine was then refurbished in order to accommodate the use of a slurry feedstock. This was done by including a peristaltic pump for slurry supply, a deposition nozzle for homogeneous slurry spreading and a solvent removal system, the latter being either preheating or vacuum draining. The importance of solvent removal during laser sintering was underscored by the presence of extensive crack patterns in rapidly dried out layers. This was especially true in cases where the laser was used for solvent removal, but also when preheating was used to evaporate the solvent more slowly. In the end, laser sintering, debinding and solid-state sintering in a furnace resulted in closed-porosity alumina ceramics with densities over 97%.status: publishe