Additive manufacturing of 3D printed SiC composites: Strengthening and densification through surface modification and use of mineral binders

Dense and porous Silicon Carbide (SiC) ceramics and composites are used in a wide range of applications that require high thermal, mechanical, and electrical stability, excellent corrosion, and wear resistance. However, manufacturing of SiC through conventional powder metallurgy technique techniques is often challenging due. Due to the covalent bonding between Si and C, they have a high melting point. Hence high temperatures, pressures, and controlled atmospheres are required during sintering to manufacture SiC ceramic with good mechanical and thermal strength. Other techniques to manufacture SiC at relatively low temperatures involve thermal oxidation, pressureless sintering, and the addition of sintering additives. Some applications like biological scaffolds, ballistic armor, space mirror substrates, and ceramic filters involve complex geometries. Manufacturing of complex geometries through the conventional route involves machining or molding. Machining SiC is a challenge due to its extreme hardness and abrasiveness. Molding a pre-form utilizes polymer resin which can cause shrinkage to the final product. upon debinding and sintering. Hence, the additive manufacturing route is considered feasible for the manufacturing of SiC ceramics or composites. Additive manufacturing (AM) enables 3D printing of complex geometries from a CAD model. Multiple direct AM methods were realized for the printing of SiC such as selective laser sintering (SLS), selective laser melting (SLM), stereolithography (SL), direct ink writing (DIW), and binder jetting (BJ). Among these techniques, the binder jetting technique was found to be easier to manufacture complex geometries of SiC as it does not require, i) polymer additives that cause shrinkage of the part upon sintering and it doesn’t require, ii) high laser power to melt SiC, and iii) ceramic slurry, where the amount of ceramic used is less. Binder jetting also allows the mixing of different ceramic particles and additives that can help in the densification and strengthening of the printed part. In this work, the following areas are addressed: i) a route for densifying and strengthening the powder bed binder jet-printed SiC through the mixing of particles of different sizes, formation of siloxane bonding, secondary surface modification, and sintering, ii) strengthening and densification of SiC composites using mineral binders using powder metallurgy technique, and iii) realizing the properties of SiC-mineral binder composites for space mirror and thermal applications. Part (ii) of the project was preliminary work done in order to realize the outcomes of SiC-mineral binder composites in strengthening so that it can be adopted into additive manufacturing mentioned in part (i). Future work will involve the inclusion of SiC-mineral binders into the feedstock in a powder bed binder jet in order to reduce the voids between the interspace of SiC particles and to have a strong interfacial region comprising of mineral binders that can fuse the SiC particles together and densify the printed part. This eliminates the need for the post-processing techniques such as melt infiltration, polymer impregnation, or chemical vapor infiltration.
SiC ceramics are 3D printed into cylindrical discs in a powder bed binder jet using a water binder. In this method, SiC of an average particle size of 40 µm was surface activated with NaOH to form a silica gel layer at room temperature, to which, 30% of 2 µm and 600 nm SiC particles were added and mixed homogeneously through milling. The presence of OH- ions in silica gel, creates a repulsion between SiC particles which eliminates agglomeration of particles upon spreading. The mixing of coarse and fine particle sizes reduced the percentage porosity by 50%. The as-printed green part was heat treated to 650 °C for 5h to create siloxane bonding which provided an improved handling strength. The heat-treated parts were then impregnated in various concentrations of NaOH to create silica gel through secondary surface activation. SEM images showed that the impregnated samples had more silica nucleation droplets that gave rise to silica nanowires upon sintering at temperatures between 800 °C – 1000 °C. The silica nanowires are responsible for fusing the SiC particles and bridging the pores. The optimum NaOH concentration for secondary surface activation, sintering temperature, and dwell time were determined. A 100% increase in the strength of the SiC was obtained in the samples heat treated at 650 °C, impregnated in 20% NaOH, and sintered at 1000 °C for 24h. Moreover, the formation of nanowires under an oxygen environment proved that silica nanowires can be formed at a temperature as low as 800 °C, and in air, the discs are oxygen deprived which hinders the growth of silica nanowires. Hence the mechanism of the growth of nanowires was found to be similar to the solid-vapor phase deposition. Cordierite and spodumene are silicate minerals that are known for their excellent thermal properties namely nearly zero thermal expansion coefficient. SiC is a ceramic with excellent mechanical and thermal properties. SiC, Cordierite, and Spodumene are materials that are considered for space mirrors, mirror substrates, and high-temperature applications. However, the glass ceramic form of Cordierite and Spodumene are less considered for space applications due to their poor stiffness and fracture toughness. On the other hand, SiC is highly considered for such applications however manufacturing them is a challenge considering their high melting point and hardness. Hence, in this work, a combination of SiC and the mineral format of cordierite and spodumene is introduced as SiC-mineral binder composites. SiC-mineral binder composites are 80% SiC and 20% Cordierite or Spodumene minerals prepared through the powder metallurgy technique. SiC-mineral binder composites were found to have good mechanical and thermal properties and can be a promising candidate for space mirror applications. SiC-mineral binders were combined with 1% of NaOH, pressed at 250 MPa, and heat treated to 1200 °C for 8 h. The SEM-EDX analysis showed a strong interfacial region of cordierite or spodumene fusing the SiC particles together. The fracture mechanism was found to be transgranular which is due to the strong interfacial bond that was created by the atomic diffusion of Si and Al at the grain boundary of SiC and the mineral interface. The characterization involves the comparison of SiC-mineral binders to the control SiC-cristobalite without mineral binders. The phase analysis from XRD showed the presence of cordierite, spodumene, and cristobalite phases. A transformation of β-SiC to α-SiC was also observed. A slight shift in the d-spacing, the lattice constants, and crystallite size was observed as a result of a solid solution of phases. The density and porosity of these composites were measured using Archimedes and mercury porosimetry. Further pore size analyses were done using SEM and ImageJ analysis. The results showed that the introduction of mineral binders reduced the pore size and the porosity percentage. The compressive strength of the SiC-Cordierite and SiC-Spodumene was 282.57 MPa and 184.58 MPa which was much higher than the control SiC-Cris, 97.45 MPa. The average compressive strength of SiC-Cord was three times higher than control SiC-Cris (p < 9.7 x 10-7) and two times higher than that of SiC-Spod (p <0.003). Moreover, the average compressive strength of the SiC-Spod was significantly higher than that of the control SiC-Cris (p <9.8 x10-7). Elastic modulus was found using the nanoindentation technique and was 380.54 GPa and 341.04 GPa for SiC-Cord and SiC-Spod composites. Thermal shock resistance is an important factor for materials to qualify for space applications. SiC-mineral composites showed excellent thermal shock resistance and dimensional stability when quenched from 1200 °C to room temperature. A thermal expansion coefficient of 3 x 10-6 /K was obtained for both SiC-Cord and SiC-Spod composites. The SiC-mineral composites were polished to a mirror finish and the surface roughness of areas comprised of SiC particles along with the mineral binder without pores measured using atomic force microscopy was 20.89 nm. The mean roughness of the SiC microconstituent in the SiC-Cord composite was found to be 2.37 ± 0.28 nm. Owing to these excellent thermos-mechanical properties, SiC-mineral binder composites are promising candidates for space mirror applications, mirror substrates, substrates for high-temperature devices, and catalytic converters. Also, porous SiC-mineral binder composites can be used as gas/fuel filters for automobile industries.