Graphene/SiC composites are widely used in the fields of filtration, biomedicine, catalysis, microwave absorption, energy storage and chemical sensing. 3D printing technology can realize the design of fine feature change and integrated molding complex geometric structure in the preparation of porous ceramic materials, especially the slurry direct writing process (DIW). The physical properties and other functions of composites strongly depend on the microstructure obtained during processing, so these phenomena must be systematically investigated to design and produce reliable materials, especially for 3D printing applications. In this paper, graphene/SiC matrix composites were prepared by DIW technology, and their printing accuracy, microstructure, pyrolysis behavior, mechanical properties and electrical properties were investigated.

SiC powder with an average particle size of 3.2 μm was used as a matrix of the composite. A graphene powder with average particle size of 8 μm, bulk density of 0.08-0.13 g/mL, thickness of 1-3 layers and purity of 99% was a reinforced phase. Polycarbosilane (PCS) with a molar mass of about 1500 g/mol was used as a precursor to prepare ceramic slurry. The diameter of extrusion nozzle was 0.55 mm, the layer thickness was 0.48 mm, and the printing speed was 6 mm/s, which was used for DIW. The morphology and distribution characteristics of graphene in the cross section of graphene/SiC_p composite extruded wire were characterized by scanning electron microscopy (SEM). The phase composition of graphene/SiC matrix composites after pyrolysis was determined by X-ray diffraction (XRD) with Cu K_α - ray (λ), the scanning rate of 10 °/min, and the scanning angle (2θ) of 20°-80°. The Raman spectra of graphene/SiC matrix composite samples were analyzed by Raman spectroscopy in a scanning range of 1000-2000 cm^-1, power of 10 MW, and laser wavelength of 532 nm. The resistance of single wire after pyrolysis was measured by inductance capacitance meter (LCR) and the conductivity was calculated. The tensile test was carried out in an electronic universal testing machine, the tensile rate was 0.1 mm/min, and the tensile strength was calculated. A universal testing machine was used to carry out compression test on the pyrolytic frame samples, and the displacement speed was 0.2 mm/min. The compressive strength was calculated.

The experimental results of single-layer printing and dot matrix structure printing show that all three slurries exhibit a good extrusion forming ability. In addition, increasing the graphene content in the slurry can suppress the spreading deformation of extruded filaments, reduce the line width, and make it closer to the extrusion diameter, which is helpful for high-precision printing. The SEM images of the extruded filament cross-section show that the graphene layers form a certain degree of orientation in the matrix. In the central region, there is a relatively random and irregular arrangement, and the orientation of graphene sheets is most pronounced in the edge region. At the central axis, the shear rate is rather low, the orientation torque is weak, and the graphene layers are randomly distributed mainly due to the non-uniform shear flow field experienced by the slurry in the extrusion port. In the edge region, the shear rate is extremely high, forcing the graphene layers to deflect and arrange along the streamline direction. The pyrolysis process forms a porous structure, and the graphene layers are well preserved. In the Raman spectra, the I_D/I_G value of 25G is the smallest, and the higher the graphene content, the lower the average defect density of graphene. The higher the graphene content, the higher the tensile strength of the extruded filament and the compressive strength of the lattice structure.

The DIW process using graphene/SiC_p/PCS composite slurry could achieve the preparation of lattice grid structure green bodies. The printing ability of the slurry improved with the increase of graphene content. In the DIW printing process, graphene layers were oriented and arranged along the extrusion direction in the extrusion filament, forming an orientation gradient structure from the center to the edge, and the higher the content, the more significant the orientation characteristics and gradient. Graphene improved the strength of composite materials via constructing gradient orientation distribution, enhancing interfacial bonding, and exerting crack bridging effect. Also, it significantly improved the conductivity of porous SiC based composite materials. At the graphene content of 25%, the tensile strength of the extruded filament and the compressive strength of the lattice structure could be the maximum values, and the electrical conductivity of a single filament reached 247.1 S/m, providing a feasible material system and technical path for the development of multifunctional lightweight ceramic materials for extreme environmental applications.